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Damaged threads

  • Thread starter Thread starter Guest
  • Start date Start date
G

Guest

Guest
This isn't in a motorcycle. A friend has a grain mill, used to crush malted barley for home brewing. I'm making a stand for it and motorizing it for her. The mill has two rollers, and at each end the rollers are held by aluminum plates, 1/2" thick, roughly 3" x 3". To mount the mill, the manufacturer tapped each plate in the bottom center, 1/4" x 20 tpi, about 7/16" deep.

The problem is that one of the holes has damaged threads at the surface. I may have cross-threaded it about one turn. The deeper threads seem OK. I'm looking for ideas on how to get past the damaged area.

> drilling: I'm afraid that drilling will always leave a damaged thread where I need to start.

> tap: I have the correct tap, but can't see any way to put it so the what it cuts would align with the existing threads.

Has anyone fixed a problem like this? I'd prefer to keep it simple, instead of drilling it out completely, and re-tapping at a larger diameter, or Heli-
Coiling it.

Thanks

Tom
 
If it's only the first thead or so that is damaged get a tapered tap. It will extend down into the hole a few threads and pick up on some of the good ones to get you started proper.
 
i'd go with the tap, though I'd be a little afraid the tap might seat in the malaligned thread and tear into the others. any possibility of getting the tap or the bolt in from the other side? -sounds like the holes are only 7/16 deep so probably not. a delicate touch and a die grinder could take down the bad thread, or even a little emory cloth on a dowel.
 
I'd agree with the above. Try a tap because there's a good chance it will work out okay. Definitely a tapered tap. If it doesn't work and tears up threads, just drill and re-tap to larger size this time using a bottoming tap. Can you just drill all the way through the plate and use a bolt and nut?
 
If the threaded hole is all the way thru the plate run the tap in from the other end. Or increase the chamfer dia.
 
Walt's right....increase the chamfer a little, and use a starter(tapered) tap.
 
Worst case scenario if the tapping doesn't work you could fill the hole with HTS-2000, re-drill and re-tap it to the original size, and all will be well.

If you go this route I'd recommend you drill the hole out to a larger size before filling it and then when you drill and tap you'll be doing so in the stronger HTS-2000 material, rather than cutting new threads at the bond point between the two metals.

Here is the link to HTS-2000:

http://www.aluminumrepair.com/

Regards,
 
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