Quick question:
I need to cut a padlock off a hasp. I went to the store but I can never remember if I have a 4" or 4.5" grinder. I bought the 4" metal cutting wheel, but it turns out I have a 4.5" grinder.
As long as the arbor is the correct size and the wheel is rated for a greater number of RPMs than the grinder can spin, does it matter?
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I can’t imagine it would. Usually I end up replacing them because they’ve worn down and they’re too small (and the guard is getting in the way). I’ve never actually measured one, but I’m quite sure well more than a half inch is worn off of them at that point. I’d guess that when I was cutting pavers, my 4.5 inch wheels were probably closer to 3.5-3 inches when I was tossing them.
I end up doing the same thing and I take the wheels very far down, just keep using them as long as I can reach anything with them. In addition I put wire brushes on with narrower diameter. I’ve never heard anywhere of a minimum size for a wheel on a grinder.
Never under any circumstances use the wrong discs on an angle grinder
It’s not just the speed of the disc that matters it’s also the amount of torque on the disc, the motor of a 4.5 inch grinder would be more powerful than that of a 4 inch, should the disc grab momentarily it can be subject to more force than it’s designed for and fail
Given the speed at which the disc rotates any failure is likely to be extremely dangerous
It is definitely not worth the risk just to save another trip to the hardware store
Po folks have Po ways.
I’m gettin kinda old ( 71 & ¾ ) & don’t do seat belts, helmets or stop using tools because some OSHA fool put a line on it.
I always been responsible for me, if I break me I don’t go cause anyone else’s insurance to go up. Want my organs before I die, you got to kill me first. Bawahahahah ( I really hope I use them up… So far so good. )
The cutting wheels are meant to gradually erode away as they are used, anyway, so I can’t see there would be a problem as long as the disk fits firmly on the spindle. If you want more specific advice, try naming the brand of the grinder and the brand of the disk.
Do you have a cite on that or did you just make it up?
Makita 4.5 inch Grinder 10 amps, 10,500 RPM
Makita 4.0 inch Grinder 6 amps, 11,000 RPM
So, the smaller one draws less amps, but spins faster. Make of that what you will. Now, lets cross over to another brand
B&D 4.5 inch Grinder 11,000 RPM, 5.5 amps
That spins faster than the 4.5 Makita and uses less than both of them.
Skil 4.5 inch Grinder 11,000 RPM, 6 amps
More power used than any of the other models I listed and spins at about the same rate as all the others.
Here’s an 8.6a/11,000RPM 4inch Grinder from Craftsman.
And here’s the first listing on Amazon for 4 inch wheels I could find that had and RPM listing for it and it’s for 11,000 which seems to be about the norm, at least for 4 and 4.5 inch grinders. Also, as has been said above, as you use the wheels they get smaller anyways.
So, like I said above, why do you think a 4.5 inch grinder uses more power. Hell, I’d guess that some brands even use the same motor in both sizes and just slap a different guard on it.
It should also be mentioned that this thread is about 4 and 4.5 inch grinders. We’re not talking about bigger ones which I see listed at amp ratings going up to 15a (but RPMs in the 6k range).
Sorry, missed the edit window for one last example:
Here’s an 8.6a/11,000RPM 4inch Grinder from Craftsman.
And here’s a 4.3 inch/11,000 RPM grinder from DrillMaster.
Basically, saying that a bigger (4.5 vs 4.0 inch) grinder has more power isn’t the case.
Using 4 inch disks on a 4.5 is not an issue.
A lot of grinders offer multiple guard sizes so you can easily accommodate different diameter disks.
My Milwaukee came with a 7 and a 9 inch guard. The guard size matters only because if you are using a 7 inch blade a 9 inch guard takes up an awful lot of space.
Cutting and grinding disks are catagorized by types and designed to operate on a huge variety of equipment. Flat cutting disks are type 1 and the ones with the indentation are typically type 27. They can operate on a .5 hp angle grinder or a 5hp multi-head machine.
The only specific limirations are type and rpm. A lot of angle grinders on the market come with type 27 backing flanges, it is dangerous to use type one blades with those.
I can safely use a 4 inch blade on my 9 inch angle grinder if need. It’s a hell of a lot easier to use them on my smaller 4 inch grinder though.
I took a look through several angle grinder manuals on-line. Not one mention of a minimum size to a disk. In addition they specifically pointed out the wire wheels, wire brushes, and sanding disks could be used. Only the maximum size of attachments was ever addressed. 4 and 4.5 inch wheels may have different arbor sizes but many grinders now come with an adapter. My Makita has a bushing that flips over to provide two different size arbors.
If you do the math, the outer edge of the 4" disk spins 89% the speed of the outer edge of the 4.5" disk, which, all things being equal all that means is that you will simply need to grind longer.
I am no expert, but I am the proud new owner of a Hitachi 4.5 G12SS grinder and have been making a few sparks the past couple of days 
Yes, it’s far and away my favorite power tool. I don’t get to use it very often, but it’s immense fun.
Thanks for the help, all.
Making sparks is the principle use for angle grinders, however they do serve some other functions.
I bought an 87 pound Peter Wright anvil at my uncle’s retirement auction last summer. It is, I mean was, covered in welding splatter and there was some chipping along the edges. I removed the splatter and started rounding over the chips. Need a bit more practice on something less valuable so that I get round edges and still have a working anvil
Go for it. I’ve used all kinds of these things, they are pretty universal.
I’m so glad I got mine. I’d been hemming and hawing about it for a while then saw and open box Hitachi at Home Depot for $40 or $50 and grabbed it. It works way better than a sawz-all when you have to cut a little piece of metal that’s welded onto something or break a lock or cut a bolt or any other small project like that. Come to think of it, it would probably work well for cutting copper plumbing as well. On top of all that, it doesn’t make your work vibrate around either. It also worked well when I put in very small “retaining wall” type thing and needed to cut the blocks. I tried using a cold chisel and failed miserably but a diamond wheel made quick (and accurate) work of them.
One tip though, pick up some ear and eye protection. They’re noisy and throw stuff around. Besides, it’s a lot easier to concentrate on what you’re doing if you’re not squinting to avoid getting hit with something. I won’t use my angle grinder or my rotozip without them. Hell, they’re just too loud.
My next toy will probably be a multi tool cutter (random picture). But I’ll probably wait until I actually need one or I see one really cheap.
