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Which angle grinder is better for cutting 7 in (180 mm) or 9 in (230 mm)?


Fido-X

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Helo everyone i am going to buy angle grinder which will be mostly used for cutting steel. Grinders smaller than 7 in (180 mm) aren't a option. Grinders larger than 9 in aren't a option neither. So 7 in or 9 in?

7 in (180 mm) has sped ~8.600 RPM and 9 in (230 mm) has speed ~6.600 RPM.

Just added:

9 in = 2.000 W (watts)

7 in = 2.600 W (watts)

And it's gonna be "BOSCH Professional", not hobby. Both are 80 M/S (meters in second).

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Power is what we want to see to make a judgement; which you didn't state.  

Which car goes faster---one with 15" wheels or one with 16" wheels?

Ok, thanks for editing in the power.  I'd go with the higher wattage given that the duty cycle is the same.  (Also the 7" will probably be less taxing on the arms. I have an old 9" industrial Milwaukee that will try to break your wrists when you turn it on; but has cut a lot of rail and welding tanks other than Acetylene that I DO NOT ALLOW IN THE SHOP, they can stay at the scrapyard!

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Personally I prefer a 4 1/2" or 5" grinder.  I would not use a portable grinder for cutting thick steel.  It's too DANGEROUS!!!!  If your stock is too thick to cut with a 4 1/2" cut off wheel, you ought to have a proper band saw or at least a hot saw!  Wait till you catch a 7" wheel in a deep cut!!!!  You'll be hearing me then!

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Momentum, inertia- however you want to phrase it.  The more mass you have moving at speed the more it wants to continue in that direction.  When it grabs you have to provide the equal and opposite force or it gets ripped out of your hands, assuming it doesn't turn the disk into a shrapnel grenade. A 9 inch disk has approximately 1.65 times the area of a 7 inch disk.  If they are made of the same material at the same thickness that's already a significant increase.  If the 9 inch disk is thicker it's even more.  The difference in RPM's does come into play, but right now I don't feel like doing all the calculations to see which one exerts more force when it grabs.  In general though, smaller = less damage when things go wrong.

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Welcome aboard Fido-X glad to have you. If you'll put your general location in the header you might be surprised how many of the gang live within visiting distance. 

If all you're doing is cutting grooves then go with as small as you can. A 4.5" - 5" disk grinder binding in a cut will break the wheel and maybe give a knuckle a rap.  9" disk grinders usually have a 1hp motor and a gear reduction drive so motor torque and momentum is a significant factor. You can NOT start or stop my 9" Milwaukee without the whole thing torquing in your hands and the motor is a large % of the weight of the tool. This means if the disk jams it WILL TEAR ITSELF OUT OF YOUR HANDS, THROW YOU ON THE GROUND AND THROW BLADE PIECES HARD ENOUGH TO EMBED THEM IN THE WALLS. Go ahead, ask me how I know I can show you the scars.

I strongly discourage the use of cut off wheels in right angle grinders but the big boys are just asking for trouble.

If ALL you want to do is cut grooves start hitting garage, yard, etc. sales for an old table saw and replace the blade with a cut off wheel. Set the depth and use it with the same fear and loathing you would cutting lumber. When the blade shatters in use it will be mostly contained in the table saw.

Frosty The Lucky.

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I use a 9" 2200 W Milwaukee for cutting everything in the shop (small or big). It has the slow-start feature (don't know the actual word for it), so it's pretty nice to use. Also not that big of a pig like the old big ones. It works nice and feels safe when using it properly. I say that knowing big angle grinders are the most dangerous tools in the shop!

I chose the 9" because around here it's easier to find discs for it than 7"-ers. Also - I think - that 2"+ makes huge difference when eating out the material. 

Sometimes I use it with thick disc, and that works miracles in seconds: grinds off big surfaces very fast.

Bests:

Gergely

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I have a 9" slow start / variable speed Milwaukee but the switches kept dying so it sits. It doesn't torque like the on/off grinders but you still want to be clear of the work before starting or letting off the trigger. 

Frosty The Lucky.

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If I could only have one cutting tool in the shop it would be a Portaband.  I have 4.5, 7,9 inch grinders, plasma cutter, big horizontal band saw, and 2 portabands.  Forget cutting with  a grinder, it's faster, easier, safer, and more accurate with a portable band saw.  

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On 08/04/2017 at 4:47 AM, Judson Yaggy said:

If I could only have one cutting tool in the shop it would be a Portaband

Love my Milwaukee Portaband.

Although, the 4" & 4-1/2" angle grinder with a "thin" reinforced cutoff disk is a tool that I go for without hesitation.

Look up Metabo I will, yessssss......

Robert Taylor

Edited by Anachronist58
addendum
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