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What size hole for 7/8-18 Tap?
- Thread starter CluelessNewB
- Start date
All,
Just throwing-out a wild data point here...
Last night, I made a special threaded shaft and nut (5/8-20) out of some drops alloy (it was one of the common 41xx alloys but not sure which one). Had I drilled the holes according to the 75% charts, it would have been darn near impossible to tap. Ended-up going about 10 thou over recommended. The fit was absolutely perfect and engagement looked better than any factory bolt by far...
As a practical matter for homeshop use, there's a lot of latitude -well indicated by the fact that the recommended drill holes are usually one size rounded-up to the closest bit size. When I know I'm making something "important", I shoot for at least 75% engagement or higher for softer metals. But every now and then, it's just not that critical and I cut myself some latitude...
Ray
Just throwing-out a wild data point here...
Last night, I made a special threaded shaft and nut (5/8-20) out of some drops alloy (it was one of the common 41xx alloys but not sure which one). Had I drilled the holes according to the 75% charts, it would have been darn near impossible to tap. Ended-up going about 10 thou over recommended. The fit was absolutely perfect and engagement looked better than any factory bolt by far...
As a practical matter for homeshop use, there's a lot of latitude -well indicated by the fact that the recommended drill holes are usually one size rounded-up to the closest bit size. When I know I'm making something "important", I shoot for at least 75% engagement or higher for softer metals. But every now and then, it's just not that critical and I cut myself some latitude...
Ray
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Thanks everyone. I didn't have a 53/64" but I did have a 13/16". I beveled the edge just a tiny bit to get the tap started and it worked great. Oh I ended up using 6061 AL. This is just a cap over an unused tachometer fitting on a Perkins 4-108 diesel. It originally had a plastic cap that had cracked. I would have been happy to just buy a replacement plastic cap but all the suppliers were out of stock and after months of promises I gave up. So for a part that should have cost about $1 I spent about $30 (I had to buy the tap). But I had fun 
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