What are you going to do with it?
First things first. Sounds simple enough, but some people may never use it for more than grinding paint. If that’s the case, your requirements won’t be the same as for someone that’s going to be beveling heavy wall stainless pipe for 12 hours a day, seven days a week. What I like to do is use different grinders, each set up for a different task. One with a cutoff/grinding wheel, one with a wire wheel, and one with a flap disc. I usually have one good grinder, and a couple of those cheap Chinese disposable grinders. I put the wire wheel and the flap disc on the junk Chinese grinders and put the grinding and cutoff wheels on my good grinder. This saves wear and tear on my good grinder, plus the junk grinders will work fine with the lighter loads of the wire wheel and flap disc. It also saves a lot of time switching wheels out.
What size Motor?
A grinder leads a tough life. It’s most always shrouded in smoke, dust, heat, and flying particles of red hot metal. Add to that the physical abuse most of them get, on a construction job site for instance, and it makes you wonder how they work at all.
The first thing to think about is that anytime that you can slow the wheel down when you’re using it, you’re pushing it past it’s maximum capacity. As you’re probably aware, it’s very easy to do this with most grinders. If you’re not sure how, put on a new grinding wheel and start pushing into the cut. You’ll notice right away that it’s very easy to get the grinder to bog down. Another way is to have an .040″ or .045″ cutoff wheel bind when you’re trying to cut something. Both of these are common occurrences, and can shorten the life of a grinder if done often.
A good analogy is that of an engine on a dynamometer. Maximum power happens at the point immediately before the engine can no longer keep up with the load. Once you start to slow it down, you’re past the maximum power. For grinders, this causes two problems – a higher current draw and lower cooling air flowing through the grinder. Two of the worst things you can do to any electric motor. Because of this, i always recommend buying a grinder that’s more than what you need. By oversizing, you’re effectively easing the burden on an already overworked tool, and in turn extending the service life you’ll get from it.
What size wheel?
I started using grinders way back in the early 80’s when Makita came out with what was then considered a somewhat revolutionary grinder. It was a 4″ grinder, and compared to a big, heavy, old black and decker 9″ grinders for instance, it was great. You could use it with one hand, and it was small enough so that you could get it in tight places. Another benefit was that it was extremely light and it didn’t wear you down after an hour of use. The main drawback, other than the first generation small diameter grinding and cutting wheels that had a tendency to explode, was that it had a 4″ wheel. You could literally burn up a wheel in seconds if you weren’t careful, or if you were too aggressive with the grinder.
Today they’re available in many different sizes; 4.5″, 5″, and 6″ (100mm,125mm, and 150mm in metric) being the common sizes. My personal favorite is the 6″. The first reason is that a 6″ wheel will last much longer than any of the smaller sizes. The jump from 5″ to 6″ makes a big difference in the life of the wheel. Also, for a given rpm, the larger wheel size will have a much faster rim velocity. This means it will cut or grind faster. Have you ever noticed that once your wheel starts to wear down it takes longer to cut or grind something? That’s because as the wheel wears down, the diameter of the wheel decreases, causing the rim speed and cutting/grinding speed to decrease. For these reasons, i suggest a 6″ or 150mm grinder. One thing to be aware of though is that in going with a larger wheel, you’ll need a grinder that can produce more torque. This translates into a higher powered grinder which also means a more expensive grinder. In grinders, like most everything else, you get what you pay for.
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