Re: [CAD_CAM_EDM_DRO] scraping beds for long tables

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

> Hello Marcus and Jon,
> Thank you for the answers.
> I will keep it as I made it before. Get as much furface area as possible
> as straight as possible.
> The idea with the surport on 3 pionts I like and possibly I will apply it
> in the future.

This was recommended for all scraping or other precision work, as a 3-point
support will not rock, and allows easy adjustment of any surface to be
level (or parallel to a surface plate it is sitting on).

>
> > 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.
> >
>
> Jon here must be something wrong. In all machines I worked on,
> It was always that the tips of the bed were worn and some spots
> in the table. Only in one ocasion (taiwanese bench mill) the bed was totally
> twisted because the casting was machined fresh. (rocked 0,015")

Well, there is some wear at the very ends of the table, as it tends to
tip the table when the center of gravity passes beyond the outermost
point of support by the saddle. This puts a LOT of pressure on the
dovetail, which is all that is holding the table from tipping off the
saddle. I try not to extend the table that far if at all possible. Only
when a large workpiece needs to be machined all over do I need to
do that. Bridgeports with the long tables get in that position more
often.

There is detectable sag in my table, but it is small. I think the wear on
the bottom is several times the sag. (Note, my machine was made in 1938,
and the table has never been scraped or ground, as far as I can tell.)

Jon