Speed / RPM Calculating Spreadsheet

bradjacob · Sep 25, 2012

I've created a handy spreadsheet for calculating actual RPMs of your machine. Just plug in the values and it calculates the spindle RPMs for each cone.

If your machine only has 3 cones/speeds then just fill in the info for the three.

Hope it helps.

Mr Majestic · Sep 25, 2012

Very cool, thank you!

SE18 · Sep 25, 2012

the flaw is you need to know the motor rpm

the tag says 1725 on mine but that's not actual speed

bradjacob · Sep 25, 2012

SE18 said:
the flaw is you need to know the motor rpm

the tag says 1725 on mine but that's not actual speed

Welp, I guess that's out of most people's hands, unless a tachometer is installed. Which, I think, is only useful if you're running a VFD to allow for those in-between speeds.

Ken-Bergen · Sep 25, 2012

SE18 said:
the flaw is you need to know the motor rpm

the tag says 1725 on mine but that's not actual speed

And how did you determine this?
If the tag says 1750rpm @ 60HZ that is what it will run unless your line frequency is off or you're using a VDP.

Ken-Bergen · Sep 25, 2012

bradjacob said:
I've created a handy spreadsheet for calculating actual RPMs of your machine. Just plug in the values and it calculates the spindle RPMs for each cone.

If your machine only has 3 cones/speeds then just fill in the info for the three.

Hope it helps.

Thank you, this will help those that don't have the origanal speed tag and or those that have non standard gearing make a speed chart for their lathe.

traditional-tools · Sep 25, 2012

SE18 said:
the flaw is you need to know the motor rpm

the tag says 1725 on mine but that's not actual speed

Dave,

Do you suspect that your hz is not accurate on your line? Or voltage is not correct and throwing off the rpm?

I wouldn't worry about it unless you wanted to test and check each machine in your shop.

Cheers,
Alan

gunsmithing1 · Sep 26, 2012

If you need a conveter for file goto Download: Microsoft Office Compatibility Pack for Word, Excel, and PowerPoint File Formats - Microsoft Download Center - Confirmation

Dave

Ken-Bergen · Sep 26, 2012

gunsmithing1 said:
If you need a conveter for file goto Download: Microsoft Office Compatibility Pack for Word, Excel, and PowerPoint File Formats - Microsoft Download Center - Confirmation

Dave

Wouldn't that require installing Windows first? :eek:

I find Libreoffice to work just fine.
Home » LibreOffice

ajclay · Sep 26, 2012

Brad
thanks for sharing. I've been wanting to figure that out for my little SB9..
AJ

vandis · Sep 26, 2012

Just one more point to confuse the issue: Most motors used on typical lathes are not synchronous motors. That is, they do not run at the line frequency speed. In the case of the 4-pole motor being discussed here, the synchronous speed would be 1800 RPM. For a 2-pole motor it's 3600 RPM. For a 6-pole motor it's 1200 RPM (For 60 HZ power) The nameplate tag RPM is the motor speed at rated load and it's less than the synchronous speed because of electrical "slip" which is how the motor can develop torque. So, if you run the motor lightly loaded, it will operate at some speed between the nameplate speed and synchronous speed. The higher the load, the slower it goes. The "slip" is dependent upon the motor design and that's why you typically see numbers like 1725 or 1750 RPM or even something different from them. Due to design factors and intended use, some 4-pole motors may have nameplate RPM spec's lower than 1725 but those motors are not typically available in the usual lathe motor.

Just say'n so we have all the facts on the table. Probably this is "information overload."

If you really want to know the motor's speed, you need to check it while it's operating.

rw1 · Sep 26, 2012

Nicely done Brad.....thank you!

You are South Bend crazy good!

SE18 · Sep 26, 2012

thx, what caused me to suspect and someone to suggest line voltage off was I used the same calculations presented here in another post titled rpms or something to that effect. I posted all my statistics, have videos and stop watches of it turning slow in backgear, calculated all non backgear rpms, and came up with different numbers (faster) than the calculations are supposed to be for the motor rpms

DaveV

Oh, and BTW, I've detailed all my calculations here if anyone is interested, bottom line, I need some sort of tachometer to get the actual speeds, but I'm just making chips now based on how the lathe reacts and knowing rudimentary data on how fast which metals should be turned):

http://www.practicalmachinist.com/vb/south-bend-lathes/math-help-needed-rpm-250313/index2.html

Ken-Bergen · Sep 26, 2012

SE18: As I said in your other thread your calculations are off because of faulty measurements of your motor pulleys.
You say the pulleys are 1.2" and 2.5" but I'm betting when measured properly they're actually closer to 2" and 3.5".
Redo your calculations using 2" and 3.5" for the pulleys and think you'll find that your motor speed of 1725rpm will give you spindle speeds very close to what you're observing.

Ken

3512B · Sep 26, 2012

thank you ! It will be vary handy
Steve

edkolt · Sep 27, 2012

Brad,

Nice job on the math. :cheers:

As noted in previous posts the RPM of the electric motor may vary from the stated spec, but for hobby machinist work I think it is close enough. VFD people would find this useful.

I would suggest adding a few line to your spread sheet. What is important to me is the surface speed of the actual part being turned in FPM. It would be nice to enter the diameter of a part and have the FPM pop up.

ED S

bradjacob · Sep 27, 2012

edkolt said:
Brad,

Nice job on the math. As noted in previous posts the RPM of the electric motor may vary from the stated spec, but for hobby machinist work I think it is close enough. VFD people would find this useful.

I would suggest adding a few line to your spread sheet. What is important to me is the surface speed of the actual part being turned in FPM. It would be nice to enter the diameter of a part and have the FPM pop up.

ED S

Couple Points Ed -

First off, SE18 made a post that inspired me to develop this spreadsheet. I knew the formulas (from HTRAL), but his "cone by cone" approach I really liked.

Second, I agree about the hobby/motor-speed thing. We can only go by what's stated on the motor tag and hope that our home's electric is correctly powering it. I guess a tach is the only (dead-sure) way to know true RPM's. But for South Bend owners, we only have 6 speeds to choose from, unless we have a VFD. And from experience, I can tell you that only having these 6 speeds, it's about "feel" and knowing your machine.

I'll modify the spreadsheet to handle the SFPM calculation.

SE18 · Sep 27, 2012

thx all, getting math burnout and will count teeth later and do calculations later. The formulas btw, are also in HTRAL and in the Hercus book. In HTRAL you have to go to several pages to find speeds of cones and then go to another page to find formulas for feet per minute for each metal. And all this assumes that you know what type of metal you just got from the scrap bin at your metal shop.

This came from the other rpm post on finding backgear ratios (to determine speeds):

Any time any one wants to pin down their back gear ratio they have but to count the teeth on the four gears and multiply the two ratios.

Example:

26 teeth on left end of cone
57 teeth on the mating gear
first ratio is 2.1923:1
19 teeth on right end of back gear quill
53 teeth on bull gear
second ratio is 2.7895:1
Overall ratio is 2.7895 X 2.1923 = 6.115:1

johnoder · Sep 27, 2012

I would appreciate being credited for the above Example in post #18

Rex TX · Sep 28, 2012

Make your life easier and buy a $20 optical tach.

New Digital LCD Laser Display Optical Tachometer RPM | eBay

If nothing else, use it to check your math

Speed / RPM Calculating Spreadsheet

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