On Fri, 28 Dec 2012 13:44:00 -0500, Joe AutoDrill wrote:
I sell to customers all the time who need to hold tolerances of 0.001"
or so while drilling, milling, reaming, etc. This is not extremely
difficult when you are talking about a single spindle device. However,
I sell multi drill spindle devices. ...sometimes with as many as 40
spindles.

One of the challenges I run across on a weekly basis is controlling
tolerances that closely on a head that has either a wide spread or many
spindles. The thermal growth of the head housing ranges significantly
from room temperature to operating temperature.

We have a few tricks to compensate for this
[big snip]
So... With all of that having been said, does anyone have any creative
ideas on how we might be able to solve this issue for some of our customers?

I think creative ideas for this will be readily available, but practical
ideas much less so. Anyhow, here are some ideas, probably ranging from
slightly to totally impractical:

1, insulate a spindle-locating frame from the heat generating parts,
eg using ceramic spacers

2, add go/nogo indicators to the heads; eg have an insulated rod or a
Kevlar rod stretched between opposite ends of head, fastened at one end,
such that out-of-limits head expansion moves the other end far enough to
trip an indicator, or such that reaching operating temp moves it far
enough to turn on an indicator

3, add thermistors or thermocouples that control go/nogo indicators

4, make the spindle-locating frame out of a thermally-stable Kevlar
composite (see paragraph about Kevlar's slightly-negative
coefficient of axial thermal expansion, near the middle of
http://composite.about.com/od/aboutcompositesplastics/l/aa050597.htm)

5, active system: add a micrometric position-changer at each spindle,
eg hydraulic or piezoelectric or a moving wedge or screw-driven, along
with accurate temperature or position measuring feedback system

--
jiw