Power drawbar foot pedal mod

The power drawbar is a great add-on for the PCNC440. It eases the toolchange – which can get quite annoying the conventional way, after opening the spindle door, release the drawbar, …, tighten the drawbar and closing the case for the umpteenth time!

Yes, it’s a timesaver! But in my view, it has one drawback: the pneumatic push-button occupies one hand. And for a toolchange it’s really «handy» to have both hands available.

A foot pedal switch is a great idea. Even though there are pneumatic foot pedal switches, I didn’t wanted to hassle with long air pipes. A wire based electrical foot pedal switch is really easier to use – and to install too! The more so as there’s one in the shop I saved for later use – very simple, but sturdy, used on a punch press for years.

The bigger Tormach machines (like the PCNC770 or PCNC1100) have an electronic control circuitry for the power drawbar. Unfortunately the PCNC440 misses this option, but the needed information is on the net or can be seen on the pneumatic push-button:

  • 5 ports
  • 4 way
  • 2 positions Air Valve
  • the machine has a 48VDC- or 24VDC-output integrated (for the solenoid)

The solenoid air valve I found was an «Airtac 4V210». All in all about 20 bucks, some silencers and connectors included. Yet, the filter regulator lubricator (FRL) was set up later on – another 25 bucks investet in persistence…

The implementation was easy. The 24VDC-output (used for the water pump relais) has of course also the right values for a solenoid. The foot pedal switch was simply clipped in between.

The connector was placed between the 5-pin-connector of the height gauge/passive touch probe and the probably future expansion slot of the 4th-Axis. The toughness of the body housing is impressive by the way!


Besides of the power drawbar being really «handy», the footpedal activation is really fast and comfortable. You can remove the actual and insert the next tool within seconds – really a big timesaver.

Open issue and warning

Even though the footpedal is only «primed» if the machine is powered – it’s always active while it’s powered! This means, that a misstep while the spindle is turning could be fatal and dangerous. Please take precautions if you think about making such a mod – everybody is responsible for it’s own and the health of others around!

I need to dig deeper in the controller scheme – I’m quite sure, that there’s a possibility to deactivate the solenoid, while the spindle is turning…

Passive Touch Probe

For some work, a touch probe appeas to me as the right approach. The most obvious here is the indexing of circular objects like holes or round stock – where one should measure one of the axis twice to get the right middle point.

The tools which are sold are quite expensive. Sure, it’d be precise – but I think with the right concept, it’s possible to make this (otherwise simple) mechanic accurate for my needs. 

The main concept consist of a closed electrical loop with three gaps, which are bridged by rods attached to the probing tip. The CNC notices the input as soon this closed loop is interrupted. Here one of my attempts which illustrates the idea behind:

Six setscrews were integrated in the housing – to calibrate the position and the orientation of the probe tip. Three on the bottom to balance it to the vertical of the CNC-Machine, and three on the side face to bring it in line the the arbor. Take a look to the following image. The top of the case was turned separate with a kind of TTS-collet. It’s mounted to the bottom part – on which you can see four of the six setscrew-threads and the hole on the bottom where the probe-tip-assembly gets through. As probe tip I used some broken ones from the Haimer 3D Sensor – therefore I don’t need a new type and can make some probe tips myself (that’ll be a separate post).

The red plate which is placed inside of the bottom part and the wiring with the 3 mm balls soldered on them is not the final solution. This was the complicated part of the project!

The used material reaches from polyoxymethylen (POM or Delrin) to polyvinile chloride (PVC), brass and copper inlays. Exept the first on the top left, all of them has issues with either the bearing ball (try once to solder them 🙂 or the mounting of the whole assembly (POM and PVC really hardheaded to glue).

The solution I’ve chosen is the milled brass type assembly mounted on «a kind of wooden» base. This version is solid and shouldn’t deform itself – but to be sure: time will tell.

The model and the G-Code for the milling machine was generated in Fusion 360. I milled it directly glued on the «kind of» wooden base – if the glue’s cured correctly, the part shouldn’t break.


This way I got two basic advantages: probably the milling machine is more accurate than glueing some stainless-steel-bearing-balls and it’s really easier to plan such an assembly with gaps being bridget with some rods:

So there we go! The finished assembly mounted in the base and wired on some standard 3.5 mm headphone jack which leads to the CNC-machine:

First test: the probe works fine. Don’t know yet about the accuracy, I need to dial it in and do some calibrations first. But I’m already satisfied, that the concept works and that the probe tip doesn’t get crushed in the first tries…


Additional edit after setup and adjustment

The adjustment is very easy to do in the Pathpilot software.  With a little patience, it’s possible to get close to some hundrets of milimeters, which is enough accuracy for my daily needs. Here you can see the output while adjusting it:

Patience! It lasted until 4/100 mm for me 😉 

The usage is simple, but for my person: not as fast as the Haimer. At least for the standard setup – for dialing in prepared stock or holes or such things, this probe will still be my first choice!