Variable Speed Bench Grinder

[H. Gregory Porter]

Is there any advantage to having a variable speed bench grinder?

Obviously, slower speed means less heat generation but longer times to grind. With the variable speed I could turn it up for jobs requiring more removal of metal. I guess the real question is how often it may be advantageous to slow it below the 3500 rpm of the fixed speed ac grinder.

I’ve got a Tormek, but need a grinder that will handle reshaping as well as lawn mower blades. I have an opportunity to pick up an 8” variable dc grinder that is capable of adjusting speed from 400-4000 (tel:400-4000) rpm. I expect to grind primarily plane blades, chisels, carving tools, occasionally carving tools, and cutters for a metal lathe. I’ll do final dressing on the Tormek or on ceramic bench stones. My main use for the tools will be to establish a clean bevel edge or reshape a tool, and to sharpen metal lathe tools and mower blades.

[Derek Cohen]

Hi Gregory

Get a half-speed 8" grinder, add a 180 grit CBN wheel for plane blades and chisels, and a 80 grit CBN for grinding. And you will no longer use the Tormek. Like me :)

http://www.inthewoodshop.com/WoodworkTechniques/UltimateGrindingSharpeningSetUp.html

Regards from Perth

Derek

[Tom M King]

I wouldn't want to sharpen mower blades on the same grinder that does woodworking edges. A mower blade needs the coarsest wheel you can find, and it makes a big mess in the process. You need real horsepower to cut with that coarse wheel too. The same wheels you use for irons, and chisels will take a long time on a mower blade, and most likely end up bluing the edge. The coarse wheels cut so fast that heat buildup is not a problem, and it doesn't matter if the edge has some striations on it.

I sharpened mower blades, for a long time, with a 9" side grinder, which makes quick work out of it, but then found a dedicated mower blade sharpener on CL. The mower blade sharpener takes 4 or 5 seconds to sharpen each end. It does make a mess, but I keep it out in the tractor shed where it doesn't really matter.

[Bill Dufour]

Buy a big old 3 phase grinder and use a vfd to make it variable speed. After a year or so report back to us. RPM is not important but FPM is what counts. that is why bigger diameter wheels must run lower RPM's.
Bil lD

[Jim Becker]

Speed and heat are related and some metals benefit from lower speeds as a result. Whether or not you will benefit from that high range of speed variability will depend upon the specific tools and metal compounds you'll be grinding. I have just a 1750 rpm 8" grinder and it does everything I find I need with both HHS and carbon steel, taking great care with the latter, of course.

I wouldn't ever use a grinder in my shop for mower blades, however. I use a hand-held grinder and leave the blades on the machine after lifting it and installing jack stands for safety.

[Jerry Olexa]

Var Speed is useful for me when SAFETY is an issue.. On a smaller piece, sometimes it is important to CONTROL the item while cleaning, polishing etc.

[Frank Pratt]

I have a 6" Delta variable speed grinder & use the speed control all the time for reasons stated by others above.

[Rod Sheridan]

I have a variable speed 8 inch grinder which sits around and collects dust.

I use a Viel belt grinder for all my sharpening, by changing belts I can go from rough removal to grits suitable for final honing..........Rod.

[tom lucas]

You are not supposed to sharpen carbon steel (mower blades) on CBN. I believe CBN is HSS variants only. I did sharpen a mower blade once on my CBN. Regretted it. It took a good bit of additional use to get the CBN back to normal. Glad I only did it for one blade one time.

[H. Gregory Porter]

Thanks Bill. I see you’re a 3 phase advocate. I looked at 3 phase a several years back when I was building my shop. It’s cost prohibitive to convert everything to 3 phase, as it is cost prohibitive to add a 3 phase grinder. If I were running a larger industrial shop, that would be an appropriate consideration.

[H. Gregory Porter]

You are not supposed to sharpen carbon steel (mower blades) on CBN. I believe CBN is HSS variants only. I did sharpen a mower blade once on my CBN. Regretted it. It took a good bit of additional use to get the CBN back to normal. Glad I only did it for one blade one time.

I nearly dismissed your reply but your experience caused me to dig a little deeper. I spoke with two distributors of CBN Wheels. Both said they sharpen lawn mower blades on CBN; one said he has sharpened hundreds over the years. Neither told me I’d have to unclog the CBN. Thanks to your post, I found several articles saying the CBN will clog but could be cleaned by grinding harder steel. I’m not interested in having to unclog those wheels. What are you using for lawn mower blades - vitrified aluminum oxide?

[Derek Cohen]

You are not supposed to sharpen carbon steel (mower blades) on CBN. I believe CBN is HSS variants only. I did sharpen a mower blade once on my CBN. Regretted it. It took a good bit of additional use to get the CBN back to normal. Glad I only did it for one blade one time.

No, that is not so. Sorry Tom. The carbon steels thrive on CBN (I have done this for years). The steel that should not be ground on CBN is mild steel or unhardened steel.

As far as I am aware, mower blade steel is water hardened, in other words, generally W1 steel. That is perfectly doable on CBN ... as long as it is hardened enough. The worst case scenario is to clean the CBN wheel by grinding with a hardened steel.

Regards from Perth

Derek

[Tom M King]

Mower blades are best sharpened with, at finest, a 36 grit wheel, whether on a side grinder, or a stationary grinder. On my rotary cutters, I use a 9" side grinder, with the blades on the cutters. I used the same grinder for years on the small mowers, until I found a commercial blade grinder cheap on CL.

Here is one of the smaller ones. Of course, it's not worth the expense for someone that has a half acre to mow, and sharpens their blades once a year.

https://www.northerntool.com/shop/tools/product_200360028_200360028?cm_mmc=Google-pla&utm_source=Google_PLA&utm_medium=Lawn%20%2B%20Garden%20%3E%20Lawn%20Mowe rs%20%3E%20Mower%20Accessories&utm_campaign=Oregon&utm_content=70041&&lid=92700043516527146&ds_s_kwgid=58700000737318873&ds_e_product_group_id=658483071033&ds_e_product_store_id=&ds_e_ad_type=pla&ds_s_inventory_feed_id=97700000001662305&gclid=Cj0KCQiAkePyBRCEARIsAMy5SctWRxXyZa315FGUoFp3 j5V4-8WvtCVFyCr6RhyhW7tERSlePV79t5caAsenEALw_wcB

Not only will a finer wheel probably burn the blade, but it will take forever to balance one while removing such a small amount of metal.

I'm not sure what kind of metal mower blades are, but it doesn't seem any different than A36 to me.

[Jon Nuckles]

I looked at 3 phase a several years back when I was building my shop. It’s cost prohibitive to convert everything to 3 phase, as it is cost prohibitive to add a 3 phase grinder. If I were running a larger industrial shop, that would be an appropriate consideration.

Because Bill mentioned a VFD, I assume he did not mean bringing 3 phase power to your shop nor converting all of your machines to 3 phase. A VFD allows you to use a single phase input to create a 3 phase output and to vary the speed of a 3 phase machine. If you are buying used, a 3 phase grinder may well be less expensive than a single phase unit of similar build. Given the already low price of grinders, however, maybe not enough cheaper to cover the additional cost of a VFD.

[H. Gregory Porter]

Because Bill mentioned a VFD, I assume he did not mean bringing 3 phase power to your shop nor converting all of your machines to 3 phase. A VFD allows you to use a single phase input to create a 3 phase output and to vary the speed of a 3 phase machine. If you are buying used, a 3 phase grinder may well be less expensive than a single phase unit of similar build. Given the already low price of grinders, however, maybe not enough cheaper to cover the additional cost of a VFD.

Jon, Thanks for the education. I knew VFD’s provided speed control. I did not remember it would convert 3 phase to single phase.

[tom lucas]

No, that is not so. Sorry Tom. The carbon steels thrive on CBN (I have done this for years). The steel that should not be ground on CBN is mild steel or unhardened steel.

As far as I am aware, mower blade steel is water hardened, in other words, generally W1 steel. That is perfectly doable on CBN ... as long as it is hardened enough. The worst case scenario is to clean the CBN wheel by grinding with a hardened steel.

Regards from Perth

Derek

I've read it several different places. Perhaps their is confusion regarding what types can or cannot be done; I don't know. As usual, there is contradicting info on this. My experience with my mower blades tell me not to do it again. It left it clogged up a bit until I sharpened several 10V steel tools on it. Because I have a high speed AO grinder, I don't chance it. Mower blades just aren't worth it.

[H. Gregory Porter]

?..

... Because I have a high speed AO grinder, I don't chance it. Mower blades just aren't worth it.


i got clarification on this. You can use CBN for softer steels if and only if you slow the grinder speed/feed rate and go slow enough to prevent overheating. Those putting CBN on fixed speed ac grinders have to be meticulously slow when sharpening softer steels to prevent heated metal from loading up the grinding wheel.

I’m going with the variable speed grinder, so My plan is to try sharpening the lawn mower blades at the lower wheel speeds, increasing it until I can begin to feel warmth near the cutting edge.

[tom lucas]

Jon, Thanks for the education. I knew VFD’s provided speed control. I did not remember it would convert 3 phase to single phase.
I thought was the other way around: single phase to 3 phase.

[H. Gregory Porter]

You’re correct. Must have Had a dyslexic moment — of a sort.

[Chris Fournier]

My thinking and practice is that the wheel grit and bond are far more important than the machine speed. I have no need for a variable speed grinder but I have spent hours getting my grinding skills up to snuff. And I'm still better some days than others...

If you want to discuss the importance of speed it should be with regards to your feed speed of the tooling. I've found this to be key on bench, cup, surface and cylindrical grinding jobs.

[H. Gregory Porter]

I received my 180 grit CBN wheel. I tested it on my lawn mower blades. The Mfgr said it was ok to grind softer steels - ie non HSS blades, as long as I ground at slow enough speed to not generate heat. The sifted hot steel would clog the CBN wheel. I placed my fingers about 1/2 to 3/4 inch behind the grinding surface and only for a brief few seconds even felt any warmth while grinding 6 blades. I ground the first 5 blades at about 1000rpm on an 8”wheel. I increased the speed to about 1400 rpm for the last blade. It was on this blade at a slower feed rate that I felt just a tinge of warmth near the cutting edge.

Although this seems adequate for lawn mower blade grinding, I would not use this if I were in the business of sharpening mower blades. When I completed 6 blades, the wheel looked every bit as new as when I received it. There was no evidence of clogging just as the mfgr stated. For the 1 or perhaps 2 times per month that I would sharpen a mower blade, I have no reservation of using the same CBN wheel that I would use on my finer plane and chisel blades.