Versatile bench grinders and grindstones

[kerisman]

I recently picked up a used 8" bench grinder of some obscure Korean brand that is desperately in need of new grindstones. Right now, it does not work well, but I expect that it will be fine after I replace the grindstones. I want to make sure it's worth doing that as opposed to just buying a new grinder.

For general use with metal, is an 8" grinder operating at around 3600 RPM fine?

Here's what I use the grinder for:
* mild and high carbon steel
* stainless
* copper and brass (cleaning up rounded edges of sheet/plate and de-burring)

What grit stones should be used? Any suggested sources for grindstones? Local hardware stores just have angle grinder discs.

[bigfootnampa]

Most (nearly ALL) inexpensive grinders come with practically worthless stones. Woodcraft sells good stones. I mostly use grinders for coarse shaping so about a 60 to 80 grit is good for me. I do finer sharpening and even final shaping with a belt sander. I like the white stones that Woodcraft sells. I can really get along without a bench grinder now as I can do most coarse shaping with a flap sander in a 4 1/2" grinder and then finer work with a belt sander and then I go to buffing wheels and compound. I wouldn't buy a bench grinder today except for use as a buffer... but I have a couple from earlier days so I use them.

[southernforge]

I picked up a course grinding wheel last fall at the local big box store and it works fine.

[Spears]

Pretty much the exact same story as bigfootnampa. The flap disk on a 4.5 is really hard to beat even for mower blades. It is unfortunate that things have gotten so global that they sell substandard wheels on working motors. You will probably have the best luck with an industrial supply store like MSC, J&L, Enco, Grainger, etc etc. Personally, Grainger is even sometimes a bit to global for me. Just remember to use a separate wheel for ferrous and non-ferrous. Copper can gum up a carbon steel grinding wheel pretty well. If I had to do it myself I would bite the bullet and order expensive wheels from McMaster Carr Supply. You'll pay a bit more but a cheap grinder can go a good distance with a better wheel. Good luck! Spears.

[kerisman]

Thanks for the responses. I like angle grinders, but I feel much more in control when doing finer work using a bench grinder. Maybe that is because a workpiece can be more accurately manipulated than the angle grinder itself. Are there stands for angle grinders? If so, the same advantage could apply with them -- you could hold the workpiece against a tool rest in front of one side of the spinning disc. Sounds dangerous, though. Maybe a special guard could solve that problem.

I found a couple of great bench grinder/wheel references:

• Popular Mechanics, 1990: Bench Grinder Basics
• Discussion of wheels, abrasive and bond materials, brands, and sources
  
  

The discussion of abrasive types is valuable -- I doubt it is common knowledge. Aluminum oxide is best for most steel grinding, while silicon carbide should be used for brass, bronze, and aluminum.

Other considerations are hardness grade (wheels are rated A-Z), grain size (grit), and grain density.

Dressing/truing an old wheel is an option. "Truing" reshapes the wheel while "dressing" cleans up the grinding surface. Star wheels can be used for truing (I believe they can also be used for dressing) and silicon carbide sticks can be used for dressing.

I'm planning to get a fairly rough (maybe 60 grit) aluminum oxide wheel for common steel work (sharpening, cleaning rough cuts, etc.) and a medium (maybe 100 grit) silicon carbide wheel for copper (and maybe bronze/brass/stainless). Anything those wheels can't do would be done using angle grinders and files.

RPM and wheel diameter doesn't seem that important. It's obviously going to affect how long a given task takes to complete and technique may need adjustment based on RPM, but the job will get done either way.

Spears, good point on ferrous vs. non-ferrous. I understand that stainless should not be used on discs that have been used to grind other steels. Is it okay to use the same wheel on stainless, copper, and brass or should each metal have its own wheel? Or maybe you could get away with using the same wheel as long as it's only for light grinding?

[John McPherson]

If you use any grinding wheel or wire brush for carbon steel, and then use it on stainless, the stainless will rust when it gets wet. When used to clean off slag between welding passes, it can 'poison' the weld. Just grinding other metals around stainless will deposit tiny particles on the surface.

Good welders will have a separate wheel to sharpen tungstens on, that never touches any other metal.

Commercial welding shops that work in different metals learn to keep them separate, and color code disks and wheels for each area. Disks are cheap, rework is expensive.

Stainless, nickel and copper also tend to be "gummy", and load up hard wheels.

Make sure the wheel you use is marked and rated for the RPM speed of the machine. There are still slow speed grinders made where not overheating the metal is more important than production. And there were hand crank grinders made in that size, stones may still be available.

One of the more common errors is buying a 12,000 RPM 4 1/2" angle grinder, taking off the guard, and mounting a 7" 6,000 RPM rated grinding disk or wire wheel. A Darwin award candidate, fo' sho'.

[Old N Rusty]

Of all the tools and equipment in the shop, the bench grinder causes the most injuries and accidents by far. Things are as dangerous as a monkey with a machine gun. I prefer to use my hand held grinders with flap wheels,or disks,either free in hand, or clamped in vise. (I always buy the grinders that can be locked on )it can be changed to horizontal or vertical easily and allows more access to the wheel.A bench grinder with an un-guarded wire wheel scares me even when it is not running.

[Dodge]

RPM and wheel diameter doesn't seem that important. It's obviously going to affect how long a given task takes to complete and technique may need adjustment based on RPM, but the job will get done either way.

I beg to differ. As the wheel/stone wears down, the surface speed decreases as it gets closer to the axis. As the abrasive slows down it wears faster than when its turning at a faster rate. This is not to be confused with RPM because that remains constant. However, each abrasive has an optimum speed. Too fast, and it heats up and just glazes over the work. Too slow and it wears prematurely and, again, has less effect on the work. Of course, many other factors come into play; pressure on the abrasive/work, temperature of the abrasive or material being worked, etc.
E.G. Where I worked manufacturing traffic and street lighting standards, we typically torch cut access holes and ground the edges smooth with a 2" dia x 4 grinding stone. As the stone wore down, there became a certain diameter where the stone wore faster than the material being ground. This was evident by the amount of dust being generated rather than actual metal filings. By the same token, we had one grinder that ran at a faster rpm than normal (probably designed for a different type of abrasive) and just didn't perform as well as the other grinders that turned at a slower speed.
E.G. My home built belt grinder was wearing 60 grit belts out faster than I thought it should and just not cutting very well. I found out the the surface feet per minute was too slow. Sped it up a bit and belts started to last longer and cut better.

[Dave Hammer]

I used bench grinders for 45 years..... Then I discovered belt grinders, which took me to a whole new level with metal grinding. Personally, I never use bench grinders anymore for anything other than polishing (and for that I have a variable speed DC unit). I even sharpen my drill bits with a belt grinder.

If you have never used a belt grinder, find a friend who has one, try it, and I doubt you will spend any more on a bench grinder....

[BIGGUNDOCTOR]

You said;
RPM and wheel diameter doesn't seem that important. It's obviously going to affect how long a given task takes to complete and technique may need adjustment based on RPM, but the job will get done either way.

If the wheel is not rated for the RPM of the grinder it can come apart on you. No Bueno. Every wheel should also have a ring test done on it to see if it is cracked before mounting. A rapid non planned disassembly of the wheel when it is in use can be disastrous to you well being. My Dad had a coworker that had a wheel come apart on him. They found one third stuck in a roof beam , one third just disappeared , and the final third was buried into his elbow. After mounting a new wheel turn it on while standing at the side, and let it run for a minute before hitting it with a dressing tool. Also bench grinder wheels are not designed to have their sides used as working surfaces, only the periphery.


As to the green silicon carbide wheels, they wear extremely fast due to their soft bond. When I say extremely fast, I mean extremely fast. For non ferrous metals I use a disc sander at work.

Too fine of a grit, and too hard of a bond will lead to overheating of the part. I have to play with these aspects when surface grinding parts and pieces at work. You have to let the wheel do the work. Speeding it up past its rated speed, and pushing harder is not beneficial. In fact it can be very dangerous.

For a supplier a big box may carry good brands like Norton, Camel, 3M, etc. We use MSC, at work. McMaster carries the same brands as MSC, but usually at a higher cost. Grinding wheels are one of those items that are constantly having a special run on them.

Belt sanders are nice, but good belts can get expensive. We had a Burr King at Jelly Belly, as well as a 20" disc sander. For hogging material down that 20" disc sander could peel some material off FAST, and one guy also found out how long it takes to remove a whole thumbnail Answer: less than 1 second.

I picked up a used JET 2hp 20" disc sander after using the one at JB. Mine also has a reversing switch.

[VaughnT]

Gotta agree with Dave on this one. I've always had a small bench grinder and it's just gathering dust because it's just too non-usable for so many things. The motor would be great for some buffing wheels, but as a grinder they are less than optimum if your doing anything more than putting a rough edge on a lawnmower blade!

Now, since I built my 2x72 "knife-maker's" grinder, things are altogether different! That thing smokes on a dozen different levels and I wonder how I ever got along without it. From a course 36 grit belt to a super fine 1200 grit belt in simple easy steps. Plenty of access to the grinding area and more power than I could ask for. I can't bog it down.

Lots more expensive than a cheap-o bench grinder, but infinitely more useful in a shop, whether you be a professional or inveterate tinkerer like me.

[kerisman]

Regarding safety, many of you pointed out the danger of bench grinders (opinions on this site and others seem to suggest that they are regarded as the most dangerous tool in the shop). Some of you prefer alternatives (angle grinders, belt grinders). Are these alternatives really any safer?

Also, what about variable speed polishers? How about using them for the soft "gummy" metals such as copper? Can regular angle grinder discs be used with them safely?

[Old N Rusty]

Well sir safety is very important and cannot be overstressed, what we do is dangerous, no doubt, but with some things a risk is built in. A bench grinder wheel CAN shatter and hurt you and all around you a side grinder with a properly rated wheel is less likely to do that and wire wheels on bench grinders with no guards, just plain scare me. A belt sander can hurt you also, and can messup your nail polish in a hurry .

[BIGGUNDOCTOR]

I don't consider a bench grinder to be a particularly dangerous shop tool, but as with any tool you have to give it some respect as to what could possibly happen.

A friend had a coworker using a belt sander pop the tip off of his finger at the first knuckle when it grabbed the part he was deburring, and pinched it between the part, and the table. It happened so fast he didn't even realize what had happened.

I have a small scar left from a 9" angle grinder that grabbed, and kicked back against my chest. The wheel went through a new flannel shirt, my Tee shirt, and gave me a road rash about 1/4" x 3" in less time than it took to read this sentence.

Tools have the potential to cause great harm, as well as death if used improperly. I use bench grinders all the time at work, and find them to be useful tools for what I do.

Not everyone can afford to buy every tool that we can use, so they have to get by with what they can afford. Would I discourage someone from buying a bench grinder if that is all that they can afford? Heck no. What I would say is to make sure that the proper type wheels are installed, that they are trued up, and that the tools rests are adjusted properly. If the tool rests are flimsy, they can be beefed up easy enough. Would I like to have a belt grinder? Sure, they do make some operations easier to do, but I can get by with what I have at this time.

He has the grinder, so lets help with getting it running correctly for him, instead of telling him that there are better choices that he may not be able to afford.

We have discussed wheels, but tools rests need to be adjusted for proper usage. The rest should be as close as possible to the wheel without rubbing. Along with that, depending on the adjustments available, it should also be set at, or just below center of the wheel. I have found for back rakes on tool bits it is easy enough to just hold the bit up at the desired angle insted of tilting the rest. Just remember to let the wheel do the work, and when you start to feel the piece heat up up cool off it in some water. With rotating equipment do not wear gloves, as they can get caught in the wheel, and drag your hand in with it. If your fingers are getting hot, cool the part.

[Old N Rusty]

Very sensible advise, Bigun. I dont want to discourage anyone from using what he has available, I can only speak from MY own experiences , and hope no one sheds a drop of prescious bodily fluids due to an easily preventable accident. There was no such a forum as this when I started working metal, we have a golden opportunity to breathe life back into a craft thay WAS dying. At the risk of sounding like "chicken little", I will continue to stress safety. Now, defend the practice of removing the guards on a bench grinder and fitting it with a wire wheel!

[kerisman]

I appreciate the wisdom on both the subjects of expensive alternatives, namely the belt grinder, because I can now add that to my "maybe someday" list. Safety advice is always appreciated as well and never off-topic.

Here's what I'm planning now (again, this is for use on general steel work, as well as some stainless and copper sheet)...
- 4.5" angle grinders with the following attachments: 60 grit disc, wire cup brush, 34 grit disc, 3" cut-off wheel, flap disc (I'll buy a few extra cheap grinders to minimize time spent swapping discs)
- something for cleaning edges of sheet and thin plate: either my current bench grinder (8", 3500 RPM) with a 120 grit wheel or a very simple belt sander (like this: http://www.harborfreight.com/1-inch-x-30-inch-belt-sander-2485.html)

I'll mostly avoid using the bench grinder, favoring the angle grinders. Eventually, it may be retired if it sits idle for a month or two.

The trickiest part for my setup is finding a way to handle fine deburring of copper sheet edges. I think a 120 grit wheel on the bench grinder or that little belt sander would be both effective and convenient. The belt sander would be safer and may even be cheaper (given the prices of 8" grindstones).

[wshelley]

I use a Scotch-Brite wheel in my grinder all the time to debur the edges of parts, ferrous and not alike. Try something like this deburring wheel

Ward

[SGensh]

Kerisman, You may want to take a look at the deburring tools offered by companies like Shaviv and Noga before you decide that the grinder is the best way of deburring your copper. The MSC catalog should show both as well as others. There are lots of unpowered hand tools that work very well and very fast for that purpose, sheet metal companies use them all the time. I've been using the same two Shaviv tools (replaceable blades) for many years myself for hole deburring. I also use a very inexpensive little 1" by 42" belt sander for a lot of edge cleanup on steel sheet metal and cut off parts. Mine happens to be an old Grainger unit I bought about thirty five years ago which is on its second second hand motor. These things are nowhere near as sophisticated as my Bader or some of the other sanders available but they are everywhere in shops and used ones are easily found in flea markets or used tool sales if you don't want to spring the few bucks for a new one. Belts (particularly in this popular size) are far cheaper than good wheels and a snap to change on most machines. Good Luck on the wheel search, Steve G

[BIGGUNDOCTOR]

Deburring copper sheet? Grab a deburring tool as noted above, or a file. Doing this on a power tool will probably give poor results, as it is very easy to put an angle, instead of breaking the edge, or just create bigger burs. At work I use my Noga tool, or a file.

As to the wire wheel and no guard. I have used them that way in the gunsmith shop I worked at. At times I was using every edge of that wheel, top,and bottom to get the rust off of a receiver, or barrel. My main concern with a wire wheel is that they throw wires, so I always have safety glasses on, and never wear gloves. Even with guards on I still end up with them stuck in my shirt at work. I also tend to stand off to one side as I wheel something. My preference is no guard, so if something does grab it is tossed away, and isn't coming around through the guard back at me, as I have had happen. Kinda sucks though when that wheel flings a small part like a Winchester 97 extractor, and all you hear is it hitting somewhere in the shop. This is MY preference, as I feel that I have done this long enough to feel comfortable doing so. Even with that said I know what that tool is capable of, and give it the respect it deserves, as well as always wearing my safety gear.

[kerisman]

Thanks for making me aware of deburring tools. Safe, simple, and portable. I think I'll try substituting them for the fine grindstones in my tool list (the list is getting simpler -- angle grinders, belt sander, and deburring tools). Now the problem is deciding which deburring tool(s) to get -- MSC has hundreds!

[ThomasPowers]

We had a bad set of belts come though the shop once and had many 220's pop the seam and thwap us. Worst danger seemed to be the shock/flinch. Don't think a shattered wheel throwing shrapnel would be as "kind".

ANY type of grinder will damage you if you feed yourself into it