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Thread: size hole to tap
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19th Dec 2018, 01:16 PM #16Most Valued Member
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Wouldn't surprise me Jordan. From the same article I quoted before:
"Multiple studies have shown that a 100 percent thread engagement is only 5 percent stronger than a 75 percent thread on pull-out force, but takes 300 percent more torque to tap. Better tapping conditions are obtained by reducing the percentage of thread engagement."Chris
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19th Dec 2018, 10:51 PM #17Senior Member
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This one seems to pop up regularly with magic figures re engagement percentages related to OD-Pitch calculation which is the general workshop formula and works for the majority of softer metals.
Has anyone actually sat down with the standards book or any reference book with the thread profiles to see what it all looks like? The maximum 100% engagement for Metric and Unified threads is 0.541Pitch. It can't possibly be any more other than a bit of metal flow which could increase the engagement to over 100%.
If we use the sample a few above of 1" UNC x 8tpi and use the workshop formula: 0.125 x 0.5 = 0.0625 which is the 7/8" drill as stated. Full depth of this thread would be 0.125 x 0.541 = 0.068" which equates to an 0.864 drill.
From this we can see that the percentage of engagement is actually 0.0625/0.068 = 92% (give or take a few pooftenths for rounding of decimals), quite a bit more than many think. Yes the drill may cut oversize but accidents shouldn't be taken into the calculation. If we want only 50% engagement on this thread we would use a 0.932" hole.
The thinking comes from thinking that the depth of the screw is what is used in the calculation. It is the amount of NUT depth which gives the percentage engagement and that can never be more than the 0.541 P as designed for 100% engagement. The only one to differ is BSW form where theoretically there can be 100% engagement of the 0.6403 P depth.
Rusty
PS. Yes I've heard the strength figures in TAFE classes but never bothered to work out those trivial things!
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20th Dec 2018, 01:10 AM #18
Hi Bob, Guys,
Go to
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Scroll to the bottom of the page and look for "Presto Counsellor" its a free printable download !
Tells you everything you want to know...Best Regards:
Baron J.
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20th Dec 2018, 06:54 AM #19Most Valued Member
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I think you might be confusing percent of thread engagement with length of thread engagement. This article explains the difference:
https://fieldfastener.com/2018/03/13...ad-engagement/
Unless you meant nut THREAD depth, in which case I'm confused, because isn't that what the tapping drill size will determine? Which is the topic of this thrread.Chris
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20th Dec 2018, 07:41 AM #20Most Valued Member
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Can you clarify this Rusty? From what I have found the agreed definition of 100% thread engagement is 0.75 times the full depth of the 60 degree thread. A lot more than your stated 0.541. I know where the 0.541 figure comes from, but it's not what the percent engagement is defined by according to my research.
Chris
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20th Dec 2018, 10:19 AM #21Senior Member
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This is the info I go on for my calcs. I can't argue with Machinerys Handbook or the Standards drawing of the Unified/Metric profile. If the 0.541P full depth of engagement is the same value as the full depth of the internal thread then that in my view is 100% engagement. Any opening of the hole in the nut reduces this percentage.
I can most likely see where a 0.75 figure comes from? The old Sellers/AN thread has a truncation of H/8 top and bottom leaving the 3/4 Height basic thread form. Somehow there seems to be some confusion by mixing these two profiles.
Now back to the workshop as there are more pressing projects to continue with now that I can see with both eyes again! Any one with cataracts, get them fixed as soon as you can. Just need glasses for close up stuff now.
Rusty
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20th Dec 2018, 10:27 AM #22Most Valued Member
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Sorry Rusty, but you don't get to invent your own definition of thread engagement. Well actually, you can do whatever you want, but you should make it clear that it's your definition and not the standard definition which the percentage engagement formula is based on.
Chris
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20th Dec 2018, 12:12 PM #23Senior Member
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