Clamping on Schallbert's Blog

Vacuum clamping

Schallbert — Tue, 02 Apr 2024

As I mentioned in the Maker Faire Ruhr 2024 article, I had major problems clamping my workpieces for the production of spirographs.

In this article, I would like to go into this in more detail, analyze the causes and finally present a few possible solutions.

What I can anticipate: All problems can be solved with knowledge of the causes and a little time, planning and material expenditure.

The problem

As usual, I had defined the path planning for the spirographs in such a way that I first had the inner areas (pockets, parts, holes) milled and then the outer areas. Cut-outs and chamfers were to be produced at the very end. I also planned to machine the parts on both sides so that the backs of the gears would also have broken edges and smooth transitions.

Vacuum loss

In my first attempt, I proceeded as I had already successfully done in the spirograph project with the material acrylic glass: The back is machined first, but not milled all the way through. After turning the workpiece, only the chamfer cutter should be used to cut out and break the edges at the same time.

This worked perfectly well for the first clamping.

Your browser does not support the video tag.

Back machining of the spirographs.

With the second one, however, so much negative pressure was lost when the surface was pierced that the material detached from the vacuum table and took on its original, slight curvature. So I had to stop the milling process and think of another strategy to hold it down.

At this point, I couldn’t really explain why so much more pressure was being lost than during production in acrylic glass.

Cling film

I tried to counteract the pressure loss with cling film. This resulted in several consequential problems: If the cutter passes through the film, it is caught by the cutter when milling closed contours and wrapped around it at lightning speed. The cutter no longer cuts, but only rubs against the material, which very quickly leads to heat marks on the wood.

But even if I attach the cling film in advance: As I cut out parts, the local negative pressure on the part is already so small when it first enters the groove already present on the underside that it can hardly be held in place.

Twisting of the workpiece

For the second attempt, I adapted the strategy: The end mill would mill through the workpiece, so I didn’t need a second set-up. I wanted to optionally realize the edge breaking on the back side later via a holder clamped on the table, into which I would be able to insert the parts.

While cutting out the parts still worked without any problems (again, milling the “inner” parts first), I ran into major problems when applying the chamfer to the front. The gears began to twist and the template with the ring gear could not be held down at all.

Too little time

I therefore had to resort to using holders specially made for the workpieces when machining the front side. Their design and manufacture took time that I had not planned for at this point.

Before the trade fair, I wasn’t able to do an in-depth analysis - a quick solution had to be found.

The causes

With a few weeks’ hindsight, I can now say that several factors led to the problems. A difficult geometry, unfavorable material properties and certain peculiarities of the vacuum table design all came together. But let’s get to the bottom of it all here.

Workpiece

The raw material is medium density fiberboard of 10mm thickness. It is covered on one side with 0.75mm thick olive wood veneer. This material is inherently problematic for processing on a vacuum table:

Due to veneering and storage, the boards are slightly curved¹, so they want to stand out from my vacuum table.

MDF is permeable to air, so the vacuum pump has to deal with larger leakage currents than usual (I only get 300mBar when clamping the panel instead of the usual 800mBar without leakage currents). The low material thickness intensifies this effect.
MDF is not only permeable to air vertically to the panel plane, but also horizontally. This greatly reduces the holding forces in the vicinity of milled grooves, as air can now also penetrate the material laterally, thus additionally increasing the leakage currents.

The video linked here Youtube: Displacement force measurements by Anton CNC clearly shows how different materials behave on a vacuum table with a porous surface. Also highly recommended on this topic is this video YouTube: Determining holding force during machining by vakuumtischDE to get an impression of the load the workpiece can be under during the milling process.

Geometry

My design for the spirographs is not particularly easy to hold down:

The template with ring gear has thin walls of only about 2cm thickness. They are thickest on the left and right sides, but have a large recess in the middle through which air can penetrate the material in all directions, thus reducing the holding forces.
The gear wheel has holes to accommodate pins. The contact pressure is reduced around the holes.
The teeth of the ring gear and gearwheel are so exposed that they can make virtually no contribution to the holding force due to cross currents of air in the material. At the same time, however, the tooth heads provide a large lever for twisting or shifting the workpiece: an unfavorable pairing.

If I take this as a basis for practice, only the vacuum holes inside the circle in the image above are capable of generating any significant clamping forces. However, these are again close to the center and therefore offer little resistance to twisting of the workpiece. In addition, the gear wheel here has several holes and therefore additional “leaks”.

Milling forces

The forces introduced into the material by the tool (the milling cutter) always act in the XY direction and, in the case of V-cut milling cutters and end mills with spiral/twist, also in the Z direction.

Forces due to feed: The faster the material is moved through, the stronger the forces in the XY direction act on the material in the direction of travel.
Forces due to cutter rotation (conventional or climb-milling): If an end mill without twist is used, forces are generated orthogonally in the plane of the traverse direction. So if I move in the positive X-direction, the milling cutter will also exert a force in the Y-direction, which is introduced by its own rotation: In conventional milling operation, the cutter wants to avoid the material, so it is deflected towards Y+ (as clearly shown in this article). In the opposite direction, the deflection direction is inverse and the cutter wants to deflect towards Y-.
Forces due to cutter geometry: If the cutter has a spiral or is angled in its geometry, additional forces act in the Z direction. With Vcut and downcut cutters, the material is pressed onto the machine bed, whereas with upcut spiral cutters, the chip is ejected as desired. However, the workpiece also experiences a force that can lift it off the machine bed in extreme cases.
If milling is carried out in the full groove (i.e. the milling cutter is engaged over its entire diameter instead of moving along the edge of the workpiece), the forces transverse to the milling direction increase even more: In the image, it moves in the opposite direction at the upper edge of the groove, which pushes the milling cutter “away from the material”, i.e. downwards in the image. At the same time, it moves in synchronization at the lower edge of the groove, which pulls it “into the material” - also downwards.

Vacuum clamping system

As described in the articles about vacuum pumps and vacuum tables, I have a dry-running rotary vane pump connected to a perforated grid vacuum table. When processing porous materials, this combination is just as problematic as grid vacuum tables with small-volume pumps:

My hole-grid vacuum table has blind holes with a vacuum channel of 0.5mm diameter in their center. This constriction relieves the pump when holes are uncovered, as it does not have to provide the full air flow that the 5mm bore allows. However, the hole can build up less negative pressure due to its limited air flow in combination with MDF.
Due to the high leakage currents, my vacuum pump is at its limit with its volume flow. Other pump types are much better suited here (see radial blower, side channel compressor).
A vacuum table generally only clamps downwards (in the Z direction). However, forces in the XY direction must also be absorbed to prevent the workpiece from shifting or rotating. These are coupled to the vacuum on the workpiece via the coefficient of static friction µ (and can be influenced by material pairing and surface properties), but if the latter is too low, the workpiece is not held reliably.

Although the vacuum channels are located in a 10mm grid, the distance relevant for the leakage flow is only 5mm. This is because the edges of the blind holes (each 5mm in diameter) are at this distance from each other, so that the air flow has to travel a shorter distance across the material than through it.

Comparison with the resistance network

The actual clamping force can be easily estimated by imagining the pump, vacuum table and workpiece as an electrical network. I have prepared a model for this and present it below.

Resistors: Vacuum table, workpiece

The suction holes of the vacuum table represent resistors for the air flow, which are connected in parallel. The workpiece is again connected in series as a resistor, which is connected with its other pole to “ground”, i.e. the ambient air pressure.

Vacuum manifold = switch, pressure gauge = voltmeter

The pressure gauge in the suction channel can be seen as a voltage meter. I take the vacuum distributor with its valves for switching the vacuum sections as an electrical switching device. If the switch is closed, it establishes the connection between pump and vacuum table. If it is open, it blocks the air flow.

Vacuum pump = voltage source

In this picture, the pump represents a voltage source, which in my case can provide a maximum of U = 8V corresponding to -800mBar. Due to the internal resistance of 0.2Ohm present in real voltage sources, it can generate a short-circuit current of I = 40A, analogous to the 40m³/h flow rate. This comparison is of course not physically correct, because I would have to work with mass flows instead of volume flows. In addition, the flow rate is not linearly related to the pressure everywhere in the characteristic diagram and the resistance values I have assumed are only reference values. In my opinion, it is nevertheless suitable for modeling the situation under steady-state conditions.

With workpiece materials such as acrylic glass or metal, the air flow is close to zero, so their “resistance” is extremely high. This causes most of the tension (negative pressure) to drop here, so that the workpiece is firmly clamped.

When using airtight materials, all is well with the world: the pressure gauge correctly indicates the pressure on the material when the workpiece is still unprocessed. Even in the case of milled holes, only the resistance of the material “in series” to the affected suction holes is set to zero. This means that the negative pressure and therefore the holding force on the part itself is hardly reduced.

If you look closely, you can see that the pressure gauge measures the pressure difference upstream of the vacuum manifold. Its value is therefore only meaningful for the clamping force if the workpiece has a very high flow resistance.

Special case of porous materials

The situation is completely different with porous materials such as MDF:

Here, the contact resistance of the material is only just an order of magnitude higher than that of the vacuum holes. I determined the values through tests on the vacuum table. They are based on pressure measurements, calculations of the workpiece surface, and the number of active suction holes covered by the material. Two effects should be particularly emphasized here, which made holding down even more difficult for me:

Pay attention to the resistances in the material in parallel to the contact resistance in the Z direction. They are caused by the fact that the air can now also move through the material “between the holes”.
On the right-hand side of the picture you can see what happens when the workpiece protrudes into an inactive vacuum area: The suction holes are connected to each other with virtually no resistance by means of vacuum channels in the table. As the workpiece allows “transverse air flow” in the XY direction, significantly less resistance has to be overcome by connecting the suction holes than for penetration in the Z direction. This means that only R29 and R22||R21 are still in series. If the workpiece covers several inactive hole fields, R22 approaches 0.

As a result, the resistances upstream of R19, namely R18||(R29+(R21||R22)), become so small that the pump practically only draws ambient air here and can hardly apply any clamping force at this point. The cross air flow also affects the pressure drop of R17, R15 via R25.

Now the small suction holes act as voltage dividers, so that a not inconsiderable part of the negative pressure “sticks” to them.

This means that the pressure displayed on the pressure gauge no longer corresponds to the conditions on the workpiece: Although I still get -200mBar displayed here, the clamping force is reduced precisely at the most sensitive points for holding down

at the edge of the workpiece
in areas adjacent to those where the workpiece is not actively sucked in
as well as at every through-milling and a certain radius

to almost zero.

For all other areas, Kirchhoff’s laws apply: A voltage divider is formed consisting of the pump’s internal resistance, the vacuum system and the penetration resistance of the workpiece. Only the voltage still on the workpiece determines the hold-down force. It is - I repeat myself now - possibly much lower than a glance at the pressure gauge would suggest.

Solutions

Quite a lot of theory for the banal realization that the workpiece is twisted on the machine bed, isn’t it? I have the following solutions in my quiver for such a problem:

Pump / vacuum table

When working with porous materials, it is important that the table itself has a low flow resistance. This is the only way to ensure that the majority of the negative pressure falls on the workpiece, resulting in a higher clamping force. According to the motto “a lot helps a lot”, a pump with a higher flow rate, a table with larger suction openings and a vacuum distributor with larger hose diameters would be appropriate here. However, this is out of question for me.

Adhesion agent

I could try using a coarser-meshed rubber mat. With a 20mm hole pattern, the resistance for the cross air flow is tripled, so that the outer areas are no longer affected by the pressure loss to such an extent. Such a mat also has more rubber surface area, which increases static friction.

Adhesive tape instead of cling film on the material surface solves the problem of catching in the cutter and closes the pores of the material locally, which can improve the holding force in critical areas.

Of course, conventional methods such as applying double-sided adhesive tape or screwing down the workpiece can also be used on a vacuum table. But I don’t do that - because then I would have to admit to myself that my vacuum table is not suitable for all cases.

Fixture and taps

This solution aims to prevent the workpiece from twisting and shifting. The vacuum table is only used here to hold down the workpiece, while a suitably manufactured holder absorbs forces in the ‘XY direction’. The picture shows holders for the template and the gear wheel, which fulfill this task. They are attached to the machine bed and require a precise zero point in order to be able to break the workpieces edges without misalignment.

I use a clearance fit with 0.1mm between the components. If the clearance is smaller, I need too much force to insert. If I use larger values, the workpieces can be twisted too much.

With this variant, I can straighten warped workpieces, continue to produce very fine engravings and still make sure that nothing slips.

The disadvantages are time and material required for planning and producing individual fixtures or templates.

Modification of the workpiece

This is obvious: I can change the properties of the workpiece so that it becomes impermeable to air. For example, I can cover the underside with a film to improve the static friction.

If, like me, you don’t like using plastic: I have found that baking paper / cling film made from unbleached cellulose hardly lets any air through. I’ll soon be trying out covering materials or covering areas affected by milling with it instead of plastic film.

If such a problem occurs regularly, grid vacuum tables with a sealing cord are an excellent solution: Here, a linear seal is used to seal the outer edges of the workpiece against the ambient air pressure so that the edges of the workpiece have no chance of curving upwards. With my perforated grid table, however, the entire surface of the rubber mat acts as a seal, so that the curvature is in the form of a contact line on the mat. However, lines do not have a surface area, and my table can only hold down through this. Incidentally, this is also the reason why flexible materials such as films can be pulled off the vacuum table so easily. ↩︎

Vacuum Pump

Schallbert — Fri, 28 Oct 2022

What is it?

A vacuum pump is an appliance that is able to evacuate a vessel, i.e., to reduce the amount of gas atoms within a container. It does not seem to be possible to shrink the number of atoms within to 0, but of course this is by far not required for our application in which a so-called technical coarse vacuum is sufficient.

There are different types of vacuum pumps with specific properties that I’ll try to shed a light on later.

How does it clamp workpieces?

For clamping with the use of a vacuum table (see my related post), we’re only interested in the pressure difference the vacuum pump generates versus ambient pressure which is (103000Pa = 10.3N/cm² = 1kg/cm²). This is the maximum clamping force a vacuum table can generate in theory.

Pump capacity vs. vacuum pressure

The key parameters of any vacuum pump are the minimum absolute pressure they are able to draw in hPa (at low or even zero throughput) and the volume of air they’re able to put through within a given time in m³/h (at low or zero pressure difference).

This is comparable to the electrical world, where open circuit voltage (pressure difference) and short circuit current (throughput) e.g. in photovoltaic cells are given parameters to calculate an optimal point of operation.

Here, you can add more cells to either get more current or more voltage from the system by connecting them together in parallel or in series. For vacuum pumps, it’s often more practical to choose a more powerful motor that drives the pump than using series/parallel operation. But still, there’s a trade-off to make. Certain pump types are very well suited to generate lots of throughput, but then are lacking vacuum pressure and vice versa, pumps that have very good vacuum ratings often have considerably less throughput than other pump types with the same power level.

If you want both, prepare to spend a lot of money. So again, it all comes down to what you need for your application.

Pump characteristics

Max. Volume and max. pressure difference can be entered as data points in a XY-diagram, and a line can be drawn between these two points which gives an approximation of overall pump behavior. While the position of these points largely depend on the pump’s motor power, the line’s slope is determined by pump type and design.

Below find a comparison of these different pump types. For this example, I arbitrarily chose a motor power of 1.5kW and selected some pumps with that rating of which I could find datasheets online.

Of course this approximation is very rough and holds true for high-throughput pumps better than for deep vacuum pump types. For the latter, the function looks more like a hyperbolic cosecant (see image below) than a linear graph due to their lower tolerance for leakage air.

We can use the pump’s characteristic curve the following way: When we have a given air volume demanded by our application (like a vacuum table), we can read the reachable pressure difference for this specific pump. This way, you can pre-select a pump type depending on your requirements.

Pump types

There are a multitude of pump types in the industry. They have different principles of operation and - of course - are suited for different applications. I’ll concentrate on the four types that are most often used for machining.

Radial blower (High Throughput)
Sidechannel compressor (High Throughput)
Piston pump (Deep vacuum)
rotary vane pump (Deep vacuum)

Radial blower

Radial blowers accelerate air between their blades. Air enters the blower at the center of the fan which creates a lower pressure here than in the surrounding air. Then, it is pressed towards the outside due to centrifugal forces as the fan rotates, and is exhausted at the perimeter.

Radial blowers are used for many applications like vacuum cleaners, inflatable castle blowers, forced air delivery for combustion processes, ventilation systems etc. They are simple, come cheap, and are well suited for applications that require very high throughput. The downsides are high noise levels and low pressure differences relative to most other compressor types. However, if you’re working with large surface sheets or porous material, the radial blower e.g. of a forced-air cooled vacuum cleaner motor might be just the right thing for your application.

Sidechannel compressor

Sidechannel compressors combine high capacity with medium pressure difference. They are built single or multi-stage (series circuit) and thus can favour higher capacity or pressure difference according to your needs. Like radial blowers, they interact with the medium only and thus do not have part to part friction that would require frequent maintenance. Like radial blowers, they tolerate vapours and can even be used in dusty environments.

If you want the sidechannel compressor to provide high pressure differences, you’ll have to buy very powerful (thus expensive) drives as their characteristic curves are shallow. Their noise can be efficiently muffled, but they often carry a higher price tag than comparable radial blowers.

In this list, they are “the jack of all trades”. The vacuum they provide can be upgraded to high-enough levels for most applications that don’t require extremely strong clamping and their price is moderate. Cost of operation is determined by the electrical power they consume.

I found this in-depth description of the sidechannel compressor helpful: GutmbH.

Piston pump

Piston pumps are commonly used in compressors and vacuum pumps alike. They provide deep vacuum levels at a low capacity, don’t use much space in your shop, and are reasonably priced.

On the other hand, their output stream is pulsating and they tend to be noisy. They also cannot handle liquids or vapours and have higher requirements on maintenance than frictionless pumps like sidechannel compressors or radial blowers. I did not find so many notes about their usage for vacuum tables, but that does not mean they’re not suited for this specific application.

Rotary vane pump

Rotary vane pumps come in two classes again: Dry-running and oil-lubricated. While dry-running pumps have similar pressure data at higher capacity than piston pumps with the same power rating, oil-lubricated rotary vane pumps can provide a much deeper vacuum level and even higher capacity with the same power rating. On the downside, they commonly need oil warmup and de-gasing cooldown runs, and have a higher maintenance cost due to their oil system.

Their principle of operation: A motor drives a slotted dish mounted excentrically within the vacuum chamber. In the slots, vanes are placed that can move freely. As the pump’s motor accelerates, the vanes are pressed against the vacuum chamber’s walls, sealing it against the other cavities. This way, compressible media can be transported only.

Rotary vane pumps deliver a continuous flow at low noise levels. The vanes are expendable parts that need replacement from time to time, which is especially true for the dry-running specimen. If you just compare capacity, they usually cost twice to three times as much as sidechannel compressors.

You should get one of theese if you often mill small workpieces at high lateral forces, e.g. mild steel. In this scenario, you can even keep your costs at bay when you’re not cutting through as in that case, leakage air is no issue of yours so you can choose a very low pump capacity.

If you’d like more detail, please find this well-made page (in German) from GutmbH.

Let the workpiece determine your pump

OK, so you chose the type and size of vacuum table you require for your jobs. Great! Now all you still need is a vacuum source and a way to feed the vacuum into your table.

As you can see in the figure, the downforce you get highly depends on the surface area of your workpiece. The smaller your workpiece is, the more pressure difference you require to maintain the same force - with the atmospheric pressure being one limit and the absolute vacuum the other. This makes vacuum tables both a very homogenuous but “weak” clamping method.

Now, what you actually want is to protect your workpiece both from being lifted off the of the machine bed by the tool, at the same time avoiding parallel shifts and rotation of your workpiece while machining. So we have to calculate the downfore created. In a second step, we want to convert that downforce into lateral forces that have to be higher than the maximum lateral force applied by your tool so your workpiece keeps its position.

Downforce

The clamping force to your machine bed is easy to calculate:

$$f_{clamp}= F_N = \Delta p A + F_g $$

It is measured in Newton and determined by the pressure difference at the workpiece top/bottom its surface and its weight that gravity forces downwards.

The value you require heavily depends on tool size and geometry, workpiece material, and feed/speed. You’ll require a lot more downforce for a scenario in which you’re milling small workpieces of sturdy materials like HPL with large tool spiral angles and tool diameters at high feeds and low spindle RPM than for cutting plywood sheets with small-diameter straight flute bits at high RPM and low feed rates.

Lateral force

To do the translation of downforce into lateral forces, we’ll introduce the coefficient of static friction $µ_s$.

Let’s assume you use a sheet of rubber between a metal workpiece and your vacuum table. We further assume a coefficient of static friction µs = 0.7 that I found online which seems really conservative for multiple reasons: First, it doesn’t take surface roughness of both rubber and metal into account, and, more importantly, we’re pressing the workpiece into the rubber which yields a form-fit bonding that increases breakaway force a lot.

Anyways, all you need to do here is to take your downforce calculated earlier and multiply it with this factor:

$$F_s = F_N µ_s$$

As you can see, there are two things you can manipulate with the same linear weighing: The material pairing of workpiece surface and machine table determine the coefficient, and the downforce created by your pump.

Rotation

This calculation becomes a bit more tricky. Interestingly, you’ll require less force to rotate a workpiece out of position than to parallel-shift it away. This is why:

When you rotate, say, your mobile phone on your desk around its center (touching it at its top-right corner), not all imaginable points on that phone have to move the same distance. Points closer to the center of rotation will have to move a shorter distance for the same angle of rotation than points at the tip of the phone. For a parallel shift, however, every imaginary point had to move the same distance.

This pheonomenon seems independent of the object’s shape once you compare travel of the outermost point of that object in relation to its center of rotation.

For vacuum clamping, worst-case workpiece and job combination is long, sleek workpieces that need a slot to be milled orthogonally to its longest dimension at the outer edge, creating the highest torque that might lead to rotation combined with little surface available for vacuum clamping.

Best-case of course are round or square workpieces where you cut close to their geometrical center.

Which pump should you buy?

Ok, let’s sum it up:

It’s always a combination of workpiece material (easy/hard to mill, low/high static coefficient of friction), minimum workpiece size (the bigger, the easier it becomes to generate high clamping forces), leakage air tolerance (porous surfaces, cutting all the way through), vacuum table type and size, and milling parameters that determine which pump to buy.

You should collect some experience before buying a vacuum clamping solution: What is your typical workpiece size and material? Do you often do cutouts? Do you require quick execution that exposes your parts to high forces?

Some recommended combinations

Aluminium, very small workpieces, not cutting through: Low capacity piston or rotary vane pump, grid vacuum table
Wood and MDF, large workpieces, cutting through a lot: radial blower or single stage sidechain compressor, porous surface table
HPL, small workpieces, cutting through: High capacity rotary vane pump or dual stage sidechain compressor, hole grid table, optimized CAM strategies for “late cut-throughs” at low depth increment

What pump I chose

I ordered a relatively large table (730 x 655 mm) with 0.5mm holes arranged in a 10mm grid. As I’m still testing a lot of different materials for prototyping purposes, my workpieces never look alike or have similar sizes. That’s why I had to swallow the pill of spending a lot of money on a pretty capable pump that would both do high throughput for bigger wooden sheets that I’d create cut-throughs in and deep vacuum levels for small aluminium workpieces.

Rotary vane pump comparison (Image courtesy of Stritzelberger GmbH) ^[1]

The pump I chose is a dry-running vane-type vacuum pump with a capacity of 40m³/h and a maximum pressure difference of 880mBar. Its air leakage vs. pressure diagram should look similar to the marked characteristic in the plot above. It weighs approximately 40kg and uses a 3-phase induction motor with a nominal power of 1.3kW.

I am using a version where the motor is operated on a single-phase with help of a capacitor due to the fact that I do not have a three-phase outlet available. I also added a motor circuit breaker to help protect the machine against overload and overcurrent.

Is it loud?

It depends on how you define loud, of course. You can compare its 68dB(A) noise level - measured at a distance of 1 meter - with a typical vacuum cleaner, just that it sounds pretty different. When the CNC machine operates, e.g. running through material with a fairly big endmill, it is still quiet and you won’t realize the pump is on.

I even made a short video for you to view and hear for yourself:

Rotary Vane Vacuum Pump - demonstration↗

CNC Vacuum Table

Schallbert — Fri, 21 Oct 2022

Let’s assume you clicked this article on purpose and didn’t just “surf the internet”. So you already know what a vacuum table is (If not, continue reading here). Its principle of operation is depicted in the figure for your reference. You probably also know there are multiple types of tables serving different purposes. And you maybe even know that you want one.

Great! This article both describes my journey towards acquiring this piece of tech and also will try to help you select the right one. Let’s start with a little interview.

Why do you want a vacuum table?

This is the most important question.

🤩 Do you want it because it’s expensive and looks great?
😪 Or because you’re tired of re-adjusting clamps?
😖 Or because you broke a lot of endmills running through the former by accident?
😨 Or rather because you experienced that workpieces tend to learn how to fly when the adhesive force of your tape is just not high enough?
😎 Perhaps because you`re going professional and you want to cut preparation and production time?
😕 Or maybe because the work results are not as good as they could (or should) be?

My Reasons: Quality and Speed

I’d go for the latter. I have the following issues I hope a vacuum table can fix:

When engraving coated materials (Dibond, HPL etc.), slight Z0 surface height differences ruin the work, especially on bigger workpieces
Cutouts from a bigger sheet have to have taps to keep them from slinging away. These taps are cut away later but are still visible on the end product.
When I don’t want to use taps, I have to leave an onion skin at the lower layer. With wood, this skin has to be thicker and I need a second job for chamfering which adds time expenditure.
Some sheets (especially wood) are concave. Thus, when trying to go all the way through, some parts are fine and others are not fully cut.
My machine suffers from vibrations, especially when using 2-flute wood roughers and high Z+ depth per pass at aggressive feeds. This might be because I use just a couple of clamps to save time and to keep travel paths clear.

Which type of table do you need?

Ok, you know why you want one, now we should find out which type is the correct one for your jobs. Note that vacuum tables are most suitable when you’re milling not too flexible sheet material as the table’s clamping force depends on the surface that is in touch with it.

There are three families of vacuum tables:

Grid tables
Hole grid tables
Porous surface tables

Grid tables

A grid vacuum table (Image courtesy of Stritzelberger GmbH) ^[1]

Grid tables have a single (and often centrally placed) suction hole. Their vaccuum area is defined by a (foam) rubber band that can be placed on the grid within the outline of the workpiece. As the rubber band is compressed by the workpiece’s downforce created by the vacuum, the seal is airtight and just very few leakage air has to be evacuated. That’s why they have low requirements on the vacuum pump’s required capacity. They can also be used to clamp materials with rough surfaces due to the rubber band, e.g. saw-cut wood. Grid tables’ designs are often simple and thus affordable.

Altough they really shine here, there are two possibly vital disadvantages: First, when you’re doing singletons, you’ll have to place the rubber band anew for each workpiece. So your XY Zero changes from part to part as most often the workpiece has to be placed centrally on the table. More sophisticated grid tables may have multiple suction holes, and the active one can be selected by the user. They may even offer fences and excenter stops to reduce set-up time. Still, you might not be able to cut any form you like as your limit is the grid.

Second and more important, you run into issues once your part has to be cut “through”. This will immediately make the vacuum collapse and the pump will be overwhelmed with leakage unless you take precautions (build adapter plates or underlay fences etc.)

In short:

Grid tables work very well if you don’t cut through your workpieces.
Grid tables have lower requirements on the vacuum pump and can even hold parts with rough or slightly uneven surfaces.
Grid tables are cost effective but may require additional care when zeroing in workpieces with different dimensions.

Hole grid tables

A hole grid vacuum table (Image courtesy of Stritzelberger GmbH) ^[2]

Hole grid tables have - as the name suggests - blind holes that are aligned on a grid. Within these holes, there’s a tiny suction hole that’s size matches the material’s properties you’re machining predominantly - tiny little holes for smooth or polished surfaces like face milled metal, and still tiny but bigger ones e.g. for hard wood and other materials that may have higher air leakages. They often have switchable vacuum chambers below these holes so that the area of where the vacuum is applied can be changed. Uncovered area around the workpiece can be covered with a foil to raise vacuum forces and efficiency. These tables often consist of multiple parts, e.g. a bottom layer for mounting to the machine bed, a middle layer to embed evacuation ducts and the top layer carrying the hole grid.

These tables can clamp workpiece of any shape that have flat bottom surfaces. When your workpieces are really small, of course, the clamping force will diminish - that’s physics. The small suction hole allows for through-cuts: Affected holes do loose their capability to hold the workpiece in place on one hand; but nearby holes on the grid are merely affected as air leakage is limited by the “lost” suction hole’s diameter even at high pressure differences. Aligning the workpiece is even simpler with hole grid tables as their grid’s holes can be used to hold locator pins that build a fence.

Their disadvantages: These tables tend to be more expensive as their design is more complex. They often have multiple vacuum circuits and thus require more connections to your vacuum infrastructure (that have to be kept leak-free). Their tolerance of leakage air and lateral inflow means that your vacuum pump requires to have a higher capacity.

In short:

Hole grid tables support any sheet shape as long as its surface is flat and big enough to meet your clamping requirements.
Hole grid tables tolerate through-cuts at the cost of higher requirements on pump capacity.
Hole grid tables make workpiece alignment a cake walk.
Hole grid tables are complex and often expensive.

Porous surface tables

A Porous surface table (Image courtesy of 'Jens´ Vakuumtische') ^[3]

These special kind of tables do not need holes or channels on their surface at all - their top surface, instead, is air-permeable. I’ve seen specimen where the top layer is made of MDF so actually it’s not a big problem if you accidentally got Z0 wrong - just place a new sheet of MDF on top and off you go again.

The tables are lightweight and - with a lot of knowledge about materials science and thin-wall milling - relatively easy to construct as a DIY project. There are many examples out there on the internet. They work best with a high-volume blower and thus often do not support large pressure differences - so as long as your workpieces are not tiny, that’s not an issue. The tables are suitable for cutting all the way through as well. They are cheap to buy and to upgrade with additional features, and a vacuum cleaner with forced cooling might already be enough to power it up.

The downsides: Might be subject to vibrations due to their low weight, best not to be operated with cooling liquids, unfavorable static friction coefficient with most pairing materials due to MDF (thus more surface needed to protect from slipping).

In short:

Porous surface tables are cheaper to have and to operate.
Porous surface tables are well suited for big workpieces and materials that don’t require cooling liquids.
Porous surface tables may not be perfect for high precision milling.

What all these types have in common

Common to all these types is, that the downforce can safely be calculated the same way for a given vacuum pressure applied to your workpiece (not necessarily identical with what you see on your gauge!). No matter if you have a grid, a hole grid table or a porous surface table, you can always calculate the full surface of your workpiece (and not just the sum of holes’ area within the workpiece’s outer bounds). This is because microscopic surface irregularities of both table and workpiece material allow a lateral airflow into the suction area of your table, making pressure differences between atmosphere and vacuum pump side homogenous.

On the other hand, this is also true for your workpiece’s outline, so there will always be a little leakage air in your system with the side effect that clamping force deteriorates the more you approach the peripheral zone.

That said, cutting all the way through your material will create extra peripheral zones that can, when situated too close to each other, even have zero clamping force left.

What else do you need?

Well, there’s a couple of things you do require. A vacuum pump, for instance, that supplies your table. But you’re in luck, I wrote an entire post on that topic as well. But there’s more, actually:

Fasteners, washers, nuts to attach the table to your machine bed
A vacuum manifold that may allow you to select chambers within your table, preferably providing a vacuum gauge to check pressure levels
Vacuum hose(s) to connect the table to your manifold
Another vacuum hose that connects the manifold to your pump
A pump motor switch and motor circuit breaker
Vacuum fleece or rubber mat (hole grid tables only)
Vacuum gasket thread (grid tables only)
porous spoilboard (porous tables only)

Conclusion

All in all, it will cost you a lot of your time and money to get and setup a vacuum table. So you should have a couple of good arguments to go through that effort. If so, chances are high you’ll be rewarded with quicker execution, better quality, and shorter job setup times.

My selection

I exclusively work with sheet material. I process wood, high pressure laminates, acrylic, MDF boards, and often have to cut out parts from a bigger sheet or use nesting for many different parts. This is why a grid table wouldn’t work for me very well.

Due to the vibration issues I have with my current setup lacking enough weight and the fact that a ready-made vacuum table didn’t exist for the dimensions of my machine, I didn’t choose a porous surface table either. This would have been the most cost-effective solution, though.

Instead, I contacted a vacuum table manufacturer to buy a couple of small hole-grid tables that I could individually switch on and off depending on workpiece size and pressure that I’d require for secure clamping. Well. I was told that a much simpler and better solution would be to buy one of their even heavier aluminium tables. Less worrying to get multiple tables supplied with airtight suction ducts, valves, connectors. Not having to access both sides of the milling machine with ducts. Not having to adjust the tables to sit together tightly or face mill for a level surface. And by the way, they’d create a customized design so it would perfectly fit my machine…

What can I say. I scraped money together for this much more elegant solution. I made a drawing of my machine bed. Just a couple of days later I got a sketch back with an initial design (this one is still missing additional holes for clamping vises, and the grid is less dense to keep processing speed high). I gave my OK for production.

Table delivered

The manufacturer is located not too far from where I’m at home. So I went there and collected it myself. When I had unmounted my old spoilboard, I thought it would be a good idea to have some heavy foam acoustic absorption panels placed in the machine frame to dampen noise and resonation from below the table.

Only afterwards, I bolted the new vacuum table to my machine with help of M6 fasteners and hammernuts. After that it took me multiple hours to get a more or less even surface. As the machine frame’s tolerance is relatively high, I initially had deviations of 3/10mm over the vacuum table’s surface 😓. With the help of some paper and thin cardbord, I was able to smoothen out the roughest dips, but I could still measure a deviation of 15/100mm.

CNC Z-axis bed leveling check↗

To get to an optimal solution, there would be three options:

Surface milling of the whole table
Unmounting the table, adding thin plastic foil where needed, remounting, adding additional foil/paper where needed…
Unmounting the table, loosen the machine frame and get to the root: the frame’s sides are rotated towards the inner just a tiny bit, and the front cross-beam seems to be bent upwards just a little, so it should be lowered by less than 1/10mm.

For the time being, any of these options seemed too much work as I don’t know yet if I need it to be that exact. So I continued by connecting the vacuum hoses.

Connecting the manifold

While happily cutting the hoses that would connect the vacuum table to the manifold, I thought “Wait a second - wouldn’t it be better to have the 9mm hoses as short as possible and rather use a longer 19mm pump connector hose for less flow resistance?” I stopped work and did some calculations:

My table has ~4900 holes of 0.5mm in diameter. Its combined cross section is $A_{table}=962mm^2$, which is equivalent to a hole with 35mm in diameter (differences in flow resistance ignored).

The seven connecting hoses have a diameter of 9mm. Together, their surface is just $A_{hose}=445mm^2$, equivalent single diameter of 24mm which is less than half than the table’s flow section.

For the pump hose and its diameter of 18mm it gets even worse: $A_{pump}=254mm$ is less than 30% of the table’s capacity.

So when I assume an even air leakage over the seven chambers of my table, it makes most sense to have the pump hose as short as possible as the flow speed is highest here and so are the losses. For a scenario where just one of the table’s section is active and there’s a lot of leakage, obviously the 9mm hose should be as short as possible.

I decided instead to never allow full leakage so I could keep my preferred, comfortable location of where the manifold is mounted and just completed the wiring. 😉 By the way, the first part I manufactured on the new table was the holder for the manifold you can see in the image below.