Grinders - is a slow speed grinder the better option?

[Colin Prince]

Hi all, I'm new here, and I hope I'm not trying to cover ground that has already been covered - I have tried searching and not found any significant discussion on the topic of grinder speed (which may just be down to my searching skills).

I (largely through reading the many threads on here) have decided to get a CBN wheel, but need to settle on a grinder first, so that I know what bush to order with the wheel. I couldn't find any slow speed grinders locally (I'm in South Africa) and bought a big solid looking 8" grinder which I managed to burn out within 30 minutes. On my second look around, I managed to find a crowd who import Creusen grinders and they have a 6" slow speed grinder in stock.

The thing is, it's not cheap - and I am :-) I see they don't have an 8" slow speed grinder, and by going to 6" you've already dropped the surface speed by 25%, so I'm wondering if the slow speed grinder is worth the extra 40% in cost.

Using 60hz, I calculate that the surface speed on an 8" wheel at a speed of 3400rpm is just under 120 fps. A 6" grinder running at 50hz is already down to a surface speed of below 75 fps and using the slow speed grinder will take that to around 36 fps.

Does using a slow speed grinder with a CBN wheel make all that much difference, or should I just go for the normal one?

Thanks in advance

[John Keeton]

Normally, if availability is not an issue, I would prefer a slow speed grinder in 8" so that the grind has less "hollow grind" to it and the speed of the wheel doesn't remove too much metal. The CBN can take off metal quickly! Even the 180 grit.

But, you may need to compromise. Neither should affect the temper if HSS is used.

Welcome to the creek, BTW!!

[Brian Kent]

Good morning, Colin, and welcome. Though it might be good evening in the southern hemisphere .

Prashun Patel has a 6" grinder that I am pretty sure is slow speed. He tried an 8" CBN wheel and has had god success with it.

I have a 6" variable speed and will buy an 8" CBN wheel for the reason John Keeton stated - a little less hollow in the grind than with a 6".

[Bernie Weishapl]

Welcome to SMC Colin. I use a 8" slow speed grinder and had a 6". IMHO 8" slow speed with a CBN wheel is well worth the extra money. Since getting the CBN wheel and this is just me I feel my tools get sharper stay sharper longer. Like John and Brian I don't like the hollow grind I got off the 6" wheel. A grinder at 3400 rpm will work but you have to be careful as John mentioned even the 180 grit wheel will take of a lot of metal so a light touch is needed.

[Reed Gray]

Well, several things to consider here. First would be how big is the motor on your 6 inch grinder? The electroplated CBN wheels are heavy, from 5 to 8 or so pounds. I would want at least a 3/4 hp motor to run one. Otherwise, you will have long ramp up and ramp down/braking times. Add to that the little motors will be pushing more weight than they were designed for, and the grinder may not last as long. You can do an 'assist' at start up by giving the wheel a hand spin before you start it up. You can use the heel of a bevel on the wheel as a brake. This may help put less strain on the grinder. There is a lot of difference in hp ratings. I have one no name grinder with a 3/4 hp motor, and it struggles with the heavier wheels. My other grinders have no problems, though both have the same hp rating.

In the high slow speed area, the main advantage to high speed is if you are trying to shape a tool. It gets done faster with a coarse wheel and high speeds. Other than that, we are sharpening so a light touch is all that is needed. The slower speeds are less intimidating if you are not used to working with a grinder.

I have never used a 6 inch grinder, so have no real idea how much difference the hollow on it effects how I turn. With my gouges, I always grind away at least half of the heel of the bevel, so on a concave surface, you are rubbing the bevel very near the cutting edge. This is used for the inside of the bowl. On the outside, it makes little to no difference because with the convex shape, you are always rubbing the bevel very near the cutting tip rather than on the heel of the bevel. It may make a difference on a skew, but that is a tool that I don't use very often either. You could grind away part of the heel here as well.

robo hippy

[Jim Burr]

And a quick use of the search function shows; http://www.sawmillcreek.org/showthre...inch-or-6-inch, http://www.sawmillcreek.org/showthre...-Bench-Grinder, http://www.sawmillcreek.org/showthre...ey-as-possible, http://www.sawmillcreek.org/showthre...-world-pricing

[George Beck]

I am a big advocate of slow speed and an 8" CBN wheel. Most people, myself included, used stone wheels for speed which also means heat. Much of the heat is generated by the stone. With a stone wheel you have to true it. you get sparks and run the risk of shattering the wheel. An 8" CBN wheel is always true and generates less heat and a good one doesn't need balancing. The other issue is many people, myself included in the past, used coarser grits to remove more metal quickly. This results in a coarse grind. A 180 grit CBN leaves a smooth, almost honed, finish even at speeds as low as 500rpm. So, an 8" CBN wheel(not cheap) is safer, quieter, smoother and cooler (although you can still burn with it). Just my 2 cents.

George

[Jamie Donaldson]

I would caution against trying to mount and use an 8" wheel on a 6" grinder. The weight would overload both motor and bearings, significantly shortening their life expectancy. I certainly recommend a slow speed grinder, for reasons of safety and tool longevity, and say that an 8" grinder with CBN wheels is a dream set-up.

[Colin Prince]

Thanks for the welcome - after spending a while reading through old threads on sharpening, jigs and CBN wheels, I hardly feel new :-)

I haven't been able to find a slow speed 8" grinder yet - I'm sure there must be someone selling them somewhere in South Africa, but I've not managed to track them down yet, which is why I'm tempted to just get the 6" one - If all I'm used to is the hollow grind from a 6" wheel, maybe I'll never know the difference...

[Colin Prince]

Thanks Brian - it was afternoon when you replied

I have considered putting an 8" wheel on a 6" grinder but would probably just buy the 6" wheel. It's an idea worth considering though.

[Colin Prince]

Thanks for the well thought out reply.

The grinders I'm looking at right now are Creusen, made in Holland so at the the power is in first-world units. The options I have are:-
8" high speed 600W (>3/4 hp) - quite pricey
6" high speed 350W (<1/2 hp) - affordable
6" low speed 350W (<1/2 hp) - similar price to the 8".

I'm not too worried about time taken to shape a tool - it's not something I do often so I can take my time over it, I'm after efficient sharpening.

The slow speed / high speed issue is what's causing my indecision right now. I'm leaning towards the slow speed 6", and may be tempted into running an 8" wheel on it - the slow speed motor is a 4 pole motor, and will have more torque (than the same power 2 pole high speed motor) and will reach it's operating speed quicker than a high speed grinder, so spinning a larger wheel shouldn't strain it too much.

[Colin Prince]

I would caution against trying to mount and use an 8" wheel on a 6" grinder. The weight would overload both motor and bearings, significantly shortening their life expectancy. I certainly recommend a slow speed grinder, for reasons of safety and tool longevity, and say that an 8" grinder with CBN wheels is a dream set-up.

I think the bearing would be under more strain from an out of balance wheel than a heavy wheel. The motor is a concern too, although I'm pretty sure that a motor that is rated 3/4 hp in China is closer to 1/2 hp if tested in Europe or the US.

I agree that the ideal setup is 8" slow speed with a CBN wheel - I just wish I could find the grinder.

[David C. Roseman]

I think the bearing would be under more strain from an out of balance wheel than a heavy wheel. The motor is a concern too, although I'm pretty sure that a motor that is rated 3/4 hp in China is closer to 1/2 hp if tested in Europe or the US.

I agree that the ideal setup is 8" slow speed with a CBN wheel - I just wish I could find the grinder.

Colin, sounds like you're thinking this through pretty well. If you'd be happy with the deeper hollow grind you'll get with the smaller wheel, then a 6" CBN on the high-speed 6" grinder you describe sounds like a good solution in your circumstances. The ideal, as you say, is the 8" CBN wheel on an 8" slow-speed grinder. Running at 60 hz and 1825 rpm here in the States, it gives us a surface speed of 64 fps. A 6" wheel at 60 hz and 3400 rpm yields 90 fps. But if you derate that for 50 hz, it drops to only 75 fps. That sounds pretty good to me, if the 8" slow-speed that's available to you is too spendy.

BTW, even if the motor and bearings would be o.k., not sure why you would consider running an 8" CBN wheel on a 6" slow-speed Creusen grinder if the grinder is similar in price to an 8" slow-speed.

David

[Colin Prince]

BTW, even if the motor and bearings would be o.k., not sure why you would consider running an 8" CBN wheel on a 6" slow-speed Creusen grinder if the grinder is similar in price to an 8" slow-speed.

David, it's a high speed 8" grinder - I haven't been able to find a slow speed 8" one yet.