Re: Machining reference surfaces on a steel frame.

Graham,
This can be done with a Blanchard. Over that size, holding an overall
flatness of .05mm (.002") shouldn't be a problem. You'll need to find
a shop with a Blanchard having at least a 60" diameter chuck, which
shouldn't be a problem. I'd recommend building the top frame with
bolt on legs, as having legs on the frame at the time of grinding
would cause stability problems with the part on the chuck. If your
top frame is a weldment, it will need to be furnace stress relieved
prior to grinding. Otherwise the residual stresses will make it crawl
around as its ground, and even if they are able to get it flat, time
dependent release of stress will make it move away from flat at some
point in the not too distant future. If you should make the frame
from square or rectangular tubing, use material with at least 1/4"
wall, as light walled materials will be hard to hold on the chuck.
Ideally, any welds on the top and bottom surfaces should be V'ed and
welded such that they can be ground flat afterward, since you'll want
the frame to be able to sit essentially flat on the chuck initially
(i.e. not resting on a few feature that protrude here and there).
Although its not necessary to grind both sides in order to get one
side flat, you may decide you want to have them grind the second side
for your convenience in attaching other parts to a true surface, but
that would be dependent on your design. Another approach to frames
like this is to weld lengths of flat stock to the top of the frame,
and grind only these flats for the reference surfaces. This is
normally done if its desirable to have the reference surfaces thicker
than the material the frame itself is built from. If you have any
further questions or need clarification of anything I've said here,
feel free to ask. I've had a 42" Blanchard for a long time, and have
spent more than a few hours (days) making things such as this flat :)

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Graham Stabler" <eexgs@n...>
wrote:

>
>
> I am building a dual gantry ultrasonic scanning tank system. It
> consists of high grade thk rails, a steel frame over a tank of water,
> linear servo motors from Copley and linear encoders from renishaw.
>
> Well actually is consists of a hole in a project bank balance at the
> moment as most of the bits are on order. But while I wait for the
> rails and motors I need to finalize the frame design and start
> getting it built.
>
> The frame is going to have 4 legs and then a square outline at the
> top allowing full access to the tank from the top and the sides. The
> reason for my post is that the renishaw encoders should have a 0.8mm
> ride height +/-0.1mm. Over the length of travel on the long axis
> this may be difficult to maintain although and changes in flatness
> should affect the rail as well as the encoder keeping the ride height
> fairly true in theory.
>
> That aside it would still be really good to get the tops of the frame
> where the rails will be mounted as flat as possible. The longest
> area is about 1135mm long and the frame will be around 800mm wide.
> This is pushing it even for the big Boko milling machine we have.
> And, the surface finsh of the milling has been a problem in the past
> and I am not sure how accurate it will be over such areas.
>
> Blanchard grinding has been mentioned in the past, this seems to be
> the manufacturers name and refers to grinding with a vertical spindle
> (horizontal wheel) and a rotary table. I have seen it refered to as
> something else in the UK as well as rotary grinding and plough
> grinding.
>
> Basically has anyone any ideas on this? What sort of tolerances can
> Blanchard grinding give and are the cutting forces similar to normal
> grinding i.e. will it be possible to machine the top supported only
> by the 4 legs?
>
> Another thought I had was to find a huge surface plate and then use
> moglice or similar to produce a bearing mounting surface.????
>
> Cheers,
>
> Graham