Re: [CAD_CAM_EDM_DRO] scraping beds for long tables

"Sven Peter, TAD S.A." wrote:

> Dear friends,
> I am pleased that it turned out that there are quite a few persons here
> who scrape bed ways.
> I have a question I have for a long time and nobody was able to
> respond satisfactory till now.
> 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.
> Because I never got an instruction about it how to make it well
> and because I saw in quite a few mills that this curve was made too
> deep.
> (with the result that the flat parts of the dovetail way are very much
> worn
> at the tips and in the centre the table not even touches the bed
> so that the oil oil pours out like nothing.)

This may not have been intentionally done that way. My aged Bridgeport
is exactly like that. There is a small bit of sag due to the overhanging
weight of the table. The center of the table wears more than the ends.
Once the table has a concave worn area in it, it causes the top of the
saddle to wear in a convex pattern. I can see the pattern duplicated
into the top of a piece when I mill it. It is only about .003" worst
case, so it is not a disaster, but something you have to know about.

>
> Therefore I never made it in any of the machines I rebuilt.
> Instead I preferred to leave the bed straight and with
> as much as possible support area. Anyway in all cases the bedways were
> so
> worn that the improvement of a wrong scraped bedway was significant
> against before the worn one.
> I preferred to have a little error than a wobbly machine that wears out
> fast.
> Who can tell me how it is done right?

If you put a curve into either part of the dovetail, then the table would
rock. The only way to make it stable would be to have matching curves
in both parts. but, then the table would describe a circle of large radius,

rather than move in a straight line.

Maybe what this is all about is compensating for sag, as the table is
scraped upside down. In the Edward F. Connelley book "Machine Tool
Reconditioning", he shows how you support the table upside down
on 3 small jack posts, so that the table bows in the same direction
as when it is hanging over the saddle. Then, you scrape it flat, and it
should remain just about as flat when put back onto the machine.

Jon