Rod Weighing Jig: Take 2

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nm9stheham

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Just an update from this prior thread on weighing rod ends with a low cost home shop jig:
Low Cost DIY Rod Weighing Setup

A triple beam mechanical scale has been tried in place of the cheap electronic scale, and STILL showed repeatability issues related to side loading on the scale. This showed up a detail of the jig construction that was inducing side loads, and which could be easily fixed: The long length of the 2 pivots on the unweighed end are causing this issue.

If the 2 pivots at the far end are much longer than the weighed end pivot, and if you draw a line from them to the weighed pivot's point, this line is on an angle. If you move the weighed end of the plate up or down any tiny bit, then the weighed end pivot will move horizontally due to this angle, which will side load the cheapo electronic scales or bind the mechanical beam scale. Lowering the rod onto the jig also causes a slight vertical movement of the scale, and the same side loading or binding. See pix below.

So steel blocks were placed under the 2 unweighed end pivots and they were shortened to approximately the same short length as the weighed end pivot and everything re-leveled. The pivots all being equal length means that as the rod is lowered onto the jig, any vertical movement results in 0 horizontal motion; this avoids a side load on the scale.

With this change, the side load issues are essentially gone. Even with cheapo scales, a rod can be taken off and put back on and get 0-1 gram difference in an end weight, which is all these cheapo scales can resolve .... no 'tapping' on the bench to get things to 'settle out'. If the jig plate if lifted in a way that makes it twist a bit and set it back on the scale, then I can force as much as 2-3 grams variation. Before, if I did this, there would be up to 20 grams variation. Also, the long pivots may have been bending ever so slightly and also causing side loads to develop on the scale. Being short now, they are rigid.

Both the cheapo electronic scale and the beam scale work much, much better with the pivots all short. I measured all the big end weights for a set of a 8 rods with the beam scale and improved jig in a 'blind' test, i.e., not having the prior measured weights in front of me. Afterwards, I found 6 of the measured weights were 0, 0.5, or 1 gram off from before; the other 2 were 2 and 3 grams off respectively. The average difference was 5/8 of a gram.

Jig-side-load-improved.jpg

Jig-side-load-original.jpg
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What is it that is between the jig plate and the scale? Better a knife edge or sharp point(s)? I'd think a wide support there would reduce accuracy if that's the word. And I stick to my "guns" although this is just a gut feeling. Seems to me the more you reduce the weight/ mass of the jig, the more accurate changes in the rod will become. A small change that is "more" percentage of the total in other words.

I applaud your efforts. I've assembled engines, but nothing involving blueprint
 
That is a cap from an old jar of interior trim putty LOL. Used it to try to spread the load and put some oil on the scale so it would slide around more easily and relieve any sideload on the scale. It's not needed now that the main contributor to the sideloading is resolved.

I had the same thought initially on the pivot being pointed. But I've found that these scales don't change their weight reading or accuracy if the load moves around on the scale surface; that is in the design of the scale, so the pivot being spread out does not change the weight. (Think of how big a rod is when laid directly on the scale.) The weighed end pivot ended up rounded just to help it slide around and not dig in be another source of sideload error.

I know is is looong and not an easy read, but it might become clearer to review the first thread. The plate is made in a way so that it is symmetical around its center along the long axis; this should make the plate have no effect on the Center of Gravity location of the rod. If that is done, then once the jig weight is zeroed out, then the rod's weight gets properly separated into each side of its CoG. I'm gonna make sure this is the case by weighing a set of SCAT rods and see if I get their same end weights. The big test!

Way to keep at it! Thanks for posting.
Thanks; wish I had seen this sooner. Like a lot of things... something shows up that you did not have any clue would be an issue, and you spend most of your time figuring that out. Makes one appreciate more the value in all this at shops and factories. I see videos on home made rod hanging setups and now wonder how good they are..... it depends so so much on having the right scale or understanding and solving this arcane issue.

Hope this is some use to those who want to develop their tools and home shop capabilities for little cost.
 
Test with SCAT rods is done...results below. Looks like the BE weights are almost 2 grams high and small end weights about 1/2 gram low veruss SCAT. (I wonder what can be wrong with their equipment? LOL)

Well, if Grumpy Jenkins says he can't get the end weights to add up to the actual total, then I guess this is not doing too badly. Some work on the registration of the big end will make it more accurate. Considering the limited resolution of the scale and the $0 price tag, this is as good as or better than I expected, and beats the tar out of factory balance accuracy.

3.2 grams off in bobweight translates to around 0.2 inch-ounces of balance error, which is about 1/10 of the factory balance tolerances. And any such weighing bias changes all the rod weights the same, and so this has no effect on one of the 2 balance factors we are trying to fix when doing a balance job.

SCAT rods on jig test.jpg


DSCN2605 (Small).JPG
 
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