Surface Grinder

[gbowne1]

My next project, not even started yet.. is to build a surface grinder that will use 8" x 1" x 1" wheels. I have seen a lot of these wheels.. and wondered how practical one would be if it's max speed was 2,865 RPM.

Motors usually turn at ~1700 RPM. There's ~1115 RPM difference. I would like at least 3 different ranges of speed. So that being said thats ~371 RPM per step.

Is 2,865 a good RPM # for mild steel, or for surface grinding in general?

What else should I have in my design.. I'm going to use a design quite similar to the one in HSM. Should I build a CNC suface grinder? Toolpost grinder?

Greg

[NC Cams]

Before you design your machine, you probably should see what sort of speeds that the proposed wheel sizes will tolerate. Also keep in mind that grit size will affect the max speed potential.

We ran into a case where a combo grit of two coarse grits commonly used for snagging could not past burst speed test with the needed margin. THis was even though a single grit version of either coarse grit did.

Granted, these were 18" wheels that we had the issue with but you're looking at higher speeds. Either way, it is the SFM of the wheel OD that needs to be considered.

Talk to your wheel suppliers - see what they say. BTW as you start going higher or lower in speed makes the wheel grind "softer" or "harder" than it really is. Ok to do if you're expecting/needing it, but a real surprise if you're not. Please have your wheel supplier explain "hard" or "soft". I understand it but can't explain it.

[gbowne1]

Alrighty, thanks so much for the input.

That 2,800 RPM I stated was on the side of the wheel I wanted to use. I also had heard that max speed depends on the bonding of the wheel, and which type it uses. Most of the ones I have seen are V for Vitrified bond. The others are Pink, White, Gray, Green or Grayish green.

I thought this particular one would be a sharpening stone for use on something like a pedestal grinder. It's made by Forney, and says "Stationary Bench Grinding Wheel".

My supplier wanted to know what machine I have?

Greg

[NC Cams]

"My supplier wanted to know what machine I have?"

Can you even wish to hazard a guess WHY????

Wheels tend to blow up when they get over spun. Grit, fire, bond, abrasive type (green, pink, etc) ALL interactiely gome together to affect the bond and burst strenth of a wheel.

If a wheel gets mis-mounted on some wierd spindle, the wheel supplier is the FIRST guy who gets his ass dragged into court because "HIS" wheel exploded. Forget the fact that it was misapplied to some un-engineered, speeded up grinder spindle.

If your supplier is smart, he'll ask MANY questions and he won't sell you ANYTHING if he thinks even a little bit that you're SWAG'ing something together. You're playing with fire. BE REAL CAREFUL.

I had a wheel blow up on me due to carelessness. Luckily I dodged the shrapnel. Don't put yourself in the path of shrapnel due to stupid SWAG's. You've been warned.

[Walt@SGS.Inc]

Someone told me 4000 surface feet per min. 5280 feet per min would be a mile a minute. I have heard people refer to grinding at a mile a minute. Somewhere
between 4000 and 5000 feet per minute you should find something that works.
You will have to start with the circumference of your grinding wheel and then determine what rpm you will want to run.
Someone mentioned, talk to the grinding wheel salesman. Try to be honest with
the salesman but don't tell him ev ery thing if you are "inventing".
Regards Walt.

[gbowne1]

Well, I don't want to have the thing fly apart on me while grinding away.

I still want to be able to adhere to ANSI B7.1 too. I belive thats the right one. ANSI B71 is for abrasive cutoff wheels.

I mostly work with 4140 and 4340 steels.. so I suggested that to my supplier and he said he'd look up a few 8" x 1" wheels to see what they typically ran at.

My supplier is also one of my best friends, Richard Brownfield. A really cool guy, and he knows about a lot of my projects. I met him because I'd started ordering a lot of different cutting tools to supply my own shop while I had some money to play with.

I measured the OD of the 8" wheel, and it's 25-1/2" (25.500"). If the stated max RPM on the wheel is 2,865.. then what? I also heard there is an acceptable margin of safety on wheel to spindle RPM. It's somewhat less than the stated rpm, If I am right.

My first inclination, after having witnessed accidents myself, is to use the strongest wheels that are safe. I saw once an improperly adjusted spindle go over RPM while grinding a piece of CRS, and blow small chunks of the wheel all over the place. This was at a high school shop. The wheel was a white 8" similar to the one I have in my hands, although narrower, monted on a pedestal grinder.

I'm still assuming I should be using a wheel meant for surface grinding, instead of a pedestal grinder wheel. Seems like a duh to me, this is just what I have on hand.

Greg

[NC Cams]

Every material wants to be cut a certain way. Depending on the grit, "fire" of the bond, binder, dress etc, you need to run a wheel at a certain SFM (surface footage per minute) to achieve the desired cutting conditions.

To calc operating SFM take OD of wheel in inches, multiply by Pi, then by RPM and finally divide by 12.

HOWEVER, the size of the wheel has to be considered too. Large wheels can easily reach high surface footage at much lower speeds and vice versal for small ones.

The burst speed comes into play as you try to achieve higher and higher SFM's for ANY wheel size.

You may want a certain SFM BUT the wheel may not be able to keep itself together at that SFM - it is literally trying to tear itself apart.

Or if the wheel is porous and coolant is used while grinding, the wheel could be heavy in 1 spot and may try to tear itself appart due to imbalance but that could be at ANY speed.

Think about it.... The TEST burst speed should ALWAYS be higher than the actual max speed of the spindle could achieve. This way, even if the speed control went max on you, the spindle could never outrev the burst RPM of the wheel. You'd hardly want the thing to even possibly fly appart below the speed you expect it to now would you???

[gbowne1]

Ok.. let's see..

Wheel OD = 12.5"
Pi = 3.14159
Answer to this part: 39.269875

Ok.. multiply 39.269875 x 2,865

Answer: 112,508.191875

Ok divide this # by 12..

Answer: 9,375.68265625

Oh yeah, provisions for coolant, which I WILL use.. will be made. Heard good and bad things about using grinding coolant.

I tend to like Norton abrasives myself.. hear a lot of things about them. Pink, Gray and White I have seen a lot of people have them in their shops around here. (I did a tour of a handful who had grinders.)

Most of the materials I grind will be 4140, 4340 and the like. Probably no or very little aluminum.. probably some cast iron though. I have seen a wide variety of SFM's listed for that material but not nessisarily for this operation. Narrow it down some for me if you could.

Greg

[NC Cams]

We grind tool steel and hard 4150. Our SFM is about 8250. We vary the infeed speed of the work to help adjust cutting/finish results.

Sorry, can't and won't give anybody wheel specs - took too much time and money to figure them out. I have at least $2K worth of "these will work for you" wheels that are nothing but useless oversized industrial cookies that now hold down shelving.

Talk to your grinding wheel supplier and see what he recomments for wheels, speeds and grits. They all vary from wheel to wheel and material to material. Sometimes you have to try different stuff. Some guys might allow you to test a wheel. But if it ain't available off the shelf, you'll end up buying a fist full as that's what the min buy is. This is when it gets expensive.

Ordering grinding wheels IS NOT LIKE CALLING IN FOR A PIZZA.
A good grinding wheel engineer is good to become friends with if you start to do industrial grinding.

He's far more reliable than ANY sage wisdom you'll get off a message board - even mine. He's especially good if he's been around a bit. The guys at Radiac in Illinois (?) have been good for us. Local distributors were clueless for our needs....

Even so, you can get into wierd situations. We had a factory guy in here recently who supports our wheel supplier. He walked out shaking his head, stumped. IT gets scarey when you hear the "...never saw that before..." comment.

Material: powdered metal tool steel cams. You could cut it EASILY. You just couldn't get a finish that didn't have minute record grooves in it. We finally prevailed.

Got help from a friend who called someone at GM who told another cam vendor to answer our questions if/when we called. Worked out well for us as he used 24" wheels down to about 18". We need 18" wheels. We get his used ones at about half price versus new - he uses his half and we use the other and we help each other out. Interestingly, the factory grinding engineers were stumped on this application.

SInce the wheel all depends on grit, structure, material hardness it is hard to give ball park specs. Each material cuts differently too.

Your question is like asking, "how much is a tire???". To which you start the "what kind of tire?" exchange of data to home in on what you want.

Finally, grinding is a true science. However, sometimes it is an art... Like riding a bike. You can read all the books but you'll never learn until you fall off a few times. Grinding is NO different.

[gbowne1]

Alrighty..

Well, thanks for your comments so far.

Greg

[NC Cams]

My attitude in post #9 may have seemed pretty shotty but it wasn't a good day.

The art of grinding is something that you can't just get out of a book. You won't just pick up a wheel that you have and start removing metal the way you want to. Different materials want different wheels. That's where the science comes in.

The same material at a differenct hardness and/or temper may want a different wheel "hardness". Ask your wheel supplier to explain hardness.

Regarding science vs art: my operator can take a 46 grit roughing wheel and dress it in a way that he can match the finish of an 80 grit finising wheel. The science says it can't be done. Once you learn the process, you'd be amazed at what you can do that you shouldn't be able to do.

The point is that grinding techniques very. What we do on our cam grinder differs from what another shop with a similar machine may do because of our coolant type, our removal rates and/or our finish requirements.

We've had cases where the same wheel part number cuts entirely different from one batch than another. The supplier says they made a minor change "that shouldn't make a difference". To which I told them to send there sorry a$$ mfg engineer down here and show me how to get these lame a$$ new wheels to perform like the old ones did .

They took the wheels back and remade them the old way - we still waited 12 weeks and were out the scrap parts... THe good part is that they did send a rep here and when he saw what we do/did, the report back to the office seemed to make a difference. We get much better service and the wheels seem to be a bit more consistant. We're now offered to "sample the wheel" (test it and if it works pay, if not, send it back with no hard feelings)..

Your wheel supplier should be able to get you going with reasonable first guess offerings. You may find something you like or it may be terrible. You have to be able to see and/or explain whats good or bad about what's happening. "It sucks" doesn't cut it as a performance describer.

If you can get some shelf stock wheels, you can see good, bad or indifference in your wheel selection. YOu should then talk to a grinding engineer and they should be able to dial in something that works better for you.

Using any old wheel that you have that fits on on a grinder and expecting it to work perfectly is like walking up to a meat case blindfolded and expecting to pick out prime rib steaks. It may happen but don't count on it.... You may end up with chicken livers.

[gbowne1]

Well, it's ok NC, I understood what you were tryin' to get across.

I sent a fax this morning to my supplier.

Greg