DIY X-Axis Powerfeed using Arduino and Stepper

[Tomzo]

Greetings,

I am putting the finishing touches on an X Axis Powerfeed for my RF-31 clone. I chose to use a NEMA23 stepper and control it with an Arduino Uno and a stepper driver. I suppose I could have made my life easier using a DC motor, but I figured I would give a stepper a go if nothing else to get a bit of experience setting these large steppers up.

I took the handwheel and X Axis coupler off the right end of the table. The coupler is secured with a tapered pin to the shaft and has a recess on the table side of the coupler to contain the thrust bearings in there. I turned a chunk of 2" round aluminum I had on hand down to the appropriate dimensions and left a recessed pocket on the table end just like the original part. I drilled a through hold in the right location and the new coupler fits exactly like the old one, containing the thrust bearings properly. I attached an image of the CAD model, although the final version was modified a bit to accommodate the pulley.

I then bored out a pulley to be a near press fit on the new coupler and put it into position on the coupler. It had set screws as well so I used those too.

I then made a couple of brackets out of scrap aluminum I had on hand with one bolted to the back of the table (with adjustment slots) and the other bolted perpendicular to that one also with slots for belt tensioning. I made this second one too long as I missed a measurement somehow but the slots work - there is just a bit more overhang than I would prefer. It is very rigidly mounted.

For the controls I chose to use a two direction momentary joystick off Amazon along with a potentiometer to vary the speed. Power for the stepper driver comes from a 24V power supply that I also connected to a buck converter to supply 9V to the Arduino Uno. I am using all inputs as pullup inputs so there is no voltage on the controls to speak of. The direction switches are normally open and the limits switches are normally closed. The latter is just in case a wire gets severed or something.

I wrestled with the code for a while before I got it to function correctly. I am not using any stepper libraries - I just have the pulse pin go high and low separated by delays that are based on the potentiometer. Getting all of the parameters right for this took some time, but it is in a pretty good spot. I have the driver set to disable the stepper unless the toggle is activated. This way I can manually make slow, fine adjustments with the power off the stepper coils. I am not too worried about sending voltage back into the driver as I will only be making slow, fine adjustments. The powerfeed can move quite slowly so I can get to pretty precise locations with the stepper.

I put my 8" rotary table and a vise on the table to approximate the maximum weight I am likely to have on the mill and the stepper moves it just fine. You can ramp from very, very slow to a very nice clip using the potentiometer. The one challenge with this version of the code is that there is no acceleration built into the code - it goes right to the speed you are telling it to from the pot. If the pot is on the highest speed, the stepper will stall when you hit the toggle switch. I just back the pot down a bit and it works fine. I can twist that pot as fast as I can with no stall, so I am working on getting some acceleration code built into a revised version (version 12?).

I 3D printed a box for the controls that has two layers - one for the stepper driver and another for the Arduino, buck converter, and a breadboard. I used a breadboard for convenience's sake while testing and figured why not just use it in the final version? If I get creative I may design a custom PCB for this and have it built, but that is a lot of work...

I also 3D printed the little bracket for the controls. I will see how this holds up over time and may replace it.

The cost was reasonable. About $35 for the stepper, $28 for the driver, and $22 for the power supply. The toggle was $10 and I had the Arduino, breadboard, buck converter, and pot laying around. I think you could buy everything for under $100, add some wire, and you are off to the races.

Anyway, here are a couple of videos showing the control box and the unit in operation. If anyone has some acceleration code for Arduino I am all ears! Comments or suggestions are appreciated.

Tom

 

[homebrewed]

You have a great start!!

Arduino code is easy to change so it should be relatively easy to come up with some acceleration code. Do some online searching for "trapezoidal acceleration" or similar phrases. Basically it's a linear increment of the step rate that ends at the step rate you want, then flattens out. Variables like the time interval for the acceleration can be easily tested by either modifying the code or, with a little extra effort, writing a little bit of code to accept different acceleration rates via the USB interface so you can try out different parameters in no time at all. Don't forget to test the code at its minimum and maximum acceleration rates (and once you come up with them, lock out "bad" values).

You probably also want to think about the visual impact of starting machining while the stepper(s) are coming up to speed. I.E. how the work looks afterwords. For a constant spindle RPM, milling while the table is coming up to speed will result in inconsistent machining marks. Just cosmetic perhaps but easy to accommodate once you see how it works. Not that this is an issue with digital drives compared to analog drives -- they will take time to come up to speed too. Provide some room for the spindle to come up to speed and your're good.

No stepper can go from zero to its maximum step rate in zero time so this is a problem that has been around from day one w/regard to CNC based machining. The problem with multi-axis CNC is that ALL the axes involved have to describe the desired path while they are accelerating. So count your blessings -- your problem is a much smaller subset of that one......

 

[Tomzo]

Thanks for the tip. One of the reasons I wanted to use the Arduino is that I can keep a "stable" version and play around with many others to test things out. I was kicking myself at one point because if some wire was landed incorrectly I would spend 20 minutes taking things apart to land it correctly when I realized that it was a LOT faster and easier to just swap the pins in the code!

There is a lot of code out there for steppers and many with acceleration libraries, but those are mainly geared towards position control and not just making the stepper spin. I am playing around with accelstepper to see if I can adapt that to this use case.

 

[rodm1]

A+!

 

[Tomzo]

I had forgotten to update this until now but I did make a small change. Running the power for the DRO off a contactor in the VFD panel created a lot of noise that made the DRO jump all over the place. I move the power for the DRO onto a different circuit and that fixed the issue. Other than that the powerfeed is working great. Of course, if I add up my time to the cost of the components it would have been "cheaper" to buy a powerfeed online, but I had fun doing it so the labor was free!