Poor Man's 4th Axis cnc

by ZenziWerken, published

Poor Man's 4th Axis cnc by ZenziWerken May 27, 2017


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In order to give tires of my model cars a final touch I was thinking of using a 4th axis for rotary engraving. For the vague idea, the intense investment and due to harsh software requirement it did not get a professional 4th axis set. It came in handy that I had a stepper motor (I changed from my y-axis) and a small chuck laying around. So I designed the pm4a.

This build was done on a desktop cnc-machine by Stepcraft from 6.5 mm birch plywood for the body and some 5 mm Pertinax for the gears using a 1.2 mm spiral toothed flute. Furthermore I used

  • 2 Ballbearings Type 6000ZZ (10 x 26 x 8 mm)
  • 1 Ballbearing Type 608ZZ (8 x 22 x 7 mm)
  • small chuck (10 mm 3/8'' x 24 UNF) with SDS-Plus-adapter
  • NEMA17 stepper
  • a sharpened M8 machine screw and a selflocking nut (for the tailstock)
  • some M3 machine screws

The two gears have an pretty uncommon cycloidal (round) teeth, which helped me to create smaller teeth, because they don't have any corners. Otherwise the teeth size would have been limited by the diameter of my milling bit.

The motor is driven by plugging it into the X-axis stepper driver. This way the machine won't move sideways, but instead the workpiece does. The distance of travel for one revolution is calculated depending of the diameter of the workpiece dived by the gear reduction of 6 : 1. For example my tires have 40 mm diameter. So the circumference is 40 mm x π (3.1415) = 125.66 mm / 6 = 20.94 mm. I entered this value in the controller settings.

When using the pm4a it'd be a good idea to use some kind of cover to prevent dust from getting into the gearing.

Visit https://www.zenziwerken.de/en/ for more interesting designs.

How I Designed This

The prototype did use a different (even smaler chuck) which had a attached hex bolt. It proved the concept, but was to shaky to be of good use.

This shows the operation of the prototype.

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Is there any way to convert this to 3D printable files, such as .STL or .OBJ? I haven't yet finished assembling my Stepcraft 2/840 and I'd like to print this in polycarbonate due it its inherent strength.

You simply can use any basic CAD tool (e.g. FreeCAD) and extrude every single part by 6,5mm.
Afterwards you can ealiy 3d-print and assemble them.

Hi Rich, I am not sure if this would make sense in case of the PM4A. You wouldn't be able to use it without a cnc anyway. Furthermore a rotary tool ain't the first thing you might want to get familiar with. Using a cnc, especially a Stepcraft, does mean you have to learn quite a bunch of (new) things.

Hi GeerGuy, I use a high quality spiral toothed bit with 1.2 mm diameter. My machine can cut plywood with a feedrate of 1200 mm/min and 2.4 mm depth per pass. The spindle is running with approx. 25000 rpm. The tool path is created using Estlcam with automatic path creation. So no fancy stuff involved.

Is the bit up-cut, down-cut or compression?

Maybe it is the quality of the plywood that you are using, but both your top and bottom edges are very clean with no break-outs or splinters.

I have tried an up-cut with similar feeds and speeds, but my top edge is not as nice. Going to try a compression bit next.


No it's a standard up-cut. I use down-cut bits for this kind of carving: https://www.zenziwerken.de/en/Haushaltsgegenstaende/Topfuntersetzer#Carving but I'd say on the pm4a parts it's the interplay of spiral toothed bit, high rpm spindlespeed and fast feedrate.

Looks great.

Can you provide some details on how you got such nice clean cuts on your plywood?

bit? tool path details?

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as always great work

I would love to see a video of this thing in action. It looks awesome!

You can see the prototype in the 'How I Designed This' section. The working pm4a you can see here: https://www.facebook.com/daniel.tobias.gross/videos/1506594349404158/