# Setting up surface grinder with VFD for slow speed honing



## Bob Korves (Mar 27, 2018)

I need to rough and finish grind some carbide hand scraper inserts.  For rough grinding I will be using my HF Baldor clone carbide grinder at high speed with a 300  (Edit: 150 grit) grit resin bond diamond face wheel.  That should work fine, with a simple jig to hold the scraper handle for grinding the correct radii.

For finish honing/lapping, I bought a 6" (150mm) diameter, 1mm thick import steel plate, with a 600 grit plated diamond coating, and plan to use my surface grinder for the slow speed work, because it has a VFD.  I had to open up the mounting hole in the lapping plate from 1/2" to 1 1/4" and then mounted it on a wheel adapter and installed it on my 1946 B & S model 2L surface grinder.  The runout is minimal, and I want the capability of running the grinder at very low rpm's to do the lapping.  Previously, I had the VFD set for 15-60 hertz, which allowed a measured 3266 rpm at the high end, and jogging at low speed to get the ISO 2 oil circulating in the plain bearing spindle.  It also allows slow acceleration and deceleration to keep the wheels from slipping on the adapters and going out of balance.  I want now to also get the maximum wheel rpm up to 3600 rpm, which is the rated limit of 7" surface grinding wheels (makes the wheels act harder when desired.)

So, tonight I reset the VFD for a range of 5-65 hz, which gets me up to just under 3600 rpm, and down to about 160 rpm, and infinitely variable between those limits.  My question is about the low speed operation.  The grinder has a 1946 1.5 hp GE 3 phase motor, which is running smoothly.  I don't imagine that 65 hz is going to hurt anything, my concern is running as low as 5 hz for short periods of time, 5-10 minutes max, light loading.  I really do not think the big old motor will overheat under those conditions for that period of time, but wonder if the VFD might add additional strain on the old motor that I should be concerned about.


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## Ulma Doctor (Mar 28, 2018)

Hi Bob,
for 5-10 minute runs, you'll be fine.


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## magicniner (Mar 28, 2018)

To allay any concerns you could check the temperature after grinding a few parts and compare that to the temperature reached with 5 minutes at low speed from an ambient temperature start. 
You're working at almost zero load so it's likely that the motor current drawn will be quite low.


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## benmychree (Mar 28, 2018)

I don't think it is a cause for worry, but why not just find a worm gear speed reducer?


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## Bob Korves (Mar 28, 2018)

benmychree said:


> I don't think it is a cause for worry, but why not just find a worm gear speed reducer?


Because the grinder is set up and working, variable speed, and designed for dealing with grit.  All I need to make is a simple -5 degree tool rest, mount it to the mag chuck, and go to work.


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## chips&more (Mar 28, 2018)

Bob, you could take an IR non-contact temp gun and check the temperature of the motor after a 10 min run. Or maybe just use the touch of your hand?…Dave


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## chips&more (Mar 28, 2018)

Bob, I have read a lot of internet chatter on this subject. People using muffin fans with temp sensors and all that. Maybe there is merit. But in my shop I can run my BP motor for most of the day on 15cps with no overheating. I have a magnetic temp gauge that I stuck on the motor. It maybe reads 10° F above ambient after I use the BP for the morning…Dave


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## Bob Korves (Mar 28, 2018)

chips&more said:


> Bob, I have read a lot of internet chatter on this subject. People using muffin fans with temp sensors and all that. Maybe there is merit. But in my shop I can run my BP motor for most of the day on 15cps with no overheating. I have a magnetic temp gauge that I stuck on the motor. It maybe reads 10° F above ambient after I use the BP for the morning…Dave


I am not at all worried about overheating.  This motor does not get warm when grinding hard.  It is 1.5 HP but looks like 5 or more HP, big heavy motor.  I was (and am) more concerned about electrical noise, spikes, or other issues from the VFD at that low cps that might hurt the motor.  To put a polish on a scraper blade takes a couple minutes and very little power.  Of course, the surface grinder has a 1.5 x 120 rubber/fabric flat belt and a shorter 1" leather flat belt, and there are half a dozen idler pulleys and a whole clock maker's nightmare of mechanical stuff that runs the grinder and the power feeds, all of which are idling when just using the spindle.  Amazingly, it all works fine after 70+ years.


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## chips&more (Mar 28, 2018)

Bob Korves said:


> I am not at all worried about overheating.  This motor does not get warm when grinding hard.  It is 1.5 HP but looks like 5 or more HP, big heavy motor.  I was (and am) more concerned about electrical noise, spikes, or other issues from the VFD at that low cps that might hurt the motor.  To put a polish on a scraper blade takes a couple minutes and very little power.  Of course, the surface grinder has a 1.5 x 120 rubber/fabric flat belt and a shorter 1" leather flat belt, and there are half a dozen idler pulleys and a whole clock maker's nightmare of mechanical stuff that runs the grinder and the power feeds, all of which are idling when just using the spindle.  Amazingly, it all works fine after 70+ years.


That’s a good question. I don’t see a problem. Can’t imagine the insulation falling of the magnet wire? Or the bearings getting brinell damage in a dynamic not static condition? Like I said, have been running my BP at the low cps since day one. And have no problems at all.


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## mksj (Mar 28, 2018)

The insulation of the motor is at risk in an older motor but I do not see that the low speed presents any greater issue than would be seen at higher frequencies, the most significant factor is the carrier frequency. The pulse voltage is the same but the the pulse width would be smaller. Ideally you would want to stay around 2-3 Khz carrier frequency. The other consideration to tame the voltage spikes to an older motor is to use a drive reactor, these are placed between the VFD and the drive. They should to be sized for the maximum motor amperage and the operating voltage of the motor. Be careful and look up the specifications as many sold are for higher voltage motors. I would look at TCI KDR drive or output reactors.  Probably recommend the KDRA28HC1 below even though it is rated for a 3Hp motor, a high Z will have a higher impedance to smooth spikes. I do not see a problem running the motor a low frequency but the Hp is greatly reduced, also at low speeds the motor over current may trip even with a low load. I had this issue when using VFDs for axis drives at below 10Hz.
https://transcoil.com/products/kdr-output-load-reactor/
High Z
KDRA25H, KDRA26H KDRA27H C1 denotes enclosure
https://www.ebay.com/itm/new-TCI-KD...amps-10-6-3ph-3-phase-protection/112477816224
Low Z
KDRA26L, KDRA27L or KDRA27L
https://www.ebay.com/itm/TCI-TRANS-COIL-KDRA26L-Surplus-New-not-in-factory-packaging/391875171464
https://www.ebay.com/itm/TCI-TRANS-COIL-KDRA28LC1-Surplus-New-In-factory-packaging/391750911526


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## Bob Korves (Mar 28, 2018)

mksj said:


> The insulation of the motor is at risk in an older motor but I do not see that the low speed presents any greater issue than would be seen at higher frequencies, the most significant factor is the carrier frequency. The pulse voltage is the same but the the pulse width would be smaller. Ideally you would want to stay around 2-3 Khz carrier frequency. The other consideration to tame the voltage spikes to an older motor is to use a drive reactor, these are placed between the VFD and the drive. They should to be sized for the maximum motor amperage and the operating voltage of the motor. Be careful and look up the specifications as many sold are for higher voltage motors. I would look at TCI KDR drive or output reactors.  Probably recommend the KDRA28HC1 below even though it is rated for a 3Hp motor, a high Z will have a higher impedance to smooth spikes. I do not see a problem running the motor a low frequency but the Hp is greatly reduced, also at low speeds the motor over current may trip even with a low load. I had this issue when using VFDs for axis drives at below 10Hz.
> https://transcoil.com/products/kdr-output-load-reactor/
> High Z
> KDRA25H, KDRA26H KDRA27H C1 denotes enclosure
> ...


Thanks, Mark.  I will look into the drive reactor.  I made a workholding jig and used the grinder today to hone 4 sides (8 cutting edges) of a Sandvik carbide scraper blank, and it worked just fine.  Looks like I need finer diamond than 600 to get the finish I really want.  I will possibly make a cast iron or aluminum lap and charge the lap with diamond grit.  There is well more than adequate power for what I am doing at 5 Hz, and the machine and motor seem to behave themselves well.  Mark, you exactly addressed the potential issues I am interested in.  Thanks!


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## webphut (Mar 29, 2018)

This is just in case what you are working on does not work as intended or maybe for the duration. I do not know how much you have invested in the project, but before you get too deep financially, check out the link below. I used one for five years on carbide, HSS, tunsten rig torches. work like a dream! If it’s too much, totally hear you, but if you find that it may be getting to the breaking point financially and to where it has gone past just being a curious interesting fun tool to build, this is very complimenting to any shop. We used one that was about $1200 new and it came with a couple different wheels. They were only available with 6 inch wheels at the time but may have bigger now.

http://accu-finish.com/


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## webphut (Mar 29, 2018)

This is just in case what you are working on does not work as intended or maybe for the duration. I do not know how much you have invested in the project, but before you get too deep financially, check out the link below. I used one for five years on carbide, HSS, tunsten rig torches. work like a dream! If it’s too much, totally hear you, but if you find that it may be getting to the breaking point financially and to where it has gone past just being a curious interesting fun tool to build, this is very complimenting to any shop. We used one that was about $1200 new and it came with a couple different wheels. They were only available with 6 inch wheels at the time but may have bigger now.

http://accu-finish.com/


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## bfd (Mar 29, 2018)

bob mksj  stated the correct worry for motors not designed to run on a vfd. the windings can flex, rub away the insulation and short out the windings either to themselves or the case. but this would only happen under significant load. I don't know a way to measure that flex, but maybe someoneelse out there might have a way to measure it bil


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## Bob Korves (Mar 29, 2018)

webphut said:


> This is just in case what you are working on does not work as intended or maybe for the duration. I do not know how much you have invested in the project, but before you get too deep financially, check out the link below. I used one for five years on carbide, HSS, tunsten rig torches. work like a dream! If it’s too much, totally hear you, but if you find that it may be getting to the breaking point financially and to where it has gone past just being a curious interesting fun tool to build, this is very complimenting to any shop. We used one that was about $1200 new and it came with a couple different wheels. They were only available with 6 inch wheels at the time but may have bigger now.
> 
> http://accu-finish.com/


I have $10 invested in the project for a Chinese 600 grit coated steel plate, plus a little electricity.  The surface grinder is regularly used as a surface grinder.  It's VFD is only being slightly reprogrammed to also work for lapping.  I am not going to go with the drive reactor, and take my chances for the little bit I will be using the grinder for lapping.  I have used the Glendo and Accu-finish grinders before.  They are nice machines, and quite good for the job, but way too costly for occasional hobby use IMHO.  I have also used a similar grinder that Ulma Doctor made from junk laying around his shop along with a Chinese wheel, and it also works well.  All that is really needed is a flat lapping wheel, a slow and preferably adjustable speed motor, and an adjustable tool rest.  Ulma Doctor's creation does all of that except variable speed, and it does the job just fine.  If I was working full time doing this kind of work for compensation, then my choices might well be different.  Thank you for the idea and link, webphut.


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