Is It Possible to Slow Down a Grinder's Speed?

[Glenn Samuels]

I am slowly learning to use hand tools and I am now considering a grinder for my chisels and plane irons. I have been looking at the 6" Jet grinder which is listed as 1/2 HP. Is it acceptable to slow down the speed (3450 rpm) with a router rheostat to help moderate the temperature of the steel? I have found a few variable speed grinders but the reviews have not been that stellar.

[Derek Cohen]

Glenn, I think that the speed on the circumference of a high speed 6" wheel is almost the same as that on a half speed 8" wheel.

Regards from Perth

Derek

[David Weaver]

I like a 6" full speed grinder, I wouldn't slow it down. It won't take long to get used to it.

[george wilson]

High speed grinders are just as good as slow speed. Just quench more often. I have never had a regular old style dry bench grinder that was slow speed. You can slow down a 1 phase motor with a variac unit,but they just cut the voltage down,and will ruin the motor. Must be 3 phase to use a VFD unit on it.

[Glenn Samuels]

How about using a 6" wheel on an 8" grinder. Any advantages there?

[Don Dorn]

I agree that a slow speed isn't really necessary. A few years ago, I bought a Woodcraft 8" slow speed that came with white wheels and now that I have it, I don't intend to buy a different one, but if I were doing it over, it would be a regular speed grinder and not even change the wheels.

[Fred Belknap]

Normal motor speed is 3450 or 1725 rpm. You can usually find a slow speed motor and rig up a slow speed grinder.

[Sam Takeuchi]

Glenn, I think that the speed on the circumference of a high speed 6" wheel is almost the same as that on a half speed 8" wheel.

Regards from Perth

Derek

6" at 3450 is 5419ft/m
8" at 1725 is 3613ft/m

That said, I have a slow speed 8", if I were buying again, I would have gone with high speed 6". But if you are afraid of burning the blade, might as well just buy slow speed grinder. It'll do anything high speed grinder will do, just slower.

[Jim Foster]

If the grinder comes with standard grey wheels, you could change one wheel from "grey" to white friable to reduce the chance of "burning" a tool.

[Derek Cohen]

6" at 3450 is 5419ft/m
8" at 1725 is 3613ft/m

That said, I have a slow speed 8", if I were buying again, I would have gone with high speed 6". But if you are afraid of burning the blade, might as well just buy slow speed grinder. It'll do anything high speed grinder will do, just slower.

That is greater than the numbers I have seen before. Did you calculate this? What is the ft/m speed for an 8" wheel at the circumference? This would give perspective to the 6" speed.

Regards from Perth

Derek

[Bill Houghton]

The router rheostats I've seen are designed for universal motors (motors with brushes is the shorthand way to think of it). I couldn't tell exactly which kind of motor the Jet grinder has, but I'm pretty sure it's not a universal motor. Split phase, probably.

[Sam Takeuchi]

That is greater than the numbers I have seen before. Did you calculate this? What is the ft/m speed for an 8" wheel at the circumference? This would give perspective to the 6" speed.

Regards from Perth

Derek

That's already surface speed (hence circumference). From one of my trusty resources (http://www.abrasiveengineering.com/spedcal2.htm).

[Greg R Bradley]

I have virtually no experience grinding woodworking tools, but am a bit familiar with industrial grinding and manufacturing.

8" is the "weird" bench grinder since it is running too fast for many uses. 7" used to be popular where you wanted something more aggresive than 6". 12" is popular as a heavy duty grinder with the same grinding speed as a 6" since they are both 5500sfm.

3450rpm is normal for 6-8"
1725rpm is normal for 10-14"

A 14" normal speed grinder is still running lower surface (grinding) speed than a normal 8".

[Mike Henderson]

As Bill said, a grinder is almost certainly an induction motor and you'd need a VFD to slow that down. Those "router speed control" devices only work with universal motors.

The speeds of 3450RPM and 1725RPM indicate that they are induction motors - a two pole motor for the 3450 and four pole for the 1725. A router speed control would not work on those. Universal motors tend to have much higher RPMs, often about 12,000RPM or higher.

Mike

[bob blakeborough]

6" at 3450 is 5419ft/m
8" at 1725 is 3613ft/m

That said, I have a slow speed 8", if I were buying again, I would have gone with high speed 6". But if you are afraid of burning the blade, might as well just buy slow speed grinder. It'll do anything high speed grinder will do, just slower.


That is greater than the numbers I have seen before. Did you calculate this? What is the ft/m speed for an 8" wheel at the circumference? This would give perspective to the 6" speed.

Regards from Perth

Derek

Well circumference of an 8 inch and 6 inch wheel is pretty easy to figure out, and RPM's make the rest pretty simple. A mile has 5240 feet, so if my math is remotely accurate, the 6 inch wheel is turning at around 62mph (or 100KMH approx)while the 8 inch wheel is turning at about 41.5mph (67KMH approx). That is a fairly substantial difference I would think but as to the effect it has on an edged tool, I am not sure.

[Sam Takeuchi]

Well circumference of an 8 inch and 6 inch wheel is pretty easy to figure out, and RPM's make the rest pretty simple. A mile has 5240 feet, so if my math is remotely accurate, the 6 inch wheel is turning at around 62mph (or 100KMH approx)while the 8 inch wheel is turning at about 41.5mph (67KMH approx). That is a fairly substantial difference I would think but as to the effect it has on an edged tool, I am not sure.

Exactly. People have used high speed 6" grinders and slow speed 8" grinders successfully for years. I think these are perfectly manageable speed for grinding steels used for woodworking tools. However, I see "high speed 6" is about the same speed as slow speed 8"" quite often, but they are not and I think such difference (33% speed difference) should be clearly advised so that the person who are considering on buying a grinder can make informed choice there. I'm not saying one is better than the other, but I think at least information should be clear and reasonably accurate.

[bob blakeborough]

Exactly. People have used high speed 6" grinders and slow speed 8" grinders successfully for years. I think these are perfectly manageable speed for grinding steels used for woodworking tools. However, I see "high speed 6" is about the same speed as slow speed 8"" quite often, but they are not and I think such difference (67% speed difference) should be clearly advised so that the person who are considering on buying a grinder can make informed choice there. I'm not saying one is better than the other, but I think at least information should be clear and reasonably accurate.33% difference:p but the point is the same... It is definitely different, but both obviously still work...

[Sam Takeuchi]

33% difference:p but the point is the same... It is definitely different, but both obviously still work...

Ha! I was mindlessly looking at the opposite side of the number. I didn't even realize that!

[Jim Koepke]

I couldn't tell exactly which kind of motor the Jet grinder has, but I'm pretty sure it's not a universal motor. Split phase, probably.

Is it possibly a DC motor? When those are series wound they have decent torque at the lower speeds. A former co-worker of mine repairs exercise equipment and found me a tread mill with a good motor and controller for my lathe. Just another project on the to do list.

Most of this is off my plate since my shop doesn't have a wheel grinder. My father's pedestal grinder was a 6" at 3450 rpm and he could sharpen drill bits or edges with an old grey wheel. The wheel was a lot smaller last time I recall seeing it.

jtk

[Glenn Samuels]

You all have given me excellent information. I think that I will go ahead and get the Jet 6" grinder and just cool off the chisels and planes continually. I am initially planning to hollow ground the back of my plane irons. I will go ahead and get the white friable wheels to help keep the temperature down.

[Danny Burns]

I have a General 8" slow speed grinder, and just recently changed the wheels that came with it to a pair of the cooler running white wheels that LV sell.
I wish I had done it years ago.
The small tools that I was grinding before, needed to be dipped in cold water quite often, and now, the same tools did not even need to be dipped once.

My guess is that you will see benefits from these white wheels on a 6" grinder too.

[george wilson]

iI just dip the chisel in water and quench it the instant the water starts to boil off. This is every 2 seconds when the edge is getting thin. NO NEED for slow speed grinders. If you want a cooler grinder that cuts faster,get a belt grinder. I use my Wilton Square Wheel grinder all the time. Grizzly puts out a decent belt grinder for MUCH less. It has no guards,but you could make some plywood ones to keep the dust down. I keep a large scoop with 4" hose right under mine,with a DEDICATED dust collector. Don't mmix wood dust with steel sparks. Right behind the dust bag I have a 2nd. shuttered wall fan to get any dust that comes through the bag.

[Gary Herrmann]

All good points, but if it makes you feel better, go buy yourself a slow speed. I had an 8" WC and it served me very well.

Sold, after I found a used Baldor in great shape.

[Brian Ashton]

I guess that depends on what you mean by slow speed grinders. Like you described mine is a 3 phase variable frequency drive set up that goes from 0 - 120htz. I grind at about 8 - 10htz and it makes a world of difference. I can grind with my eyes closed, literally, by simply listening to the sound and the feel given off by the tool as to whether I'm grinding in the hollow bevel or not. At those sorts of speeds it's harder to burn a tool than not - you only need to quench if you get heavy handed. And! what no one can believe till I show them it's actually much quicker to grind at such low speeds - about 40% faster. I think it has to the metal being able to absorb and pull the heat away much faster, speed as a function of heat buildup and being able to remove larger amounts of material at slow speeds... Over all it's by far the best way to grind bar none. Probably the biggest benefit is at such speeds you can take your time and carefully grind some extremely complex shapes without EVER worrying about burning or over grinding the tool. YMMV

[Aric Krueger]

I can't remember who said it other than it was a well known woodturner, but, "everyone has access to a slow-speed grinder; just use the 'off' button. "

I think if you use this method, it might be prudent to keep an extra power switch on hand... on off on off on off... :)

[Jessica Pierce-LaRose]

Every grinder manual I've seen (yeah, I read the papers that come with even the most basic of tools) cautions against turning the power on and off rapidly, particularly if the wheel hasn't come to a stop yet. I guess it could eventually loosen the grinder wheel. I don't know if that's actually true or not, but I try to abide by it.

Found the PDF of my manual - here's what mine says, at least:


DO NOT TURN THE MOTOR SWITCH ON AND
OFF RAPIDLY. This could cause the wheel to
loosen and create a hazard. Should this ever
occur, stand clear and allow the wheel to come to
a complete stop. Disconnect your grinder from the
power supply and retighten the wheel nut securely.

[george wilson]

Probably mostly over uses the switch,wearing it out. Especially with these cheap Asian grinders which we mostly have,I'd be careful about over use of the switch. My old 1964 Craftsmans are still working fine. I like the old flat front motors,especially for grinding tools like draw knives. The motor doesn't get in the way. At the time,we considered these grinders cheaply made (still are,compared to GOOD old ones!). They even have real glass windows in their guards(don't scratch up so easily). How things have gone down hill!!

[allen long]

Both my grinders are 8" one slow, one high speed. I actually prefer the high speed one for grinding chisels and irons. I have found if you can pretty much grind without quenching unless you get heavy handed. For quenching (when I do get heavy handed) I use a water-soluble cutting oil concentrate from Grizzly.

[george wilson]

Why would you do that? Water is a more effective quench.

[Frank Drew]

A mile has 5240 feet...


5,280 ft., no?

[allen long]

Well, the water soluble oil is specifically made as a coolant for machining operations. The CNC and Machining instructor also used a similar oil for grinding opertions including lathe cutters (tool steel). Supposedly it draws the heat away bettter. It also won't rust your tools.

Again, I try to do my grinding with a very light touch when I approach the fine edge of the tool.

My question is why would you want a fast quench, won[t that make the edge brittle?

[george wilson]

You want a fast quench if your cutting tool suddenly gets too hot! No oil quenches as fast as water. Water by itself is hard on machines,so they found ways to slow down the corrosiveness of water.

Unless you get the tool up to red hot,cooling it fast will not make it brittle. It's too late to save the temper by then anyway.

[bob blakeborough]

5,280 ft., no?Yes... Typo or brain fart... Either is not surprising! lol!

[allen long]

The water soluble oil is actually mixed about one or two parts oil to to ten parts water, So it really is not strictly an oil coolant. My apologies for not making that distinction clear. I can certainly see why water would be a better quench or coolant than pure oil!

[Glenn Samuels]

Since I started this thread, I should say that I went to Woodcraft and purchased a slow speed 8" grinder (on sale). Took it home and it hums beautifully. It even comes with 2 friable wheels so I don't have to buy new ones initially to replace the gray ones.

[george wilson]

If it only HUMS,it will soon burn out!!!:):):)

[David Keller NC]

By the way, it is possible to slow down a single-speed permanent magnet, split-phase or other single-phase motor with a modern VFD. They work by slowing the 60 Hz frequency of the AC power and keeping the voltage constant. Here is an example (the GS1 model):

http://www.automationdirect.com/adc/Overview/Catalog/Drives

The more tradtional way of doing this is with a gear motor, though the addition of a transmission makes a gear motor considerably more expensive.

[george wilson]

I do not think that they will operate 1 phase motors,David.

[David Keller NC]

Yeah, my bad - that was a poor example. But you can get VFDs that are 3ph in, 3 ph out, 1 ph in, 3 ph out, and 1 ph in, 1 ph out. Here's a better example:

http://www.anaconsystems.com/text/opti_e2.html

[george wilson]

Now,this is some useful information. Are these something new? I might get one for my 1 phase Oliver wood lathe,minimum speed is 500 RPM. Need it to go slower.

[David Keller NC]

Now,this is some useful information. Are these something new? I might get one for my 1 phase Oliver wood lathe,minimum speed is 500 RPM. Need it to go slower.

Yeah, they're fairly new, at least at relatively low prices. The electronics used to be really expensive because the small semiconductors couldn't take the power/current. But with the advent of high-power integrated circuits, they've gotten a lot cheaper.

But I would think that a person with the skills that you have might prefer a mechanical gear-down of your lathe. I would think it would be simpler, and my personal preference is "simpler=better", despite being an engineer that loves playing with high-tech toys.

[george wilson]

My lathe has the motor connected to the spindle with 2 variable diameter pulleys only. One on the motor and one on the spindle. I'd replace the motor with 3 phase,but it is a special shaft motor,I think. The low speed on these type lathes is never low enough 500-3000 RPM. If I really had to do a large outboard spindle turning,I could do it on my 16" metal lathe. I've got it rigged to do 30 RPM. However,it is not collected too the dust collector,and not in an area I want to get dust all over. I try to keep my precision machine tool area(for metals) clean as possible of dust.

[David Keller NC]

Well, how about replacing one of the variable diameter pulleys with a larger diameter one? I've bought such things from McMaster/Grainer/MSC before to change the rotational speed on HVAC units.

[george wilson]

Then I'd lose some top speed. VFD is an easier solution. I do a lot more small work than large,and don't want to lose top speed.

[Glenn Samuels]

Since I started this thread, I'd like to say that I went ahead and purchases a slow speed 8" grinder (on sale at a large tool store). It even came with 2 friable wheels so I don't have to purchase replacements. Turned it on and it runs very smoothly. Thanks for everyone's input.

[Mike Henderson]

Yeah, my bad - that was a poor example. But you can get VFDs that are 3ph in, 3 ph out, 1 ph in, 3 ph out, and 1 ph in, 1 ph out. Here's a better example:

http://www.anaconsystems.com/text/opti_e2.html
Thanks for the pointer to those VFDs. They are expensive, though. For many tools, the VFD could cost more than the tool.

Mike

[David Keller NC]

Thanks for the pointer to those VFDs. They are expensive, though. For many tools, the VFD could cost more than the tool.

Mike Yeah, the solutions offered to the OP of this thread are a lot more practical than a single-phase VFD. But they might be useful to those interested in building their own powered lathe. With one of these, the mechanical alignment issues with multiple pulley diameters and wooden pillow blocks aren't an issue.

[Curt Harms]

Yeah, my bad - that was a poor example. But you can get VFDs that are 3ph in, 3 ph out, 1 ph in, 3 ph out, and 1 ph in, 1 ph out. Here's a better example:

http://www.anaconsystems.com/text/opti_e2.html

And my woodworking related bookmarks got one longer. Thanks!

[Mike Siemsen]

I find that all a slow speed grinder does it make take longer to burn your tool. Grinding takes practice, dress you wheels so they are sharp.