Thread: Grinder speed

I vote slow because I have a TORMEK watercooled grinder, but to some extent I guess it depends what is being done. Fast speed for very short periods is acceptable to some - but fast speed without a cooling pause could loose the temper in the steel.

soth

Originally Posted by jmk89

There seems to be a real division of opinion here. Should we use high speed or low speed? High speed advocates say it does the job faster and you can tell if it's too hot because your fingers will tell you. Low speed say, you don't need that much speed and why risk burning the steel. So what do you do?

Basically you should use what ever you want. There is no right or wrong speed. But there are better methods than others.

When it comes to grinding high carbon and other tool steel chisels... I can tell you high rpm is not faster than grinding at low rpms. I grind my tools at or below 300rpm and I can remove steel at least 10% faster than an equivalent high speed grinder. If all you're trying to do is remove steel and heat build up isn't a concern then a high rpm grinder is faster. I've not, as of yet, tried to determine an ideal rpm range, that might happen if I can get my hands on some sacrificial chisels.

Low rpm also offers other benefits. It gives the operator more time at the wheel to get the job done right before the need to cool the steel. The lower the rpm the more time you have to work. Not the best way to describe what I'm thinking - it's late and my brain cells aren't firing all that well.

With a coarse, clean, friable wheel and a light touch, a high speed grinder is a safe tool.

With a heavy hand and/or gunked up wheels and/or high grit wheels, a half-speed grinder will burn a blade very easily.

It is all in the way it is used.

Regards from Perth

Derek

It depends on the job. Some I get better results by hand (knives mostly). Somethngs I use a jig on a slow speed wheel or belt. For stuff it doesn't matter if it get hot I may use high speed. Another thing to consider is how much material is removed? High speed often takes more material than necessary reducing the life of the tool when sharpened on high speed.

More important than RPM is SFPM (surface feet per minute)

ron

For knives I use the Scangrind (Record in my case) with a jig to maintain a straight edge, for gouges I use a 80 grit white wheel on the bench grinder for about 2 seconds because thats about all it takes.

I'm with Iain on this... where is the 'both, depends' button?

I use a coarse grit wheel on a high speed grinder to take chips out , reshape, or change bevel angles. (tools get a regular dip in a bucket of water). Then onto the waterstones.

As I only have a 150mm fast grinder that is what I use. As an apprentice I learnt to grind on a fast grinder and if I have to grind extensively I keep dipping the tool in water to cool it. As I have had plenty of practice I reckon I do alright.

For the last three years I have had a wire wheel on one side of the grinder and on the other side I have had a green stone for grinding carbide drill bits and this what I have been sharpening my chisels, plane irons, turning chisels and HSS drill bits with.

So sharpening tools can be a fly by the seat of your pants thing, or you can make it an exact science. I suppose both ways work, it's just that one is quicker than the other but the result is usually the same.

Don't . . .

Originally Posted by Toolin Around

When it comes to grinding high carbon and other tool steel chisels... I can tell you high rpm is not faster than grinding at low rpms.

and

Originally Posted by Toolin Around

If all you're trying to do is remove steel and heat build up isn't a concern then a high rpm grinder is faster.

. . . sort of contradict each other?

If you go slow enough or use inbuilt cooling there should be an ideal RPM where you shouldn't need to stop to cool the blade. If you go faster you just have to stop more often and apply extra cooling to the blade. It then comes down to how much time you save at slow speeds because you don't have to stop and cool down the blade versus the faster grinding you get at higher speeds. It would be interesting to see the difference between the two methods in removing metal. Personally I'd have my money on the high speed.

As Derek says, the wheel type and cleanliness are important factors in removing metal. The other factor is how game you are at using your fingers as temperature sensors to decide when its time to stop cool the blade. Once you have mastered this I'd be saving the $ needed for a low speel sharpener for something else.

jmk89's first sentence sounds spot on

BobL the tone of your reply sounds like you're taking this a bit too serious Why is it sharpening threads get all serious anyways

It's only sharpening - relax. Oh that's right you grind at high rpms - you can't take your time ad relax

OK lets get serious Be more specific as to how what I said was contradictory. I thought it was pretty clear. Anyone else confused?

As for the cost... My first grinder cost around $150 - about the same as a half arsed grinder at bunnings.

Speed - shouldn't and doesn't matter. I satisfied my curiosity as to which is faster some time ago with a side by side trial. Here's a tip for ya... bet on the turtle

Toolin, I agree with you on never taking sharpening threads too seriously.

Unfortunately I've found another inconsistency in your previous post, if high speed is slower then surely I'm the one that can take my time and relax - right?

In a nut shell what I think allows me to grind faster at low rpms is: The steel is a great conductor of heat. At low rpms the heat being absorbed by the steel is significantly lower than at high rpms. The tool's able absorb that heat and carry it away from the edge so I can keep the tool on the wheel for a much longer period of time. At really low rpms it's possible to keep the steel pressed fairly hard on the wheel almost indefinitely with little or no heat build up. At high rpms the heat's able to build up rapidly and most can't be absorbed and conducted away from the edge so the tool must be removed much more and cooled. These combined factors allows me to grinder tools faster at low rpms than at high rpms.

If all I'm grinding is mild or untempered steel where there is no concern for damaging heat build up, then using a high rpms grinder is faster. Because I would be pushing the steel into the wheel as hard as the grinder could handle. And only cooling when it got to hot to hold in my hands. Not something I would attempt with a tool that was heat sensitive.

[quote=Toolin Around;452419]In a nut shell what I think allows me to grind faster at low rpms is: The steel is a great conductor of heat. At low rpms the heat being absorbed by the steel is significantly lower than at high rpms. The tool's able absorb that heat and carry it away from the edge so I can keep the tool on the wheel for a much longer period of time.
[quote]

I used to think like you and I can still recall hours wasted reshaping old handtool blades that I bought from markets. Having made a proper tool rest and guide for my 8 inch grinder, I now disagree. It takes me a minute to reshape a blade of a 2 inch chisel that would have taken a few hours of frustration to do on a slow grinder. The only time I blued a blade and needed to regrind was when I was getting too lazy and too greedy trying to get the job done in a few seconds.

BTW you should not be applying a great deal of pressure to the wheel either, you are putting the grinder and wheel under unnecessary stress. With a high speed grinder and dressed white wheel you should not have to apply much pressure at all...

Using the grinder gives me a hollowground blade and I create a microbevel with waterstones. It works for me. Others have said that the hollow grind reduces the strength of the blade, I have not found this to be the case. Most importantly I can touch up the microbevel in less than 20 seconds. This way I ALWAYS have razor sharp tools. As I said it works for me and I only sharpen handtools and woodworking blades, I do not sharpen knives, scissors, etc.

regards

Marios

Originally Posted by BobL

Once you have mastered this I'd be saving the $ needed for a low speel sharpener for something else.

mastered ?......there's really no mastering to be done......when your finger feels hot, you instinctively pull it away.

One of my first grinders was a slow one......just a workhead pullyed at slower revs......think it would have been around 1200rpm........

Worked it that way for a while, till I started using a regular faster grinder for a few years as well........since realised that it is actually quicker done on the faster grinder, and as safe.....yes, including the time it takes for the occational water dip if you feel you need it.

So my slow ones been repulleyed back up to around 3000rpm. I'll never go back to a slow one.