VFD on a surface grinder
I've just bought a small manual surface grinder that is 3 phase and I would normally simply throw in a VFD to convert from single phase. However as I'm not at all familiar with using surface grinders I wondered if there is actually any advantage in being able to vary the wheel speed?
You could speed it up to compensate for wheel wear... But then can the spindle bearings handle the extra speed?
I run my T&C grinder on a VFD for the same reason just for the phase conversion. The motor on my grinder is built into the head and was a bit of an odd voltage (500V 60Hz), so swapping motors was not a choice. I used a VFD to get the 3 phase conversion and a couple of small transformers to step the voltage up.
Try as I might, I could think of no good reason to vary the speed of the grinder. It was suggested to me that one could vary the speed to compensate for various wheel sizes to fine tune the SFM. In the end I just put the VFD in a closed box at the rear of the machine to keep the grinding crap out of it and brought out a switch to turn it on, off and switch direction. I'm no grinding guru, so I may be overlooking something here.
It seemed a good idea to limit the output of the VFD to 60 Hz to avoid any nasty surprises from over speeding wheels.
In any case, my grinder works great set up like this.
When I purchased my Harig manual grinder it had a three phase motor. For my home shop I purchased a balanced single phase motor and made the conversion. The grinder work OK, but was never smooth running. I have since changed to a VFD, and it really made a difference in the performance of the machine. Works much better. Although I can change motor speeds, I have it set to run at the original designed speed.
Don't sweat it!
Setting a VFD like Alan posts would be a fine way to go, IMO.
So too would allowing yourself the ability to compensate speed for shrinking wheels...
It's not as though your bearings are going to implode at a certain rpm, more so the expected life on them goes down with higher loads and speeds. Think of it this way, from the factory the bearings have 40,000 hours on them at rated speed and load.(thses are all statistical estimates: one of those bearings may have a flaw and be only good for seven minutes!) If you overload the spindle for a short period you may chew up some more of those hours... if you run at a higher speed you'll chew up more of those hours as well. As long as you're not running at astronomical speeds, you're just going to see imperceptably increased wear rates.
Mind you, when the wheel gets smaller, the load goes down somewhat, so there's a little compensation there if you were to bring up the speed.
It's a used machine, so you may have one 2000 hours left on the bearings anyway. The inevitable bumps and bruises that come with learning to use the machine wil take up far more of that than overdriving the spindle will.
In reality though, once a machine goes into service, the maintenance and cleanliness of the bearings counts for way more than any designed-in bearing ratings. I think your best bet would be to set up the VFD, learn the machine, use it, get some nice work out of it, and then plan to give it new bearings somewhere down the line when you have some spare time.
I have a VFD on my KO Lee grinder to convert it to single phase. I never adjust the speed and don't plan to.
I think the biggest advantage of a vfd on a grinder would be the ability to ramp up the speed at power on. This should help keep the wheel from slipping and losing balance from the high acceleration forces of plug starting.
A major advantage the VFD provides is the use of a 3-phase motor which does not have torsional vibration like all single-phase induction motors do.
I use a Cofer VFD .(bought from HSM Member Dave Cofer) It is used on my old Norton Surface Grinder to adjust the hertz range to tune in the actual spindle speed for the specific wheel i am using. I use a Tachometer and just adjust the RPM (by varying the Hertz setting on VFD) until I get the surface feet per minute that particular wheel calls for. As I have a collection of wheels,,. diamond ,,.and others this adjustability comes in Handy to Optimize wheel Performance and also Lifespan .
I would think that one of the advantages of a VFD is being able to do a rapid Deceleration and not have to wait for the Wheel to spin down on it's own.