Fasteners

Are there any good books on the development and use of fasteners, particularly machine screws and nuts and bolts etc?

And another question on the topic, why do countersunk insert screws have a different angle to normal countersunk screws. I’m assuming it’s more secure but why aren’t all countersunk screws this angle?

If you are talking the difference between wood screw counter sink and those used as metal fastenings maybe it is the type of material that dictates the different angles. Just a thought no scientific reasoning on my part

There are 100 or 110 degree heads on some screws and there are 90 degree heads on others.

Metric machine screws have 90 degree heads as a general rule.

USA and Imperial wood and machine screws typically have 100 or 110 degree heads.

Why did DIN and ISO in Germany standardize on 90 degree heads? They were right, of course, and rest of world was wrong, as usual! Just ask them! (joking of course)

In theory a 90 degree head would give a higher force centering and retention action, but of course this does not matter a) in soft materials and b) if the head is eccentric to the shank, as they often are on inexpensive commercial countersink head screws.

Bottom line - if you need to locate two plates or parts together accurately and repeatably, don't depend on countersunk head screws to do it - use a couple of dowels for location, one in a hole one in a slot. Use good quality socket head cap screws to hold it together. For flush fitted heads counterbore the screw holes. Lock washer or Loctite on threads to retain the screws.

US machine screws have 82* heads. Fasteners fir aircraft may differ.

If you were wanting information on real world sized fasteners I could probably find several references. Unfortunately, I am not aware of similar information on model size fasteners.

Tubal Cain's book "Drills, Taps And Dies" has interesting historical information about early thread development and subsequent unification in addition to general information about the theory and practice of the book titles subjects and their application for the model engineer.

Not read that one but none of the other books I have give a theoretical explanation for the various thread angles used. Or if they do it's in the maths and goes over my head.

Whitworth adopted 55 degrees as a result of sampling British industry best practice circa 1840. He measured thread samples from across the trade and averaged their measurements. Then he rationalised the results and wrote his famous standard. 55 degrees seems to have evolved for practical rather than theoretical reasons. It's a heavy engineering thread well suited to cast-iron and the other softish materials of the time.

Later, the British Association standardised threads for instrument work. They appear to have adopted 47.5 degrees based on Swiss practice. No clue as to why the Swiss chose 47.5 but it may have been for practical reasons. As much instrument and clock work was done in Brass, it could be that 47.5 suits that material. It seems strange that the BA thread angle is such an odd number - what's wrong with 48 degrees?

About 1865 Sellers in the USA reviewed the Whitworth Standard against American needs and decided it could be simplified to speed production. At the same time he switched to a 60 degree thread. My theory is that by 1865 improvements in materials made 60 degrees a stronger general solution for most fasteners than 55 degrees. Later the Sellers thread was adopted as the US standard thread. In the same period metric threads went the same way. Again, it's not clear to me why metric settled on 60 degrees rather than any other angle.

The relationship between thread angle and the strength of materials may be related to the requirement that the male threads of fasteners should fail before females. It is easier to replace a stripped bolt than to drill out a broken stud left inside a casting and then retap the hole. Not sure the idea holds water, because male and female have the same angle, and therefore the same strength.  Or do they?

I think thread angles were decided more for practical and experimental reasons than theory. Other aspects of thread design have a more obvious mathematical rationale. Like much of engineering 'practical bloke' was first to find what worked, and his solution was later refined and improved by brainy chaps.

Dave

Edited By SillyOldDuffer on 21/12/2018 11:45:07

Yes they do, but it's not the whole story. An external threaded part can fail in two ways. One, failure of the core material in tension or two, failure of the threads in shear. Generally the shear strength is greater than the tensile strength so the core fails first. That's why you don't necessarily lose strength when you don't have the full thread depth. Of course the exact failure mechanism depends on the materials used and the thread dimensions. I know from experience that 3/16" BSW is particularly prone to failure of the bolt in tension.

It's also why you can use ordinary nuts with high tensile bolts. Years I ago I did some experiments at work. A high tensile SHCS in a hole tapped in aluminium alloy 6082T6 broke in tension before the thread stripped, even when the hole was drilled for a nominal thread depth of 50%.

Andrew

It might just be me, but I think Vic's questions was about the countersink angle on screws used to retain indexable inserts. These are normally 60 degrees.

Question is why? (I assume its to give better lateral rigidity)

Ian P

That would be my assumption too, as well a slightly better location accuracy. The screws on my dividing head that hold the division plates in place are also a 60 degree countersink; presumably for the same reasons.

Andrew

The answer to Vic's original question is this: Carroll Smith's Nuts, Bolts, Fasteners and Plumbing Handbook. It's American, but fasteners don't know that.

I don't know but suspect the 60 degree angle on insert screws is because there is a small element of wedging the insert into its seat in the holder as well as pulling it down tightly.

Chris,

That is exactly why the 60* head is used on insert screws. The holes in the toolholder are set back just a bit so when the screw is tightened it tends to pull the insert into the back and side of the pocket.