Bench Grinders: 8 inch or 6 inch?

[Darren Brewster]

I am wondering what everyone's thoughts are on bench grinders for sharpening woodworking tools? Rob Cosman says in his videos that he uses 8 inch because there is too much hollow grind on a thick mortising chisel. Other sources I have read while researching say that the 6 inch is better because it runs more slowly at the surface of the wheel, and the greater amount of hollow helps register the chisel flat on your sharpening media, not to mention 6 inch grinders and wheels are cheaper. Does anyone have an opinion on what they would choose and why?

[Chris Griggs]

6 inch grinders are great. Yes, the surface speed of a 6 inch wheel running at 3600 rpms is slower than the surface speed of an 8 inch wheel running at the same speed. My 6 inch grinder is variable speed but I run it almost exclusively on its highest setting. For an 8 inch you would definitely want the low speed option. I don't have a lot of experience with mortise chisels, but my understanding was that they shouldn't be hollow ground anyway - hopefully someone more knowledgeable will chime in. Anyway, unless you're planning on grinding turning tools, I'd go with a 6 inch.

[Jessica Pierce-LaRose]

I like a 6", because that's the only grinder I've access to. If the mortising chisel is the only tool holding you back from a 6", but you like it in all other aspects, I guess I'd still go with the 6".

For my Narex mortise chisels, (which have a similar blade to a pigsticker style), I find I prefer rather than hollow-grinding the whole thing, to do a largish secondary bevel. Originally this meant that I needed to use an honing guide, but I find that I can get it good enough freehand if I lock my arms and am careful.

I like a shallow, 20-25 degree primary bevel - I feel like this lets the chisel penetrate the wood more when really hogging away. This ends up being quite a long bevel because of the low angle and thickness of the chisel. Then I grind a steep 35-ish degree (probably more as the bevel sometime gets a little steeper everytime if I'm not careful freehand honing) secondary bevel on the tip, so the edge doesn't break down in use. I often round the transition between the two bevels, and between the main bevel and the top of the chisel, because so many mortise chisels I've seen have been like that. I have no idea why that makes me want to do it, but what the hell. In practice, I probably end up only honing the very tip of the 35 degree bevel by adding a third bevel. I don't really think to much about all these bevels - I simply eyeball the shape that I like, and make sure the tip is sharp.

as the steeper 35 degree bevel eats up the shallower bevel, I grind back down the 20-25 degree bevel some more. This takes a long time to get to this point. I still leave the 35 degree tip quite thick by the standards of secondary bevels. I usually do this grinding on sandpaper on a flat surface, but you can do it with a grinder too - and I look at it as hollow grinding just the bevel behind the steeper secondary bevel, not hollow grinding the whole tool. A nice mortise chisel is probably thick enough, you could do kind of what Derek proposed for bevel up plane blades, and actually do separate hollow grinds, one for the main bevel, and one for the secondary bevel that you'll actually be honing. Splitting the difference like this, it seems like the hollow grind from a 6" wheel still wouldn't remove too much excess material from the edge.

[Kent A Bathurst]

I think the real issue is tip speed/surface speed of the wheel. It is a function of wheel dia. and motor rpm, so the answer is "it depends".

[george wilson]

The use of a grinder with satisfactory results has more to do with the technique of the user than the speed of the grinder. I dip my chisels in water. Some droplets stay on the chisel. When those droplets sizzle off,I immediately dip the tool again before it turns brown,blue,etc..

[Bob Strawn]

The use of a grinder with satisfactory results has more to do with the technique of the user than the speed of the grinder. I dip my chisels in water. Some droplets stay on the chisel. When those droplets sizzle off,I immediately dip the tool again before it turns brown,blue,etc..

I take a plant fiber bristle brush and wet it. This brush sits on the back of the chisel while I grind. Just like George, the second it starts to sizzle, I wet it again. this way it never gets much past boiling temperature.


Bob

[george wilson]

There are some very experienced and expert machinists who say that the slow speed (1725-1750) RPM grinders are a huge hoax. I have to agree. You need to learn proper technique more than hoping the grinder will do things right for you.

Bob,the bristle brush sounds like a great idea. that temp. goes up very fast after the water sizzles. Tool must be dipped EL PRONTO!!!

[Salem Ganzhorn]

8" or 6"? Yes one of both please :)For lathe tools I like the 8. For bench chisels and planes I wish I had a 6 to give more of a hollow grind.Salem

[Tony Shea]

Bob, what is a "plant fiber bristle brush"? I have never heard of such a thing, maybe it should be odvious though. Sorry if its a stupid question.

I really like the idea of holding something wet behind the blade to tell you when you reach the "sizzle" point. I also use a 6" grinder just because that is what I have. If I had an 8" I would probably stick with that. In fact the 8" size I probably would prefer for a more gradual hollow, but the difference is probably neglidgable. Most wet grinders such as Tormek use 10" wheels and I believe they still produce a sufficient hollow grind to help with freehand sharpening.