[cnc_is_fun]

First, I'd like to say hi to all CNC Zone members, my name is Stefan. I've been following this forum for a while and finally decided to register to participate in some very interesting discussions. I am going to build my CNC milling machine and already have some idea of how it is going to look like but would like to get your feedback and advice.

Here are some requirements for the machine:

- it will be used for machining plastics and soft metals including aluminum. It may be used to make steel parts on some occasions but I'm not sure if the machine would be capable of doing it
- accuracy of 0.01 - 0.02 mm
- precision ground ballscrews
- 4Nm motors
- work envelope still to be decided

1. Frame design.

I was considering a moving table vs moving gantry designs and decided that a moving table will be better for what I need. Moving table design would provide way more rigidity. I am aware that the machine will take more space but precision is my priority.

2. Frame material.

I am considering two options: aluminum extrusions or steel bars. Would aluminum extrusions be rigid enough to machine aluminum / stainless steel parts with the machine?
I could order Misumi 45 series aluminum extrusions which I reckon should be good enough (extrusion spec attached).
It actually may be a good idea to do FEA analysis on the two type bars and compare the results. I might do that and share the results if anyone is interested to see that.

I know that steel bars would be more rigid but it's more hassle with welding and then stress relieving the frame. It would have to be machined then to achieve perfectly straight bed.

3. Motor type and torque

I've made some torque calculations assuming the cutting force to be 500N and it worked out to be just over 1 Nm but it doesn't seem to be right, I would expect it to be more than that. Could anyone give me an idea of how big the cutting force could be when machining aluminum and stainless? For now I assumed 4Nm motors on all 3 axes but please correct me if I'm wrong, maybe i should use 6 or 8 Nm motors?
Another consideration is what type of motors should I get. I'd like to use a closed loop system to avoid motor stalling but servo motors and drivers are quite expensive. Would a hybrid stepper motors (i.e. with embedded encoders) be something worth considering?

4. Precision vs ballscrew pitch vs microstepping

I'd like to get very high accuracy on the machine and this could be achieved by changing a number of different parameters. Ballscrew pitch would be one of those parameters and I was thinking of getting a 5mm pitch ballscrew. On the other hand I don't want my machine to be slow and 10mm pitch ballscrew would make it twice as fast but less precise. Would it be better to get a 10mm pitch ballscrew and use microstepping to increase precision or use 5mm pitch ballscrew and use full steps?

I guess that's all of my questions for now, will surely have more in the future. Once I have a better idea of what I want, I will start designing it in Solidworks and share my progress with you.

Thanks in advance for all your feedback and advice.

Regards,
Stefan

[pippin88]

Aluminum extrusions are not precision and will require machining also.