Preventing warpage when only one side can be welded

I am building a new hub for a 8" rim I use on my mini chopper. I
turned down a tube to about 3/16" thick, and bored each end to fit a
new bearing. I then CNC machined 2 mounts out of 1/8" thick steel, one
for my rim and one for a disc brake. They each have four mounting
holes on them.
I then attempted to weld the two mounts onto the tube. The mounts fit
very snug on the tube so I was able to get them perfectly square before
I started welding. Though for the hub to work properly, I can only
weld on one side of each mount becuase the other side needs to sit
flush against the rim and the disc brake. This is how the original hub
was designed.
I began by making small tacks 180 deg from eachother. Then I stitch
welded small portion 90 deg apart until I had a bead all around the
tube. But both mounts ended up warping up towards the welded side. So
now my mounts are not perfectly square to the tube, and I have about
.030" run out on my disc brake and rim when spinning on the axle.
So basically I have to start all over again, but I really don't know
how to avoid this warpage while welding on one side of a part. I
could use less heat, but I really want to make sure I penetrate the
1/8" steel. This is a wheel hub which has a lot of weight on it, 100's
of pounds.
Any suggestions in preventing this type of warpage would be greatly
appreciated.
Thanks.
Reply to
Terry
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Change the sequence of operations. First weld, then machine (the holes for the bearings).
Nick
Reply to
Nick Müller
Bevel prep the inside bore of the mount so that you have a 45 degree bevel 3/32 nds deep. Make up a dummy flange/disc to mimic the wheel and the disc. Bolt them to the mounts in order to hold the mounts flat during welding. With the preparation you only need to put in enough weld to come out a sixteenth of an inch maximum beyond the mount surface. You will have almost full penetration with the bevel prep. The more weld you locate further away from the centreline of the mount the more distortion you will have. I am guessing the mounts are just small flanges with bolt holes. Can you modify your CNC program to create several ribs on the open face of the mount. It would add fancy detail and create a more rigid disc mount. Randy
I am building a new hub for a 8" rim I use on my mini chopper. I turned down a tube to about 3/16" thick, and bored each end to fit a new bearing. I then CNC machined 2 mounts out of 1/8" thick steel, one for my rim and one for a disc brake. They each have four mounting holes on them.
I then attempted to weld the two mounts onto the tube. The mounts fit very snug on the tube so I was able to get them perfectly square before I started welding. Though for the hub to work properly, I can only weld on one side of each mount becuase the other side needs to sit flush against the rim and the disc brake. This is how the original hub was designed.
I began by making small tacks 180 deg from eachother. Then I stitch welded small portion 90 deg apart until I had a bead all around the tube. But both mounts ended up warping up towards the welded side. So now my mounts are not perfectly square to the tube, and I have about .030" run out on my disc brake and rim when spinning on the axle.
So basically I have to start all over again, but I really don't know how to avoid this warpage while welding on one side of a part. I could use less heat, but I really want to make sure I penetrate the 1/8" steel. This is a wheel hub which has a lot of weight on it, 100's of pounds.
Any suggestions in preventing this type of warpage would be greatly appreciated.
Thanks.
Reply to
R. Zimmerman
With welding you can reduce but never eliminate the warpage. If it's critical, weld first, machine second. Sounds like you need tolerances in the .001" to .003" range, no choice but to weld first. In fact, you would be well advised to stress relieve the part after welding and before machining. If you don't, the vibration from use will stress relive it for you, your mounting flanges will drift.
Terry wrote:
Reply to
RoyJ
Thanks for the information. Yes, the mounts are small flanges with 4 bolt holes. I will bevel the inside bore of my flanges, and build a mount to keep the flanges from bending. Some 1/4" steel should work pretty well. I'll probably step down the current one notch and take a little more time in between each weld to minimize the heat.
Do you think that even if I sucesfully weld the flanges on the tube without warpage (while mounted to some sort of support piece), that when I remove the support piece from the flange, it will have some sort of memory and warp anyway? It would probably be best to let the piece completely cool down before I pull anything apart.
I could add some ribs to the flange, but I would have to start out with a thicker piece and take off a lot of material. How tall and how many ribs would you recommend for an 1/8" thick flange. It is about 4" diameter with a 1 1/8 center bore.
I don't have the option to weld first and then machine. My mill does not have the Z travel to accomodate my hub.
Before I begin machining a new hub I will try the old hammer and dial indicator trick to see if I can true up my flanges. Maybe I'll get lucky this time! Thanks.
Reply to
Terry
It will.
Doesn't help that much.
I would (if you insist on machining then welding) weld it in a different way that makes less warpage.
Drill holes into the flanges (diameter 6..7mm) and countersink them (on the outer side). The holes have to match the center of the tube's wall. Weld into those holes with a MAG (quite high current). These welds tend to pull the flange flush to the tube. Of course you have to clamp your work while welding. Trick is to make the welds in a short time with high current that assures that you melt the tube. You won't have succes with stick welding and less success with TIGing. Make some trial welds to get the trick. It is not very difficult. There are even special gas nozzles (for MIG/MAG) for that job, but you don't need them. I have one but never use it. Holes can even be bigger -to some amount- than the tube's wall thickness
Nick
Reply to
Nick Müller
First, I doubt that the mount has to be welded all the way around. They seldom do in HSM projects.
*Assuming* that they do not, here is what I would do (it's what you might call a vertical plug weld): on your mount, before you cut out the hole for the tube, drill holes (3/8 diam +-) around the circumference of the hole, centered on the circumference. Maybe 8 (every 22 1/2 deg). When you then cut the hole for the tube, it will have semicircular notches around it. Fit the mount to the tube and fill in the notches with weld.
Let us know if you try this and how it works, Bob
Reply to
Bob Engelhardt
I would try something like an extra one eighth rib on the outside rim. It is only four inches in diameter so that should do it. It will locate your weld closer to the neutral axis of the disc. I agree with one comment about overwelding. Can you shrink the assembly on and just put a couple of locating tacks in??? I worked for a company that build wheels for ore trucks. Their original design called for simply shrinking the rim onto the steel centerpiece. No weld was required at all. Later designs had just a little bead to locate the rim on the centerpiece. These were big wheels handling hundreds of tons payload. Your solution will end up being the combination of all the contributions here. Yes you are right about letting your assembly cool after welding. Warm to the touch is OK. Randy
>snip
Reply to
R. Zimmerman
Oops, that would be every *45* deg.
Reply to
Bob Engelhardt
I like the idea of shrinking the flange onto the tube. I can easily machine the flange I.D. to be a couple of thousands smaller than the tube O.D. Then I could use a hotplate to heat up the flange then slip it into place and let cool. I could then put some very small tack welds in place.
How much undersized would you recommend making the flange I.D. compared to the tube O.D. for a permanent fit with no chance of slippage?
If I can't true up my existing assembly, I will go this route on the next one. Thanks.
Reply to
Terry
I was so taken with this idea that I had to try it. View from the outside (front):
formatting link
from the inside (back):
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The shaft is 1" diam, the notch was drilled with a 5/16" bit. A 3/8" notch would have made it easier to get weld deposited in the back/inside.
I know, the weld is cold. But that was the welder's fault, not the technique's.
Bob
Reply to
Bob Engelhardt
For steel the coefficient of linear expansion is about 12 x 10e-6 per degree C. Figure the max temp difference you can achieve while assembling the worst case fit (oversized shaft tolerance with undersized hub tolerance. If you put a 1" shaft in some liquid nitogen at -196C and heat the hub to the max your home oven will go (say 270C) you will get .0055" of total expansion/shrinkage. The max interferance fit you can get in practice has a lot to do with how accurate your machining is plus how well you can assemble the parts before they start normalizing their temps.
We did one aluminum prototype in two pieces that was supposed to have .002" clearance with the temps we were using. We wound up using a press to assemble since the aluminum conducted the heat so fast. But the parts were bonded together1
Cheers.
Terry wrote:
Reply to
RoyJ
idea:
cut slight taper on part to be inserted (less than a morse taper, like a degree, or maybe even half a degree), with matching taper in the axle end. freeze spindles, heat outer tube, slam together "pronto-like" (wood block, vice, huge hammer). bonded forever, no welding, no add'l fasteners.
reamers -might- be available in sizes correct for the outer tube, to simplify things...
Reply to
dave
Using a taper will work but makes it very tough to get the longitudinal position correct.
dave wrote:
Reply to
RoyJ

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