A new kind of tolerance

[Tin Falcon]

Well sort of. I am finding myself in an unusual situation. An interesting situation. At work we have a load tester that we set weights on a platform and it translates to hydraulic pressure, up to 2000 psi the smallest weight is for .1 psi it is missing so I said I would make one. I have a .2 weight that I measured out . the weights have a male and female boss and socket to keep things stacked neatly and have a hole in the middle to store them on a shaft.
so I know how much the .1 needs to weigh now I just need to figure out how thick to make it based on the other needed dimensions.
I drew up the .2 weight with Alibre cad and set the material to 60601 aluminum and came up with about 200mg to heavy. Hmm need to double check the part.
Enough rambling has anyone else here had to make a part within a couple milligrams or am I the only crazy here lol.
Tin

 

[joeby]

Been there, tried that. I had a very nice old scale given to me one time, but it was missing a couple of the weights.

I made the replacements out of brass, the same as the existing weights, and got the last little bit of adjustment by trial and error.

The time spent on making the weights was probably more than they were worth, but the satisfaction of bringing a nice old tool back to usefulness was worth it.

Kevin

 

[shred]

Somewhere I have a lab-spec 1-lb weight. Accurate at one time to some tiny tiny fraction of an ounce. What they did was make a stainless steel weight a little under the target weight with a hole drilled in the top. With a calibrated scale, determine just how much weight to add. Drop that additional weight, in the form of small granules of something (lead, I suspect, it rattles) into the hole. Close the hole with a lead slug tapped into place (of course you've taken into account the weight of the lead sealing slug in advance). If done right, no weight is lost from the lead slug on the way in and it seals the calibration grains in place. Can be re-done if the weight goes out of calibration for some reason later on as well.

 

[rake60]

Tin
I have no experience with the tests you are attempting.
I do recognize two factor, male and female.

Past experience has taught me that female ALWAYS wins.
If the male element fails to yeild first there can be NO positive
resolution to the test.

Maybe I should re-read the post.
My reply may be a reflex response.
Rof}

Rick

 

[hammers-n-nails]

would it be possible to make te weight too heavy then file, drill or otherwise remove a small amount of material to get the desired weight?

 

[Tin Falcon]

I will probably go a little heavy in this case. Then adjust with sanding . The bigger weights in the set were adjusted down with drill holes this is another common method. Cast iron weights have the adjustment cavity then in them and use a lead seal plug with a steel backer typical stainless weights have an adjustment cavity and an aluminum plug. a lot of the small tiny weights just get a little sanding to lessen the weight then get tossed and replaced if too light. I have worked with the state guys several times adjusting the 50 pounders.
The higher class weights are all stainless now brass is no longer accepted as a standard.
After posting the thread a light came on and i realized i only have to make the .1 weight 1/2 the thickness of the .2 due to the fact that the boss and socket effectively cancel each other. Sometimes a drawing helps one think. anyway the drawings will be a good addition to the equipment file.
Thanks for the helpful comments and the interest in the little project. rick thanks for you help as well lol.
Tin

 

[steamer]

Hi,

I think I'm answering your question....sorry if I missed.

Machine a cylinder of the material you wish to use.

Make it exactly .75 diameter and 1 inches long. ( got some TGP hanging around?)

That will yield a nominal volume of .44178 cubic inches.

If you hold +/- .001" on the machined dimensions, the volume will range from .44016 to .44340 cubic inches, IT MATTERS! Keep the corners sharp!

Weigh the part. write it down.

Now the math

Weight /volume = density ( in this case specifically for THAT bar of material)

If you have it modeled in Alibre already, you should be able to input a custom density in the custom field. Do that and then adjust dimensions accordingly. Otherwise just measure the volume of the model and do the math.

If its a cylindrical weight, keep the diameter as large as you can as the variation of volume for a cylinder goes as the square of the diameter but is linear with length. One or two thou on a small diameter has a larger effect than on a larger diameter..