Re: [CAD_CAM_EDM_DRO] Hot rolled router frame

I have done this a bit too, and I agree with Doug
about the stress in ASTM A-500 welded structural
tubing. It is not as bad as cold drawn solids, but it
will warp a good bit if you take a bunch off of one side.

I have not found the tubing particularly straight...
1/8" over six feet is typical. I don't know if there is a spec.

I have had good luck with finite-element designed tubing space frame
structures by doing the following:

1) carefully cut pieces to length as accurately as possible

2) tack weld structure up on a leveled surface using a surveyor's transit
for guidance.Complete welds using minimum heat in an alternating pattern.

3) use hot rolled ASTM A-36 solids for critical areas like linear rail way
bolting surfaces. Much less residual stress.

I found MIG welding much much easier to do than a
pinned or bolted structure.

Warping? I was concerned about it on the last one.
It was too big to heat treat stress relieve economically. I considered
vibratory stress relief,
but was very dubious about the claimed results. I
had done a study at work on the subject and found
no scientific proof that it does anything at all.

So I did no stress relief, other than careful welding sequences.. The way
surfaces were scraped/grouted to +/- 0.0005" initially. This may result in
less induced
stress than Blanchard grinding. My local grinders at
the time ( American Grinding in Chicago) said that
they could hold no better than +/- 0.007" in due to
grinding and other induced stresses. I am sure a way grinding firm could do
much better.

I found that drilling and tapping blind linear way bolt holes warped the six
feet of solid A-36 about 0.002".

That was corrected, and the frame sat for about a year while I worked on
other stuff. After a bumpy
1000 mile ride in a truck and reinstallation, I found
only 0.0015 of additional warping had occured.

So, by careful welding practices and using A-36
hot rolled in critical areas I seem to have gotten
away with little residual stress in the structure, but
I did have to correct a couple thousandths.

Les

----- Original Message -----
From: <vrsculptor@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Friday, February 28, 2003 10:08 AM
Subject: [CAD_CAM_EDM_DRO] Hot rolled router frame


> I'm still thinking about building a good size router. My 12X48 mill
> is too small for some of the architectural projects I'd like to
> tackle. I've found a firm in Miami (Smith Hamilton) with a large
> blanchard that could grind the frame for a 60" (or larger) router. I
> was considering a moving table design using bolted 2X6 heavy walled
> hot rolled tubing for the table rails with linear slides. I've never
> attempted anything precision on this kind of scale and would
> appreciate your input:
>
> 1. Is common hot rolled tubing likely to be straight enough as it
> comes from the manufacturer to use or do I need to order something
> special? Is a 5' or 6' piece likely to be straight withing .020-.025"?
> 2. With a bolt together frame there are going to have to be some
> weldments and holes added before grinding such as end plates and
> bushings through the 2" width for bolts. Will the tube still be
> straight enough after arc or mig welding to grind?
>
> All input welcome... I prefer to learn from others experience rather
> than my mistakes.
>
> Roger S