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Thread: Drilling and tapping (tight)
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4th Feb 2021, 04:55 AM #1New Member
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Drilling and tapping (tight)
Hi folks.
I have a good range of high quality taps, I don't as yet have any thread testing tools as a set in the range I'd need would be pretty expensive. I've noticed all the bolts I've bought in the past, cold, stainless, etc are all cold rolled to be slightly under nominal. There are obvious reasons for doing this, but what do you do if you need a very tight fit, one that will turn easily but with no slack.
I've tried a few tests with delrin, drilling undersize then tapping normally and the results aren't all that different, so I'm guessing I 'm just asking the tap to do more work.
Before I go breaking a load of expensive taps I thought I'd ask the obvious here, how can I tap a tight hole? Or do I need to find a supplier of perfect bolts?
Thanks.
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4th Feb 2021, 06:31 AM #2Philomath in training
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I suspect that it is better bolts. I remember when working for a defence contractor that we had a particular grade of bolts specified on the drawing, just to ensure they were the right fit.
You may have to go to a specialist fastener supplier for 'good' bolts. I would not guarantee the goodness of fasteners from a general hardware shop.
Michael
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4th Feb 2021, 08:19 AM #3Most Valued Member
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4th Feb 2021, 08:39 AM #4Member: Blue and white apron brigade
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4th Feb 2021, 09:17 AM #5Most Valued Member
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You can also buy a different fit class of tap (effectively undersize taps) in the EU the tap will have a fit number h1, h2 etc. I have some undersized taps from sutton but I dont know if they are easily order able.
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4th Feb 2021, 09:34 AM #6Most Valued Member
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What’s the application that requires Aerospace tolerance?
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4th Feb 2021, 09:57 AM #7Most Valued Member
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Who said anything about aerospace tolerance?
Chris
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4th Feb 2021, 12:43 PM #8Most Valued Member
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Other option is to try a form tap. I find they produce a marginally tighter hole (I only have experience with m10 form taps and only from one brand so ymmv(
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4th Feb 2021, 05:43 PM #9Gear expert in training
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You can buy different tolerance taps, but the only real way to ensure a neat fit is to make your own bolts (or have them made to your required tolerance)
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4th Feb 2021, 06:23 PM #10Most Valued Member
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Expression of speech, wasn’t meant to be taken literally..
Although once you move beyond ISO/SAE type tolerance you pretty much find yourself in NATO/MIL spec.
My experience is generally when people start asking about tight tolerance thread it normally means they want to locate the parts using the fasteners rather than locating them with dowels and clamping them with fasteners.
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4th Feb 2021, 06:34 PM #11Senior Member
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Threads are for clamping not precision locating.
You could use or make shoulder bolts. Threads to clamp and precise shoulder to locate
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4th Feb 2021, 07:20 PM #12Most Valued Member
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Threads can also be used for things like a vernier depth stop or a fine adjuster on a table saw fence. I recently made these and cut my own male threads to give almost backlash-free movement.
Admittedly I have no idea what the OP wants to do with his snug fitting threads. I’m just spit-balling.Chris
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4th Feb 2021, 09:24 PM #13Most Valued Member
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Welcome to a TOP FORUM. Sounds like what you need are shoulder bolts.
Shoulder screws are essentially a socket cap screw that is only partially threaded with a shoulder. ... These bolts are often used on machinery to provide a smooth surface for bearings to ride on. Shoulder bolts are measured by their diameter of shoulder x length of shoulder x thread diameter.
Shoulder bolt.jpg
HTH
KrynTo grow old is mandatory, growing up is optional.
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4th Feb 2021, 09:27 PM #14Gear expert in training
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Where did this shoulder bolt stuff come from?
All OP asked about is how to get tight mating threads from off-the-shelf bolts...
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4th Feb 2021, 09:38 PM #15Most Valued Member
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