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PeteF
12-06-2010, 06:51 AM
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?

Pete

.RC.
12-06-2010, 07:16 AM
You could speed it up to compensate for wheel wear... But then can the spindle bearings handle the extra speed?

alanganes
12-06-2010, 07:19 AM
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.
-Al

928gene928
12-06-2010, 09:27 AM
Pete,
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.

RobbieKnobbie
12-06-2010, 09:58 AM
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.

Waterlogged
12-06-2010, 11:38 AM
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.

gzig5
12-06-2010, 12:31 PM
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.

RWO
12-06-2010, 02:01 PM
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.

RWO

madman
12-06-2010, 03:04 PM
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 .

mayfieldtm
12-06-2010, 04:29 PM
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.

Tom M.

Mcgyver
12-06-2010, 04:46 PM
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.

+1 i have 3 phase so haven't bothered with a vfd but for fussy work the norm with grinding is you don't want to turn the wheel off....I bet a vfd would remove that concern, something i've thought of adding (number 342 on the list)


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.

that plus the motor on a good grinder should be balanced better than a run of mill motor

Jim Shaper
12-06-2010, 06:07 PM
I use a VFD on my grinder for simplicity in making it a single phase device. The switches are all wired in so it acts as though it was plugged into 3ph, but isn't.

Doesn't matter when the coolant pump is switched on or off either - just hit the switch and she starts pumping.

PeteF
12-06-2010, 06:38 PM
Thanks guys, as far as the wheel itself I couldn't see any massive advantages but the soft-start/braking was something I'd forgotten about and good points. I should have thought of that myself as it's one of the advantages of a VFD I often mention when asked why to use them in the first place.

J. R. Williams
12-06-2010, 06:46 PM
I have a VFD on my surface grinder and I like the slow ramp-up to speed and have found the unit runs smoother at 57 HZ compared to running it at 60 HZ.

JRW

tdmidget
12-06-2010, 06:54 PM
RC, Robbie, and Madman are working on their Darwin awards. Go for it!!

lazlo
12-06-2010, 07:05 PM
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 can confirm that. I've run two Harigs, a 6x12 and now a 6x18 on a rotary phased converter, and when I switched the 6x18 over to a VFD, it runs noticeably smoother, with better surface finish: near perfect 3-phase on the VFD versus the wild leg on a rotary.

My rotary is an unbalanced Arco (?? -- JTiers has one), so a balanced rotary would probably have less of a difference going to a VFD.

Doozer
12-06-2010, 07:57 PM
A faster wheel acts harder. A slower wheel acts softer. This is a major factor to be able to tune in what you are grinding. You need to swap out wheels less to get use of different hardness and frability characteristics. A good grinder man will appreciate the variable speed feture. --Doozer

PeteF
12-06-2010, 09:03 PM
A good grinder man will appreciate the variable speed feture. --Doozer

Hopefully the same principles apply to a crap grinder man with NFI what he's doing! :p

jep24601
12-06-2010, 10:09 PM
I have a VFD on my surface grinder and I like the slow ramp-up to speed and have found the unit runs smoother at 57 HZ compared to running it at 60 HZ. JRW
I have found exactly that same thing!

madman
12-09-2010, 10:30 AM
In one of the die shops I worked no one ever balanced a small grinding wheel. I can see the reason on larger diameter ones. But we wore them out so fast ... I never had finish or accuracy problems stemming from a small unbalance grinding wheel. I would at times grind to one tenth or even less if ihad too but usually we kept stuf to 2 tenth regularily (.0002)