DIY SMA torque wrench

[rhb]

  I decided to build one this morning.  It took a *lot* longer than I expected.  I still haven't calibrated it yet.

It's made of material on hand:

 1/2" EMT (electrical conduit)
1/4"  - 20 5" carriage bolt
two 1/4" - 20 Tee nuts
#10 - 24 screw and 3 nuts
small piece of cold roll 1/4" x 1" steel
spring and a glass marble

A couple of mistakes I made to watch out for.  When you squeeze the tubing in the vise, pay attention to the weld and make certain it is on the minor axis of the ellipse.  When fitting the jaw, use an actual SMA connector, not a #10 - 24 nut.  The offset on one side is the result of my cutting the jaws too wide and then forcing them back together with a hydraulic press.  It worked a bit more than expected.  The pivot was not quite properly centered so the action is asymmetric.  It has the best release when the offset of the jaw is on the bottom.

All the work was done with a drill press, hacksaw and files.  I made threaded spacers by rounding #10-24 nuts with a file using a screw in the chuck as an arbor.  I did the same for the Tee nuts to get them to fi the tubing.  The innermost is a close fit and the outermost is a press fit.  I drove it in with a deep socket and a hammer as it was too tall for my manual arbor press.

While the EMT works, it's rather heavy. Aluminum tubing would be nicer.  This was all done by eye.  The distance from the pivot to the marble should be smaller.  That will allow more error in the centering of the pivot hole.  Once I'm sure I don't want to tak it apart again I'll center punch the EMT either side of the outermost Tee nut.

Edit:  In playing with since posting, I think the marble needs to be a closer fit in the tube and that that is what is causing the asymmetric release.  The marble is 0.560" and the EMT is 0.620" ID

[mcinque]

very clever, thanks for sharing 

[tggzzz]

I took an easy approach that was sufficient for my needs:

• an ordinary spanner to fit the SMA nut
• a spring balance

It is easy to measure the "lever's" length, and knowing the required torque leads to the value the spring balance should show.

Obviously that presumes there is suitable access, which isn't always the case.

[rhb]

This a convenience item.  With a modest amount of practice you can tell the pressure with your fingertip.  And I have a force guage, though I don't recall if it goes low enough.  I know my 1/4" torque wrench does not.

After playing with this morning's build I'm going to redo it with aluminum tubing and a steel ball which is a closer fit in the tube.  Also shorter distance from pivot to ball and a shallow V notch for the ball.

I'm also going to use an internal die when I upset the tube so that the internal dimension is more tightly controlled and the sides flatter.

I have a lathe and mill, so I could make these very quickly.  I did this one by hand for the practice and also so people would understand that you don't need a machine shop.  I saw an HP wrench on eBay for $125,   I'll see how Mark II goes, but I might do a small manufacturing run just for fun.  I think I could make a decent wage selling them for $35.  Of course, I thought it would only take me an hour when I started.  I was only off by a factor of 5x.

In any case, a successful prototype.

[TheSteve]

I cheated and bought a pair of 5/16th inch torque wrenches off ebay for $12.50 each shipped. I then had some fun with weights etc to perform a DIY cal for SMA on one and 3.5mm on the other.
A full on DIY solution wasn't something I considered, I do like your design, thanks for sharing.

[rhb]

I'm afraid it's *rather* passe.  Check this out:

https://www.ebay.com/itm/0-5N-m-3N-m-opening-8MM-SMA-torque-wrench-RF-connector-opening-spanner/292516601321

What did you get for $12.50?

[Alex Eisenhut]

I'm afraid it's *rather* passe.  Check this out:

https://www.ebay.com/itm/0-5N-m-3N-m-opening-8MM-SMA-torque-wrench-RF-connector-opening-spanner/292516601321

What did you get for $12.50?

I got one, it's OK. Not the greatest build quality but it's serviceable.

[TheSteve]

I bought a pair of Utica A5 wrenches with 5/16th heads.

[tggzzz]

This a convenience item.  With a modest amount of practice you can tell the pressure with your fingertip.  And I have a force guage, though I don't recall if it goes low enough.  I know my 1/4" torque wrench does not.

More than that, it is a way of calibrating your fingertips.

[rhb]

This a convenience item.  With a modest amount of practice you can tell the pressure with your fingertip.  And I have a force guage, though I don't recall if it goes low enough.  I know my 1/4" torque wrench does not.

More than that, it is a way of calibrating your fingertips.

A great practice item would be a lever on a pivot  bearing from a scrap hard drive with the appropriate weight at one end.  Not quite as repeatable as a torque wrench, but you'd get good at it pretty quickly.  Well calibrated fingertips are very useful, they're permanently attached, so you can't misplace them.

[CopperCone]

I wonder how much metrological error the drift in this crudely manufactured instrument adds over the ware and tear error caused by reconnections.

It seems pretty good but i wonder how a higher grade of pipe i.e. stainless uncoated and a reamed hole using a machine pin as the pivot would add to stability. There will be some deformation on the joint due to the fact that its drilled if it was reamed or better yet rotary lapped it would probobly jiggle less and the center of axis would change less due to repeated use. I also wonder how much deformation there is in the pipe vs a real handle.

Cast iron might be good for this application due to its rigidity, easy machinability with hand tools and the fact that it will be used in a metrological setting with a low corrosivity enviroment.

[CopperCone]

Can a crude analysis be done o severity of torque error introduced by various factors so we can unserstand ehich parameters can be improved with minimum workmanship and cost and isolate error terms that cause low error but require precision machining techniques to reduce?

[EVS]

Got today Keysight 8710-1765. From Aliexpress !!! . Like New. For $38 only  .

[CopperCone]

Do people actually sell real things on aliexpress like ebay?