Re: [CAD_CAM_EDM_DRO] scraping

Smoke wrote:

> Some of you mention scraping curves into the way surfaces to allow for
> material sag.
> Others have mentioned building an angle into the vertcal column to allow for
> weight of the vertical head.
>
> Never saw any machine tools designed or made this way!
>
> No matter what the size of the machine....if it has a curved surface built
> into the ways (due to scraping or any other means), it will be physically
> impossible to machine a true flat surface.

Well, I think the whole thing here is that when the part is ON the machine,
it is actually straight. It is scraped with a curve carefully calculated so
that when the part sags under it's own weight (or the weight of parts it
carries) then it becomes straight or in alignment.

> Also, tilting the column to
> allow for weight of the head will prevent the head from traveling
> (vertically) perpendicular to the table.

On the manual Bridgeport, the heads generally have a 2-axis knuckle
joint, and so it is adjustable. On many CNC Bridgeports, and a large
number of other larger machines, the head is fixed in place. If you were
to scrape a flat perfectly parallel or at right angles to the table, and then
mount the head, it will deflect the arm, ram or whatever the head mounts
to enough so that the head will not be perpendicular. So, the scraping hand
needs to put a tiny angle into it to compensate for the sag.

> I believe the real solution is building the frame strong enough to eliminate
> the suggested "solutions".

Not always. The big, Cat-50 and larger, variable speed, liquid cooled heads
on some of the gantry machines weigh several TONS! There is no way to
avoid deflection when these massive heads are mounted. On gantry bed
mills, the gantry has to raise an lower the head. Adding another 20 tons
to the gantry beam is just going to cause more deflections somewhere
else. Some older Cincinnatti gantry bed mills do, indeed, use the brute
force approach, and they are so big and massive they have to be moved
a piece at a time on railcars! These machines require multi-million $
foundations to properly support their weight, and just aren't practical
for many shops. making an intelligent compensation to allow the machine
to be lightened by several tons may be a good choice, but you have to
be sure that once you compensate for the sag, it is going to STAY in
alighment.

> Another solution is to design the machine
> properly in the begining to eliminate the problems...such as building the
> "base for the table much longer so as to allow the table to be fully
> supported thruout its travel (eliminating the possiblity of end sag.

The best CNC (and other) machines do, indeed, do it that way. And, they
last longer, as well as perform better initially.

Jon