The other factor, percent of thread


[Follow Ups] [Post Followup] [Dodge Power Wagon Forum]


Posted by David Sherman on Tuesday, July 28, 2009 at 13:58:13 :

In Reply to: Drill size? posted by Bob in N. Ga. on Tuesday, July 28, 2009 at 07:31:23 :

Rather than worry about a difference of 7 thou, which is only 3%, remember that standard tap drill tables are set up for a partial thread, typically somewhere around 75%. That's because if you try to get a 100% thread, you have to remove so much material that you're likely to break the tap unless all conditions are perfect (sharp tap, free-cutting material, good lube), and yet you'll only gain a little bit in strength, since the screw doesn't have perfectly sharp threads either. The main difference between US (UNC and SAE) and British Whitworth threads is that on US threads, the roots and crests are squared off and on Whitworth they're rounded. In theory, the Whitworth should be stronger, but in practice it doesn't make much difference because it's the middle part of the thread that does the work.

The upshot of this is that you can often safely go quite a ways either side of the recommended tap drill size if you take into account the material you're tapping, and you use a good cutting oil (except on cast iron, which should be tapped dry, and only with coarse threads). With free-cutting materials like brass and cast iron, there's no problem with going one size smaller on the whole, or rounding down to the nearest fractional size you have on hand if you don't have the "correct" tap. With aluminum, you can probably get away with it too, but you need to keep it lubed so it doesn't gall up. A dry lube stick like Tapmatic also works and is neater. Soft copper is tricky, and needs a sharp tap and careful work. Tapping in steel, unless it's a part that clearly needs maximum strength, I'd drill to the next size larger rather than smaller if I didn't have the right size drill. Any tighter than standard, and you're liable to break taps. Also, pay attention while you're tapping. If it seems to tight, and the tap is twisting noticeably, stop and figure out what's wrong (dull tap, no lube, too small of a hole, tap started crooked, etc) before you break the tap.

Remember, with the correct drill, you're only getting a ~75% thread depth anyway, so if one size tighter or looser changes that to 72% or 78%, it's not a big difference.

Another factor to consider is the thickness of what you're tapping. If it's good and thick, you can get away with a shallower thread (bigger starting hole) than if it's thin and there are only a few threads to do the work. I'm amazed at how shallow the threads are on a lot of modern manufactured products. I've seen factory made machined parts with threads cut to only 50%. I'm sure it's pure economy -- if they can save on machining time and increase tool life, or depending on the part maybe even save on material cost, they'll do it, so long as what's left is strong enough to hold the thing together long enough for someone to buy it.



Follow Ups:



Post a Followup

Name:
E-Mail:
Subject:
Message:
Optional Link
URL:
Title:
Optional Image Link
URL:


This board is powered by the Mr. Fong Device from Cyberarmy.com