Shopsmith Forums

Tiny differences in measurements will make a surprising difference in calculated results. I am talking third decimal differences. Measuring accurate
operational radii is very difficult.

Latest speed table and graph. This one shows what happens when belt length is measured along different locations - along belt inside, outside, and centroid. Same belt, but we get different lengths and different calculated speeds. As before, all the curves cross at 2156 RPM, which is at 1:1 sheave ratio. Which location gives us chart speeds closest to reality?

All of them predict sub-700 RPM speeds, and now I think what is happening is there are physical stops (motor spring going solid, sheaves fully closing etc.) that in reality are preventing Shopsmith from reaching those "theoretical" min/max chart speeds. I have not laid out all the mechanicals required to quantify where/how such stops are acting.

Stops are good and bad - good because they could prevent interference problems (which the high speed setscrew does) and prevent belt derailment (when the motor spring goes solid and stops further motor sheave opening). Bad because in practice we can't seem to get to 700 RPM.

But it's not the Shopsmith belt's fault.

Here is a copy of the data collected on my 520. Since last report I put a different motor belt on it, lowering the slowest achievable a bit. I had taken the speed control out for cleaning, basically threw another belt on it when I reassembled to see what would happen. Instead of the traditional setting the speed indicator dial at FAST I set it at K. Now that I have the speed data I could adjust the dial for a closer overall fit.

Notes:
Motor speed decreases slightly with higher headstock setting due to increased load. As expected.
Columns C thru E are measured. F is projected/calculated.
I set the high speed stop higher than normal, belt approximately flush with rim of sheaves.


- David

Very interesting. Thank you.

My machine numbers are much different . I have not taken a thorough set of readings like yours but I have enough to see that I have work to do. Maybe my machine can get to 700 if I do a better job of maintenance. Thanks again.

Dave

I need help that I hope you can provide. I am attempting to understand the differences between your numbers and mine. If you would do the following for me it would help me greatly in getting to the understanding.

What I would like to know is this:

With the speed dial set so that your idler shaft speed equals the motor speed (3587rpm) what does the speed dial read.

Why: Because I have been considering that as "Mid Range" at which time I expect the idler pulley and the motor pulley to be equal diameter. Measured with a tool similar to your use of a t-nut.

dusty wrote: ↑Fri Aug 05, 2022 10:37 am Dave

I need help that I hope you can provide. I am attempting to understand the differences between your numbers and mine. If you would do the following for me it would help me greatly in getting to the understanding.

What I would like to know is this:

With the speed dial set so that your idler shaft speed equals the motor speed (3587rpm) what does the speed dial read.

Why: Because I have been considering that as "Mid Range" at which time I expect the idler pulley and the motor pulley to be equal diameter. Measured with a tool similar to your use of a t-nut.

Mine measured 3574 RPM today at Idler = Motor, slightly lower motor RPM than when I collected data. Estimated 'K.6' slightly more than midpoint between K and L. Motor sheave rim to belt measured 0.76". I can't get a quality measurement on the Idler for belt depth, but presume it is equal.

- David

Thanks. That is about as close as I would hope given what we are dealing with. But I still don't understand the big differences.

I discover that RPM is an old and much-discussed topic on this forum (viewtopic.php?t=7538), with many of the same participants then as now. No resolution to the mystery.

Last week I too went out and bought the Harbor Freight tach http://www.harborfreight.com/digital-ph ... 66632.html. I think it is junk. The numbers jump around. I am using mine outdoors, so maybe daylight screws it up. If anyone here owns one and has had success with it, please advise before I take mine back for refund.

I think the best type of tach is the strobe type. With this type, you tune the strobe frequency (which is a well-defined quantity) to match the optics of the rotating object. When the object appears to stand still, you will have either the actual RPM or some multiple of it. Same technique as with using an engine timing light. So that should be a highly reliable method. Lowest price for a strobe tach I have found is $90 for a "TYEMUI" on Amazon. The serious pro strobe tachs cost plenty.

Unreasonable to expect accuracy with a $10 product. Some favorable talk on this forum about a "VBI" brand tach but I can't find it online.

You may observe that almost all 'discussions' involving trying to gain greater understanding of the beast will include the same participants roaming here. Not all come to the same conclusions however.

But all gain by their involvement.

SteveMaryland wrote: ↑Mon Aug 08, 2022 9:16 am
Last week I too went out and bought the Harbor Freight tach http://www.harborfreight.com/digital-ph ... 66632.html. I think it is junk. The numbers jump around. I am using mine outdoors, so maybe daylight screws it up. If anyone here owns one and has had success with it, please advise before I take mine back for refund.

The one I bought is a NEIKO brand 20713A off Amazon. It is more than adequate for the task, about $20 including shipping. I didn't try it outside and would have limited expectations of it working in sunlight conditions because it uses an optical sensor. I tested it against the PowerPro built in tach, and it worked fine. (PP evidently measures motor RPM and then scales it to main spindle RPM, which is what is displayed.)

- David