Your Tentative Guide to not Making Stupid Mistakes About Belt Grinders

I continue learning things about belt grinders, sometimes too late to capitalize. I might as well keep documenting my discoveries so other people will benefit.

To get everyone back up to speed:

I bought an Oregon Blade Maker 2″ x 72″ grinder. I set it up with a VFD and an 1800-RPM motor. I found out that you need an enclosed motor (TEFC or TENV) to do it right, because the crud the grinder throws will get into open motors and kill them. I bought a new TEFC motor, and the Post Office broke it. I bought a second motor, and this time I went to 3HP. The other motors were 2HP.

I might as well point out that I use “1800-RPM” to describe any motor that has a speed anywhere near 1800. The actual speed of a 3-phase 4-pole motor running at 60 Hz will be somewhere below 1800, but the difference isn’t worth getting worked up about.

Okay.

I did a lot of research, trying to choose the right motor. Some guy who is a big banana on a knife-making forum claimed 1800-RPM was the way to go. Something about 1800-RPM motors being built better with regard to the stress of high speeds.

Right away, I should have realized that didn’t make sense. But I trusted him.

I don’t know a whole lot about electric motors. When I was getting my degree, we went through the principles of electric motors, but the things they teach you in physics are generally a long way from practical. That’s why we also have engineering classes. Not the same thing.

I was too lazy to sit around studying the 3-phase motor; I figured any guy who has the awe and admiration of a whole bunch of people on a knife forum ought to know which motor to use.

A person with common sense would have looked at the motors that come with turnkey grinders; they’re 3600-RPM motors. Dohhh!

Arbitrarily, I had decided I wanted the belt to move at 5000 feet per minute (FPM). With a 4″ drive pulley, that required 4775 RPM. With slippage, that’s probably not quite accurate, but that was the figure I chose. To get 4775 RPM, I would have to run the motor at 167 Hz, or almost 3 times the motor’s rated speed.

I didn’t think this was a problem. I knew the bearings were probably good for over 14,000 RPM. The armature–the thing that turns inside the motor–was probably made to the same specs as a 3600-RPM armature, and at 4775 RPM, it would only experience something like 1.7 times the centripetal force. I didn’t think it would fly apart.

The 2HP motors I had ran around 40 pounds. I found a wonderful deal on a 3HP model with nearly the same NEMA frame as one of the 2HP jobs. “Hey,” I figured, “same frame…can’t weigh much more.” WRONG. It’s 84 pounds. Also, NEMA frame specs don’t tell you the overall length of a motor casing. This motor is maybe four inches longer than the others.

The new motor was a real joy to carry to the garage and hoist onto the grinder cart.

I got the motor going, with a lovely jury-rig wiring job, just to make sure it was okay so I could kiss the Ebay seller goodbye. It ran fine up to 150 Hz, but after that, it decided to slow back down on its own. The 2HP motors hadn’t done that.

I went to Practical Machinist, a forum I dread. These guys can be ruthless to hobbyists. But they know a lot. They told me I was lucky the motor hadn’t blown up. I think they were wrong about that, but they also said the motor would probably act as a frequency filter at high speeds, killing the speed. They said the torque would also drop off pretty badly.

I can’t have that. Daddy needs his torque.

No word on why the 2HP motors didn’t act crazy.

What do I do now? I can get over 3500 FPM as it is, no problem. I can also make a new 6″ wheel, which will get me to whatever 3/2 of 3500 FPM is. Okay, I’ll work it out, since it’s a two-second math problem: 5250 FPM.

If I go to a 6″ wheel, surely the force applied to the belt will be 2/3 of what it was (at a given RPM figure) at 4″. That’s just basic physics. On the other hand, it will be going 2/3 as fast at a give FPM figure, and if torque drops with speed, then there ought to be some compensation.

You can see why I need an engineer. They have little books with tables that answer questions like this. Physicists have to derive the answers using calculus and tensor analysis and God knows what else.

One reason I went to 3HP is that I wanted to start with something strong, so it would make up for any losses I had due to the VFD or overclocking or…whatever. I believe 2HP is the most common size motor for a 2×72, but people who use belt grinders seem unanimous: get 3HP if you can.

Now I have to make a decision: leave it alone, make a 6″ wheel (cost of metal: $33), or get a 3600-RPM motor.

I think I’ll just use what I have and see what happens.

The weird thing is, my second motor helped pay for my third one. The seller refunded my money, included shipping, and the Post Office paid his claim. Now I have a free 2HP motor. The Post Office isn’t coming to take it away. That means I can put it on Craigslist, where I can surely get $50 for it.

The big motor only cost about $130, including shipping. It’s a magnificent motor; the unit itself was only $65, which is ridiculous. If I choose to sell it, I can fully expect to recover almost all of what I paid.

Summing up, it doesn’t really matter what I do. In the end, changing motors might run me $100, net. By that I mean the most I can expect to put into a motor, after all the deductions and whatever are included, is about $100. I would probably make money by selling the motor I have and buying a new one.

If you want a 2×72 grinder, here is my advice:

1. Don’t build one unless you’re incredibly handy and have free metal. A nice grinder costs only slightly more than the parts required to make it.

2. Get a 3-HP TEFC or TENV motor.

3. Use a VFD.

4. Make sure the motor’s rated speed is 3600 RPM.

5. Don’t worry about getting an inverter-rated motor, because nobody else does. If the added cost is not much, sure, go ahead and get one, but in practice, you will probably never have a problem with a random 3-phase motor of good quality.

If you have an OSHA shop, I’m sure they don’t do what I did. I mounted the grinder on a board and put the board on a foam shop cart which has a 250-pound shelf capacity. I don’t plan to fasten it down. It’s heavy, and it’s not going anywhere. I could push it over if I really wanted to or I was just stupid, but those things don’t apply, so I don’t intend to worry about having a top-notch professional imbecile-proof installation.

Sadly, I have even more advice.

I looked into crowning, which is what they do to at least some of the pulleys on a typical belt grinder. This will sound crazy, but if you make a pulley bigger in the middle (with a “crown” like a crowned road), belts will try to move toward the crown, not off to the side. This helps them track well. If you have a belt grinder, and it’s not made with tremendous precision, you will want to have at least one crowned wheel.

There are people out there who do great business selling crowned aluminum wheels to knife makers. A lot of folks can make a non-crowned wheel, but making a crowned wheel is intimidating, so the appeal of having it done for you is obvious. I fell for it myself.

If you have a lathe, you can make a crowned wheel, fast. The guys who sell crowned wheels tend to use CNC to make wheels with a rounded profile (radiused from one end to the other). That’s not easy to do on a manual lathe. But you don’t have to do it. In fact, you can make two very shallow straight cuts on a wheel and take it straight to your grinder.

For a 2″ wide wheel, you want the radius (or maybe it’s the diameter; look it up) to be about 0.030″ less on the sides than it is in the middle. I haven’t checked, but I have read that you need about a 0.5-degree angle to do this. That’s simple; just use your compound slide.

If you want, you can pretty it up with a file so it looks round.

You do not want to cut all the way to the middle so the pulley has a pointy ridge in the center. It’s unnecessary, and it doesn’t work as well.

Here’s a jim-dandy link that will tell you what you need to know, while correcting whatever errors I made while trying to paraphrase it above: CLICK.

If you have a VFD, you will want to make sure flying crud can’t get in. You may want to build a box for it, or just move it away from the grinder.

I think that covers it.

Oh…also, don’t buy a grinder with a platen as your first tool. Get a contact wheel 8″ or 10″ in diameter. That’s what most people use most of the time when making knives.

I may be mistaken about some of this stuff, but it’s a lot better than what I thought I knew a week ago. You can’t trust knife makers when it comes to tools, because they don’t know much about them. They make wonderful knives, but that doesn’t make them tool experts.