Lathe speed formula

[Raymond Sprouse]

I am looking for a formula to determine lathe speed. I just did a motor upgrade from a 110v 1/2 horse motor to a 3ph 220v 2 horse motor. I am working on building a remote with run/stop, Fwd/Rev, and Pot switches. I want to add decals on my remote indicating min/max speeds on the Pot switch.

I know the motor speed, motor spindle diameter, and spindle diameter. I can also see the freq readout on my vfd. Is there a formula I can use to determine the speed of the spindle at any given frequency?

[Dick Strauss]

Raymond,
If I understand correctly...you take the motor speed and multiply by the ratio of motor pulley/spindle pulley diameters to get the 100% spindle speed. Assuming you got a 5k linear pot as recommended by the VFD manufacturer and are running at 60hz on the VFD, you need to set the 100% and 0% marks on the pot controller (these points may or may not be at the end of the pot rotation). Then you basically start dividing in half, divide in half again, ... until you have 4 or 8 speeds not including the zero.

Once you change the vfd frequency everything changes. You may want to make marks at the prescribed pot rotations for three sets of speeds for say 20hz, 40hz, and 60hz. So you might have speeds marked 200, 400 and 600 at the same pot mark to represent 20, 40 and 60hz speeds.

[Reed Gray]

The only lathe I have had that had any way to tell exactly what speed I was using was a 4 speed Atlas. Since I never had a read out, I never developed a need for one. You can easily turn by feel, and don't really need to know what your exact speeds are.

robo hippy

[Paul Atkins]

You could get a digital tach.

[Brian Brown]

If you are mathematically challenged like I am (I am also lazy), you can just use something like this. aachteeeteepee://Dubdubdub.blujay.com/item/New-Digital-Photo-Tachometer-RPM-Model-Airplane-Engine-15070100-1725384&keywords= . Just make a faceplate with a hole drilled near the outside edge, and put a light source behind the face plate. Hold the meter up to your face plate where it can see through the hole, and it will count the RPMs. If you want to determine the actual speed the wood travels past your tool, you'll have to use that math stuff. Something about square pies. I don't think it matters if it is apple or banana cream.
rpm-plate.jpg

[Dave Mueller]

Raymond,
I believe you have an 1800 RPM motor, so it will turn at 1800 RPM when the controller frequency is at 60 Hz. If your spindle and motor pulleys are equal in diameter (i.e. 1:1) the motor RPM and spindle RPM are equal. If not, multiply by the ratio of the two pulleys. Then, wait until you have the vfd controlled by the external pot. You actually don't even have to have the motor working. Just adjust the pot until the controller reads 60 Hz and then mark the pot position as 1800. Do the same for as many other speeds as you want on your pot legend.

[Dennis Ford]

If you have MS Excel, type this info in as shown in attached PDF file. Excel will do the math for you.

[Raymond Sprouse]

Thanks for all the help. All good advice.

It is true that the true speed doesn't matter when turning. I just want to know out of curiosity.

Dennis, thanks for the formula, that is exactly what I wanted to do with it. This will allow me to choose multiple freq to give me multiple speeds.

Thanks again everyone.

[Jerry Marcantel]

Ray, this link will give you the speeds you are seeking. Drive (a) pulley size, motor rpm, and pulley (b), hit calculate, and your actual RPM is displayed...

http://gadi.agric.za/software/renting/pulley.php
Jerry ........(in Tucson)

[Art Jordan]

The digital tachs (21.00 from Ebay) really work well. Checked mine with a new 3520B Powermatic and was within 20-25 rpm. Also works on small Jet.

Plenty of stick-ons for multiple tools

[Peter Fabricius]

Hi Ray;
The VFD should be able to show you the actual RPM instead of the Frequency. The set up should be in the user manual.
Peter F.

[Dick Strauss]

Peter,
Unfortunately the VFD will not show the result of the pulleys driving the spindle...it will only show rpms based on the motor rpms.

[Roy Turbett]

I remember in my 8th grade shop class the instructor used several different colored tapes on the end of the faceplate that gave off a different pattern depending on the speed the lathe was turning. This way he was able to tell at a glance from across the shop how fast each of the four lathes was turning. Does anyone remember the color scheme? I've been thinking about decorating my PM 90 outboard faceplate.

[Bill Noce]

Go to this link from google...its for Woodturning Design by DAle Nish explaining lathe speeds it might help you
http://www.woodturningdesign.com/askdale/5/5.shtml