Grinder at 2850 rpm for sharpening

[Arifi Koseoglu]

Hello everyone.

As the selection of the forum may suggest, I am *very* new to turning. I received my lathe only a couple days ago. Before starting to turn, I assumed it would be good to also be able to sharpen the tools as needed (and I believe it is going to be needed a lot). So I went looking for a decent grinding machine similar to the ones I saw in the numerous YouTube videos I watched.

I reside in Turkey, and here the selection in hardware far more limited than what is available in the states. It seems that *all* grinders that I can find/buy are 2850 rpm, the motor power varies between 400 and 600 Watts, and the weight varies between 22 lbs and 50 lbs. The one I liked most has a 600Watt 3-Phase motor, runs at 2850 rpms (fixed) and weighs approx. 30lbs (16kg). The wheels are 36 and 60 grid, 200mm (8") in diameter and 30mm (1.18") thick.

In the videos there is very frequent mentioning of "low speed" grinders and their advantages. Is 2850 rpm with 8" wheels too fast for sharpening my turning tools? Is the motor too week?

Best regards,
Arifi Koseoglu

 

[Hugh Cowart]

Wise to Inquire

You were very wise to inquire about what equipment to purchase to sharpen your woodturning tools. The slow speed machine is preferred due to the amount of heat generated with high speed machines. However, having made that statement I know a few turners who still use the faster machine. There is so much to talk about and I will only cover the bench grinder per your question. The high speed grinders come with grey carborundrum wheels which is another name for silicon carbide and is not recommended for high speed steel. DO NOT use the silicon carbide wheels on your tools. I would recommend that you purchase two new wheels. There are several US retail online stores where you can order the Norton wheels with an 80 and 120 grit. The Norton Blue wheels contain cobalt and is one of the most used wheels. It is a fifty percent mixture of Seeded Gel (SG) grain ceramic abrasive. This is not to say that white or pink, which are aluminum oxide wheels, will work for you also. Do some research and then decide which wheel most suits your need. Good luck and enjoy turning.

 

[Owen Lowe]

Just my two kuruşâ€¦ I would buy the grinder with the highest quality appearance (heavier can also equate to quality and in use heavy is better than light). The speed is of lesser importance with today’s high-speed and powdered metal steels. You’d be hard-pressed to heat these steels to the temperatures that would affect the temper (>1000F/540C). If you have a number of high-carbon steel tools then a light touch is a must since you can easily heat metal to blue or hotter and that will soften H-C steel.

The gray wheels that come on the machine are certainly usable but are generally of low quality and may not be the most suitable regarding hardness. Most turners use aluminum-oxide wheels in the 36-120 grit range with medium-soft to medium hardness. (Look up the grading specification systems for replacement wheels to become familiarized with the nomenclature.) There seems to be a trend toward CBN wheels that use a crystal abrasive on a metal wheel. These are long-lasting and maintain the diameter throughout use - unlike standard grinding wheels.

 

[Bill Boehme]

Since you have 50 Hz power, the grinder speed will be different from the grinders used in the US where we have 60 Hz power. 2850 RPM is fine, but if you can find 1440 RPM, it might be slightly better, but not enough of a difference to make it worth paying much more money. A 400 W motor is approximately equal to a US 1/2 horsepower motor and a 600 W motor is approximately equal to a US 3/4 horsepower motor. For woodturning tools, the 400 W motor is completely adequate. As others have mentioned, the wheels that come with a grinder are usually the gray silicon carbide wheels which are not the best for sharpening high speed steel woodturning tools. It would be better to replace the wheels with aluminum oxide wheels or ceramic matrix wheels.

 

[Arifi Koseoglu]

Dear Hugh,
Dear Owen,
Dear Bill

I thank you very much for your responses. They helped me to get a grip on this matter. I will combine all of your advice and get started without wasting time. Once I get the grinder and get used to the physical feel of the tool, I will look into the various disk types you mentioned.
Many thanks again,
-arifi

 

[Dean Center]

Arifi,
My experience has been that my turning skill improved in direct proportion to my skill at sharpening. Not entirely because I was sharpening better, but it was a big factor. My suggestion is to pay as much attention to learning sharpening as you do to learning turning. Also, use a very, very, very light touch with whatever grinder and wheels you buy. If at all possible, having a teacher is extremely helpful as you get started.

 

[Arifi Koseoglu]

Arifi,
My experience has been that my turning skill improved in direct proportion to my skill at sharpening. Not entirely because I was sharpening better, but it was a big factor. My suggestion is to pay as much attention to learning sharpening as you do to learning turning. Also, use a very, very, very light touch with whatever grinder and wheels you buy. If at all possible, having a teacher is extremely helpful as you get started.

Thanks for the advice Dean. Sadly, there is no one I can use as a teacher for sharpening turning tools, except for the helpful woodturners on YouTube. I have been watching lots of videos on sharpening and woodturning lately, trying to get accustomed to the process. From what I gather, sharpening is very important, as you mentioned, specifically on three aspects: 1. Sharp tools cut cleaner 2. Sharp tools are safer 3. In the sharpening process one can alter the tool to fit the exact need. My current set of cutters is quite cheap, so I will exercise a lot on sharpening.
Best,
-arifi

 

[Odie]

Just quick note, in case anyone else if following this thread......

Surface speed is faster with a larger diameter wheel turning at the same speed as a smaller wheel. It's not the diameter of the wheel, nor speed of the motor, considered separately, that determines the surface speed. Arifi's choice of 8" wheels is about optimum, but larger, or smaller wheels can produce equally good results. It's mostly a matter of learning to use the tools and equipment that you have to their best level of performance. If I were Arfi, I'd consider Bill's advice of 1440rpm as a better choice, but the faster 2850rpm can certainly be used for the purpose.

Keep some water handy for cooling the tool shaft. This isn't a matter of overheating, and subsequently altering temper.....but, is mostly for comfortable turning. However, overheating can certainly be possible with aggressive grinding. From time to time, a little "bluing" can pop up quickly, and isn't an indication that temper is lost, except for at the very thinnest part of the cutting edge. Since it's strictly on the surface, it's relatively easy to quickly grind the bluing away and put that tempered steel right back on the leading edge. Bluing is to be avoided, but there isn't a turner who hasn't done it at one time, or another......

ooc

 

[Bill Boehme]

.... From time to time, a little "bluing" can pop up quickly, and isn't an indication that temper is lost, except for at the very thinnest part of the cutting edge. Since it's strictly on the surface, it's relatively easy to quickly grind the bluing away and put that tempered steel right back on the leading edge......

Just to clarify a small detail. Bluing is surface oxidation that is an indication of overheating. Since it is oxidation, it will only appear on the surface, but you can be sure that the metal beneath the surface got just as hot, but it won't be colored. Removing the surface color doesn't make the issue beneath go away. HSS is more tolerant of bluing than carbon steel, but the often heard statement that you can't hurt the temper by overheating while grinding is somewhat of a mistaken belief. The good news is that with the type of grinding wheels used by turners and good technique, overheating is unlikely to be a problem.

 

[Odie]

Just to clarify a small detail. Bluing is surface oxidation that is an indication of overheating. Since it is oxidation, it will only appear on the surface, but you can be sure that the metal beneath the surface got just as hot, but it won't be colored. Removing the surface color doesn't make the issue beneath go away. HSS is more tolerant of bluing than carbon steel, but the often heard statement that you can't hurt the temper by overheating while grinding is somewhat of a mistaken belief. The good news is that with the type of grinding wheels used by turners and good technique, overheating is unlikely to be a problem.

Myth 7 – Blue while grinding and lose temper.
If you have high-carbon steel and you get the edge so hot during grinding that it turns blue, then you will draw the temper and have to grind back beyond the blue. But if your tool bit is high-speed steel (HHS), then you have to get the tip red hot before you will lose temper. The rule of thumb for grinding is to hold the tool in your hand and, if it is too hot to hold, it is time to cool it off. While I don’t advocate that you never cool your tools while grinding, you should not panic if it gets a little hot and turns blue. If it turns blue and will not hold an edge, then it is probably not HHS.

http://www.neme-s.org/shapers/columns/shaper_column_41.html

What is the difference between High Speed Steel (HSS) and Carbon Steel tools?

High Speed Steel (HSS) tools are prized because they will NOT lose their hardness (temper) if "blued" during sharpening. HSS will retain its hardness even if brought to red under heat.

Carbon Steel will lose its temper if blued. Typically it is the thin, sharp edge that will blue first. You want your edge sharp and hard, not sharp but soft.

Given it is very easy to blue steel while grinding, particularly if you are heavy handed or have a high speed grinder, you can see the attraction for HSS tools. It doesn't take much to overheat a knife-thin wafer of metal taken to a razor-thin edge. I've heard stories of inexperienced woodturners grinding away at a carbon steel tool, continually chasing a bluing edge, trying to get an edge that was sharp, but not blued.

http://voices.yahoo.com/steel-wood-turning-tools-7185952.html

I think you've got characteristics of carbon steel and HSS confused, Bill.......

Although I can probably count the times I've blued a HSS turning tool in the past decade on one hand, I've been to the understanding that these definitions shown above are correct......

I do believe, from my experience, that the very thinnest portion of the grind, along the cutting edge itself, can lose temper in HSS, and will have problems maintaining an edge. The fix is a quick re-grind that doesn't produce the bluing, and is all that is necessary to restore the temper to the thin leading edge. I haven't noticed any temper loss if re-ground as usual......

As I understand it, it is possible to lose temper, but it takes quite a bit more heat than that which creates a bit of bluing along the edge. Very aggressive grinding could do it, I suppose, but I don't believe we're discussing the kind of overheating that is relevant to the average turner......with the possible exception of carbon steel tools. I still have a few of those, but haven't used them since the early 80's.



ooc

 

[Bill Boehme]

I do believe, from my experience, that the very thinnest portion of the grind, along the cutting edge itself, can lose temper in HSS, and will have problems maintaining an edge. The fix is a quick re-grind that doesn't produce the bluing, and is all that is necessary to restore the temper to the thin leading edge. I haven't noticed any temper loss if re-ground as usual......

That is basically what I said. I wasn't implying anything drastic -- merely pointing out that the bluing can only appear on the surface that is exposed to oxygen.

And, as you probably know, HSS is tempered following quench hardening by heating it to a much lower temperature than where it starts to glow. Heating it to the point of glowing is going to have some kind of effect on the very thin portion right next to the edge. As you pointed out it would take some really heavy handed abuse to do that. Another thing about HSS is that instead of the hardness decreasing, the very fine edge can become brittle from cooling too rapidly even if it isn't quenched in water. The result of this shock hardening is that the fine edge can break away sooner than it normally would get dull.

 

[Odie]

That is basically what I said. I wasn't implying anything drastic -- merely pointing out that the bluing can only appear on the surface that is exposed to oxygen.

And, as you probably know, HSS is tempered following quench hardening by heating it to a much lower temperature than where it starts to glow. Heating it to the point of glowing is going to have some kind of effect on the very thin portion right next to the edge. As you pointed out it would take some really heavy handed abuse to do that. Another thing about HSS is that instead of the hardness decreasing, the very fine edge can become brittle from cooling too rapidly even if it isn't quenched in water. The result of this shock hardening is that the fine edge can break away sooner than it normally would get dull.

OK, Bill......good enough! I thought you were implying that if bluing occurred, that temper loss under the surface was happening. My misinterpretation of your words, and a good thing we have clarified the issue.......

ooc