Thread: Lathe motor/drive conversion

Maybe you can fit a little 1hp 3 phase motor and a Huanyang VFD ?

That would give you variable speed and a digital readout of rpm in one go

MIKE

I am interested in a digital rpm readout for my lathe as well, is there anything out there that is reasonably priced?

I agree with morrisman that the vfd would be the way to go, if you keep a look out on ebay etc you should be able to piece together a motor & vfd for under $200.

I'm not sure how the vfd displays the rpm though, any more info morrisman as I have a vfd on my lathe and it would be handy if it could tell me what rpm I am doing. I thought you would need some sort of pickup on the pulley and additional hardware.

I've also come up against the problem of changing belts/gears to adjust speed on my small lathe so I'll watch this thread with interest.
I'm one step ahead in that I already have 3 phase connected

The Huanyang VFD's display RPM in one of the display options , but I don't think this is an actual RPM display taken from a sensor . Rather, it is a theoretical RPM calculated by the processor in the VFD ,if that makes sense

You can buy hand held digital tachos for a few bucks on EBAY , they use a reflective tape for a sensor e.g., the tape is attached to the spindle or chuck or whatever .When I tested my bench 3000 rpm grinder , the hand held tacho measured the rpm at about 50 rpm lower that the specified 3000. Ive also noticed a 1 to 2 % lower reading than specified rpm on other machinery . I put it down to friction in the bearings maybe . Or, maybe the cheap Chinese tachos are not accurate anyway

Mike

I have one of those hand held tachos, I was after a more permanent solution.

I noticed some of the hares & forbes lathes now come with a display fitted, something like that would be ideal.

http://images.machineryhouse.com.au/L565D/5/700

They must only be a few $$$ from china.

This may work for you

Low cost tachometer - mikesworkshop

Originally Posted by morrisman

The Huanyang VFD's display RPM in one of the display options , but I don't think this is an actual RPM display taken from a sensor . Rather, it is a theoretical RPM calculated by the processor in the VFD ,if that makes sense

You can buy hand held digital tachos for a few bucks on EBAY , they use a reflective tape for a sensor e.g., the tape is attached to the spindle or chuck or whatever .When I tested my bench 3000 rpm grinder , the hand held tacho measured the rpm at about 50 rpm lower that the specified 3000. Ive also noticed a 1 to 2 % lower reading than specified rpm on other machinery . I put it down to friction in the bearings maybe . Or, maybe the cheap Chinese tachos are not accurate anyway

Mike

Mike, Your bench grinder would have an induction motor fitted. These have a theoretical speed of supply frequency (cycles/second) x 60 (seconds per minute) / no of magnetic pole pairs in the motor windings, (typically 1,2,3 pairs or 2,4,6 poles).

So for a 50Hz mains motor, they would run at 3000, 1500, or 1000 RPM nominal speed. However to develop torque and do usefull work, they 'slip' or run slightly slower than nominal speed. The amount of slip is normally in the order of 2 to 4% at rated load. The ID plate on the motor typically indicates expected speed at rated power, eg, 1440 RPM at 1HP. This decodes to 4 magnetic poles in the windings (2 pairs) so 1500 RPM nominal and 4% slip (Nominal RPM - Rated RPM)/Nominal RPM or (1500-1440)/1500 or 60/1500 = 0.04 or 4%.

This would apply to all induction motors, single phase, 3 phase etc, and explains how the VFD's alter the speed of a 3 phase motor. They take the mains voltage, convert it to DC, filter it to a reasonable extent, and then generate an output as 3 phase variable frequency for the motor. Won't work for single phase motors as they are not inherently self starting and need auxilliary windings to be incorporated for starting and then switched out once the motor is almost up to speed. If the motor is deliberately slowed down with a VFD, there is a possibility of the start windings being reactivated and burning out.

Frequency based VFD speed indicators are never dead accurate as the degree of slip varies with load, so there is not a constant slip value that could be programmed into the display logic. They can however indicate the nominal speed from knowing the output frequency, and having the number of motor poles programmed. They should then indicate with accuracy within the slip range.

Yes I forgot about the induction motor slip .

These are cheap enough ..no wires needed

Wireless LCD Cycling Computer Bicycle Bike Meter Speedometer Odometer | eBay

Hi,
The taco wouldn't be that hard to do with an arduino/pic, a screen and a hall effect sensor reading on of the gears. still cost $40-50 though.
I already suggested to H to think about a treadmill motor, or go DC so at low speed you still have good torque. Not needing to use VFD's to run my 3 phase stuff means i don't really know how gutless the motors get at low speeds.

DC motors are good especially if you can find a compound wound one some have a tachogenerator which gives a voltage output proportional to speed. My 2 pedestal drills have DC motors, they are real stump pullers. I am too lazy to to change belts. I have sold DC motors on ebay but they never got much interest so you might get lucky there.

The VFD option is a good one, many VFD's have the option for RPM readout as an alternative to frequency , slip is not much of an issue 3% at full load and if slip compensation is enabled that gets offset.

But the motor rpm isnt the spindle rpm.

Also depending on the speed range of your lathe it would be asking a lot of the VSD and motor to do all the speed control. For example my lathe goes from 75rpm to 1500rpm. You'd need to run the motor at 10Hz for 75rpm(not much power), 200Hz for 1500rpm. Yes it will cut down greatly on gear changes, just letting you know it wont remove the need completely

Stuart

Morning Gents,
There is an old post of mine from 2010 titled "tacho update" , the P.C.B. used is still available from Kitstop and reflective sensors similar to the Sick I used are available from HK for a few dollars.
Regards,
Martin

Originally Posted by Stustoys

But the motor rpm isnt the spindle rpm.

Also depending on the speed range of your lathe it would be asking a lot of the VSD and motor to do all the speed control. For example my lathe goes from 75rpm to 1500rpm. You'd need to run the motor at 10Hz for 75rpm(not much power), 200Hz for 1500rpm. Yes it will cut down greatly on gear changes, just letting you know it wont remove the need completely

Stuart

The VFD allows an arbitrary multiplier so assuming the drive has minimal slip then spindle speed will be a fixed multiple of the motor speed. Even early 90's VFD's had this feature, they also had analogue output meaning that a voltmeter can be hooked up to them and calibrated in terms of spindle rpm. Spindle speed measurement on industrial machines is so common only a foolish VFD maker would fail to include it.

75 - 1500 is quite a wide range for a VFD and you would not configure one that way, 150 to 3000 (5 to 100 Hz) is a better option because top end torque is not usually required. Using a larger motor is mandatory to get decent low end torque, that is why modern CNC machines have 20 or 30 Kw spindle motors.

Originally Posted by Crossfeed

75 - 1500 is quite a wide range for a VFD and you would not configure one that way, 150 to 3000 (5 to 100 Hz) is a better option because top end torque is not usually required. Using a larger motor is mandatory to get decent low end torque, that is why modern CNC machines have 20 or 30 Kw spindle motors.

I don't quite understand this paragraph.

First, the speed range is the same so the VFD range is the same and the same problems.

Second, if you want to cut threads to a shoulder or the like, you'd better have razor sharp reflexes to do it at 150 rpm so I still can't see what's going to be achieved.

Better to use a g/box to get the torque multiplication and use the VFD to cut down on but not eliminate the belt/gear changes.

My Chipmaster has a Kopp variator drive with a 10:1 ratio. I'd ditch it for a VFD if I could (Kopp variators are notorious for problems) but even if I go to a 5 HP motor, I'm still going to lose on that range as I need the low end for 2 totally different reasons - thread cutting which generally doesn't take a lot of torque, and turning large sizes, which most certainly does.