Mechanical scale calibration

[DLT]

I have an older rcbs 502 scale. It is pretty repeatable. I use it to double check charges. I have a set of Lyman check weights but my scale does not weigh what the weights says. The blades on the beam are pretty sharp. Is there some one on here who works on scales that’s trustworthy besides Scott Parker. I have not dealt with him personally but have read many posts with bad experiences. Thank you in advance

 

[JMayo]

Doesn't the pan have a bottom that has bb's in it ?
Add or take away from them.

 

[K22]

Define "pretty repeatable" and "does not weigh what the weights says".

 

[DLT]

Doesn't the pan have a bottom that has bb's in it ?
Add or take away from them.

I have did that.

 

[DLT]

Define "pretty repeatable" and "does not weigh what the weights says".

Sometimes it will read a previous weighed object the same. Say I weighed a 50gr bullet and it read dead on 50gr. The next time I weigh it it might be 50gr or a tenth one way or the other. And the same with the check weights, a 100gr will read 100.3. The higher the weight of the check weights go the more my scale seems off. 150gr will read 150.5. I calibrated it with lead shot and it will zero and stay zeroed. And yes I make sure the ac is not running. Could be a chance the check weights are not exact. They read dead on with a 505 scale I have.

 

[justinp61]

Doesn't the pan have a bottom that has bb's in it ?
Add or take away from them.

All that will change is where the scale zeros, at least that's been my experience. I've added or taken weight out of the bottom pan when I've used a different pan.

I have gram and milligram test weights with a conversion chart for grains. When I set up my RCBS 505 I choose the test weights that are closest to the charge weight I'm going to use, then zero the scale to the test weights. My test weights were verified on the lab scales at the chemical plant I retired from.

 

[1066]

Sometimes it will read a previous weighed object the same. Say I weighed a 50gr bullet and it read dead on 50gr. The next time I weigh it it might be 50gr or a tenth one way or the other. And the same with the check weights, a 100gr will read 100.3. The higher the weight of the check weights go the more my scale seems off. 150gr will read 150.5. I calibrated it with lead shot and it will zero and stay zeroed. And yes I make sure the ac is not running. Could be a chance the check weights are not exact. They read dead on with a 505 scale I have.

You have two different problems here.. Your scale is not repeating reliably. It really doesn't matter what the numbers say for a starting point. When you place your 50gn check weight in the pan it might register, say, 50.5 grains, not a problem at all, it can easily be corrected. What is a problem is if it doesn't ready 50.5gn every time - remove and replace the pan a dozen times, should always ready 50.5gns. If it doesn't then no amount of tinkering with the weights or re-zeroing will make a difference.
Only when it's repeatable will you be able to progress from there.
If it's not repeatable I would first check the knife edges again with a good eye glass looking for any burrs or dull areas. Remove and scrub the agate bearings with a toothbrush and alcohol/lighter fluid and try again. If no improvement, remove the bearing side plates and try again, this will eliminate any problems being caused by the knife edges contacting the side plates.
If still no difference, check the panhanger stirrup. The panhanger swings in it's own little set of knife edges, again give them a scrub and ensure the stirrup is free to swing in the center of the slot
Next thing to check would be the damper system, check that the damper blade is not loose on the beam and shine a flashlight in the damper slot to look for fine hairs of wire wool/ dust sticking to the magnets.

Only when it repeats every time can you progress to any sort of calibration.

(The increasing error the higher up the range you go is due to the main poise being too heavy, possibly due to an accumulation of dust or grime if the scale has not been kept covered.)

 

[DLT]

You have two different problems here.. Your scale is not repeating reliably. It really doesn't matter what the numbers say for a starting point. When you place your 50gn check weight in the pan it might register, say, 50.5 grains, not a problem at all, it can easily be corrected. What is a problem is if it doesn't ready 50.5gn every time - remove and replace the pan a dozen times, should always ready 50.5gns. If it doesn't then no amount of tinkering with the weights or re-zeroing will make a difference.
Only when it's repeatable will you be able to progress from there.
If it's not repeatable I would first check the knife edges again with a good eye glass looking for any burrs or dull areas. Remove and scrub the agate bearings with a toothbrush and alcohol/lighter fluid and try again. If no improvement, remove the bearing side plates and try again, this will eliminate any problems being caused by the knife edges contacting the side plates.
If still no difference, check the panhanger stirrup. The panhanger swings in it's own little set of knife edges, again give them a scrub and ensure the stirrup is free to swing in the center of the slot
Next thing to check would be the damper system, check that the damper blade is not loose on the beam and shine a flashlight in the damper slot to look for fine hairs of wire wool/ dust sticking to the magnets.

Only when it repeats every time can you progress to any sort of calibration.

(The increasing error the higher up the range you go is due to the main poise being too heavy, possibly due to an accumulation of dust or grime if the scale has not been kept covered.)

I have checked the blades even lightly touched them up with a diamond sharpener. I don’t see any flat spots on them using a flashlight. The is no dust or debris around the magnets and the damper is tight on the beam. I do need to check the bearings and dust that could have accumulated along the beam. Other than that I don’t know what else I could do. If I do get it repeatable how can it be tuned to read right from low to high weights ? Thank you

 

[JFrank]

I’m still learning but I don’t like Diamond sharpening the preference for me is a fine stone with a few careful strokes, I follow the technics @1066 has posted. I also keep alcohol and Q tips handy to clean not only the V grooves and beam itself but swipe that Q tip into the pan once in a while and check out the build up of powder stuck to the pan.

 

[DLT]

I’m still learning but I don’t like Diamond sharpening the preference for me is a fine stone with a few careful strokes, I follow the technics @1066 has posted. I also keep alcohol and Q tips handy to clean not only the V grooves and beam itself but swipe that Q tip into the pan once in a while and check out the build up of powder stuck to the pan.

Thank you. I sharpen knives very regularly and I did use a very fine diamond hone although a ceramic might be better

 

[1066]

One of the most important things to consider when touching up the KE's it to make sure the edge is in line with the point. (RCBS scales, not Hornady/Pacific). As the beam rotates, if the point of the edge is touching one of the bearing end plates there should be very little friction as the beam is rotating round a fixed point. If the blade and the point is not in line or the tip of the point is chipped, the point of the KE will scribe an arc on the bearing end plate and cause friction.

 

[K22]

Sometimes it will read a previous weighed object the same. Say I weighed a 50gr bullet and it read dead on 50gr. The next time I weigh it it might be 50gr or a tenth one way or the other. And the same with the check weights, a 100gr will read 100.3. The higher the weight of the check weights go the more my scale seems off. 150gr will read 150.5. I calibrated it with lead shot and it will zero and stay zeroed. And yes I make sure the ac is not running. Could be a chance the check weights are not exact. They read dead on with a 505 scale I have.

1066 says relatively the same I was going to say.

 

[cheek9075]

I've worked in the industrial scale industry for over 20 years. First the scale has to repeat zero and weight reading, don't care what the weight reading is. If it doesn't do those two things then a calibration will not correct anything. Scales also weigh in a linear line, meaning if you put 5lb on a scale and reads 5.5lb and repeats every time when you put 10lb on the scale it should read 11lb. The error will double when you double the weight. If it does and repeats then the scale is working correctly and a calibration will fix the issue. If it doesn't then the scale in non linear and there is an issue which can not be corrected with a calibration.

With the balance you have check a couple things.
1. Repeat zero.
2. Repeat weight reading.

If it does then all mechanics are working. If it doesn't look for binds and also the knife and pivot, make sure they are clean and sharp.

Once that is done you will need to check each slide weight if it has more than one. Put a know weight in the pan (50 grams) and put the slide weight on 50. If it doesn't read 50 grams then the slide weight will need to be adjusted. Add or take away weight from the slide Weight until it reads 50 grams. Do each slide Weight that way, that's how I adjust mechanical scales.

Its is all a simple lever ration 1:10 as an example. Place a 10lb Weight one inch from the pivot and bearing it takes 1lb 10 inches on the opposite side to counter balance the 10lb. If the 1lb weight doesn't really weigh 1lb then lever will not counter balance. If the 1 lb weight was .9lb then the scale will read heavy due to not being an exact 10 inch from the pivot. The 1lb weight is actually 1.1 lb the scale will read light.

 

[Bronsin]

This is how I fixed mine. You have already done the cleaning and deburring, so I took my scales and put them on flat board, (actually its a case for Brown and Sharp depth mics) and made sure the case was level, then zeroed the scales. Take the hangar off and disassemble it. Lay the hangar wire on a flat surface and tweak it until it lays flat. Put it back together and set it back in place on the scale. Then using one of those circular bubble levels, put it on the hangar plate. It should be level side to side, but end to end probably won't be. Just tweak the hangar wire until the level is sitting as centered as you can get it. Put the pan on and recheck. My scales wouldn't repeat at all until I did this to them. Now it always works and shows the smallest change. Good Luck!