Product on Schallbert's Blog

passtdas - a helper for your CNC

Schallbert — Mon, 08 Sep 2025

The Problem

A friend of mine wrote to me. He uses a CNC gantry milling machine as a hobby, just like me, and often produces unique pieces. However, the resulting leftovers are sometimes too valuable to throw away. To maximize material utilization, he tries to use these offcuts as blanks for future projects.

“Do you know how I can find out on the machine whether my milled part fits on the blank?” - Anonymous

This happens: According to the dimensions, the workpiece just fits on the blank. If you set the zero point even slightly incorrectly, the milling cutter will move beyond the workpiece boundaries, and scrap it is.

The Solution

It would be great if the machine followed the contour of the workpiece before starting the milling process, wouldn’t it? This way, you could easily find out whether the blank is actually completely within the workpiece boundaries.

Since I can’t find a quick solution online right away, I’m thinking about writing my own software to implement this. So I’m getting to work.

TODO: ADD VIDEO LINK

The Software

For a quick prototype, I choose the high-level language python. Why? I have a fairly good command of python, libraries for the necessary geometric calculations are available, and the written program can be easily compiled into an all-in-one file.

The program is designed to read machine instructions (G-code) and analyze the milling paths. The positions at which the maximum and minimum X and Y values are approached in the milling program are to be saved and output as a motion command in a path file.

If this program is loaded onto the machine, an “extreme value” determined in this way is approached and the Z-axis is lowered from the safety height to the measuring height. The program is then paused so that the operator can make adjustments to the workpiece position. Only by entering another start command will the next extreme value be reached, and so on.

This program will be called passtdas.

Program type

I’m choosing a console program with few parameters:

Shortcut	Parameter name	Description
`-f`	`--file`	The source file to be analyzed. Required.
`-s`	`--zsafety`	Safety height the machine will do flyovers with. Optional.
`-p`	`--zprobe`	Probe height the machine will pinpoint to. Optional.

The two height parameters are given default values, so they don’t always have to be specified.

Input

passtdas reads files and searches them line by line for motion commands like G00, G01, G02, G03. While the analysis is simple for linear movements like G00, G01 - linear movements can be expressed using point-to-point connections whose extreme values cannot lie between the points - it can be quite challenging for circular segments.

The height information on the Z-axis is also read and saved. This allows the machining technician to quickly check whether the maximum milling depth matches the desired one.

If the G-code file contains no instructions or incorrectly formulated instructions, passtdas displays corresponding error messages and aborts the analysis.

Definition of Circular Segments

Background: Circular segments can be defined in two ways in G-code.

[...]
G00 X5.660 Y-0.000 Z10.000
G02 X-5.660 Y0.000 R5.660 F1600
[...]

Command:

“Move a circular arc clockwise from point X5.66 Y0 with radius 5.66mm to point X-5.66 Y0 with feed rate 1600mm/min at height 10mm.”

Drawing these commands creates a hanging semicircle. Circles can be defined using three points. However, only two points are given. The third point must be calculated from the specified arc radius using the start and end points.

Task: Find the extreme points of this arc in the XY plane.

But an arc can also be defined like this:

[...]
G00 X-2.5696 Y5.0431
G03 X-4.6258 Y3.2616 Z-0.5000 I2.5696 J-5.0431
[...]

Command:

“Traverse a circular arc counterclockwise from point X-2.56 Y5.04 with center coordinates I2.56 J-5.04 to point X-4.62 Y3.26.”

Drawing these commands creates a short circular segment in the upper left quadrant. Although three points (start and end points, as well as the center of the circle) are given, it can sometimes be difficult to determine whether an extreme value is being reached somewhere along the arc.

Task: Find the extreme points of this circle in the XY range.

Calculating the extreme points of circle segments

A circle segment can have up to four additional extreme points, depending on its range: X+, Y+, X-, Y-. For a full circle, these are always the coordinate intersection points as seen from the radius.

def get_extremes_from_arc(arc, coordinates):
 # [...]
 x_plus_radius = [center_x + radius, center_y, xyz[2]]
 y_plus_radius = [center_x, center_y + radius, xyz[2]]
 x_minus_radius = [center_x - radius, center_y, xyz[2]]
 y_minus_radius = [center_x, center_y - radius, xyz[2]]
 extremevalue_order = [x_plus_radius, y_plus_radius, x_minus_radius, y_minus_radius]
 if (arc[1] - arc[0]) < 0:
 # crossing the 0° line (x-axis), handle overflow with nested if below
 arc[1] += 360
 result = []
 for crossing_angle in range(0, 721, 90):
 if crossing_angle in range(int(arc[0]), int(arc[1] + 1)):
 i = int(crossing_angle / 90)
 if i > 3:
 i -= 4
 result.append(extremevalue_order[i])
 # [...]

If the circle center and radius are known, the swept angle can be determined from the start and end points. This makes it clear whether the circle’s start and/or end points themselves represent extreme values, or whether additional extreme values arise from sweeping over a coordinate origin. It is precisely what the above code example does.

Data table

All possible extreme values (two for a line, up to four for a circle segment) are now collected in a data table. Once each G-code line has been evaluated and the list is thus complete, the maximum and minimum values for each axis are determined from this list.

This ultimately creates four XYZ coordinates, which describe the maximum extent of the workpiece for each spatial direction.

At the same time, the program searches all commands to the Z-axis and remembers the maximum plunge depth.

Output

The command-line program has no visual output. Therefore, I’ll limit myself to creating a simulation program for the milling machine in which the workpiece extension is approached and paused at each point:

G90

# Go to coordinate zero
MSG "Zmin of this job: -1.0"
G00 Z40
G00 X0 Y0
G01 Z15 F1200

MSG "PathPreview: Hit START to go to Ymin: ['0.0', '-5.66']"
M00
G00 Z40
G00 X0.0 Y-5.66
G01 Z15 F1200

MSG "PathPreview: Hit START to go to Xmin: ['-5.66', '0.0']"
M00
G00 Z40
G00 X-5.66 Y0.0
G01 Z15 F1200

MSG "PathPreview: Hit START to go to Ymax: ['0.0', '5.66']"
M00
G00 Z40
G00 X0.0 Y5.66
G01 Z15 F1200

MSG "PathPreview: Hit START to go to Xmax: ['5.66', '0.0']"
M00
G00 Z40
G00 X5.66 Y0.0
G01 Z15 F1200
# [...]

The above example traverses the extreme values of a circle with r=5.66mm. It always moves to a safety height of Z40 to cover the points and, upon arrival, slowly moves the Z axis at F1200 to the target height of Z15. It is important that the zero point in Z has been correctly measured beforehand - otherwise, a collision with the workpiece may occur.

Testing and Verification

To see if the program works as expected, I created three test files:

Unit tests: Tests individual functions in the program, e.g., whether line and circle segments are correctly recognized
Integration tests: Tests whether functions function correctly in conjunction, e.g., the correct calculation of extreme values based on input values
End-to-end tests: These tests check the entire function chain, e.g., whether incorrectly formatted input files are recognized

Finally, I create several test files with the program and move to the extreme values generated from them with my CNC.

Open-source software

The source code for passtdas is available for free on GitHub, but must be interpreted using Python.

I’ll build a user-friendly and easy-to-use program once enough people have expressed a desire for it.

Klingenbutler

Schallbert — Mon, 14 Oct 2024

Why Klingenbutler?

After I had solved the problem of the lack of a shelf for solid shampoos in my shower - the result was Seifenbutler - I noticed razors flying around there more and more as a nuisance.

If they lie on the edge of the shower, a puddle forms around them. This creates a lot of limescale stains and makes cleaning more difficult. If you put them on the fittings, they will fall down sooner or later and get damaged. The same applies to the wobbly shelf on the edge of the shower cubicle.

It makes sense to reuse the Seifenbutler’s clamping mechanism. This means that only the “front part” needs to be redesigned and I have a stylish shower tray for razors - the Klingenbutler.

The design

I like the round shape of Seifenbutler and want to design Klingenbutler in a similar way. However, the razors should not be touched too easily by accident. So I decide to attach them as “inside” as possible. To do this, I create a ring with an opening, which in turn has three notches for razors. I design the radii of the notches so that all the wet razors I have available fit in there and even toothbrushes can be hung on the Klingenbutler without the bristles touching.

I design the opening (in the drawing on the left) for upper and lower shells so that the upper shell always covers the lower shell in all possible closed positions. This avoids protruding edges with particularly large or small shower rod diameters.

Snapping the razors into place

It was more difficult to plan snap-in aids so that the razors cannot come loose on their own. In the first draft, I decided on a notch in the shape of a double tetrahedron. In the picture above, the notch is labeled “drain”. It has a gradient of 3mm over a length of approx. 15mm, allows splash water to run off and enables the razors to dry quickly.

G2 curves

When I showed one of my designs to a friend of mine shortly before starting production - he is a 3D designer by profession - he immediately noticed that I had not planned for a smooth transition between curve radii. He said that in such cases “G2 curves” are used in CAD and that the model would have much more harmonious transitions.

In technical jargon, this seems to be referred to as geometric continuity. I immediately asked the software company behind my CAD tool whether they supported something like this and lo and behold - there is the tool Create tangent continuous connection. From now on I will use this tool when I connect radii.

The tool creates a Bezier curve whose endpoints are tangentially connected to the two circle segments and thus eliminate the kink.

In addition to the more pleasing appearance, this approach also has an advantage in production: When traveling along bends, the CNC machine has to make sudden changes in direction, similar to connected line segments that meet at an angle. This means that it has to slow down and accelerate at the new curve radius. By using tangent-continuous connections, however, acceleration peaks are significantly reduced and the machine runs at a more consistent speed.

If you have projects with lots of “bends” in the drawing, like with the Klingenbutler, you can quickly add up to a few seconds of production time per part, which can easily be saved - perhaps more on this later.

First prototype - version 1.0

I started production with my “classic” prototype material: cast PMMA (acrylic glass) with a thickness of 5mm in opaque white. If you use reasonably suitable parameters, the material behaves extremely well. At the same time, it mercilessly reveals all errors: here a bevel a little too wide, there slight chatter marks caused by the machine accelerating out of a corner, chips caught in the bottom of threads - all of this is clearly evident in the slightly translucent material composition and strong reflections on the material edges. This material is therefore ideally suited to improving the design and production of products.

Thanks to my experience with Seifenbutler, holding it down during production and assembly of Klingenbutler went smoothly. From now on, toothbrushes and razors can be found in a fixed place in our shower 🙂.

Version 1.1 - Production in bamboo

I actually want to get away from plastic. Over the course of the year I have already switched the production of my Seifenbutlers to “greencast” acrylic glass - i.e. PMMA, which itself is made from 100% recycled material - but it would be even better not to use any plastic at all.

For this I had a few samples of “Moso bamboo” sent to me: stick glued wood cross-glued in three layers, similar to plywood, just under 7.5mm thick. For production in bamboo, I made the fitting tolerances a little more generous (0.1mm undersize) and simply converted metric threads in the plan into simple holes for wood screws.

Omitting unnecessary things

When assembling the first prototype, it became clear to me that the third bayonet had become superfluous. Together with a few other smaller changes, I developed version 1.1:

Omitting the third screw connection in the locking circle
Locking lugs/drain channel: Continuous engraving without stopping at “keyhole”
Bayonet: Chamfering the insides on the back and thus breaking through the surface

Speed and feed

For full groove milling, I use a DLC-coated (diamond-like-carbon) 4mm two-tooth milling cutter without twist with 12mm cutting length. I operate it at S30000rpm with a feed of F3000mm/min. I go into the full material depth with Z+ 7.5mm and drill holes and material ramps at 20°.

I make bevels using a 90° engraving cutter, three-edged, diameter 10mm. I operate it at F5000mm/min at S15000rpm and a center distance of 2mm, which for a smooth edge with a width of 0.5mm already means that it needs 2.5mm of space “downwards” in the Z direction.

For holes, pockets and small recesses I use a relief-ground 2mm two-cutter fishtail with 15° twist and 20mm cutting length. I operate it at a cautious 1700mm/min at S30000rpm and a depth of only Z+1.5mm. Here, too, I dive into the material at 20°.

Production

Working with the material “bamboo”, which was completely unknown to me until then, was surprisingly easy. Thanks to my new and very sharp cutters, there were no tears to complain about and the material itself also had hardly any defects or damage.

Making of Klingenbutler↗

During processing, quite fine chips were created, which are a bit reminiscent of dust from processing MDF boards. I am not yet sure whether I should reduce the speed or whether this is due to the straight cutting edges of the cutter. When milling full slot, the cutter may have to carry chips with it for several revolutions before it can leave them behind.

In any case, drilling with this cutter and unchanged parameters is not recommended. All of the drill holes have charred edges, which suggests that the speed was far too high. I also drilled in one piece instead of retracting the cutter several times. Let’s use the rule of three to make this obvious: At a feed rate of FZ+2000mm/min, S30000/rpm and Z7.5mm, the cutter makes 112.5 revolutions in the drill hole, so the cutting edges rub senselessly along the wall of the drill hole 225 times - only the center cutting edge at the bottom removes material. In addition, the chips are not ejected but are carried along the whole time.

For the next prototype, I will either make drill holes with retractions and a drastically reduced speed or simply use a smaller cutter.

Result

This is impressive. I took the bevel for edge breaking from the production in acrylic glass. It is very narrow at 0.5mm. Nevertheless, the detail is really clean. The upper and lower shells of Klingenbutler can be smoothly moved against each other and the clamp, my problem child from this video, could also be milled without tearing.

Assembly is problem-free and the wood screws can also be easily screwed into the pre-drilled holes. The bamboo Klingenbutler variant holds on to the shower rail even better than its acrylic glass counterpart - thanks to the rougher surface and greater material thickness.

It remains to be seen whether the material is durable enough in damp rooms. It is still popular for cutting boards in the kitchen, so I guess it will be just fine.

Outlook

The use of bamboo has left me very motivated: a great material. Hard and strong but light, easy to machine, dimensionally stable and hardly any material defects: that’s how you want to work with natural materials!

This prototype is now in our shower for endurance testing. In the meantime, I’ll continue to refine the design.

Version 1.2

I have the following small improvements planned for V1.2:

Drilling: Choose a smaller cutter (at the expense of the cycle time)
Fastening: Mill a cutout so that the knurled wheel no longer protrudes
Remove superfluous recess on the inside of the upper shell
Allow different radii to be merged with G2 curves
Replace locking lugs with a more harmonious, round drainage channel
Align the grain: Manufacture all parts “vertically”
Mill the clamps in the first setup and make them 0.04mm thicker

Here too, the aim is to achieve a good product through continuous improvement.

Seifenbutler - Pilot run!

Schallbert — Thu, 08 Feb 2024

My Hardware project Seifenbutler is complete. After more than a year of development, I have manufactured some in white, black, transparent, and white translucent. They even made it to a first online shop.

How do I determine the sales price?

As I use high-quality materials and the production and assembly time is long, I don’t have much room for maneuver when it comes to pricing. On the other hand, I can’t ask moon prices for a commodity like Seifenbutler - unless I use precious woods or other unusual or complex materials. There is a lot of material online for such calculations. I used a YouTube video from the “Cutting It Close” channel as a guide.

My price calculation is made up of the following items:

Final price = material price
 + machine time (investment, wear, tools, consumables)
 + production and assembly time 
 + packaging costs 
 + fees (postage, payment service providers, stores)

As you can see, I have not included my own time for product development in the calculation - if I had to do this, it would easily add the equivalent of 250h of working time.

Material prices

As of the end of 2023

Ok, let’s quickly list the expenses per part:

quantity	item	price/unit	unit used	price
2	PMMA 5mm XT	90€/m²	0,03	2,70€
1	Knurl M4 A2 22mm	7,54€/25pcs	0,04	0,30€
1	Washer M4 A2 12mm	0,94€/100pcs	0,01	0,01€
1	pin M4 A2 12mm	6,70€/50pcs	0,02	0,13€
5	screw M4 A2 8mm	12,40€/200pcs	0,025	0,31€
			Material cost/part	3,45€

Machine time

I estimate the machine time for this project at 100€/h. The lion’s share of this is the investment, maintenance and wear costs of the machine at €50/h plus my own working time, which I simply estimate at €30/h.

The tool costs are quite high at around €15/h, as I have to use four different tools, all of which are small in diameter and therefore subject to high wear.

Consumables are vacuum fleeces, positioning pins, cut protection gloves and adhesive tape. I need the latter to fix the small milled parts and the spiral. Their share of the machine time costs is around 5€/h.

The machine time is currently 8min/part. This means that the item machine time amounts to 13.30€/piece.

Assembly time

It takes me about 5 minutes to assemble a soap holder. If I apply an hourly rate of 30€, I arrive at 2.50€ per item.

Packaging costs and purchase of materials

I pack the soap holders in simple DIN A6 envelopes after wrapping them carefully in wrapping paper. The costs here amount to just 0.20€ per item. However, if I now include my working time for packaging and shipping labels, we are already at 0.90€ per item. Added to this are the costs for postage when purchasing materials and the costs for paper and printing the operating instructions, which I calculate at a further 0.50€/unit - the small quantities produced are weighing heavy on this.

Fees

Fees are incurred for shipping and payment service providers, for example. Here the prices vary depending on the provider, so I plan a shipping surcharge of 3.50€ (shipping by large letter) - this does not apply to sales “at the counter”.

The total price

If I add up all the items, I would have to call up a store price of 20.15€ (cash) for the unpacked goods and 24.75€ in the case of shipping with cashless payment. And then the company would still not have made a cent of profit.

I’m not sure whether the soap butler can be sold for such prices. But if you’re honest and don’t want to end up paying more, such calculations are unavoidable, at least roughly.

The assumed merchant with a store

If I were a merchant¹, I would still have to add VAT and would also want to see the development costs amortized somehow. I’m also not sure whether I would get through the door “as a merchant” with an hourly rate of 30€.

Phew. If you now include things like store rent, costs for storage space, workshop rent, electricity, waste disposal, membership of the “Green Dot” in accordance with the Packaging Act, contributions to the Chamber of Industry and Commerce, the employers’ liability insurance association as well as taxes and duties, then even with a further 20% mark-up, a whole heap of soap holders would have to be sold in order not to end up poorer than before!

Let’s assume I can rent a workshop and open a tiny store.

A fictitious store

Let’s do a quick calculation with a few assumed values.

Item	Price/Unit	Used	Final price
Shop	16€/m² per month	40m²	640€
Storage space	4€/m² per month	8m²	32€
workshop	8€/m² per month	30m²	240€
Service charges	per month	1	200€
Taxes	IHK / BG / insurance	1	125€
Fees	20€/month	3	60€
		Total	1297€
		Staff	1600€

And if I were to hire someone to work in this store for 20hours/week, I would incur an estimated €1600 in costs.

Summa summarum

Ok, let’s summarize: I start with a mark-up of 20% on the production costs. As an assumed merchant, however, I would have to pay VAT, which I would have to add on again.

Retail price = production price 
 + contribution margin (20%)
 + value added tax (19%)
 = (1,2 * 20,15€) * 1,19 = 28,77€

Oh well, that puts us at almost thirty euros per item.

So how many soap butlers have to be sold per month before I would make a profit as a merchant with a store? The contribution margin is 4.03€ (20% of the production price). If I now divide the costs generated by production and sales by this amount, I get my minimum sales quantity: 719 units/month.

So how many hours per week would I have to spend at least at my CNC milling machine or assembling and packaging soap butlers? If I take the values from above, I get a total of 8 + 5 + 2 = 15min/part.

With the minimum sales volume, that would give me a working week of about 45 hours! Oh, 60 hours a week is easy, isn’t it? So I could produce 960 units per month - and assuming they sell excellently, I would earn an extra €971 to amortize the development costs (estimate 250h*€30 = €7,500). That’s a bang!

I would then march into the profit zone with the company after just eight months (with 60 hours per week, of course). Oh, or do you still have to pay taxes? And when was my machine paid off?

Conclusion on profitability

I realize that the calculation is not that simple. Some parameters cannot be scaled up or down arbitrarily and linearly and I cannot realistically estimate the machine costs, for example, because I simply don’t have any experience in continuous operation - keyword wear and tear.

Nevertheless, the amount of work I have to do is enough for me to say:

It won’t work. Nice project, but not worth it, at least not with my own funds and on my own initiative. It remains a pilot series and my joy at a successful, functional product.

Fortunately, this is all “just my hobby”, so I’m not forced to draw the disastrous conclusions 😉

Careful. My vocation is not to be a trader or a businessman, but to be an engineer. That’s why I only have a very rudimentary knowledge of business administration (just that one semester at university when nobody really wanted to listen). All my comments here are rough estimates. They should therefore be taken with a grain of salt. However, I am very happy to receive corrections and constructive criticism! ↩︎

QR-codengrave V1.1 / V1.2

Schallbert — Sun, 12 Feb 2023

In a nutshell

Faster Engrave algorithm
Several bugfixes
Usability improvements

Demo video

QR-codengrave Quick Demo↗

Version 1.1

Engrave algorithm upgrade

The algorithm now scans the lines horizontally
One line “Left-to-Right”, next line “Right-to-Left” etc. to reduce machine travel
prefers lines over dots, and longer lines over shorter ones for time optimization
draws lines vertically or horizontally
rapid move positioning now is packed into vectors that can be called to new Points(X, Y) simultaneously, not only one dimension at a time
New algorithm just needs 80% of the time for engraving compared to the old one

Bugfixes

splash image (qruwu.jpg) is not shown on startup when Persistence.dat is not available
Crash when pressing Stop Draw although drawing has never been initiated before
Crash when adding a tool on an empty selecion list
Initialization of application with no tool selected disables dropdown menu
Fix a calculation error in QR-code size estimation
Fix an error pixel width calculation for tapered tools
Actually the currently selected tool is not shown “selected” in the dropdown on startup
QR-codes can grow larger than the display area

Usability improvements

Change default tool name from name to default for clarity
Make non-tapered (V-cut) tool the default tool for better usability
Setting a new XY-zero offset became more intuitive (Y down is negative, X right is positive)

Version 1.2

Minor change over 1.1, most importantly a Bugfix that could lead to tool change macro not being called. In addition, tapered tool angles are now taken into account when calculating track width and QR-code engrave size.

Bugfixes

Tool select Tx and tool change command M06 are now in the same line of the G-code to correctly trigger tool change macros
Actual selected tool not shown on startup, instead the first tool of the list was shown
QR-code drawing could grow larger than the screen. This is fixed now.
Crash fix when trying to update status although no engrave parameters available

Usability improvements

Add application version to G-code as comment
Add qr_ prefix to G-code save-to dialog’s default name
Improve job duration estimate

Availability

The latest release is available for download here, free of charge for home and private use. For more info on the project, please find the readme and the background information linked here.

Migration from an earlier version

It is possible to copy the persistence file to the new version’s directory. This way, saved tool lists, engrave parameters, and XY0 offsets can be ported to the new version. How it’s done:

Navigate into src/assets of your QR-codengrave folder.
copy Persistence.dat to a temporary folder or e.g. your Desktop.
Remove the current installation by deleting the folder.
Download the latest release, move it where your previous release was deployed.
Move the Persistence.dat into src/assets of your new installation and start the application.