Re: [CAD_CAM_EDM_DRO] re:Levelling large table

----- Original Message -----
From: <ballendo@...>

> Doug,
>
> Which do you have more of; time? or money?

Never enough time, but certainly not enough money to buy an already built
machine.

> You may find that .625 cast alum has a 'tighter' tolerance for
> flatness (from the mfr) than .500 . So using it will get you closer
> to start with.

I originally planned to use .75" or 1" cast plate until I noticed that
others were using .500". If .625" is better, then is .75" mobetta?

> What level of 'dead nuts' are you REALLY after? do you mean to accept
> the basic flatness of the plate alum and just hope not to inroduce
> ADDITIONAL inaccuracies?

The basic flatness of the toolplate is sufficient. Just want to get the
twist and sag out of it.

> Surely you realise that a half-inch thick 60x120 plate will not be
> anywhere NEAR flat.

As long as I can count on about .005" tolerance between the tool and table
at any XY coordinate I will be happy. Large aluminum sheets will drape with
the table, so some table unevenness is tolerable as long as the tool to
table relationship is reasonable. Big, stiff parts like daggerboard foils
won't require this much accuracy.

> Is this a bridge or Gantry machine?

Gantry.

>Thin wire stretched
> across the table at many points (held above the surface by feeler
> gauge stock thick enough to 'bridge' the high points) will give you a
> reference to get the table parts straight.

Excellent idea! Can I ignore caternary droop of the wire over this length?

> work, but WILL not be (or stay) FLAT! Other times , ONLY
> the 'bearings' or 'rails' (of the linear bearing system) were bolted
> to the .500 plate! IT was 'supposed' to be flat, right?!? NOT!!!

Attaching the rails to the plate would at least reduce the tolerance buildup
associated with attaching them separately to the frame. Get the plate flat
(within my requirements) and the Z axis should be close enough, assuming the
gantry is done right.

> P.S. BTW, a 'leveling' jackscrew 'should' be designed as an
> ADJUSTMENT, NOT a SUPPORT (IMO).

I totally agree!

Thanks;
Doug