Help me avoid distortion pls

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I have these parts I've  made.  The OD is 2.5" the wall thickness of those cups is 0.250"The bearing races are very snug fits.  That's my worry.  Deforming the races.The steel  can be welded.  It is neither free machining ( oh boy is it not free machining~!!) and it is not leaded. I want to weld a 3/8" thick  1 1/2" wide by 3" long bar to the exterior of the cups.The stresses  on the finished product are not great.  Maybe the load might be as high as 40 pounds.These parts are is a lot bigger than  it needed to be  because I designed the whole thing around the dirt cheap tapered roller bearings I found.So it looks like it's  got to take a ton of loading but it isn't.   The final product is an articulated hollowing system for a wood lathe. Mechanical attachment is pretty much out of the question. The welder is a Thermalarc 211i with Mig Tig and Stick  The welding skills are not great.I am thinking that if I use the MIG operation I can get a setting that  gives me sufficient penetration I can sort of "spot" the welds in small increments letting the parts cool between each weldment. Pre heat  may be possible.  I have to jig this all up for welding and the Jig material  is going to be  Maple (wood)   and there will be a fair bit of handeling to get things in positin  I can pre heat the parts ot maybe as mush as 400F.Weld Beauty is not a requirement.  I am unafraid to use Weld Improvement compound (bondo) before paint. What do you think? Should I pre-heat or not? Welding as described  do you think that I avoid distortion?Hey~!! It's a hobby. It's not supposed to make sense~!!
Reply:Pre-heat is not necessary on 1/4" cup wall or 3/8" bar. Can you reduce bar thickness from 3/8" to 1/4"? It will allow a lower voltage setting. Are the flat bars to be coped to radius of cup od? A tight cope would allow for a low heat tig fuse versus high mig heat. Can the bar be welded to cup prior to race/bearing assembly?I'm interested in the design of the hollow rig your building. Any chance we can see pictures or drawings? I've built a few different styles in the past.Miller TrailBlazer 251Miller HF-250-1Miller MaxStar 150 STLHyperTherm PowerMax 380 plasmaLincoln PowerMig 180Millermatic 252Miller Diversion 180
Reply:Originally Posted by waginPre-heat is not necessary on 1/4" cup wall or 3/8" bar.
Reply:As for pre heat, you have studied the tech. spec more than I. But I would if pre heated, the weld the bar to cup, the bar would act as added heat sink, possibly causing distortion. If you plan to fuse tig the pieces, I would not pre heat.For coping, if you have a drill press and the right hole saw, coping is easy.Miller TrailBlazer 251Miller HF-250-1Miller MaxStar 150 STLHyperTherm PowerMax 380 plasmaLincoln PowerMig 180Millermatic 252Miller Diversion 180
Reply:If the bearing race is a tight fit, you may have to preheat the welded assembly and/or freeze the bearing races to fit anyway.
Reply:Constrained expansion with rapid heat always happens to a degree, whether it's enough to affect a bearing fit is on a case by case basis I'd guess. When rapid heat is applied the normal effects of thermal expansion don't have time to come into play, the metal in the heated spot has to expand somewhere, is restrained by the metal of the larger area around it, so it has to 'push out' into the surrounding area away from the plane of the metal. When the cooling begins the metal in the surrounding area, being still under normal compression itself starts coming out of compression towards the heated spot that has now expanded (and is still hotter) compressing it a small bit and filling the voids left by the hot active molecules (uncompressed heated weld area). This leaves a bump where the rapid heat was applied. If this were on a flat plate possibly both sides would have this bump. The same will happen on a tube. We're not talking about the shrinkage of the molten weld metal at this point yet (that exacerbates distortion) just the heat effects. An experiment would be to take a 1/4" piece of plate and a fine tipped torch. Heat a dime sized spot red quickly, only one half to two thirds the way through if possible .. Let it cool plenty then hit it with a sanding disc. The bump left from the heating should be able to be felt with the fingers but the sanding will definitely bring it out. Flip it over and check the back side. Good chance of there being a raised bump there too. Sanding will show it easy. Heat another spot to red all the way through and there will will dern sure be a small bump on both sides. Check it out for yourself as an experiment 1st and prove me wrong here. I'm saying this is going to happen on a tubular structure too. So if it is a precision machined inner diameter with a press fit bearing I would plan on turning or honing out the finished size when all is said and done. Heat or weld a button sized spot on the outside of a nice smooth tube. My guess is there will be a corresponding bump on the inside.Not sure how much, if any preheat will reduce the effects of the rapid heat expansion. Trial and error."The things that will destroy America are prosperity at any price, peace at any price, safety first instead of duty first, the love of soft living and the get rich quick theory of life." -Theodore Roosevelt
Reply:If you don't need super strength, what about brazing?  That should pretty much cut out any heat distortion.KevKevin / Machine_Punk from The Aerodrome Studio - Lincoln PowerMIG 210 MP - Meco N Midget w/custom welding station - Vintage Victor 100Current Projects: The Aerodrome Studio
Reply:thanks guys.   After long  consideration I decided to revisit the "mechanical attachment" scenario.I ordered some 3" dia A-36 steel cut in 1 & 1/4 lengths.   I'll put that on my itty bitty Elgin Jeweler's lathe and see if I can turn the guts out of them  to get an ID that matches the OD of my parts (about 1.490" + or - about 002")   Then I'll cut those rings in thirds.  This will give me  ring segments about a quarter inch thick,  Then I'll   clearance drill and spotface them for 4  1/4-20 Allen screws drill and tap corresponding holes in the knuckle cups and weld 1.25" wide x 3/8" a-36  bar stock to them using 6010 stick rod.   Even a welding illiterate like me can get that done.  Then I can just bolt the welded-up part to the cups and avoid ruining  my very snug bearing races. I'll use locktite on the threads to keep the grease in. I think this is for the best no matter because  after I tried to cut up the test welds on coupons of the metal I used for the Knuckles with the Bearing race cups  I discovered that  the material all around the weld area was super hard.    I though this was tough stuff to machine.  Turns out it is most likely 4041 steel which though weldable,  demands a 600 F  soak in pre heat and post weld heat and still  presents issues with cracks on the weld.  If welding with soft rod  hardens, the steel then most probably  I'd need to anneal it after welding to avoid stress cracking and I just can't do that.  All those heat cycles  on bearing races?  Plus each knuckle assembly has three O ring grease and dist seals which all present  dimensional sensitivity.Last edited by Raul; 09-04-2015 at 11:35 AM.Hey~!! It's a hobby. It's not supposed to make sense~!!
Reply:I have welded many parts where machining tolerances have to be kept. We usually do the welding, then the machined parts get re-machined after welding to bring them back to tolerance.Rich
Reply:Originally Posted by steelsurgeonI have welded many parts where machining tolerances have to be kept. We usually do the welding, then the machined parts get re-machined after welding to bring them back to tolerance.Rich
Reply:Originally Posted by Machine_PunkIf you don't need super strength, what about brazing?  That should pretty much cut out any heat distortion.Kev