How to weld plate to tube at 90 degrees?

[coolman]

I want to weld the face of a 1/4" mild steel plate (6" x 6") to the end of a mild steel tube (3" x 3" x 1/4" x 48"). This is for the gantry on a metal CNC mill I am building. My question is how do you weld the two pieces using MIG and have them at exactly (or as close as possible) to 90 degrees or at right angles? The flat plate will be bolted to the floor or a frame so that the tube will stand upright vertically.

There is or course the welding magnets, but I would rather clamp the two pieces together using C-clamps so that any warpage from the weld can be compensated for until I get all welds done. Is there a trick to using the magnets?

The method I have primarily used is to use a piece of angles where I clamp one side of the angle to the tube and the other to the plate and since the angle is at perfect right angles, the plate and tube should also be at right angles after the weld. However, I have found that if the plate is not exactly flat then you will not get the perfect right angle. Maybe i can use thicker plates to get the flatness I need.

Any suggestions of recommendations is welcomed. I want to do this stuff right the first time to save time and money.

Thanks,

[kong]

Why not weld a smallish piece of steel plate to the end of the tube, then it can be shimmed and clamped to the main plate before final welding. It must be far easier clamping two flat pieces together ?!

[HuFlungDung]

Kong's got a good suggestion there.

I'm not sure how you'll be able to deal with the inevitable warp of the plate, no matter what you do, it will end up convex on the face, and should be milled into square and flatness. I'd opt for something thicker, to reduce the warp factor and also to give a little extra material for facing.

Or, if you have access to a large enough lathe, you should be able to center drill the plate, and then face it between chuck and tailstock after welding.

[cncwhiz]

What if you tree pan or cut a groove a little bit bigger than the tube in the plate. Then slip the tubing into the groove and weld it? I would think that it would still warp but should be an even amount around the plate. When it is bolted to the floor it should pull the same. The other problem that I see is weather the surface you are sitting on is flat as well?

[trilect]

When your ready to weld, buy or borrow 3 magnetic welding clamps. Space them radially around your pipe and stuck to your plate. these should hold your workpiece very very close to true while you tack weld it in a few places.

[ESjaavik]

I don't like welding magnets! They pull the weld. A lot if you're spray transfer welding, but considerably when globular transfer and short circuit welding too.

Rest the tube on the plate with 3 thin nails between, about the thickness of the tube. (OK, thinner when tube is 1/4") Tack it close to, but not including one nail, then remove that one. Check straightness and tack next to one of the remaining nails. Repeat for the last nail. Any remaining error should be small, and can usually be corrected by the pattern used to complete the weld. It should be completed as a series of tacks to minimize warping of the plate.
Go symmetrically if the straightness was fine after the 3 first tacks.

With a bit of training an angle grinder can be used to correct any remaining warp. Use it as you would with a scraper: take out small scallops by just touching the part slightly.

[trubleshtr]

how long is the bar you want to weld? You could buy "bar clamps" (for schedule 40 black piping) alot of hardware store sell them? You would need to own or have access to threading equipment to thread the pipe ends for the clamps.

[Verboten]

Hello,

If you need it to be exactly perpendicular to the ground and the other parts of your machine then you might want to consider making it adjustable.

This might sound like a lot of work but it goes fast:

You could buy a larger piece of thicker plate to bolt to the ground (say 1/2 or 3/4 x 8 x 8) , center your smaller plate and tack the two together pretty good. Then you could drill 4 holes through both near the corners of the smaller plate (say 5/16 ths to tap the bottom with 3/8 nc). Then mark the pieces with a punch or something so that you know how to assemble the parts then seperate them. Now you can thread the bottom plate and drill it with 4 more holes near it's corners to bolt it to the floor. Then enlarge the holes on the smaller piece.

Then you could decide if you wanted to use shims to adjust or you could drill and tap the smaller plate twice more per side for setscews to use for adjustment (probably wait until after welding to tap). Then weld on the pipe as straight as you possibly can (most likely it won't be "perfect") and assemble.

I like ESjavik's suggestions - magnets and clamps can only hold steel straight until you start welding after that its technique.

Good luck with your project.

[sol]

A common technique that is used with lamp posts and commercial construction:
The plate is welded reasonably close to square.
Nuts are put on the anchor bolts on BOTH sides of the plate.
The nuts are used to adjust the column to plumb. Simple, and allows for a multitude of sins, from a non-level floor to warped plate or bowed columns.

When sitting at a stop light check out how the metal lamp post is connected to its concrete footing....the double nut method is everywhere.

[ccm]

Maybe I'm slow, but is what you're attempting like figure 1 or 2 ?

Weld examples

-Art

[coolman]

Thanks for all your replies.

CCM, I am trying to create the gantry section of my CNC using mild steel square tube 3" x 3" x 0.25" x 48" and a flat mild steel plate 6" x 6" x 0.25". The flat of the plate is connected to the end of the square tube. This is like a post where the plate is bolted to the ground and the square tube stands upright.

Sol, I like the double bolt method. Since this is widely used in commercial projects, it must be a proven method. So basically for one bolt the trick is to use two nuts and adjust the height and tighten both nuts to each other, correct? The plate goes on top of the nuts. In a CNC where vibration happens alot, how can I be sure that the two nuts per bolt will not loosen? Are these two nuts welded fixed to the bolt or something?

Thanks,

[sol]

Here is a GIS for the system.
http://www.brantacan.co.uk/LampPostBaseEA.jpg
There are nuts underneath the base (not visible here) which are used to level the post. The top nuts (2) are tightened against the plate and each other.
This pic is overkill, one anchor-bolt in each corner of your square base plate will do.
Washers and lock washers recommended as well.