Friday, February 7, 2020

SMA Torque Wrench for the NanoVNA (uncalibrated)

I have been using SMA connectors on most of my projects, and have occasionaly gotten a slightly different reading than I had expected.  Using the NanoVNA, this has shown up a little more often.  After a some checking, it appears that having the connectors  'finger tight' is not quite good enough for getting consistant readings. After watching several Youtube videos and reading soome instruction manuals on much more expensive VNAs , I decided I need some form of torque wrench to help eleviate the problem.  

Looking around I found that even the cheapest torque wrench for SMA connectors cost nearly as much if not much more than the NanoVNA.  I wondered if it would be possible to 3D print someting that would be usable.  I looked around Thingiverse.com, and found a couple examples of torque wrenches that looked easy enough to model one on for SMA connectors.   Looking as the specifications for SMA connectors, I found that they shold be tightened to 5 in. pounds of torque.  For my use it would be adequate to have it somewhere near that value as long as it was consistant.

SMA connector nuts are 8mm. across the flats, so I went 8.2mm. to make it easier to get on and off.  I also put a wide enough slot in the end of the wrench to allow it to slide over cables easily.  While I was working on the design I decided to taper the other end of the wrench so it could be used as a stylus for the touch screen.  I also added a small hole 2" from the center of the wrench opening, so you can use a small luggage scale to compute the actual torque.

I printed a couple with different printer settings and tried them out.  Just place over the connector nut and turn until the wrench slipps around to the next flat. I measured how much force I had to apply for this slip to happen. Then by adjusting the number of top and bottom layers, along with number of perimiters I got the torque to be somewhere around 4 in. pounds.  With this value I appear to be getting more consistant readings than I hade before, and have a handy stylus for the touch screen.  It also makes it easier to get the cables on and off than just using my fingers.

My settings for the print is for PLA filament.  Three perimiters (wall line count), four top and bottom layers, and 40% infill.
The .stl file is located at.  
https://www.dropbox.com/sh/qt816x30fujt3kl/AABbUj5DdeNC5n3VE0RyJD9ra?dl=0

Not sure how long these wrenches will last, but only take 15 minutes to print and use about 3 grams of filament.




6 comments:

  1. Interesting topic Duwayne. I found issues with sma cables while working on our SNA project and attributed the issues to bad cables. Now I'm wondering if the problem is as you have outlined.

    One thing I gave in the shack is 3d printed knobs that fit over the sma fitting so you have a better quip while tightening. Cant measure torque that way though.

    73, Peter

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  2. Hi DuWayne. I finally got around to printing a couple of your wrenches. I used 0.2mm resolution, TacTink filament and the parameters you specified. Calibration check was done with a cheezy 1000g spring scale.

    This is an awesome little wrench. The first couple of pulls with it are a little strong but then it settles in and seems pretty repeatable. I'm getting just shy of 3 in. pounds and that is tight enough that I can't remove the connector with my fingers. I could probably mess with the layer thickness but this is good enough I think. Much better than my finger tight connections. Thanks for a great 3D design.
    73 - Jerry - W0PWE

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  3. Thanks for helpful article! https://ufo3d.com

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  4. Why not bolt your wrench to a 5kg load cell (https://www.adafruit.com/product/4541) hooked to a Nano? You could calibrate it super-accurately. You'd have to print the wrench head a bit tighter to stop it slipping, or you could use a cheap steel wrench.

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  5. The wrench is desogned to slip. Actual load at which it slips is not really important, as long as it is withing a certain range. Important thing is that it slipps at a consistant value. This is to get a consistant tightness on the SMA connector to elminate that as a possible error.

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