Best way to repair a casting with Devcon?

[bhowden]

I am fitting stepper motors to my lathe and needed to drill and tap a hole very near the edge of the cross slide casting. The hole was fine but the tap broke through the edge of the hole. The bolt threads in fine but clearly it is not as strong as I would like. Is it worth building up the edge of the casting with DevCon? If so, can I clean the casting, oil the bolt and thread it in the hole and then build up (slather it on) the side of the casting with Devcon and still expect the bolt to unthread? I cannot go over size and I won't be able to re-drill and tap the hole again. Any other suggestions? Are any of the Devcon (steel, aluminum etc.) better or stronger than any other or is it just cosmetic?

Brian

[JoeLee]

Unfortunately repairing something like that isn't going to offer any added strength to the threaded hole. If all your looking for is a cosmetic fix then have at it. I've done it many times. As for weather or not you should oil the bolt, I say not.......... reason being when you thread the bolt into the hole the oil is going to crawl all over the interior of that hole and may spread to the areas where you don't want it resulting in a weak bond.
What I've done in the past is make sure the bolt is free of oil thread it into the hole, plaster up your Devcon patch and when the stuff just starts to set back the bolt out slightly just to break the bond on the threads. Even if you left it there you would still be able to turn it out with out doing any harm to the patch.

JL...................

[Glug]

You could give us a bit more information. A photo saves a thousand words. What size and pitch? How much engagement? What sort of loads does the bolt see? Bending, shear, tension? How much installed torque? How well is the load shared with other fasteners? How much has broken through?

I'm not sure if it would help, but you could bolt a piece of material there and try and re-tap. Helicoil? Maybe sink a dowel next to it? Could you replace the bolt with a stud? That's often a good choice for a compromised hole.

If you are trying to get strength out of the devcon, I'd go for a metal filled variety. You can compare the properties and specs of the various formulations, though it may not actually matter much in this case. It sounds like you'll want it to be thick so it doesn't run, so additional filler may also be helpful.

[bhowden]

Thanks for the input. It is a 6mm X 1.0 threaded hole (call it a 1/4" NC bolt) about 10mm (3/8") deep. The load is mostly tension with no shear on the bolt. I can get a second fastener in nearby but it is not an optimal location for supporting the motor. The two will be fine so I suppose the patch is really more cosmetic than structural but we all like to think we are helping when we fix things.....

Brian

[TGriffin]

Devcon is good in compression due to the metal fillers, but it sucks in tension (2800 psi for Devcon vs. 40,000 psi for aluminum). I used it once to repair worn brake calipers on DC8's (an approved repair) and was amazed at how well it performed, but it was entirely in compression where the pads rode on the calipers.

Your application would be entirely cosmetic.

Tom

[Cuttings]

If you don't want the Devcon to stick to the bolt rub the bolt up against a block of Para wax.
The wax won't get into the bond area and once the Devcon has gone off you have perfect thread.
This is an old trick people working with fibre glass and Epoxy use all the time.
Another good product is Marine Tex.

[bhowden]

That was exactly the info I needed. The compression vs tension makes perfect sense and is almost intuitive. The wax worked perfectly and I now have a good cosmetic fix. Turns out I can work in a third fastener in a location that will supply the necessary strength.

Brian

[Glug]

"A little bondo will cure us of this deed" - Macbeth, Act II, Scene II.

[Tilaran]

Go to this site and look around.

http://muggyweld.com/all-products/alloy-1