Centering for welding issue 3

[bssem]

I need to TIG weld two machined (cylindrical) SS316L parts. Both parts have a identical central bore which has to be perfectly alligned after welding. I have considered using various centering features and clamping options but was unable to get the allignment of the central bore after welding. Is press fit an option? If so could you pl. direct me to such source / offer guidance?
thank-you for your help.
bpss

 

[MikeHalloran]

Parker and others offer tube fittings intended for assembly by automatic orbital butt welders, also commercially available. Take a look.

If you have access to the bore after welding, you could use a consumable insert for the root pass, and locate the parts with a drift stuck into both parts, having a central relief on the o.d. to clear the insert. If the insert melts correctly, it should be possible to pull the drift past it after welding. If it doesn't, you'll wish you'd used a collapsible drift, like a collapsible core for a mold, which is expensive to make nice, but can be done.




Mike Halloran
Pembroke Pines, FL, USA

 

[MintJulep]

Would it be possible to manufacture the two parts with a smaller than needed central bore, weld them together and then bore out the finished weldment to the desired final diameter.

That is the only way to them "perfectly aligned".

 

[BillPSU]

bssem
"perfectly aligned" what is the tolerance you want? What is the tolerance on the bores?

Welding by its nature adds stresses to the part. I assume you are doing a circular weld around circumference of the part. In the area of the weld the bore will probably be distorted smaller than the orginal size. Depending on how you locate the two parts they will be pulled together by the cooling of the weld. The locating surfaces must be perependicular to the bores or the finished weldment will have the bores out of alignment.

The approach I would use is (2) expanding mandrels held in a fixture in alignment. Put the parts on the mandrels and then clamping the parts together making sure the faces are square to the bores.

The approach I try to take whenever possible is the machine the part after welding. Welding is not a precision process unless many variable are very repetitive; meaning under machine control. Hand welding of any kind is not a precision process and unexpected results can occur because of changing welder, changing welder setting, changing wire or simply by doing a partial weld and restarting.

 

[bssem]

Thank-you All for your feedback. BillPSU you are correct with your assumption that we are doing a circular weld around circumference of the part. The dimension of the bore is 0.339 with tolarence of +/- 0.8 thousandth of an inch. As you have said the welding is dependant on many variables, will press fitting the allignment feature to aid welding be any better?

 

[MetalsInc]

Press fitting then welding will not keep a tolerance of 0.0008" The heat will most certainly distort the piece. I agree with those who say make the ID smaller than needed and then bore it out. Hopefully it's not a very long piece to the weld.

Consider the steps in fabrication to making post weld machining an option if it isn't already.

 

[BillPSU]

If you "have" to weld this assembly there are somethings you can do to limit the impact of the weld distortion.

Move the weld away from the critical area. Make the critical bores in one part and weld it to a non-critical part.

Move the direction of the weld. Make flanges on the parts and plug weld the flanges together hopefully limiting the distortion.

Change welding process. Use laser or EB welding and limit heat input and heat affected zone size.

Utilize a pressfit guide between the parts with a flange for added cross section to keep alignment and limit distortion.

Don't weld, press fit, screw or bolt the assembly.

There are many different techniques to reduce or control welding distortion. Give us a better idea of what you are trying to accomplish with this part.