Boy, THIS is a DIY...

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bluedogz

bluedogz

Minister of Fire
Oct 9, 2011
1,245
NE Maryland
As some of you know, I sometimes wear a prosthetic arm. This consists of a thermoplastic socket that slides over my residual arm, ending in a ball-and-socket connection threaded to receive a manufactured "forearm." Looks like pictures below.

Anyway, the most expedient "forearm" is actually a piece of 1/2" aluminum or titanium tube- light and strong. Problem is, the socket is threaded to receive a 1/2"-20 thread, and it seems exceedingly difficult to mate a bolt or chunk of threaded rod to the tube to thread into the socket.

Does this question make sense? Any ideas from the peanut gallery to put this thing back together?
 

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Wow I did not know that, are you saying it is hard to get it to line up and start with out cross threading?
 
If thats the problem, one is fine thread and the one course as daksy says.
 
I don't fully understand your question but I will give my opinion as to what I think.
Threading a bolt into a nut or casting can be made much simplier by relieving the threads of the bolt.
By this I mean to machine or file about 1/8-1/4"of threads on the starting end of bolt. Machine or file only the threads, leaving the "minor" diameter.
What you are trying to accomplish is to be able to insert the bolt into the nut this "relieved thread distance".
If done properly, it will be in perfect alignment with little chance of "crossthreading"
I hope this helps!-----Giles-----
 
Are you trying to make your forearm longer or shorter? You could look up a local welding shop to mig or tig one.
 
Ya, I had to go find 1/2-20 bolts aren't "rare", but they're less common. Are you saying that you cross thread or that you just can't find the right rod for it?

Should be simple to make a 1/2-13 tapped piece or an adapter.
 
I can find 1/2-20 bolts easy enough. Problem is that the 'forearm' is a tube, and obviously has no threaded end, meaning I need to find some way to attach a bolt or threaded rod to the tube.

So, yeah- cross-threading and all that is ground-level stuff. The threads are obviously pretty coarse and not hard to thread properly. I just gotta attach the bolt to the tube.

Oh yeah, it is imperative to use the 1/2" tube, because that is a standard size for the attachments like hooks and hands.

Fifelaker: Are you trying to make your forearm longer or shorter? You could look up a local welding shop to mig or tig one.
Click to expand...

Actually, this is about attaching it at all. This photo was taken the day I got it, the threaded joint broke about 2 weeks later. The tech tried to attach a bolt using 2-part epoxy, and it didn't work.
 
Can you get a 1/2 20NF tap and tap the tube? Is there enough meat on the tube to run a tap into it?
 
You could machine a plug that is a press fit into the tube and then reduce the diameter to 1/2" and run a 1/2-20 die down it. Or another approach is to make/find the plug and drill a hole down the center. Then tap the hole out with 1/2-20 threads and run a long bolt into that from the "inside" of the plug. Use blue Loc-Tite to secure the bolt in the plug. Should leave you with a nice 1/2-20 stud out the end once it's pressed into the tube. You could even use a Grade 8 bolt for extra strength.
 
MasterMech said:
You could machine a plug that is a press fit into the tube and then reduce the diameter to 1/2" and run a 1/2-20 die down it. Or another approach is to make/find the plug and drill a hole down the center. Then tap the hole out with 1/2-20 threads and run a long bolt into that from the "inside" of the plug. Use blue Loc-Tite to secure the bolt in the plug. Should leave you with a nice 1/2-20 stud out the end once it's pressed into the tube. You could even use a Grade 8 bolt for extra strength.
Click to expand...

If I'm going to open the yellow pages and find someone who can do this, where do I look?

Is this a complex job for a machinist? I am running low budget (which is why a prosthetics tech is not doing this- insurance only covers after I meet a catastrophic deductible.)

Generally, the tube does not have enough "meat" to hold threads. Weight is hyper-critical in this application- remember whatever I screw onto this socket is hanging on the distal end of my "arm" with no muscular support at all. So, tube stock is all I've been using.
 
Should be ultra simple for any competent machinist to make a cylindrical plug to an exact diameter and drill/tap a hole down the center of it.
 
in aluminum tubing you should be able to get different wall thicknesses
allowing you to cut threads without sacrificing strength
a good machine shop should be able to give you some options
another thoughtbeing it is pipe it might not be a true 1/2-20
could be a pipe thread
also maybe try lowes and look at fan down rods for ceiling fans
come in all different lengths and are hollow
and I think 1/2"
 
looking closely at the picture
I think if you went to a solid aluminum rod
you would not notice the difference in weight
it appears to be about 8" long overall
what are the exact dimensions??
 
What's the ID of the tube? You need the fabricate a threaded rod with
whatever thread you can get into the tube at one end (ie 3/8-16 or 7/16-14)
& the 1/2-20UNF on the other end. Tap the ID for the smaller thread, assemble
the smaller thread into the tube, securing with loctite or even superglue, then
screw the entire assembly into the 1/2-20 thread...
Any small job shop should be able to make you what you need in an hour or so.
 
Maybe you could contact a vocational technical high school that has a machine shop? They might like the opportunity to work on this for you. The local tech school around here does all sort of projects for towns labor free if a town buys materials for it.
 
another option : plug the end of the tube for support and thread the outside, then use an elongated nut like they have for joining threaded rod.

Ehouse
 
taping a tube shouldn't be too hard, and shouldn't take too much time at all. The hard part is finding the right tap. I've been looking for a 5/8 x 11 tap for forever and can't find it. A machinist could proably do it for you right quick.
 
bluedogz said:
MasterMech said:
You could machine a plug that is a press fit into the tube and then reduce the diameter to 1/2" and run a 1/2-20 die down it. Or another approach is to make/find the plug and drill a hole down the center. Then tap the hole out with 1/2-20 threads and run a long bolt into that from the "inside" of the plug. Use blue Loc-Tite to secure the bolt in the plug. Should leave you with a nice 1/2-20 stud out the end once it's pressed into the tube. You could even use a Grade 8 bolt for extra strength.
Click to expand...

If I'm going to open the yellow pages and find someone who can do this, where do I look?

Is this a complex job for a machinist? I am running low budget (which is why a prosthetics tech is not doing this- insurance only covers after I meet a catastrophic deductible.)

Generally, the tube does not have enough "meat" to hold threads. Weight is hyper-critical in this application- remember whatever I screw onto this socket is hanging on the distal end of my "arm" with no muscular support at all. So, tube stock is all I've been using.
Click to expand...

PM me the dimensions of the tube you're working with and I'll crank some adapters out of aluminum - if you have a sample I can measure it out for a good tight fit

Don't think you would need titanium unless the part is going to see a lot of threading/unthreading and a heli-coil would take care of that at the expense of a couple of grams

Aaron
 
Aaron Pasteris said:
PM me the dimensions of the tube you're working with and I'll crank some adapters out of aluminum - if you have a sample I can measure it out for a good tight fit
Click to expand...
Ding, Ding, Ding. An aluminum plug with 1/2" threads turned on a lathe. Piece-o-cake for a lathe man.
 
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