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View Full Version : Torque for a 1725 rpm motor vs. 3450 rpm motor?



pgmrdan
03-21-2006, 09:20 AM
Should a 1/3 hp bench grinder with a 1725 rpm motor have approximately twice the torque of a 1/3 hp bench grinder with a 3450 rpm motor?

I asked this in the middle of another thread but I think it got buried.

I'm asking because I've seen a 1/3 hp 6" 1725 rpm bench grinder. The horsepower concerns me but it should have the torque of a much larger 3450 rpm motor.

I can stall out my current 6" 3450 rpm bench grinder if I'm not really careful. I think it's a 1/4 hp. I only use it for wire brushing and buffing.

I figure the 6" wheels would also be better than 8" wheels at keeping the thing from stalling.

Sound logical?

Thanks,
Dan

Evan
03-21-2006, 10:40 AM
It may have more torque but the wheels also cut slower. The wheels should run close to maximum. As the old saying goes, there is no substitute for horsepower.

FYI, the wheels my wife sells must withstand 1.5 times rated rpm during spin testing. They are all spin tested.

[This message has been edited by Evan (edited 03-21-2006).]

johnny o
03-21-2006, 11:01 AM
Dan
Its interesting to note grinders vs. buffers.grinders with wire wheels are usually 1725 as buffers always seem to be set up on 3450 motors. Hmmmmmmmmm! I run 8" ped. grinder(1725) and a Buffer 6" (3450) but I've never been able to stall them.I think motor amperage would equate to h.p. here and quality of motor,(leeson,baldor,import,ect.)
just my .02 I guess.
Johnny O :rolleyes

pgmrdan
03-21-2006, 11:10 AM
Evan,

In this case (sharpening woodworking tools), slower is better.

I still may go the route you suggested last week with a slow speed wet grinder but I'm still considering other options.

As for no substitute for horsepower, horsepower in a motor is related to rpm AND torque. To reduce stalling I'm more interested in torque than rpm. If my thinking is correct the torque should be about double on the 1725 rpm motor vs. the 3450 rpm motor. That is, it should have about the same torque as a 2/3 hp motor running at 3450. Yes?

If yes, that should be plenty of torque for my purpose.

[This message has been edited by pgmrdan (edited 03-21-2006).]

Forrest Addy
03-21-2006, 11:21 AM
Buffing and wire brushing requires a bit more HP than grinding. If you're stalling a wimpy 1/4 HP grinder, 1/3 HP won't be much improvement. Maybe you should consider a half HP or 3/4 HP grinder.

As for the torque Vz HP question it goea like this: about 1.5 ft lb torque per HP for 3450 RPM; 3 ft lb for 1750; 4.5 ft lb for 1160 RPM; and so on. T = 5252 / (HP x RPM).

The other thing you have to look it is efficiency. Long industrial experience and scientific experimentation has shown that aluminum oxide grinding wheels should run at about 5500 peripheral feet per minute. this works out to a 6" wheel running at 3450 RPM almost exactly. 6" wire and buffing wheels are much more aggressive at 3450 RPM.

While I'm on the subject, there's a stupid fad sweeping the woodworking side where so-called "slow-speed grinders" is hald speed prevents overheating the edges of fine tools. That is nonsense. The reasons people overheat fine edges is they either are getting too agressive with their grinding, they aren't paying attention, they are not cooling the tool at frequent intervals, or most likely they are not dressing the grinding wheel.

Grinding done efficiently is a cutting process. A grinding wheel will dull after a time in ordinary shop use. The grains from which the wheel are made are sharp meaning the dulled ones on the surface of the wheel have to be removed to expose the sharp grains. This is done with a diamond dresser by hand. A grinding wheel is a consumable item, its diameter will be reduced a trifle with every dressing.

Some tool steels used in a machine shop are extremely abrasion resistant. I refer to M42 cobalt tool steel, T-15, and the like. They will dull a wheel in a few minutes.

[This message has been edited by Forrest Addy (edited 03-21-2006).]

pgmrdan
03-21-2006, 11:23 AM
Thanks Forrest! Right to the point. http://bbs.homeshopmachinist.net//biggrin.gif

1/3 hp at 1725 rpm should be plenty for my sharpening/grinding needs (which is the topic).

Since I'm stalling out the 1/4 hp 3450 rpm motor while wire brushing and buffing I'll need to correct that problem next. Sounds like wire brushing and buffing need 3450 rpm and in my case I could probably use at least 1/2 hp, like you said. That takes care of looking into that later.

[This message has been edited by pgmrdan (edited 03-21-2006).]

pgmrdan
03-21-2006, 11:50 AM
Wow Forrest! You edited your response and added some great information.

I know the usual wheels that come with a grinder are typically garbage for my use so I had planned on replacing them with aluminum oxide wheels. I also planned to get a 1/4 carat diamond dressing stick.

I didn't realize that the 1725 rpm (or slower) fad was a bunch of bull hockey.

Evan
03-21-2006, 12:31 PM
Dan,

Spend a little more and buy a 1/2 carat diamond dresser. It will last much longer. Diamond conducts heat about 6 times better than copper but starts to decompose at around 600F. The hotter it becomes the faster it reverts to graphite. A larger diamond will conduct the heat from dressing to the holder much better and last much longer.

BTW, diamonds are not forever. Diamond is a metastable form of graphite. At standard temperature and pressure diamond slowly converts back to graphite with a half life of a few thousand years. That is why placer diamonds always tend to be very small. It isn't because they have worn down in a stream or river.

Dr. Nick
03-21-2006, 12:56 PM
pgmrdan,

The slow speed stuff isn't hooey, it's for people who don't know to keep their tools from overheating which can happen much faster at 3450 than 1725. If you're worried about stalling your machine while sharpening, I'd suggest a lighter touch, rather than a more powerful machine. You're going to lose a lot less tool steel that way. The only reason to go at it that hard is if you're reprofiling a tool and even then, what's the hurry? Stalling while buffing or wire brushing I can see but not while grinding a tool.

My .02

Rob

pgmrdan
03-21-2006, 03:16 PM
Sorry Evan. Leonard Lee of Lee Valley says in his book that a 1/4 carat dressing stick should outlast me. I think I'll stick with 1/4 carat.

Diamonds may not be forever but neither am I.

pgmrdan
03-21-2006, 03:21 PM
Dr. Nick,

I'm not so worried about stalling during sharpening but, like I said above, it's going to be for sharpening and grinding. Plus, I didn't know it's harder to stall out while grinding than while buffing and wire brushing. I've been stalling out my grinder during buffing and brushing.

Fasttrack
03-21-2006, 05:45 PM
What do you guys think, as long as we're almost on the subjec http://bbs.homeshopmachinist.net//biggrin.gif, about the dressers that come with the grinders? I suppose most don't come with dressers, but when i bought my sears one it came with essentially a handle with a nice flat surface to rest on the tool rest, and then it had maybe ten or more (dont know, never counted them) star shaped wheels. They come to sharp points and are allowed to spin so that when they come in contact with the wheel they spin but the points bite into the wheel. It throws up a prodigous amount of dust but seems to get the job done quite well. Any thoughts about these type? What are the downfalls - maybe not a square of a surface??

TECHSHOP
03-21-2006, 08:30 PM
Wondering about what Forrest Addy posted, wouldn't it be to'ther way. That the electric motor's advent drove the engineering developement of a grinding wheel to work propery at its "speed"(3450rpm)?

I have a few bench grinders, different sizes, speed, and purposes. What I don't have use for is the "consumer grade" electronic variable speed grinders.

Fasttrack:

I have one of those star wheel dressers. I is ok for "quick cleaning" work, but mine has no "accuracy" for anything beyond spark testing, deburring, or "speed filling, metal wasting" type grinding. I need to make a support for my diamond that I can use on all my grinders.

------------------
Today I will gladly share my experience and advice, for there no sweeter words than "I told you so."

DancingBear
03-21-2006, 08:50 PM
Fasttrack, once you've used a diamond dresser you'll never go back to the star wheels. I got a 1/3 carat for about 10 bucks a few years back, and I don't think the price has gone up that much. They may wear out eventually but mine's lasted longer than star wheels ever did and still going strong.

As far as a guide, I just used a 1/2" thick chunk of 1" round aluminum stock, bored to fit the diamond shank and drilled and tapped 10-24 on the edge for a locking screw. Works great.

Walt

wierdscience
03-21-2006, 09:28 PM
Slow speed grinding in woodworking tools is not a fad.As I wrote on another post woodworking tools don't tend to be made from HSS or anything else the least bit exotic.They are most often made from plain vanilla carbon steel.

High speed grinders came about at the same time as HSS cutting tools,before that low speed machines ruled the roost simply because carbon steel tools burn much more quickly when grinding.HSS by definition has a high red hardness which carbon steel does not.

sch
03-21-2006, 09:51 PM
recall a huge cloud of dust thrown up at school with the star dresser and worried about the grit drifting over the nearby lathe beds at the time. Nothing wrong with star dressers, they do leave a flat sharp wheel. Diamond dressers work well but single point dressers run the risk of slightly grooving the wheel if you bobble a bit running the dresser over the wheel. They do a great job of shaping wheels for grinding profiles though and that is their intended use originally.
Steve