# Thread: Bridgeport spindle RPM calculation for non-standard motor RPM?

1. ## Bridgeport spindle RPM calculation for non-standard motor RPM?

I have a Bridgeport with a J1 head that has a non-original 2HP motor on it.

My motor runs at 1720 RPM, which I believe is different from the speed of the motors that originally came with the machine - which means that my actual spindle RPMs at any given pulley and gear combination are different from the numbers written on the machine and in the machine manual.

So, does anybody know of a spreadsheet or online calculator (or simply know the formulas) so that I can make a spindle RPM chart which is accurate for my motor?

2. The error should be consistant, that is the same factor (probably in the range of 0.5 to 2.0) multiplied by the machine plate RPM should result in the actual RPM. However no belt driven machine (and some argue any machine) is totally accurate for all RPMs.
If no tachometer is available, mark the spindle and count the RPMs on a slow speed for a minute to get an actual RPM and then compute the factor.

3. Originally Posted by PhoenixMetal

My motor runs at 1720 RPM
Most US step pulley machines had an approx 1700 rpm motor. Possible that there was a 3400 rpm motor on it originally. If that was the case, then just half the motor speeds listed in the manual. Nothing hard to calculate out.
You can turn the motor and see how many revolutions the spindle spins when in high speed. That will tell you what motor speed you should have had. IF it is set up for the faster motor, and the current motor is 3 phase, then run a VFD at 120 hz and the motor will spin at double the original speed. Might want to make sure the bearings in the motor are in good condition first.

4. Thanks for the suggestions to manually count the RPMs at low speed and multiply out from there... That sounds like a good solution. I'll post up my results when I do it.

Also, that's interesting that according to polaraligned's post that most US machines came with a 1700 or so RPM motor. If that's the case, then maybe my motor is more or less "normal" RPMs for the machine. I have been basing my assumption that my motor is not standard on the specs from a manual I found online (Page 14 of the PDF at http://www.truetex.com/bridgeport-manual.pdf) which states that the machine came with a 2 speed motor that output 1400 RPM at low speed and 2800 RPM at high speed. The RPM chart printed on my machine matches the chart in that manual.

Anyhow, it sounds like counting the RPMs at low speed is the way to go to figure this out... I'll post up my results later.

• Originally Posted by PhoenixMetal
The RPM chart printed on my machine matches the chart in that manual.
2 speed motor.

• I finally got around to counting the spindle RPMs. Using a stopwatch, and with two trials, I counted 90 RPM on the spindle at the lowest speed.

I made a spreadsheet to calculate the spindle RPMs for a given motor RPM. I calibrated its calculations based on a stock motor of 1400 RPM and the spindle speeds which are printed on the machine and in the manual. I have attached the spreadsheet to this post for anybody that may need it.

Interestingly enough, when I enter 1720 RPM for the motor, it comes up with different spindle speeds from the speeds from what I observed. So, I had to adjust the motor RPM in the spreadsheet until the calculated lowest speed matched my observed speed. This came down to me needing to enter 1580 RPM for the motor's speed. This could mean one of two things: Either the motor is actually running at a lower speed than what is written on the face plate (or I misread it, which is possible because its badly scratched up), or the machine's stock motor wasn't really 1400 RPM. But in any case, the ratios entered are still correct, so as long as you adjust the entered motor RPM to match your observed spindle RPM, all the other numbers will be correct and good to use.

Thanks for all of your help, and I hope that my spreadsheet helps someone out some day!