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bench grinders 1750 vs 3600 rpms

kenking2

Plastic
Joined
May 4, 2009
Location
Philadelphia, PA
Is there any reason to prefer a 3600 rpm bench grinder versus a 1750 rpm bench grinder? I thought the 1750 might be a better tool since it might not overheat the tools i'm trying to sharpen and also, that it could do all that the 3600 does... in some cases slower though, not by much as far as I can tell.

Also, from what i recall, alot of buffer wheels are recommended to run at 1750 vs 3600.

Let me know if there's anything that the 1750 can't do as well as th 3600.

thanks,
Ken
 
Generally larger wheels have to spin slower, as they are not rated or safe at a higher speed. A larger slower wheel has the same surface feet per minute as a smaller faster wheel. HTH, Jim.
 
Need more info - what diameter wheels, what materials will you be grinding?

1750 grinders are nice for woodworking tools made of carbon steel, less danger of blueing them and losing the temper.
 
Ahh, wheel diameter!

The 3600 rpm grinder is 8". the 1750 grinder is 10".


From what JB is suggesting... sounds like these grinders might perform similarly.

Ken
 
Depends on what you are grinding as well. I have an 800 rpm 8" that is very nice for smaller tools that do not have a lot of heat capacity. I would hate to have to remove serious material with it though.
 
I'm going to be deliberately insulting:

1750 RPM bench grinders have been sold to gullible woodoworkers for several years now. They are hawked as the cure for over-heating cutting tools while grinding their edges. As grinders they work OK but not as efficiently as the "regular" speed grinders running at 3450 RPM. My complaint with these low speed grinders is they are sold to as a solution to a problem they do not solve.

The reason for the success of the newer slower speed grinders (1800 Vs 3600 RPM) is ignorance and pandering. The object of this wasteful purchase is ostensibly to prevent the overheating of edged tools when grinding them. The result is when purchased the problem recurrs as the original factory dress on the grinding wheel dulls.

Dispelling the ignorance:
The reason the tools overheat while being ground is friction. When the grinding wheel is clean and the abrasive grains in its surface are sharp, they cut cool and efficiently. Any edged tool dulls in use and that's as true with the abrasive grains in a grinding wheel as it is with a lathe tool or a plane iron. When the abrasive grains become dull from use they tend to rub not cut and ever-increasing force on the tool against the grinding wheel is necessary to sharpen it it. Thus a carbon steel tool overheats, turns blue and its temper is lost. A tool will overheat as readily on a 1800 RPM grinder as it will on a 3600 RPM grinder.

Solving the problem:
Learn to dress your grinding wheels. Dressing a grinding wheel "sharpens" it by knocking the dull grains out of the bond to expose fresh sharp grains. Incidentally dressing a grinding wheel removes the roughness, grooving, loading, and makes them round.

Make/buy a diamond wheel dresser on a hand shank so you can dress your grinding wheels. A suitable diamond dresser would be a small cluster diamond such as found in the the MSC catalog and the import tool catalogs for less than $40.

A suitable shank could be made from a piece of 7/8" round bar 12" long by drilling a 7/16" hole in the end 1" deep and cross drilling and tapping for a set screw. This is a life time tool when used in the home shop for the weekly grinding wheel tune up. Your grandchildren will pass it almost unscathed to their grandchildren and they will use it to dress their grinding wheels into the 22nd Century.

Exposing the pandering: when a perceived need is fulfilled by selling something useless by flattering the mark willingness to believe plausible nonsense, that is pandering: magnet therepy, herbal remedies for impotence, beauty products, and slow speed grinders are all in the same class of pandering.

When new, the slow speed grinders work great and why not? The wheels are fresh and sharp and they agressively cut while the work stays cool. You are pleased with the money spent for the problem is solved - until the wheels grow dull and then you're burning the edges again. Then you have to buy another grinder, don't you? Why not? Thats how you solved the first problem.

Nope. There's good reason why grinding wheels turn as fast as they do. That's the optimum speed established almost a hundred years ago for vitreous bonded aluminum oxide grinding wheels in professional shops. You'll never find a slow speed (half speed, actually) grinder in a professional sharpening shop. They're not productive.

Times haven't changed much since 1912. The abrasives and the bond and the process controls for making grinding wheels are much more refined today but the interaction of the grain with the steel edge is still the same 5000 to 5000 surface feet per minute is the optimum speed for aluminum oxide against hardened carbon or high speed steel.

You are looking for 5000 to 5500 peripheral feet per minute (FPM) for aluminum oxide against carbon and high speed steel. FPM = wheel diameter in feet times pi times the grinder RPM. Let's dissect this a little. A 6" wheel is just about perfect for a 3450 RPM grinder at 5419 FPM. 7" is a little fast but very usable. When a 7" wheel wears, it passes through the 6" optimum diameter. An 8" wheel is too large for a 3450 RPM grinder (getting up to the unsafe zone) and on a 1750 grinder is too slow. A 10" wheel is almost unsafe on a 3450 RPM grinder but a bit on the low side for 1750. 12" wheel is likely to burst on a 3450 RPM grinder but the peripheral speed is amlost perfect for 1750.

A grinding wheel wears away in normal use. Ideally, the grinder would have some means of varying the wheel RPM to mantain a constant peripheral speed. A variable or step pulley, a DC motor and variable speed control, a VFD and three phese motor are all suitable drives for those whose budgets or curiosity allow such exploration. There are in fact everal variable speed grinders now on the market but in the very cheap or the very expensive price brackets. A three phase Baldor #500 face wheel grinder equipped with a 3/4 HP VFD would seem to be the most plausible option but if purchased new would be quite expensive.

However single speed grinders have been around for generations and they work very well as is.

Exhortation:
The reason you may want to buy or make a slow wheel grinder is because of deliberately perpetuated ignorance and possibly because the other guys are getting them. Well, you're smarter now. Are you going to make/buy what you don't need and will work properly for a short time? Or are you going to learn dress your grinding wheels and solve the overheating problem forever?

No, you HAVE to use a diamond dresser to make a clean smooth round sharp grinding wheel for the precision grinding of fine edged tools. There are no low cost options. Trust me. The wheel dress obtained by use of Norbide sticks, star dressers, the abrasive stick are all a distant second best to the silky cool cut of a sharp well dressed wheel dressed not too smooth with a diamond dresser.
 
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Baldor makes very high quality bench grinders with real motors, as opposed to the HF and similar junk. They know what they are doing and have been doing it a long time..

Baldor Products Page

All their 6" grinders run 3600 RPM. I know, it is really 3450 or so, but their catalog says 3600. Their 7" and 8" grinders can be ordered either 1800 or 3600, and one is even a two speed for those who can't decide. All their 10" and 14" grinders are 1800 RPM. It is a wheel size (SFM) issue, but the 7" and 8" wheels are apparently in a fuzzy zone, so the customer gets to decide which speed he wants.

Now we can put a cheap VFD on a bargain-priced used three phase grinder and dial in any SFM we might want, adjusting for wheel wear.

And don't forget the dust collector and mask when dressing the wheels.

Larry
 
A big thank you to Forrest Andy.

I really learned a lot. I guess a fell into the whole marketing ploy with slow speed grinders.

Just got my first bench grinder, an old Queen city 7" or 8" 3450 RPM. In the process of replacing the bearings, repainting, making new wheel covers and getting wheels for it. Also need to get a top notch wheel dresser.

Chuck
 
Sea Farmer. You see Norbide (boron carbide) dressing sticks used for wheel shaping in tool and cutter grinders where soft white (A46-H is common) cup wheels grind only on a narrow edge. While they are hard enough to wear away an aluminum oxide grinding wheel, their factory new sharp edge rapidly wears to a radius incapable of knocking out abrasive grains as does a diamond dresser. You can true the wheel and shape it but actually dress a unifform surface of fresh sharp abrasive grains? No, not with a Norbide stick a little wear on it.

The 1 x 1 x 6 abrasive dressing sticks have no earthly use that I can determine.

Star dressers are excellent where rounding, wearing away loading, and rapid shaping are primary concerns. A wheel prepped wth a star dresser is well suited for rough freehand grinding like weld preps and shaping irregular parts. A wheel prepped with a star dresser is great n a fab shop but nearly useless for prepping cutting tools for the final edge.

A diamond will wear too. A diamond dresser has to be re-oriented from time to time otherwise the pointy end will take on the contours of the end of a baseball bat and crush the abrasive back into the wheel instead of actually dressing it.

Here's a test of a sharp wheel. A lead pencil thrust into the grinding face for no more than 1 second should wear down as though against a sanding belt. The air should be filled with the aroma of fresh cut cedar. Scorch, blue smoke, etc are signs of a dull wheel.

Pat: You're correct. Nowadays , woodworkers and other seekers of the keen cutting edge use a bench grinder to generate the finished shape only. The final cutting edge is acheived using powered or armstrong wet stoning sometimes followed with a final lap or stropping.

Us machinist types go from dry grinding on a bench grinder or machine grinding on a tool and cutter grinder often followed with careful hand stoning. It should be noted that tools for metal cutting are deliberatedly blunted as a means of securing better tool life and finishes - particularaly with carbide tools that edge crumble easily. Think of the granular microstructre of carbide powder sintered in a cobalt matrix and compare it to the edge of a concrete sidewalk. An edge radius of 1/6 to 1/10 the feed rate is a common starting figure. This is not a universal practice. Many tools work best dead keen.
 
There is also the issue of the hardness of the wheel bond, and the effect that the SFM has on the apparent hardness of the wheel. So for an 8" grinder, running at 1800 rpm, you'd want a harder wheel bond than for 3600 rpm. A hard 8" wheel running at 3600 will resist shedding dull grains and need frequent diamond dressing. The same wheel running at 1800 rpm may work much better, in that grains will break out more readily as they get dull, before the diamond is needed.

Hard wheel bonds are okay for deburring where rather sharp corners are being continually presented to the wheel face, but for tool sharpening, you should opt for the softer wheel grades (keeping your spindle speed in consideration), and dress them frequently to straighten up the wheel face.
 
I thought the 1750 might be a better tool since it might not overheat the tools i'm trying to sharpen and also, that it could do all that the 3600 does... in some cases slower though, not by much as far as I can tell.

It is twice as slow. The rate at which it will grind is directly proportional to the speed, all other things being equal.
 
I have a big old serious Cincinatti Tool Co 3hpase 1hp 10" 1750rpm grinder. It's mounted on a baghouse dust collect and has the big lighted guards... not plastic lensed thing with a night light, but 6x6" safety glass with a pair of full size 100watt light bulbs on either side of EACH lens. You do sacrifice some surface speed, but in trade you get GOBS of torque. I have leaned on a bushog blade with all my weight and never slowed the wheel. As for heat generation, I don't really think there's any difference, but the sheer torque difference between a 1750rpm and 3400 rpm grinder of equal hp is pretty amazing.

I have a 60# on the right side for roughing and geveral grinding. I have a beige 120# of some kind that produces an amazing finish on the left. For just touching up, and final finishing of HSS tools, I have a 6"x1/4" white wheel I keep on my K. O. Lee cutter grinder with a small guard over the top. No rest, no obstructions. Man, what you can do with that wheel.
 
i have two B&D industrial 9inch angle grinders, one is 5krpm the other is 6krpm

the 6krpm is superior for everything i have used both on, the wheels cut faster, run cooler
and are just less work to use.

me, i would want a bench grinder that would turn as fast as the stone i am using can be safely used at.

1750 (1800 nominal)rpm might be fine on a 12 inch stone, but too slow for my likes for a 8inch, and likely too slow for a 10inch as well,

i do know it is horrible using one with 6inch stones

personally i like fast, clean, sharp wheels, and have a coolant dip cup handy to keep the work cooled off.

bob g
 
Works fine on 10" stones! Agree that a 1750 6" would suck.

I almost didn't bid on my grinder. We had a 3/4hp older Craftsman 10" 3450 bench grinder at the museum. It was nice, but not overly powerful. I wasn't sure this 1hp would be much better... heheheh. The difference in the torque of the lower rpm motor is well worth the swap in surface speed. The Craftsman took about 3-4 seconds to come up to full speed when the switch was flipped on. The big Cincy is at speed by the time the switch clicks. the 1/2hp blower fan motor on the baghouse takes longer to come up than the wheels.

If you just want a grinder for sharpening tools, you need to be running a white cutter grinder or surface grinder type ALOX wheel and you might as well get T&C grinder so you can make even more use of it on your endmills and such. If you want a general puropse grinder, a big low rpm grinder is unbeatable for heavy grinding and can also make VERY short work of roughing HSS lathe tools, drills, and other cutters.
 
Forrest Addy said:
Make/buy a diamond wheel dresser on a hand shank so you can dress your grinding wheels. A suitable diamond dresser would be a small cluster diamond such as found in the the MSC catalog and the import tool catalogs for less than $40.

A suitable shank could be made from a piece of 7/8" round bar 12" long by drilling a 7/16" hole in the end 1" deep and cross drilling and tapping for a set screw. This is a life time tool when used in the home shop for the weekly grinding wheel tune up. Your grandchildren will pass it almost unscathed to their grandchildren and they will use it to dress their grinding wheels into the 22nd Century.

Forrest, I presume that you are talking about something like the following from the MSC catalog:

8154747-11.jpg


MSC Item Detail

What Carat size do you recommend?
 








 
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