In article ,
Tim Wescott wrote:

On Fri, 28 Dec 2012 16:57:47 -0500, Joseph Gwinn wrote:

In article ,
Joe AutoDrill wrote:

Let me add that one of the greatest challenges is keeping the guide
rods from binding up in situations such as I've mentioned above. I may
sell a 36" wide head with 1" guide rods at the ends... When the head
grows a few thousandths of an inch, the guide rods bind. Roller ball
bushings help, but still disintegrate quickly under such loads.

In optical design, design for temperature independence is called
athermalization. There is a huge literature, part of which could be
relevant in that it involves using materials with different temperature
coefficients of linear expansion to passively adjust critical
dimensions. (The non-relevant part involves picking optical glasses
that have contrary variations in optical properties with temperature.)

In a linear array of axes, one would use aluminum and steel in
mechanical combination such that the inter-axis spacing doesn't change
even as the temperature of the assembly changes. One way to do this is
to have a steel frame with the spindle bearings attached to one long
wall by flexures, pushed via a second set of flexures from two aluminum
plates attached to the two short walls and parallel to the other long
wall. If one dimensions things correctly, the two expansion effects
will just cancel.

I considered mentioning something like that, but you'd need to make sure
that everything heats up the same, or at least in the same way.

Contrary scenario: You mount your spindles in aluminum, in a steel
frame. The dimensions stay the same over temperature as long as
everything is the same temperature. Now run the thing: The spindles heat
up, the aluminum expands, the spindle-to-spindle dimensions actually
_shrink_ before everything comes up to thermal equilibrium.

So -- your athermalization suggestion isn't to be dismissed, but it
should be approached with care.

Yes. The devil is in the details. DoN's discussion of athermal
pendulum rods is a parallel example of mechanical athermalization.

To keep temperature uniform, I would fill the assembly with hydraulic
oil like Mobil DTE24 (thin enough that the operation of the mechanism
will keep the oil well stirred) such as used in the headstocks of
Clausing 5900-series lathes.

I also like the idea of a thermostatic heater to keep the assembly at a
constant higher temperature. Simple enough, but does require electrical
power.

Joe Gwinn