First CNC Mill Design - General Questions

Hi all,

I've been toying with the idea of building a CNC Mill. And with that, I must start with design! I've been lurking around the net on CNC Hobby projects and forums. Before I can model my design, I have some general questions about equipment that i think if answered would save me a lot of hassle and time.. perhaps even money.

Some basic info of what I am envisioning

Goal: 3 axis Steel and Aluminum milling machine. Repeatability the best I can get. Would be very happy with +/- .0012"

Budget: Undecided entirely. If I can reach good repeatability and power, a few thousand USD.

Work Envelope: 20" x 12" x 8" (X,Y,Z)

Motors: NEMA 34

MOTION CONTROL: Ebay ball screws, ebay HSR Linear Rails, 2 per axis, Z-Axis considering going with rod style ways to save money.

Spindle: Was looking at ebay spindles for a few hundred.

[B]Frame:/B] Custom machined steel frame design (bolt and assemble type design)

Table / Mounts: I may go with Aluminum for the table as a substrate and bolt a thinner (5/16") steel top to it for lessesning load on the X and Y axis motors. Aluminum mounts for different pieces where acceptable.

Control Gecko drive, not worried about electronics at the current time


With that said, here are my questions for the experts:

1.) With the HSR linear rails, I found that pricing for larger HSR rails gets pretty steep the wider you go. HSR25 has a major rail width of 23mm. Do you believe this is rigid enough (2), or would stepping up to a HSR 30/35 be worth the 50-100% increase in price?

2.) Assuming ebay parts and the best rigid design possible with this hardware, do you believe my repeatability is realistic with amazon/ebay ballscrews?

3.) What is the proper term for finding "accordion" chip guarding. I don't know how to word that. I am referring to the chip covers that stretch and contract with the table to cover the ways and screws.


Any other comments/tips on my design are welcomed. Thanks for reading.

Regards

If I understand correctly, you want to use this machine for milling steel and aluminum, and you have a budget of a few thousand USD.

Just buy a used milling machine. Preferably a CNC, that way the hard work is already done. Then upgrade as needed. No point in reinventing the wheel.

For instance, I paid $1000 for my 32x12.5x12 (10x52 table) CNC machine that was in nearly perfect mechanical condition. I did a controls upgrade later. A friend of mine picked up a similar, but heavier and with a tool changer, machine for $2500, and we did an upgrade on it also.

From what I have seen over a few years it's the best advise buy a great used machine , building something is a lot harder than some think.