Squaring up box girder

Rainbows · Jul 29, 2016

I am more of a welding type of guy so I don't really know what the monetary cost of machining is.

I have a 100x100x5 girder 900mm long (offcuts I have lying around). I want it milled on each to have 4 flat surfaces square to each other.

A girder isn't too easy to ship back and forth so if there is no one on here nearby could someone could give me an idea of what a reasonable price is? That way I can work out if local companies are giving a fair price or not.

johnoder · Jul 29, 2016

I'll asume you really mean structural square tubing

Impractical - unless you envision very loose tolerances for flat and square

The 5 is too thin - it will holler on every face milling pass and though "shiney" will in no way be flat due to the milling pass releasing stresses.

If he tries some more you will have practically no wall left

In a word, you will be disappointed and they will find they under bid the damn thing

Rainbows · Jul 30, 2016

Ah thats a fair point.

fusker · Jul 30, 2016

John is right of course.
Why don't you tell us what you need this thing for and how 'square' and 'flat' it has to be?
I suspect there are better ways to achieve your purpose than milling.
Just my two 'øre'!
Regards, fusker

SMT · Jul 30, 2016

Perhaps a big surface grinder?

Illinoyance · Jul 30, 2016

Rectangular tubing has a lot of internal stress. Removing material from one side will cause it to deform. Each time you turn it and surface another side it will move again. Chances of ever getting it straight and square are really small.

Matt@RFR · Jul 30, 2016

Illinoyance said:
Rectangular tubing has a lot of internal stress. Removing material from one side will cause it to deform. Each time you turn it and surface another side it will move again. Chances of ever getting it straight and square are really small.

To a machinist. To a welder it may be exactly what he needs.

Rainbows · Jul 30, 2016

In this case it has to be machinist sort of square. The vague plan was machine the girders, fill with epoxy concrete and use then as the ways of a cnc milling machine or the bed of a cnc lathe.

SteveBausch · Jul 30, 2016

If you insist, I guess you can make a silk purse of your sow's ears, but you should attempt to visit a successful attempt of this enterprise.

johnoder · Jul 30, 2016

Rainbows said:
In this case it has to be machinist sort of square. The vague plan was machine the girders, fill with epoxy concrete and use then as the ways of a cnc milling machine or the bed of a cnc lathe.

A way to get there is resequence and add necessaries

"Rebar" crossing both ways (6 mm all thread) screwed thru tapped holes both walls and welded in c'sink outside

Plugs/bungs for later tapped holes as needed - all in planned for locations - similarly welded

THEN epoxy concrete

THEN machined for flat and square

DDoug · Jul 30, 2016

Rainbows said:
In this case it has to be machinist sort of square. The vague plan was machine the girders, fill with epoxy concrete and use then as the ways of a cnc milling machine or the bed of a cnc lathe.

If your going to build a real C.N.C. machine, you spend real money on proper materials to build a proper machine.
Cobbling with what you have laying around sounds like A hobby machine.

And there is a forum for that, one that cannot
Be listed here.

BTW I have designed fixtures doing just what you intend, it didn't turn out good, and I redesigned
It using different materials and techniques.
(Eliminated machining the box tube)

it then ended up being a economical, repeat job for my employer.

goldenfab · Jul 31, 2016

digger doug said:
If your going to build a real C.N.C. machine, you spend real money on proper materials to build a proper machine.
Cobbling with what you have laying around sounds like A hobby machine.

...And if you are doing this as a hobby and trying to go the cheap route, then having machine work done for you is not going to fit in your definition of reasonable price.

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