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Calculating Lathe Speeds Using Step Pulleys

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Forum topic by Comteck posted 05-17-2020 02:49 AM 288 views 0 times favorited 13 replies Add to Favorites Watch
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Comteck

5 posts in 10 days


05-17-2020 02:49 AM

Hi Everyone,

I’m brand new to this forum, so I hope I’m posting this in the correct section. I wish to build a lathe using a 1725 rpm motor, and step pulleys to vary the speed. I’m looking for a formula to calculate the different speeds of each position.

I have found many formulas online, but the numbers don’t seem to work out. My drill press uses step pulleys and there’s a chart showing the different rpm’s at each position of the belt. I measured the diameter of each pulley, entered them into the formula along with the rpm of the motor (which is 3100rpm), but it gives a totally different number than the one on the chart.

Any help would be greatly appreciated.

Comteck


13 replies so far

View Sylvain's profile

Sylvain

1027 posts in 3230 days


#1 posted 05-17-2020 11:28 AM

The linear speed of the belt is the same while running on each pulley.

Linear speed is 2 X Pi X D X rpm in inch/min if D is in inches and rpm is rotation per minute. In cm/min if the diameter is in cm.

so 2 X Pi X D1 X RPM1 = 2 X Pi X D2 X RPM2
after simplification: D1 X RPM1=D2 X RPM2

or RPM2 = RPM1 X (D1/D2).
Units used to measure the pulley diameters doesn’t matter as long as you use the same ones for every pulley.

Now, with a V belt, the diameter to use is not the bottom of the pulley groove.
That might explain why your drill press chart gives (slightly?) different numbers.
I would have to search on the web what diameter to use for a V belt.

-- Sylvain, Brussels, Belgium, Europe - The more I learn, the more there is to learn

View MrUnix's profile

MrUnix

7991 posts in 2930 days


#2 posted 05-17-2020 03:33 PM

Easy way – use this:

Arbor RPM Calculator (at the vintagemachinery site)

Cheers,
Brad

PS: Your drill press motor is 3100 rpm? What kind of motor is it?

-- Brad in FL - In Dog I trust... everything else is questionable

View xeddog's profile

xeddog

296 posts in 3738 days


#3 posted 05-18-2020 07:44 PM

I was curious what diameter to use. What if your belt is down in the groove a bit. Then the “effective” diameter would be less. So would you use the diameter of the innermost belt contact, outermost, or middle?

Wayne

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Comteck

5 posts in 10 days


#4 posted 05-20-2020 03:21 PM

Thank you all for your replies.
First of all, one thing I discovered with my drill press is, that the 2 step pulleys aren’t the same size, which changes the numbers. However, the numbers still don’t seem to work out.

Sylvain, I’m using the same formula that you posted.
And Wayne, I was wondering something very similar to what you said. The formula takes into account the diameters of the pulleys and the RPMs of the pulleys. But doesn’t the RPM of the motor pulley change with different sizes of pulleys? There’s nothing to take the RPM of the motor itself into account.
Sorry to keep asking about this, but it still isn’t making sense to me.

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Comteck

5 posts in 10 days


#5 posted 05-20-2020 03:40 PM

Brad, it’s a 3-phase motor.

View MrUnix's profile

MrUnix

7991 posts in 2930 days


#6 posted 05-20-2020 04:05 PM

But doesn’t the RPM of the motor pulley change with different sizes of pulleys? There’s nothing to take the RPM of the motor itself into account.

No – the motor RPM and the motor pulley RPM will be identical regardless of pulley size – unless your pulley is slipping on the shaft ;)

The link I provided above uses the motor speed and pulley sizes to calculate the arbor rpm and should be exactly what you need.

Cheers,
Brad

-- Brad in FL - In Dog I trust... everything else is questionable

View Zort's profile

Zort

20 posts in 497 days


#7 posted 05-20-2020 04:10 PM

In a perfect world, the rpm’s of two gears (pulleys) are inversely proportional to their size. That is to say that the product of the speed of a gear and its size is the same as the product of the speed of a second gear and its size.

It is pretty easy to measure the size of a gear but not so much with a pulley, outside diameter, inside diameter or something else. It really is the something else—it is sometimes called the pitch diameter. This is the point where the belt meets the pulley in such a way that there is little to no slippage. Sometime you can find evidence of this point by examining the belt.

Generally, we don’t need an exact rpm for the second pulley—close is generally enough.

I can’t remember where but there was a post about a paper design that could be placed on the second pulley and when viewed with a black light or fluorescent light markings around the paper would align and yield the true speed of that pulley. About all I remember about the post was that it was about determining the turning speed of a lathe. It worked like this—find the design on the web, print it out, attach it to something that will spin. Turn on the machine and view the spinning disk. Sorry but I can’t find the design on the web.

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Comteck

5 posts in 10 days


#8 posted 05-20-2020 04:19 PM

Thanks again for the prompt replies. I will have to buy a tachometer to see if the RPM numbers on the drill press are accurate.
A couple of other questions I have.
1. What speeds should I consider when building a lathe?
2. What type of motor should I get? AC or DC? RPM for hardwoods and softwoods? HP?

I want to be able to turn both bowls and spindles.

Comteck

View MrUnix's profile

MrUnix

7991 posts in 2930 days


#9 posted 05-20-2020 05:00 PM

Generally, we don’t need an exact rpm for the second pulley—close is generally enough.

This ^^^^ We aren’t talking sending someone to the moon, just spinning some wood. For pulley size, the generally accepted diameter is the O.D., and that is the size listed when they are sold. For the marginal difference between O.D. and pitch diameter, it really makes no difference. Throw into the mix that for induction motors, the RPM of the motor is NOT what is listed on the data plate. A standard 4 pole motor has a synchronous speed of 3600. The data plate will typically list something like 3450 or 3475. This is because of the inherent slip of the motor along with the load, and the actual speed will be somewhere between the two numbers. Same with a 2 pole motor – which has a synchronous speed of 1800 but will be listed as 1725 or thereabouts.

1. What speeds should I consider when building a lathe?
2. What type of motor should I get? AC or DC? RPM for hardwoods and softwoods? HP?

Depends on what you want to do. Larger stuff like bowls really benefit from a low speed capability. Spindle stuff benefits from higher speeds. If you look at what is available out there, you will get a feel for what a typical speed range is. For example, my PM45 can run between 300 and 2800 RPM. My Jet does around 500 to a little over 3000 rpm. For really large, out of balance blanks and also sanding, a very low speed is desirable – and I’ve found that 300 is sufficient but still wish it could go down a little lower.

AC or DC? Heck that is like asking if you prefer Chevrolet or Ford ;-) Too many combinations and configurations and each one has it’s merits and downfalls.

Cheers,
Brad

-- Brad in FL - In Dog I trust... everything else is questionable

View Zort's profile

Zort

20 posts in 497 days


#10 posted 05-23-2020 02:55 PM

I have been trying to find the post I referenced above with no success but I did want to post this pattern. It works like the view of wagon wheels in old westerns, where the wheel appears to go backwards. I found this image by searching for stroboscopic paper pattern.
Somewhere there is a website, in which you enter the cycles of your electricity and the minimum and maximum rpms you want the disk to display and then print the paper disk. However, I can’t find it.

View Lazyman's profile

Lazyman

5182 posts in 2118 days


#11 posted 05-23-2020 03:10 PM

LJ Woodknack has a blog about using a similar strobe tachomenter that he designed for use with fluorescent lights or a strobe.

-- Nathan, TX -- Hire the lazy man. He may not do as much work but that's because he will find a better way.

View Comteck's profile

Comteck

5 posts in 10 days


#12 posted 05-23-2020 04:33 PM

Thank you all for your replies. I have a lot more information now than I did before. I really appreciate all the help.

View Zort's profile

Zort

20 posts in 497 days


#13 posted 05-24-2020 02:26 PM



LJ Woodknack has a blog about using a similar strobe tachomenter that he designed for use with fluorescent lights or a strobe.

- Lazyman

Thanks, that looks familiar. It works pretty well.
A search of your terminology of “strobe tachometer” revealed some interesting phone apps that use the flashlight and a white marking on the pulley to calculate rpm. Might be just the ticket for determining a lathe speed.

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