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| threaded inserts for holding take down archery limbs |
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| jcs0001:
I have made threaded inserts in the past from low grade steel 1/2 in. x 13 tpi bolts - cut one inch of the threaded portion then drill and tap for 5/16 - 18. They work ok but will rust if there is any moisture. I very recently bought some 304 stainless 1/2 in. rod and used it today to make some inserts. Threading the outside to 13 tpi under power in my lathe worked well and gave decent threads. However drilling it with an F drill then tapping it was a real chore. It was so obnoxious that I don't want to use the 304 again for this purpose. Inserts are just under 1 in. long and are pictured with a 5/16 - 18 bolt: They are installed along with a pin for each limb of the takedown recurve or longbow: The insert and pin are 2 in. apart and the limbs are about 22 in. (in a straight line) from the insert to the where the string goes around the tip of the limb. The length along the limb is a couple of inches longer because of the curve. The draw weight of the bow will never exceed 70 lb. I can get stainless 303 in 1/2 in. rod and possibly other types of stainless locally and I can also get brass (do not know what alloys are available at this point). 1) Is 303 likely to be much easier to work with (particularly with drilling and tapping for the 5/16 bolt) than 304? 2) Is there any other stainless alloy that would be easier to work with and fairly readily available? 3) Would some type of brass alloy be strong enough for this purpose and if so what should I ask for? Note: even though making these three inserts was a real bear, the positive part was that the 36 tooth change gear I made a couple of days ago for my lathe, worked like a charm and I learned how to use the threading indicator. Thanks for any information you can provide. John. |
| DaveH:
John Excellent piece of work, really well done :thumbup: :thumbup: :thumbup: :clap: :clap: :clap: 303 ss is much much easier to drill and tap than 304 ss. The trouble with the 304 ss it has a nasty habit of work hardening especially with drilling. 304 ss is best cut with lots of coolant and sharp tools. You will find the 303 sometimes called free cutting stainless a lot easier. As for "strength" and this is a guess the weakest part seems (to me) the insert pulling out of the limb material - not sure what the limb material is. :beer: DaveH |
| jcs0001:
Dave: Thanks for the reply. I suspect you are correct - the riser (handle) into which the insert is "inserted" is wood - in this case laminated "actionwood" which is really a fancy name for birch/maple plywood. The riser is drilled and tapped for 1/2 - 13 and the insert is screwed in using epoxy to provide better holding power but there is only so much that can be done for holding power. Some bows I have seen actually use an insert with a wide bottom and it is screwed into place from the opposite side. Sounds like I need to go out and mortgage the house for some ss 303 rod. Is it fairly corrosion resistant? John. |
| jcs0001:
Just to be clear - I build the mold for the limbs, laid up the bow limbs and made the hardware however my friend did all the shaping and finishing on the riser and limbs. Don't want to take credit when it isn't due. John. |
| Tony Wells:
303 will be fine for what you are doing. Easy to machine, and is corrosion resistant. From one online source: Resistance to Corrosion The addition of certain elements to stainless steels to impart better machining characteristics also slightly lowers corrosion resistance. For dry conditions, and in most mildly corrosive environments, the free-machining grade is practically comparable with its corresponding unmodified type. Where moist atmospheres are involved, some free-machining grades may tend to form a rust film, and in certain severe environments, they may show somewhat increased corrosion as a result of the free-machining additions. In a majority of cases, they will perform nearly the same as the basic parent composition. Because the free-machining grades have a slightly reduced corrosion resistance compared with unmodified basic stainless steel, bright machined parts may be susceptible to surface dulling or etching by final treatment with nitric acid solutions. Caution in the use of such treatments is suggested. For mild action, mixtures containing as little as one or two percent by volume nitric acid, with additions of an inhibitor, may be allowable for short periods of time at 120 to 140°F (49-60°C). A solution of 12 percent nitric acid and 4 percent copper sulfate is also satisfactory where mild action is required. In some instances, the complete elimination of treatment with nitric acid solutions may be desirable. |
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