Subject: Tips on assembly of steel frame for best precision

From: [email-masked] (Vjekoslav Balas)
Date: 5 Dec 1996 12:47:55 GMT

Hi,
I want to put together a steel frame (made of U profile steel)
on 4 legs (eg. square steel profile) as support for some linear
guides. I'd like this to turn out as precise as possible (flatness
etc).

I know there are different gradings of steel (hardness etc)
eg. here in Germany St37 (construction steel), St 44.3, St59 etc.

My questions:
(a)Do the different gradings have some standard tolerance of
flatness e.g.. x mm/meter ?

(b)I have been told that one method of getting best flatness etc
is to bolt precisely machined steel plate (eg 15mm thick) to the frame (instead of
machining the whole frame once it is assembled (welded or similar).

I guess that the principle behind this is that you
assemble the frame such that the top surfaces of the 4 U frames
mate precisely. Thick machined steel plate eg. 15 mm thick is bolted on top
ensuring flatness/accuracy. Any inequality in flatness of bottom
surfaces of 4 U profiles + length of legs is then counteracted by
screw adjustable legs - is this the correct understanding of the
principle - any other tips ?

(c)I'm intersted if I can use a number of 
precisely machined plates per each U frame (just because
I can get them in shorter lengths more readily) - I guess
this depends on (a) and correspondingly what sort of "step"
I get at the plate to plate joint.

Thanks,
Vjeko

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From: T Lipton <[email-masked]>
Date: 10 Dec 1996 03:00:42 GMT

> guides. I'd like this to turn out as precise as possible (flatness

Vjeko, Depending on just how accurate you want your frames to come out
there are several ways to proceed. To answer your first question, yes
there are many diffrent manufacturing specs for standard construction
materials. They material you mentioned in your post is called Channel 
here in the US. 
  Channel strikes me as a sub-optimal material for
construction of a precision weldment. I would start with a heavy wall
square or rectangle section tubing and attach your plates to that.
  I think also that you may have mis-interpeted earlier instructions about
the way the plates work. Generally when fabricating a frame that will be
used for a precision application the frame is built to resonable and
achievable tolerances. Then if high precision surfaces are required additional
plates or bosses are added to the frame in the nessesary places and machined
or ground afterwards. 
  It is important to note that if your frame is not rigid the accuracy of your
plate surfaces will be wasted. It is actually surprising how inexpensive precision 
grinding if the weldments are properly designed for these secondary operations.
  I would also recommend a full stress relief prior to any machining or grinding.

    Good Luck  TML