Damaged threads

[themess]

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

[Nessism]

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.

[smagnusen]

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.

[onchiman]

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?

[walt]

If the threaded hole is all the way thru the plate run the tap in from the other end. Or increase the chamfer dia.

[hp1000s]

Walt's right....increase the chamfer a little, and use a starter(tapered) tap.

[Planecrazy]

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,

[themess]

The tapered tap worked well!

Thanks for the suggestions.

Tom