View Full Version : Calculating lathe RPM

Sunblade

05-16-2009, 01:41 PM

Hi All:

I have a benchtop lathe with a 4 step pulley drive. The rough diameter of the step pulleys are: 3"-2.25"-1.6"-1", this is as close as I can measure with a small caliper. The motor has a no-load speed of 1725 RPM. My rough calculations show that the speeds would be 575, 1104, 2695, and 5175 RPM.

Am I doing something wrong here or these figures right?

As I am a complete novice at wood turning I was wondering if 5175 RPM would be way too fast a speed for anything I would be doing even on a small diameter piece? Do I need a different set of pulleys or maybe I'm calculating wrong.

Any help on this would be greatly appreciated.

Thanks,

Kerry

TDHofstetter

05-16-2009, 02:16 PM

Wow - that's a pretty surprising ratio! Your numbers are right, provided your measurement was taken at the right spot... and I'm just a little suspicious there.

You see... a 1"-diameter pulley is LEETLE and really unusual in a v-belt pulley. Are you measuring the RIM of each step, or the HUB diameter of each step? You measure across the rim to get the pulley size.

5175 is pretty fast for anything. Probably fine enough for really small stuff, but not with any sort of chuck!

-- Tim --

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Sawduster

05-16-2009, 03:14 PM

Other than you high and low numbers, I come up with different numbers than yours assuming that you have two sets of identical pulleys but in reverse order on the shafts.

If you add even 1/4" to the largest and smallest, the turning speed drops to a more realistic 4485 RPMs.

[link:www.csgnetwork.com/pulleybeltcalc.html|Here is link] to a belt and pulley calculator. I came up with the same numbers for those two middle pulleys as this calculator.

TDHofstetter

05-16-2009, 03:18 PM

Dang. I didn't even think to check but the first & last numbers to get the full range.

-- Tim --

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arcticfox46

05-17-2009, 08:56 AM

There are several machining RPM calculators in the web,

Here is one I found just off the top

It tells you how to figure out RPM to use for a given material.

http://www.fignoggle.com/library/speedfeed.htm

Sunblade

05-17-2009, 09:16 AM

Let me get this right...I need to measure across the largest diameter of the pulley? That's where the problem is, I was measuring across the inner groove. I'll remeasure and recalculate.

Thanks guys!!

FrankC

05-17-2009, 10:09 AM

For an accurate calculation you need the "pitch" diameter of the pulley.

The pitch diameter of a pulley is not the same as the outside diameter, in fact, the pitch diameter is very difficult to measure directly. If you cut a belt and look at the end, you will see a row of fibers near the outside surface, the pitch diameter of any pulley is measured where these fibers contact the sheave.

That being said for most applications the outside diameter measurement puts you in the ballpark.

Sawdust Making 101

http://sawdustmaking.com

TDHofstetter

05-17-2009, 11:04 AM

Correct on all counts, Frank. VERY few people ever use the true pitch diameter (or the belt's true pitch length, which is also hard to measure directly without cutting the belt)... but that's more precise.

It's not TERRIBLY hard to measure the pitch diameter, though, at least to a very close figure. To get it done (if you ever need it), you need two pieces of steel square bars the same section size as the pulley's nominal width (1/2", 5/8", etc, depending upon the "type" of belt it's intended to use). Lay one across the groove on each side of the pulley, and mike across 'em. Subtract twice the section size of the bars (once for each bar), and you have the pitch diameter.

Pitch diameter changes if you use a different size belt, which complicates matters. Use a 5/8" belt instead of a 1/2" belt and the pitch diameter increases, or use a 1/2" belt to replace a 5/8" belt and the pd decreases.

-- Tim --

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arcticfox46

05-17-2009, 10:16 PM

Difference between a scientist and an engineer - exactness.

An engineer will not be as exact as a scientist.

Pitch diameter:

Measure the outside diameter

Measure the inside diameter

find the mid number

Close enough ----

I betcha dollars to donuts - you will be within 5-10 percent - and for what you are trying to figure out - that will be fine.

KISS !!

TDHofstetter

05-17-2009, 11:15 PM

Aw, heck. If we're gonna' KISS, the outside diameter's close enough. :) :) :)

-- Tim --

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arcticfox46

05-20-2009, 04:29 PM

Yep - it is

FrankC

05-21-2009, 09:41 AM

What really counts is how fast the surface of the object being turned meets the cutting edge of the tool, with any step pulley system this will decrease as the diameter of the turned object decreases so you have to work in one of four ranges in this case.

With a variable speed system the range is limitless.

Sawdust Making 101

http://sawdustmaking.com

PastorPaul

05-26-2009, 07:28 AM

I miss doing it the easy way. Back in either high school or college, we had a variable speed strobe. You chalked a line across whatever you wanted to speed check, aimed the strobe at it, adjusted the strobe speed until the line appeared to stop, and then just read the speed setting on the strobe... instant RPM reading.

Pastor Paul

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