Re: Large machine construction and temperature control

Hi Ken,

Yes, big machines move big amounts if not properly dealt with. A
good example is real wood furniture. Wood expands and contracts
(substantially) with changesin humidity. Furniture designers always
have to be mindful of those constant changes in their designs.

With CNC machines (any big machines for that matter), you have to
design "Cooling" into some of the components. A C frame mill for
example can be a bear to control thermal expansion as the headstock
generates lots of heat and is reference to the machine from one
side. To deal with that, many builder use recirculating oil and
chillers to keep the headstock at a constant temp.

Ball screws are often "Bathed" in oil, either recircultating or
stagnent baths.

Working on the design of components, a designer can build into the
machine which way the expansion occurs. For example, on a lathe,
you wouldn't want the expansion to cause the machine to grow along
the X axis. Deisgners have resolved that by creating slant bed
lathes which create expansion at 90 degrees to the X axis (what
would be called Y axis on a lathe - tool moving above and below
center). With this type of thermal expansion control, as the machine
heats up (and grows) the tool moves above and below center as
opposed to towards and away from center which would throw off all
your diameter control.

This is just some of the controls I am aware of and I am sure there
are many more. I do know that it is a very tough challenge for
machine designers.

Chris




--- In CAD_CAM_EDM_DRO@yahoogroups.com, Ken Jenkins <kjenkins@b...>
wrote:

> The following reference in a previous post got me thinking
>
> >>> http://www.scaled.com/services/cms_mill.html
>
> A friend of mine works with the aerospace industry doing QC

fixtures for 3D

> tubing. He is constantly telling me about the tolerance these

giant milling

> machines work to. The thing I don't understand is, for a machine

that large

> one would thing that just expansion due to temperature fluctuation

would

> cause the machine to go back and forth several thousands over a

whole tool

> path over time.
>
> Can anyone address how the manufacturers of these kind of machines

manage

> this issue (other than spec'ing that the machine must run in a

controlled

> environment?
>
> Do those of you who have built larger (NOT as large as the above)

CNC

> routers, plasma tables find any problems as a result of heat

expansion of

> machine parts?
>
> Just curious,
>
> Ken Jenkins