Re: [CAD_CAM_EDM_DRO] ACME or Ball?

ghidera2000 wrote:

>I'm looking at putting together a small mill/drill for PCBs and
>doing cutouts of plastic or sheet metal enclosures and front panels
>( <= 1/4" thick). The mill working area would be a maximum of about
>18" by 18". Traces would be pretty fine, between 0.0125" and 0.01"
>

This is the kicker! If a trace is .01" wide, and there happens to be some
problem with backlash and which side the software approaches for doing
each side of the trace, then a .005" backlash could cut the trace to half
thickness, maybe worse at inflection points. If you can get the backlash
under tight control, then it might work.

>and drilling for all the through-hole parts.
>
>Think an ACME screw setup would do ok at this or are ball a must for
>this level of precision?
>
>Also, think the small ballscrews would be good here? In the reid
>catalog I was looking at the TBS-17 screw and the precision nuts for
>them. Would these be strong enough? I just can't see my application
>requiring much in the way of brute strength...
>
>

You don't need a lot of linear force for PCB milling. Plastic is a
problem, as you have to keep moving fast or the work melts. Sheet
metal starts to get into higher cutting forces, so you have to take that
into account. But, with small tools turning at high speeds, cutting forces
on .031" aluminum can be in the range of 15 Lbs. If you are talking
about 1/4" aluminum panels, then you would have to cut them in a
number of successive depth passes to keep the axis load down.

Very small screws 18" long start to run into problems with bowing,
unless the structure can keep the screws under some tension.

Jon