Re: [CAD_CAM_EDM_DRO] How well does your CNC retrofitted desktop mill performs?

Hi Marcus,

Thanks a lot for the info and advise. I guess the biggest question is how to
define tight tolerance.

The small parts I have seen are not for the mass-market end products of the
semicon industry but mostly parts for some jigs, dies, and not overly
delicate or sensitive parts. These parts to my inexperienced eyes do not
seem to have high or tight tolerances to the extend of +/-0.0005". They are
mostly being produced using manual knee mills with DRO and I was told with
backlash problem in the leadscrew.

Considering that ball screws can have "practically" no backlash or about
0.01mm backlash with proper pre-loading, is it not possible to do a CNC
retrofit mill that will match these manual knee mill clones?

Your additional inputs and the experiences of others in this list will be
much appreciated. Thank you.

WT

----- Original Message -----
From: Marcus
To: CAD_CAM_EDM_DRO@yahoogroups.com
Sent: Sunday, May 06, 2007 12:03 AM
Subject: Re: [CAD_CAM_EDM_DRO] How well does your CNC retrofitted desktop
mill performs?

Hi WT:
You have no chance of working to tight tolerances in a production
environment on a retrofitted manual mill regardless of the origin or
vintage.
If your output needs to be within +/- 0.0005" or better, you need a purpose
built machining center and will still have a significant inspection burden.
If your output is forgiving enough to tolerate +/- 0.002" or looser, you may
have a chance with 100% inspection and a 10% or worse reject rate.
You will have no joy if your production quantities are more than 10 parts
per run, and if you need to remove more than 10% of the raw material to
produce your parts.
You will have no chance of commercial success if you need to use cutters
smaller than 0.125" diameter or bigger than 0.500" diameter for any part of
your projects.
You will have no chance of success if you need to routinely mill 304 or 316
stainless steel.
Retrofitted CNC mills certainly have their place...in the R&D lab and in
the hobby machine shop, but they are very poor candidates for production
shops, particularly jobbing production shops.
You'll be far better off with a single decent quality VMC than with a whole
bank of retrofit hobby class machines.
Cheers

Marcus