Angle grinder efficiency question

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All I've used up to now is a Dewalt 4 1/2 inch, maybe 1.5 hp angle grinder. Fairly cheap one that cost under a hundred. I'm buying a second one, a Dewalt 7" grinder that puts out 4 hp, 3K watts. I wanted one bigger so I could take more metal off faster. How much faster are we talking here? Can I expect a big improvement for the money? Safety is another issue. These things can grind off a lot of meat in a real hurry.
Reply:its all about wheel surface speed with a grinder. Figure out how fast the wheel is moving across the piece at the point of contact and you get some idea of how much faster it will be, keep in mind that more power will let you push harder on the grinder and keep speed up which will help remove material a little faster.Assuming you use the outer rim of the wheel and both are 10krpm grinders. the 4 1/2 will move 141,300ipm of wheel past the metal, the 7" will move 219800imp of wheel surface, my math make it about 65% faster which should give you some idea of what your dealing with. You will need to know the speeds of the actual grinders but the math is pretty simple, its just circumference of the wheel X RPM of the grinder
Reply:Interesting by the numbers response. I like that.Miller Bobcat 250Syncrowave  200Millermatic 252Spoolmatic 30ASpectrum 625 X-TremeMaxstar 150SVictor O/A torchesDewalt angle grindersWelded on/off for over 40 years.
Reply:my point of view, I have a 7-9" grinder Dewalt wild cat type, and a few 4" to 6" Dewalt grinders, yes the big grinder is great if your  working at wast height and flat, and have a lot to remove, you can lay in to it and grid away.but I end up grabbing the smaller grinders 95% of the time, for the things I do, but I seldom have a 8 foot long seam or weld to grind down,  it may be the thing I do or it may just be the convince of th smaller grinder and ease of handling of it, when I started out I had a sears 7" grinder for everything, but IF I had to have one grinder I would go with the medium (Dewalt has a 6") just a touch larger frame than the 4.5" grinder and a little bit more powerful, but normally I use the smaller wheels on it but can put larger if wanted, (NO I WILL NOT SELL OFF MY WILD CAT) for the times I need it it is great, but it is not my first choice, normally.
Reply:I really have to agree with the smaller grinder idea.My largest grinder is the Dewalt 6" http://www.dewalt.com/tools/metalwor...s-d28144n.aspx  This is a good lightweight grinder with lottsa power, and a PADDLE SWITCH.  You'll appreciate a paddle switdh the first time you drop a grinder and it doesn't shut off  I had one with a cup brush mounted on it buck out of my hands, and the non-paddle switch kept it running until it took a sizeable chunk outta my knee before I could yank the cord to stop it.My 4 1/2" grinder is also Dewalt http://www.dewalt.com/tools/metalwor...s-d28114n.aspx  I like these for pure ease of use.  AND you can buy gear kits to rebuild the head for around 20 bucks at a repair facility.Pair any grinder with the Sait 24N grinding wheel, and you have a real metal eating monster.  I GUARANTEE IT  [ame]http://www.amazon.com/gp/product/B000FEHKH2/ref=olp_product_details?ie=UTF8&me=&seller=[/ame]    With these wheels you don't have to lean into the grinder to make it work.  Leaning into grinders is the worst thing you can do.  Number 1.......it eats up the gears  Number 2..........It causes the grinding wheel to form a glaze on the leading edge which makes it not grind worth crap, and you have to lean into it harder to do the same job.  I switched to Sait because the weight of the grinder will do the cutting without any real force applied to the grinder.  CAVEAT   (There's always a caveat )  These are very soft wheels designed for extremely rapid material removal, and they don't last as long.  But if you can remove a lot of material in a short time it's more economical to use a softer wheel..........faster and easier on the grinder.Did I say I hate grinding??  Well I do   Anything making the chore easier gets my vote and my bucks."Any day above ground is a good day"http://www.farmersamm.com/
Reply:If the small grinder will remove X amount of metal running balls out at 1.5hp, then a 4hp grinder will essentially remove 2.67 more running at balls out. If you put more power into the workpiece, more work is being done.Welding/Fab Pics: www.UtahWeld.com
Reply:Originally Posted by ironmangqits all about wheel surface speed with a grinder. . . Assuming you use the outer rim of the wheel and both are 10krpm grinders. . .
Reply:Now we're talking feed rates.Fpm varies according to cutter diameter.  Smaller can turn faster, larger has to turn slower.Feed rates directly relate to power and cutter toughness.Power is required for higher feed rates, and the bigger grinder does have more power.Cutter toughness when grinding is a more complex problem.  A hard grinding wheel with fine grit will last longer, but cut slower.  A soft wheel with larger grit will cut faster, but wear out faster.To be honest, I really feel that a well wheeled (can't believe I said that ) will remove just about as much material as a large grinder with a crappy wheel, or a glazed wheel due to some moron leaning into it with all his 350lbs But put a good aggressive wheel on the jumbo grinder, and it performs well.  And it appeals to the cave man in all of us   Wouldn't wanna have your psyche messed up by grinder envy   Bigger can be better............unless ya gotta hold the stupid thing over your head all day long "Any day above ground is a good day"http://www.farmersamm.com/
Reply:Geez...........I forgot about work hardening.  Another can of worms"Any day above ground is a good day"http://www.farmersamm.com/
Reply:Amazing what I go on blabbering about just because I don't want to go to work.  It's gonna be another hot one today"Any day above ground is a good day"http://www.farmersamm.com/
Reply:FedEx just delivered my larger angle grinder. This thing is a monster. I will have a use for it. Like grinding dinosaur toenails and things like that. Lol. Buying things on the Internet can be deceiving, picture made it look smaller. I still have my 4 & 1/2 incher, but this one will be useful too.Miller Bobcat 250Syncrowave  200Millermatic 252Spoolmatic 30ASpectrum 625 X-TremeMaxstar 150SVictor O/A torchesDewalt angle grindersWelded on/off for over 40 years.
Reply:I have a couple old big 7/9'' Milwaukees, 6,000 RPM. They will remove metal ,rust , and skin,rapidly.Great for truing up a torch cut. Heavy, but sometimes needed . A plus is that I bought  NEW 7/9"  discs at the flea market for $1.00.
Reply:5" grinders are for cleaning up, small jobs and awkward positions9" grinders are for grinding. They are made for it, the smaller ones will not last if used for hard grinding, and you will exert more energy using the small grinder rather then letting the big one do the work for you.
Reply:I really like 7" grinders, they get stuff done fast and usually aren't too big and heavy. I don't like 9" grinders however, because when they come out it's usually a sign that the job's not going to be fun lolWelding/Fab Pics: www.UtahWeld.com
Reply:Your abrasives also make a big difference. Cheap wheels usually don't grind anywhere near as well as top quality abrasives of the same grit in the same tool.I have 2 7" grinders, one with a large flap disk and one with a wheel. They are used for large surface or heavy stock removal. The small grinders have a wide variety of sanding disks, slitting wheels, wire brushes etc on them. The right tool for the right job..No government ever voluntarily reduces itself in size. Government programs, once launched, never disappear. Actually, a government bureau is the nearest thing to eternal life we'll ever see on this earth! Ronald Reagan