Re: [CAD_CAM_EDM_DRO] scraping beds for long tables

Smoke wrote:

> Hmmm...one of you experts should explain all this to the machine tool
> builders in the auto industry. According to what I've been reading (here)
> they've always been building machines incorrectly.
>
> Smoke
>
> >> When my father initialized me in scraping and later in a practice
> >> I was told that for a bed of a long travelling table (example x table of
> >> a Bridgeport that has a short bed) you should scrape into the bed
> >> a curve so that the curve counter arrests the weight of the table
> >> when it travels far out.
> >
> >Sven;
> >
> >This is an accepted practice. Both Edward Connally (Machine Tool
> >Rebuilding) and Wayne Moore (Foundations of Mechanical Accuracy) explain
> >the process in their books. What is required is either lots of experience

What my understanding of the comments on this in the Edward F. Connelley
book is that you want to PRESERVE the existing bow in the table when you
do the scraping. Just turning the table upside down on a flat surface will
cause it to start relaxing the "set". You'd scrape it flat, then put it back
on the machine, and it would take the "set" from the overhanging ends
again, within a week. So, now you'd have the bottom of the table accurately
scraped into a curve, thereby guaranteeing the work you did would have
curved tops!

You preserve the existing set in the casting by supporting it at 3 points
near the ends. This is where experience comes in, to know exactly how
far in from the ends to support the table to get the best reproduction
of the stresses on the table in its normal position. Now that the table
is supported upside down, without releasing the strain in the material,
you can scrape it perfectly flat, and it should go back onto the machine
and STAY very nearly that flat until wear develops again.

Scraping the top of the table, if you should want to do that, is another
matter. I assume you have to support it roughly at the same location
as the ends of the saddle. Maybe since the bottom is now very flat, you
can just have it ground.

Jon