threaded inserts for holding take down archery limbs

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jcs0001:
Tony and Jonny:

I've never used this style but have used some other thread in ones that may be some type of aluminium alloy (maybe with zinc). I'm not keen on using the aluminium/zinc ones due to possible strength limitations and the internal thread seems a bit loose on them.

Tony - The ones you provide the link for appear to be 0.43 in. long in the 5/16 in. bolt size. I feel that closer to 1 in. in length gives better gripping power in the wood. Going to a larger than 5/16 in. bolt to hold the limb on would cause other difficulties.

Fortunately I don't need a lot of these, so given easy machining stainless I can make up a few here and there as needed.

Thanks again,

John.

Lew_Merrick_PE:
John,

I've been somewhat busy over the weekend with a new (being paid for it) project that had to be ready for presentation today. However, (1) if you go to http://www.scribd.com/Lew%20Merrick you will find the paper UN Thread Data Chart that will give you values for the shear area of internal and external UN primary threads listed as square inches per inch of engagement. If you multiply that value times the shear strength (stress, in reality) of the material times the length of engagement (in inches), you end up with the pull-out strength of that side of the threaded joint.

I suspect that you will find that aluminum is much stronger than the crushing strength of the wooden laminate -- but that is a gut feel rather than a researched fact. Key-locking inserts (Keensert is a brand name) do not do well in wood. Among other things, the thread on the OD of key-locking inserts is actually only a 50% thread.

304 ss work hardens (as you discovered). The trick is to be aggressive enough with your feed to cut away the area that is work hardening as you cut it. Few home-shop machines are rigid enough for that type of machining. 303 ss is the better choice as it is much harder to work harden than 304 ss. 400 series ss (such as 410 or 416) actually machine better and more easily with a light heat treat (say Rc 32-38) as the chips will form better.

The companion paper to UN Thread Data Sheets is Pilot Hole Considerations (also posted at ScribD.com). They may help you evaluate things.

jcs0001:
Lew:

Thanks very much for the information.

I did up a quick spreadsheet with figures for yield strength found on the net (Online Metals) along with the figures in your paper for area. For all the metals I looked at, the "ultimate tensile strength" was higher so the relationship between them (noted below) would hold if that figure was used.

Interestingly enough it appears that 303 has a higher number than 304 and brass (c360 and particularly c464) has even higher numbers than ss 303 and 304. I won't go into details on the actual numbers as it is the comparison that is important to me.

Does this make sense in comparison of brass vs. 303/304?

John.

Lew_Merrick_PE:
John,

Developing good material properties values is seriously non-trivial. Most of the information floating around the web is typical properties rather than minimum properties -- which may vary greatly. Next, nearly all material property values are based on normalized condition of metals. They are not accurate for work hardened materials or, in most cases, heat treated materials.

The thing to remember about thread pull-out strength is that it is based on the shear strength of the material. In many cases, this value may be approximated as being .632 * UTS (ultimate tensile strength). The thing to remember is that this is not always the case. Most brasses have a shear strength in the 36,000 psi (36 ksi) range. Normalized 304 ss has a shear strength of 46,250 psi. Normalized 303 ss has a shear strength of 56,800 psi. Minimally work hardened (25%) 303 ss has a shear strength of 79,000 psi. Work harden it a bit more (50%) and the shear strength grows to 94,800 psi. The problem is figuring out how much work hardening you have done.

I have about 30 inches of bookshelf devoted solely to properties of metals. Getting actual values is far from trivial without taking samples and testing them individually. Just remember, we are doing this for fun!

jcs0001:
Lew:

I think I would die of old age before learning all that is available on this topic. I will use my time for more productive activities :D Since I am dealing with at most a few 100 psi force (say even 1000 psi just in case) it certainly appears that the wood is the weak point by far.

I've ordered some 303 and when it's in will give it a try. On my own bow I may use brass just to add a bit of bling. Otherwise I will use stainless as it should be a bit more durable.

It's been an education and I certainly appreciate your responses and those of the other posters.

John.

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