Technique for reducing weld distortion

[DanielW]

I'm startring to build my CNC and the bed of the CNC will be created in mild steel and the gantry in Aluminium plate.
Anyway as people will know about CNC mild steel orientated machines the welded sections must be kept to a minimium distortion so I was wondering about this technique with a view to the more experienced welders in the group for their valued input.

As I'm not a seasoned welder I've devised this clamping method below consisting of 50mmx5mm thickness at 600mm L shaped clamp to which the 25mm box section will rest on the bolts at a depth of 23mm so that the box section will have a 2mm height above the clamp,enough to get a weld at the joins.

Will this help in maintaining miniumum distortion on the welding of the box section?

 

[overload]

Welding on top of the box section with the proposed clamping set-up? Then yeah you will get distortion. The box section will have a bow in it.

You need to put a stiffer box section under the 25mm box section and clamp them together at the ends with a feeler gauge blade or thin spacer in the middle to force the small box section do an opposite bow. When welded, it should straighten itself.

 

[TechnicAl]

Whatever you do on one side repeat on the opposite side

If you cant, either offset it and hope or strongbacks (clamped or welded)

 

[DanielW]

Welding on top of the box section with the proposed clamping set-up? Then yeah you will get distortion. The box section will have a bow in it.

You need to put a stiffer box section under the 25mm box section and clamp them together at the ends with a feeler gauge blade or thin spacer in the middle to force the small box section do an opposite bow. When welded, it should straighten itself.

So what you're saying if I got some 25mm solid bar and clamped it under the 25mm box section top and bottom and using my above clamp method,you say in the middle? is that in the middle of the length of the box section or in the middle of the 25mm end to end where I'm welding?

 

[DanielW]

Whatever you do on one side repeat on the opposite side

If you cant, either offset it and hope or strongbacks (clamped or welded)

thats what i was thinking,do I let it cool first then turn it over and do the opposite side to the first weld.

 

[rikrobson]

you'll never clamp the angle like that either. the bottom flanges will go narrowe than the box and the tops flare out

 

[TechnicAl]

thats what i was thinking,do I let it cool first then turn it over and do the opposite side to the first weld.

yes

 

[Mr Fro]

you'll never clamp the angle like that either. the bottom flanges will go narrowe than the box and the tops flare out

Not if he threads the bolt side and uses a lock nut on the inside of the other bit of angle or just uses locking nuts on the inside of both pieces.

 

[Welderpaul]

Is this just to weld two lengths together end to end? Can't you just use longer lengths of box?

 

[DanielW]

Box frame bed,the clamp I thought I might be able to reduce the heat by using it as a heatsink somewhat and also at the same time using it to keep the corners together at a 90* angle to get a weld in place

The clamp shape is L shaped

 

[Richard.]

Clamping and mechanical restraint is a mean of distortion reduction but only to a point. Distortion is created by localised heat input and usually effects are worse with low conductivie metals. Aim to reduce it with welding technique and use your clamp as a secondary measure. Turn your unit up nice and high and blast it down at a fast travel speed. Heat input is always devided by travel speed so the quicker you travel the lower the heat input and distortion is reduced. Sounds crazy but a tig run at 120 amps going veeeery slowly will create worse distortion than a 3.2mm sub arc wire at 450 amps traveling at 600mm/min.

 

[DanielW]

Thanks for the info,guys...guess I'm going to chicken out on the welding and nut and bolt it together with angle iron on its corners.I'm not to bad on welding with the mig but on this type of welding I don't want the CNC bed warped and the 3 metre length 25mm cost me £25? so don't fancy buying another length if I ballsed it up.

I may attempt to tack weld it at certain areas but I'll see how rigid it is first with nut&bolt points.Will throw a pic up when I've cut all the pieces and bolted it together.Should be ok?

Cheers.

 

[DanielW]

So I had to phone back up for clarification at me local steel stockist as internet was double the price.

Angle iron 50x50x3mm 6mtre length... internet £48 odd give or take £2.
Local stockist £24 odd.
£6 extra for cutting it in 4x1mtre lengths.

That'll do me

 

[Wallace]

Are you building this from plans or on the fly?

 

[Kent]

So I had to phone back up for clarification at me local steel stockist as internet was double the price.

Angle iron 50x50x3mm 6mtre length... internet £48 odd give or take £2.
Local stockist £24 odd.
£6 extra for cutting it in 4x1mtre lengths.

That'll do me

50x50x3 rsa is unlikely to be dead straight once it's come off thier racks and been delivered anyhow- check it out!
On a job like this I might look for something a bit beefier and in bright
Always find angle a lot harder to get welded up at dead 90 personally and if it were me I should look twice or thrice at the design
Shs with lightly stitched in supports perhaps?

 

[DanielW]

RSA, whats that?

 

[DanielW]

Are you building this from plans or on the fly?

Yep on the fly,always in me head me plans,never on paper but the brains going ten to the dozen thinking out any problems as it transpires.....even with watching the telly.

 

[DanielW]

I'm gonna use bolted angle iron for the bed lengths and for the corner uprights 25mm box section.

 

[Rocketman]

Yeah in machine terms structural steel will be as flat as the sea. Really depends on what kind of tolerance you want on the frame. Personally I would look at the final tolerance required and then reverse engineer back from that with all the known tolerances and stack them up.

If you want anything accurately bolted together you need to use dowels. You can get dowels with a hole through the middle for a bolt to pass through.

 

[DanielW]

Yeah in machine terms structural steel will be as flat as the sea. Really depends on what kind of tolerance you want on the frame. Personally I would look at the final tolerance required and then reverse engineer back from that with all the known tolerances and stack them up.

I know the accuracy will suffer with rolled mild steel angle iron and there would only be two points to consider when bolting it together,the bed where the job gets clamped and the bars supporting the supported rails.
If I can overcome those two points I suspect it will be fairly accurate,a guy in america on you tube outlined this when he constructed his bed in mild steel,the bed he got round by getting it to a decent level then machined it even whilst in situ.