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Lathe motor question "Variable Speed"
« on: April 18, 2008, 07:22:41 AM »
Ok  more advice needed.

I am starting converting my lathe shortly, it is single phase 240 volt. Harrison M250

I want some control over the spindle speed, what is the best way to get this regardless of cost. Cheaper options will be considered also.

Any links to circuits for spindle speed encoders also welcome.

Thanks
Phil_H
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Offline Hood

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Re: Lathe motor question "Variable Speed"
« Reply #1 on: April 18, 2008, 08:55:39 AM »
Best option is an AC Servo, torque from zero to full revs, certainly not cheap though unless you find suitable drive and motor on eBay.
Hood
Re: Lathe motor question "Variable Speed"
« Reply #2 on: April 19, 2008, 03:01:05 AM »
Thanks Hood.

What am i looking for, can you be more specific ?

Phil_H
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Offline jimpinder

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Re: Lathe motor question "Variable Speed"
« Reply #3 on: April 19, 2008, 06:35:55 AM »
I will suggest the alternative  - which is  - replace your spindle motor with a three phase motor (about twice the power) then use a single phase to 3 phase inverter to control the speed. PWM and M3 and M4 signals are readily available from Mach 3.

I say "twice the power" becasue you do loose torque at lower speeds with an inverter - but I am assuming you have a gearbox on the Harrisson

I only say this becasue I've done it, and I know Hood has done the servo option - the choice is yours.
« Last Edit: April 19, 2008, 06:37:45 AM by jimpinder »
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Re: Lathe motor question "Variable Speed"
« Reply #4 on: April 19, 2008, 08:15:10 AM »
Thanks for the answer Jim.

What i want to know is how much power is lost, i know that is a really tough question to quantify.

Can you give any more clues or real world examples, in the main i will be setting the speed via the gearbox and using the speed control to fine tune the speed.

Phil_H
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Offline Hood

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Re: Lathe motor question "Variable Speed"
« Reply #5 on: April 19, 2008, 08:51:05 AM »
I have a VFD and induction motor on my mill. It is a good quality Mitsubishi drive but just the standard Bridgeport 2HP motor so I dont know if its an inverter duty motor. I have heard that the inverter duty motors dont drop off so much in torque but how good they are I dont know. What I do know is with my VFD/motor combination I lose too much torque if I go down below 50% of the motors rated frequency. I still use the vari speed pulleys on the mill to get to the rough speed range and then fine tune by the VFD (external potentiometer). I was considering hooking the VFD to Mach but after using the mill for a while I decided there was no point as the drop off in torque would be no use for the varying types of work I do.
 As Jim has said you could go up a few HP on the motor and that would help things a great deal.

An AC Servo for the spindle however is a different thing alltogether, the torque is almost constant from 0RPM to Full speed, it does drop off slightly but rather than it dropping off at low revs it is high revs it drops off at. This is perfect for both a lathe an mill as it is the lower revs you are wanting the real torque anyway. However when I say the torque drops off at higher revs I am not talking a lot, below is the torque curve of the motor I am about to fit to the new mill I am retrofitting. It is a 5.5KW motor and its continuous torque rating is 18Nm (160lbs in approx), as it goes above 3000RPM it starts to drop off a bit but as said the torque is not so important way up there. One other advantage is no brake is needed, the AC Servo will have full rated torque when at 0 RPM so when you tell it to stop it will go to zero speed at the acelleration you have tuned it and sit there. I have been testing the motor I have and although its not connected to the spindle it still is impressive. I have tested by running the motor at 4000RPM then gone to -4000RPM with the acelleration set to 40,000RPM per second. That means from full speed CW to full speed CCW is only 2/10ths of a second and the motors out of position error doing that is zero counts and that is with the 8000count encoder (2000line) I have not tried the acell higher as it takes a bit to hold the motor down on the floor when it changes direction :D

Hood

Offline Hood

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Re: Lathe motor question "Variable Speed"
« Reply #6 on: April 19, 2008, 09:24:23 AM »
Just a quick update, I decided to get brave and have just about dislocated my knee LOL
 Anyway I have just done a full speed CW (4000RPM) to Full speed CCW (-400RPM) with an acell set to 160,000rpm/second. It did that no problem and it is impressive to see the motor go 4000rpm CW to 4000rpm CCW in 0.05 seconds :D
 Obviously with it hooked up to a spindle with a big face cutter it might not do so well but in reality this kind of acelleration is probably not needed.
 Away to rest my knee now

Hood
« Last Edit: April 19, 2008, 09:26:25 AM by Hood »
Re: Lathe motor question "Variable Speed"
« Reply #7 on: April 19, 2008, 09:43:09 AM »
Thanks Hood

I did a search for that motor, one link that came up was on ebay a guy called HoodScotland bought it at good price, is that you ? ;D ;D ;D

That doesn't seem to be a lot of power for a spindle motor, or does it, what is the equivalent in HP

Would it be feasible to run it with timing belts to make use of the positioning feedback.

Phil_H
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Offline Hood

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Re: Lathe motor question "Variable Speed"
« Reply #8 on: April 19, 2008, 10:17:59 AM »
Yes thats me :)
5.5KW is approx 7.33HP this is going  on my mill which currently has a 4HP inducion motor, I think it will do fine :)
Yes I am planning on timing belt, main reason I want an AC Servo spindle, other than the speed control and continuous torque, is I am planning on making a toolchanger in the future and so will need positioning accuracy. Also I am hoping that rigid tapping should be possible with a controlled spindle but not thought too much into how that would be done yet.
Hood

Offline jimpinder

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Re: Lathe motor question "Variable Speed"
« Reply #9 on: April 20, 2008, 05:54:59 AM »
Yes - Hood is about right - much under 50% frequency and torque drops off quite quickly - which is why I asked if you had a gearbox. I am just having to make one to turn wheels for my miniature railway.

6" wheels at 100 ft per min mean something in the order of 70 to 100 rpm. My motor runs up to 1300 rpm so I could do a 10 or 12 to 1 reduction to keep the torque.

The figures for Hoods motor are pretty impressive. He's a Scott - make him an offer for it  ;D ;D ;D :D :-\
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