Raspberry Pi Forums

As a project this year I want to try and build a RPi desktop CNC mill. For the most part I will be machining wooden blocks however, in some cases for short periods of time I will need to machine metals such as mild steel (20mm max thickness) which do not require a high precision cut.

- What size motors are recommended for such tasks (ie Nema 17 75oz??).

- From what I have read up till now, the stepper motor driver plays a large role in power/torque output of the motor. I see lots of people using easydriver's which seem to be very cheap (around £0.99 on ebay) and some people using driver which cost as much a £20 each. What would you recommend for this application and is the difference really that big? I would also appreciate it if anyone has any recommendations or guidance on power supplies.

- Long lead times are not a problem (parts can be shipped from China), I found this website (http://www.omc-stepperonline.com/3-axis ... p-188.html) it seems to be FAR more cheaper than anywhere else I have seen however, seems too cheap. Firstly, are such parts from China really that cheap?! And is the quality decent? Secondly, would this kit not be ideal for the application?

If anyone has any recommended suppliers for any parts or have any experience with omc-stepperonline then please let me know (long lead times are not a problem).

As this is really my first "big" build I would appreciate it greatly if anyone has any guidance on this, even if it has nothing to do with the questions above please let me know.

The NEMA 17 will work for Wood, plastics and light metals. You will have to take it slow in mild steel. If you need to cut 20mm deep mild steel I would recommend at least a NEMA 23 or better. My Sherline mill has the NEMA 23's and they work pretty well. You just cannot take a huge bite. Slow feed speeds and shallow cuts. This is also going to depend upon what you are going to use as a headstock and spindle.

The stepper motors rated torque goes down as the speed goes up. Steppers do not provide feedback. If you push them too hard you will start losing steps. This is especially true on Mill headstocks. The weight of the headstock will cause lost steps if you continually use rapid positioning (G0) or high feed rates when lifting out of the work.

You can get very accurate work from small Mills and lathes. Make sure to set your Backlash Compensation.

There is a thread on LinuxCNC for the Raspberry Pi below;
http://www.raspberrypi.org/forums/viewt ... nc#p631881

Thanks for all the information, I didn't even realize that tools such as LinuxCNC existed! This will come in very handy. The answers have now created more questions:

-I made a mistake in my earlier post, maximum mild steel thickness is 10mm which could be cut a very low speeds for long periods of time (not many parts required), does this change things?
- What sort of torque rating would be recommended for what I require?
- What driver and power supply options do we have for this? Any recommendations?
- Does anyone anyone have any experience with any suppliers from China/Hong Kong (long lead times but cheap parts)? If so, could you please provide any websites/links?
-Is it required that the stepper only has 4 wires or can 6 wire steppers also be used with the RPi for a CNC application?
-For my spindle/headstock I was planning on using a simple dremel in a custom build support, are there any recommendations for this or any existing setups that can be purchased?

Everything I have read says that dremel-type tools are good for wood and plastic but not for steel. The rotation speed is too high and the power is too low. You can engrave hard metals, but not cut them at any practical rate (except with an abrasive tool like a cutoff wheel, which is very fragile and wears quickly, not suited for CNC machining).

Thanks for informing me of this, for the most part I will be machining hard woods however, if I do eventually need to machine mild steel, I will have to re-look at this.

I have found the following guide for a small DIY CNC Mill which has dimensions that are perfect for my application (https://wiki.hackmanhattan.com/images/4 ... ok_1-4.pdf) this can be modified to suit NEMA 23 steppers however, the question still remains, what size would be required and how would I drive these? Please let me know your thoughts and suggestions.

Thanks

Those are very detailed instructions; looks good. Here are two other, similar Kickstarter projects:
https://www.kickstarter.com/projects/14 ... cnc-router
https://www.kickstarter.com/projects/ma ... p-cnc-rout

And there is also one sold as a kit, "Shapeoko" at http://www.shapeoko.com/
and you can check what users say about cutting metal: http://www.shapeoko.com/wiki/index.php/Materials#Steel


People converting manual machine tools like lathes and mills typically use NEMA-23 size steppers. The available torque also depends on the voltage and current limits of your driver electronics. The data sheet for the stepper shows you plots of torque vs steps per second, for a given drive voltage and current (assuming there is a data sheet; eg. you buy from a respectable supplier, not ebay Anyway I haven't attempted to make such a thing myself, so I can't advise in more detail.

I have have spent the past 2-3 days almost constantly reading up on this and now understand the scale of the machine that is required to meet my initial manufacturing requirements. Understanding this, I think it would be best if I took a step back and made something a bit smaller which can be used to machine less aggressive materials.

So, what I think may be more achievable is to build a DIY router to machine wood (around 40mm maximum depth) and also engrave aluminium. Do you guys think this will be a good starting point? It's uses would be to prototype tooling and mainly for hobby carving (at small depths) such as (http://www.youtube.com/watch?v=NeBQQG0bxn4) although I think for something as good as I would need a much better machine!

For the motors, was thinking some decent torque Nema 23's (or high torque Nema 17's if I can get away with it) please let me know if you have any recommendations on size and type of motors. Lob0426, you mentioned that you are currently using Nema 23's on your mill, can you please share with us what drivers you are using for this? I ask because I see everyone saying "Gecko drivers are a must" but when I look at things like Polulu drivers they seem to be doing the same job for a fraction of the price.

I also started looking into the RPi real time limitations and saw that the PID add on board in the LinuxCNC link Lob0426 had provided solves this however, it costs almost £40 for the board. Is it worth the money? is the Raspberry Pi controlling the steppers directly really that bad? In terms of accuracy, how much difference would it make (ie, how far out would dimensions be)? I ask because for the most part, I will not be machining components with super high precision therefore, am not sure if these are necessary for now and if I could instead just use a python script directly from the Pi to control the steppers.

sur4j wrote:Is the Raspberry Pi controlling the steppers directly really that bad? In terms of accuracy, how much difference would it make (ie, how far out would dimensions be)?

No experience on this field, but my feeling is that RPi is fine if the machine does not need to be very fast. Linux is a ready system for any slow control application.

The precision mechanical parts on the other hand can be expensive and that will probably be the highest cost in the project if development and testing hours are taken as free hobby time without any cost.

The Shapeoko I posted a picture of above, comes in a kit starting at $300. If you really can build that machine for that price, I think it is about the best value I've seen for a machine of that capability (CNC-Dremel for carving wood and engraving aluminum). But I don't have one myself, just working from the info online.

I think the R-Pi can probably do an OK job of driving steppers, especially if you are careful about system setup (eg no other processes running), and if you are willing to turn off interrupts (kills USB / network) you can get even better waveforms.

On the other hand, an Arduino can give you basically perfect waveforms, and did you know a basic ATmega328p based Arduino-compatible board is under $3, including shipping, from ebay? That's like the price of one connector cable. Search ebay for "Arduino pro mini" Given how cheap that is, it may not make sense spending too much time trying to tune a R-Pi to do "mostly OK" what is so much easier for a cheap microcontroller to do perfectly.