Grinder question

[Bryant]

Last year for Christmas I was given an 8" bench grinder. At the time I did realize that what I wanted was a slower speed(1750). Is it possible to put in an inline circuit to slow down a 3500RPM grinder? I though about putting a rheostat inline with the power cord but do not know if this will do any harm. Can someone give me some insight here? Thanks, Bryant

 

[KEW]

Your grinder is probably an induction motor which is not compatible with a rheostat. The rheostats are for universal motors like routers.
Oneway recommends the normal (not slow) speed grinder. I wouldn't sweat it too much and just use what you have with a gentle touch.

 

[Gordon Seto]

Bryant,

Can't answer your technical question.

The problem is not the speed; it is the smoothness of the grinder. I could get a better control at my friend's good high speed grinder than on my own slow speed grinder (now dead and replaced). Light touch is all you need. When the tool is bouncing in and out, the 1750 rpm is still too fast.
Try balancing and truing the wheels and see whether that helps.

BTW, Oneway recommends high speed grinding.

Gordon

 

[john lucas]

I prefer to grind at 1750 but 3450 on an 8" grinder is supposedly the correct speed for High Speed Steel. You need a light touch which is a good thing to learn anyway no matter what speed you grind at.

 

[Mark Mandell]

Better Investment

Last year for Christmas I was given an 8" bench grinder. At the time I did realize that what I wanted was a slower speed(1750). Is it possible to put in an inline circuit to slow down a 3500RPM grinder? I though about putting a rheostat inline with the power cord but do not know if this will do any harm. Can someone give me some insight here? Thanks, Bryant

Your grinder's fine; don't fall for the "Slow-speed" hype (NTM variable speed) used to sell novices who don't know any better. More important is the wheels, and maintenence thereof, that you put on your machine. There's no substitute for good wheels (I use Norton) that are kept "dressed" and "unglazed". While it's true that a higher speed machine will generate heat in the steel faster, you can control that with how much pressure you put on the grind and the stone used. The friable wheels wear more quickly, but they run cooler so long as you don't stand there for 10 minutes with the steel on the wheel wondering which will wear down faster.

Trying to grind on a wheel that's clogged with metal will blue up your tool in short order at any speed. Get a good diamond dresser and give each wheel a light touch before you turn the grinder off each time. It takes all of 3 seconds on each wheel to keep them clean and cool-running..

Remember, grind wheels, like your turning tools, are "consumable" shop items. They'll work much better for you if you treat them properly.

mm

 

[Bryant]

Thanks for the quick replies. I plan on getting a wheel balancer soon and I'm sure that will help out.

 

[Hugh]

You might want to look back about a month ago or so in the "Forum" - there was a thread about balance on grinding wheels. It had some good info in it. One reply was from someone who makes a tool to help you make the grinding wheel round (not lopsided). I know there are a couple of these types of tools out there. I made one up and holly cow did my grinder work better.

A couple of months ago we had Cindy Drozda do a demo for the club. We did it in my shop. Cindy worked on my grinder wheel and made it run so smooth. I had to duplicate that. That is where this wheel dresser comes in. I just used a diamond dresser mounted in a block of wood with a lip that runs behind the tool rest to keep it even. I now have smooth running grinding wheels.
Hugh

 

[Gordon Seto]

I plan on getting a wheel balancer

Look at your wheels before ordering. Not all wheels can use a wheel balancer.

Gordon

 

[MichaelMouse]

Last year for Christmas I was given an 8" bench grinder. At the time I did realize that what I wanted was a slower speed(1750). Is it possible to put in an inline circuit to slow down a 3500RPM grinder? I though about putting a rheostat inline with the power cord but do not know if this will do any harm. Can someone give me some insight here? Thanks, Bryant

Cutting the current won't do it unless it's a DC or universal motor. Not likely it is. Your alternatives are to to generate a different frequency at the same voltage or live with it.

Lots of mythology about scuffing steel with a stone. Twice as fast in RPM means twice the removal is possible in the same amount of time, all other factors being equal. Period, end of sentence. What anyone else, regardless their manufacturing affiliation may say, is simply silly. If your stone is a full 8, you're already moving more grit in the same amount of time than a 6" stone at the same RPM.

That said, a low speed is more forgiving of momentary angular deviations and imbalance. These I count as plusses, and since I'm not interested in removing much steel (grinding) to change shape, but taking a bit off the shape I have to sharpen, I like slow and 6.

Friction and the heat produced does not depend on stone speed, but pressure, and time, when you consider that the metal has to conduct the heat away. Once again, no big deal, because you're not normally recontouring, which wants coarse rather than long.

 

[KEW]

Friction and the heat produced does not depend on stone speed, but pressure, and time.

I'm not sure this is correct. Much of the heat being developed is from the metal being abraded away. Since a faster wheel is abrading the steel at a faster pace, wouldn't that cause faster heat build-up?
However, I don't see it as an issue for HSS as long as you're not trying to heat the metal!
I'm not sure why, but I know for a given amount of pressure and duration, it sure gets hotter when I sand at higher RPM.

 

[Richard Baker]

Kurt,
You are not abrading steel at a faster pace IF you are using much lighter pressure.

 

[Bill Boehme]

Much of the heat being developed is from the metal being abraded away. Since a faster wheel is abrading the steel at a faster pace, wouldn't that cause faster heat build-up? ..... I'm not sure why, but I know for a given amount of pressure and duration, it sure gets hotter when I sand at higher RPM.

I don't know where you would find a rheostat that huge, but that is irrelevant because it will not work anyway. The speed of an induction motor is governed by frequency and not voltage. The only thing that happens when you lower the voltage is that the efficiency gets lower and the current goes up which means that the motor runs hotter. Lower the voltage enough and the motor will run so hot that it will be toast in short order.

The difference between running a grinder at 1750 RPM and 3550 RPM isn't enough to amount to a hill of beans. Use of a proper grinding wheel (aluminum oxide -- the cheap soft white ones are good they just wear down faster) and proper technique are about the only important factors. If you happen to get a lopsided wheel take it back and get a decent one -- a good wheel ought to be reasonably balanced out of the box. There are some cheap hard aluminum oxide wheels and I don't like them. Also, get a diamond dressing tool -- the T-handle type sold by Woodcraft are great.

Don't equate tool sharpening with sanding. Your technique may not be correct -- you should be using almost zero pressure -- barely touch the tool to the grinding surface -- you are not shaping metal -- just restoring a razor sharp edge -- if you are sharpening for more than a few seconds, then something is wrong.

 

[John Nicholas]

Want to slow your grinder down? Turn it on, let it spin up. Then turn it off and grind as the wheel slows down. Everyone has a multi speed grinder. Full Speed and as it slows down.:cool2: BTW: That is not original, IIRC Bill G said it first.

John

 

[Bill Boehme]

.......... IIRC Bill G said it first.

Even if he didn't, I think that we out to pin the blame on him, anyway.

Bill

 

[Ziffy]

If you really want to lower the speed, you can always 'gear it down' with pulleys. Makes a bit of a mess at your sharpening station, but it will slow down the wheel without slowing down the motor.

Ziffy

 

[Bill Boehme]

If you really want to lower the speed, you can always 'gear it down' with pulleys. Makes a bit of a mess at your sharpening station, but it will slow down the wheel without slowing down the motor.

Ziffy

I would strongly advise against doing that unless properly designed and properly installed pulley and belt shrouds are an integral part of the system. Open pulley and belt drives present an extremely hazardous working environment.