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Excel Formula Question

**Ian.M**:

I assume this is related to your previous drill press topic: https://www.eevblog.com/forum/mechanical-engineering/calculating-total-number-of-speeds-on-drill-press/

Its all about the ratios.

For each belt, divide the pitch diameter of the driving pulley by the pitch diameter of the driven pulley to get the factor it transforms the speed by. Multiply the factors for both belts together to get the overall factor, then multiply the motor speed by it to get the spindle speed.

N.B. it is almost certain you have measured the pulley diameters wrong. A V belt drives on its flanks. Unless worn out, the inside flat surface across the tip of the V doesn't even touch the pulley. The pitch (effective) diameter is approximately where the belt reinforcement sits in the V groove. Measure how far in from the back of the belt it is (preferably on a new belt), then subtract twice that from the diameter of the pulley at the back of the belt when its held tightly round the pulley. You can measure that straight off the larger pulley of any pair with the belt in place (assuming you've got large enough callipers), but for the smaller pulley you'll need to slacken it enough to hold it in to get >180° wrap.

Finding the pitch diameter with a link belt will be significantly more difficult. I'd probably measure it with a compatible profile V belt wrapped round each pully, then adjust the fudge factor (was double the thickness between the back of the belt and the reinforcement) you subtract from the measured diameters (over the belt) which should be the same for all, till the measured spindle speed comes out right.

**IanB**:

Measuring the gear ratio of a belt drive can be done more easily using the belt itself to help with the measurement. Make a mark on the belt lined up against a point on the first pulley. Now turn the pulley through one full revolution, and make a second mark on the belt. The distance between the two marks is the circumference of the pulley. Repeat the same procedure with the second pulley. Now the gear ratio is the ratio between the two measurements.

**Ian.M**:

Noted for future use! You've just removed the need for big callipers, and improved the accuracy as well as you will be measuring the circumference as the belt sits under tension.

Also, there's no need to convert to diameters, (as long as you use *all* circumferences) as the Pi drops out when you calculate the ratios.

**bostonman**:

--- Quote ---I assume this is related to your previous drill press topic:

--- End quote ---

Yes, you're correct. This wasn't meant to deviate towards repeating the drill press thread. This is what I've done that led me up to this thread. I measured speeds on every belt combination (on both the idler and chuck pulley along with the motor) using a meter and the majority are extremely different than the tag on the press (the high speed should be 2800 and I"m getting around 3900 - I don't have the numbers with me at the moment). Now, I went ahead and measured the circumference of each pulley by tightly wrapping a string around the base of each one and then measuring the length of string on a tape measure. This is the circumference, and, from that, I could also (obviously) get the diameter if I cared to use this or have a need to use it.

Trying to find the "pitch diameter" and/or measuring error factor, I entered all the measurements into Excel and created a "fudge" number cell that will allow me to enter a number that adds a larger (or smaller) circumference and all the cells will update accordingly. This will allow me to see if I can get all the calculated numbers equal to the numbers I measured.

As an example: if one pulley is 3" circumference, and I have all the cells calculating the ratios from the motor, speeds, pulley ratios, etc... Then in my "fudge" number cell I may enter 0.3" and see if that makes the drill press speed what I measured.

What led me to post on this thread is that my Excel formula wasn't working and I thought it was an Excel formatting issue caused by me. What I learned is that one of the ratios was canceling, and it wasn't a formatting issue in Excel that I caused, so the ideal mathematical way of calculating is still eluding me.

I don't have the numbers handy, but I can post them to the original drill press thread I began and cease this thread as it's pertained to Excel - which has been resolved - and confirmed it was something I was clearly doing wrong.

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