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View Full Version : Cracked Rotary Table Mount - Fix it?



Professor
07-30-2008, 09:51 AM
I have a 15" Bridgeport rotary table that exhibits some lateral play when the direction of rotation is reversed. I disassembled it and found the casting that holds the table bearing to be cracked.:(

I don't have a way of supplying enough heat to braze it back together (nor a good way of remachining the diameter afterward to closely fit the bearing). Any suggestions as to how I could stabilize the crack to hold the bearing securely?

Thanks in advance,

Larry

http://www.gamesandexercises.com/photogallery/Crack1a.JPG
http://www.gamesandexercises.com/photogallery/Crack2a.JPG

dfw5914
07-30-2008, 10:02 AM
If there is sufficient clearance, perhaps a steel ring around the outside of the bearing boss?

JCHannum
07-30-2008, 10:03 AM
Ouch. How did they do that?

Mount the table on the mill, and using a boring set up bassackwards, machine the OD of the first step of the boss to remove the cast gussets and clean up the casting to some measurable diameter.

Turn up a steel ring with an ID under the OD and shrink fit it to the boss. The home oven can be used to heat the ring sufficiently to get a significant shrink. Look up shrinks in Machinery's Hand book to determine sizing the ring.

derekm
07-30-2008, 10:04 AM
After drilling the end of the crack and suitable surface preparation , layup carbon fibre and epoxy around the the outside of the central boss. Lay the tows of carbon fibre around in a complete circle around the bearing boss to use the tensile strength of the carbon to prevent the crack opening. As always YMMV

Derek

rotate
07-30-2008, 10:22 AM
Why is silver soldering not an option?

JCHannum
07-30-2008, 10:42 AM
Why is silver soldering not an option?
The table still has to be raised to red heat for silver solder to work. That coupled with the fact that the crack is probably oil soaked, makes any braze type repair problematic unless proper cleaning and V-ing out of the cracked area is done.

If the heat shrink sleeve is used, it would be a good idea to drill the bottom of the crack to help stop further propogation.

Bruce Griffing
07-30-2008, 11:28 AM
yeabut..... on the shrinking ring approach.... If the internal bearing surface has a small step at the crack when the machining is done, the ring will just stabilze that step. It needs to be held in alignment during the machining somehow... but I am not clear on how that could be done.

JCHannum
07-30-2008, 11:29 AM
yeabut..... on the shrinking ring approach.... If the internal bearing surface has a small step at the crack when the machining is done, the ring will just stabilze that step. It needs to be held in alignment during the machining somehow... but I am not clear on how that could be done.
Install the bearing. It is a rotary table, any minor inaccuracy at that point has little, if any, effect on the outcome.

Bruce Griffing
07-30-2008, 11:42 AM
Well - maybe this is just a theoretical problem, but installing the bearing may just open the crack rather than stabilize the step. If the bearing surface is slightly sprung, it is not clear to me that installing the bearing will help.

dsergison
07-30-2008, 12:03 PM
intall the bearing with epoxy or locktite and forget about it.

it'll take up the slop. and you'll never stress it enough to grow that crack any more.

gnm109
07-30-2008, 12:07 PM
Ouch. How did they do that?

Mount the table on the mill, and using a boring set up bassackwards, machine the OD of the first step of the boss to remove the cast gussets and clean up the casting to some measurable diameter.

Turn up a steel ring with an ID under the OD and shrink fit it to the boss. The home oven can be used to heat the ring sufficiently to get a significant shrink. Look up shrinks in Machinery's Hand book to determine sizing the ring.


I think that's a superb idea. It would require some precise machine work but it should be a trouble-free repair.

I bought a "G" Scale (Taiwanese) model of an American Diesel freight locomotove and upon running it, discovered that all of the wheels wobbled in an annoying manner. The dealer refused to make good in the problem. When I disassembled both trucks I discovered that the plastic wheel mounts were cracked on each side where the short axles pressed in. After removing the pressed-in wheels, I machined aluminum collars sliightly undersized and pressed them onto the wheel mounts with a vise. This drew the cracks together so that the wheels ran straight after they were pressed back into the plastic axle mounts. That was about 7 years ago and the locomotive is sitll running on my layout.

Using a collar on the Bridgeport mount should bring the bearing hole back to original size. Once a ring is shrinked onto the machined area, it should be a permanent repair. Great idea!

JCHannum
07-30-2008, 12:08 PM
This is not a jillion RPM jet turbine, it is at most a 1 RPM rotary table. If the bearing mount becomes deformed, it can be cleaned up with a boring bar. The worm engagement is adjustable, so the bearing mount can be somewhat off center of the original location with no effect, it need only be parallel to the table and at the proper height.

The entire boss could be machined away and a complete replacement made that would bolt in place from the bottom.

dfw5914
07-30-2008, 12:23 PM
Wouldn't the crack self align upon closing due to the grain structure of the iron.

Scishopguy
07-30-2008, 03:17 PM
intall the bearing with epoxy or locktite and forget about it.

it'll take up the slop. and you'll never stress it enough to grow that crack any more.

I have used Loctite's bearing seating compound before and it would do a fine job on this type of problem. It adheres the outer race to the pocket to prevent rotation in oversized pockets. It should stick that race firmly enough to keep the crack from opening. And, unlike epoxy, you can heat it and it will let go if you need to take the bearing out in the future.

hardtail
07-30-2008, 07:17 PM
http://www.locknstitch.com/

A proven crack repair option that works, many engines etc have been saved with this one............

oldtiffie
07-30-2008, 07:44 PM
The absolute last thing I would do is to apply any significant amount of heat as there is no way of being sure that it will not be distorted when cooled. Irrespective of whether the "weld" can even be done or not.

Even if it is brazed, bronzed, silver-soldered or cast-iron welded etc. it may well be impossible to restore the table back to reasonable specifications/limits.

If I recall correctly, there are "cold weld" processes that have been used to repair car engine blocks, cylinder heads etc. I am sure that some here know a lot about this from first-hand experience. There will/may be specialists in your area who do or can do this type of work.

Putting the original bearing back in and heat-shrinking a ring onto the boss after turning it may be an option. Drilling a hole at the end of the crack - as suggested - will be a good idea.

That must be one big mother of a rotary table!! How big and heavy is it? How do you get it on an doff the mill table? It must be one big mill as well!!!

Fasttrack
07-30-2008, 09:20 PM
intall the bearing with epoxy or locktite and forget about it.

it'll take up the slop. and you'll never stress it enough to grow that crack any more.


That was my initial reaction too. Locktite makes some really awsome bearing epoxy for loose fits. Its not cheap though. I've got a small bottle of the stuff and it cost 23 bucks. Damn good stuff, though.

How do you not have a way to machine the ID? Make your own if you don't have a boring head small enough for the job. Make a simple fly cutter, center the spindle over the bearing bore, then use the mill table to set the diameter of the cutter. Just make sure you return your table to the zero posistion before taking the cut!

If it were my table (and me being a bit of freak when it comes to my tools) I'd machine down the OD and shrink fit a ring, as suggested. Then, I'd scratch the inside clean if needed (to clear a "step" or "ridge" from the crack) and install the bearing with some epoxy. A wee tiny bit left/right/forward/backward of center won't hurt anything at this point, as JC says.

Professor
07-31-2008, 12:02 AM
Thank you for all your helpful suggestions. As many suggested, brazing/Silver Soldering wold not be feasible in this case. I took JCHannum's suggestion and modified it a bit. I wasn't comfortable removing all of the gussets - I felt they were providing significant support to the bearing - so I only machined down 5/16" and made a ring out of 1/4" aluminum.

I didn't have a boring head large enough to machine the outer diameter (it ended up being 3.752") but I did have a trepan that ended up working just fine as long as I took it slow.

When the bearing hub was machined I carefully measured it and turned a ring out of 1/4" aluminum with an inner hole targeted at .0135 smaller (3.75" x 300F x .000012 in/in F.). I put it in the oven at 350F and after about 15 minutes, took it out and it pressed easily over the hub. The crack drew shut although it remained visible, primarily due to the dark oil that remained in it. (The crack is barely visible in the last photo at the extreme far end of the bearing.) The bearing fit snugly in the resulting hole. I reassembled the table and I can see no horizontal translation as I reverse direction.

Thanks JC!

http://www.gamesandexercises.com/photogallery/Mach.JPG

http://www.gamesandexercises.com/photogallery/Ring.JPG

http://www.gamesandexercises.com/photogallery/Fixed.JPG

By the way, I DID clean out the base before reassembling it (in case anyone was wondering).

JC: I have no idea what caused it to crack. It had play in it when I acquired it. But, as you say, it must have been a big OUCH when it happened. Luckily it didn't affect the operation in any other way.

Old Tiffie: The table weighs 170#. It lives on a standard Bridgeport. I get it up there usually with the help of a friend or with a hoist.

Again, thanks to all for your excellent assistance. :D

Larry

Richard-TX
07-31-2008, 12:50 AM
Why is silver soldering not an option?

Because silver solder and cast iron don't play nice together sometimes.

Fasttrack
07-31-2008, 12:54 AM
Nice job! Looks good. Thanks for posting pics of the steps along the way.

I hadn't pictured the ring being near that big ... just a simple collar was what I had in mind.

hardtail
07-31-2008, 03:07 AM
If you haven't already I would still drill the end of the crack for precaution.

Professor
07-31-2008, 09:08 AM
I did drill a 1/16" hole where I imagined the crack ending. I couldn't really tell where it stopped since the shape of the structure at the visible end was complex. So I eyeballed it, drilling just past where I could see it end from the top and the bottom. Of course, the hole I made would then allow the oil to leak out so I drooled a bit of epoxy into it to plug up the leak I created.

I suppose I should have included a picture of that, too, but forgot to shoot one when it was visible. I get involved with the process and forget to do the the documentation.

Larry

JCHannum
07-31-2008, 09:14 AM
All's well that ends well, and that looks like a proper repair. Good job.

Loctite products are excellent for their intended purpose, which are usually thread locking, bearing retention or press fit applications. They are not epoxies, but anaerobic adhesives. They produce a high shear bond, but most do not add mechanical strength. That is, Loctite would prevent the bearing from spinning or pulling out, but would do little to keep the cracked casting from spreading.

gnm109
07-31-2008, 12:10 PM
That's a very nice repair. You can probably forget about it now. Good job!