New Makita SJSII angle grinders

[General Geoff]

I've been doing some research into angle grinders because they would drastically speed up metal restoration processes that I've been doing lately. I picked up the cheapo 6 amp 4.5" Harbor Freight angle grinder, but upon reading the instruction manual, they say it should not be used for cutting. Isn't that a big part of what an angle grinder is supposed to be capable of?

Anyway, I returned the HF one and decided to set my sights higher, and go with a variable speed job so that I can get a more controlled, slower rate of material removal when I need it. I've got my eye on the Makita GA4542C, which I believe has just been introduced to the market this month. Anyone get their hands on one of these new SJSII grinders? I'm going to be grinding, cutting, sanding, wire wheeling/cup brushing, and polishing bare metal with it.

Also in the users manual for the SJSII grinders, it says it's not recommended for polishing operations, despite metal polishing being exactly the same as grinding, just with a finer grit abrasive. Am I missing something here, or are they referring to polishing painted/clearcoated finishes, for which this tool spins too quickly (even on its lowest setting)?

 

[neophyte]

For cutting disks you're supposed to use a cutting guard. A cutting guard isn't open on the bottom like a grinding guard. The closed bottom helps contain cutting disk shrapnel in case the thinner cutting disk breaks. Most people probably just use the regular grinding guard with cutting disks anyway.


Angle grinders typically run at 7,000 to 12,000 RPM depending on the disk size the grinder was made for, with 10,000 RPM being typical for a 5" angle grinder. Some angle Grinders made for stainless steel and specialty alloy processing might be slower with a max speed around 7,000 RPM.

Polishers usually have variable speed and run 1,000 RPM to 4,000 RPM, or 500 RPM to 1,500 RPM, or even 150 RPM to 500 RPM for some specialty low speed gear driven models.

The polishers also may use a different, and in some cases far more robust, bearing and/or gear system from the grinders.


The abrasive pads, finishes, and other accessories used have to be compatible with the speed of the grinder/polisher the accessory is used on.

Even if a grinder/polisher is equipped with variable speed, you're not supposed to use an accessory on a tool whose maximum speed is greater than the maximum speed listed for the accessory. The reason for this is that the speed dial could accidentally get changed causing a speed increase above the maximum safe speed, or the speed control could fail which tends to cause the speed to jump to the maximum level, which can cause the grinding/polishing accessory to explode or shatter.

There are finishing disks made for higher RPMs that should be perfectly safe to use on angle grinders. For certain types of finishing it's better to have both a high speed grinder and a lower speed polisher.

 

[General Geoff]

Thanks for the info, neophyte!

This particular grinder has variable speeds from 11,000rpm max, down to 2800rpm at its lowest setting. If I ever find myself needing to polish a painted finish, I will definitely get a dedicated orbital polisher. But I think the grinder will do just fine for bare metal to bring it to a mirror shine with the right finishing discs.

 

[sam.coll]

I have the Aus/NZ version of the grinder that you are looking at, I cant recommend it enough had it for just over a year plenty of power and the variable speed comes in handy for wire wheel cup brushes etc.

 

[General Geoff]

Thank you for your input, sam!

 

[Mccool]

For cutting disks you're supposed to use a cutting guard. A cutting guard isn't open on the bottom like a grinding guard. The closed bottom helps contain cutting disk shrapnel in case the thinner cutting disk breaks. Most people probably just use the regular grinding guard with cutting disks anyway.


Angle grinders typically run at 7,000 to 12,000 RPM depending on the disk size the grinder was made for, with 10,000 RPM being typical for a 5" angle grinder. Some angle Grinders made for stainless steel and specialty alloy processing might be slower with a max speed around 7,000 RPM.

Polishers usually have variable speed and run 1,000 RPM to 4,000 RPM, or 500 RPM to 1,500 RPM, or even 150 RPM to 500 RPM for some specialty low speed gear driven models.

The polishers also may use a different, and in some cases far more robust, bearing and/or gear system from the grinders.


The abrasive pads, finishes, and other accessories used have to be compatible with the speed of the grinder/polisher the accessory is used on.

Even if a grinder/polisher is equipped with variable speed, you're not supposed to use an accessory on a tool whose maximum speed is greater than the maximum speed listed for the accessory. The reason for this is that the speed dial could accidentally get changed causing a speed increase above the maximum safe speed, or the speed control could fail which tends to cause the speed to jump to the maximum level, which can cause the grinding/polishing accessory to explode or shatter.

There are finishing disks made for higher RPMs that should be perfectly safe to use on angle grinders. For certain types of finishing it's better to have both a high speed grinder and a lower speed polisher.

Very good info. If you're new to angle grinders, watch some videos and read the safety precautions. They are very dangerous tools that can ruin your day in the blink of an eye. Cutting disks are especially dangerous without the proper safety gear.

 

[General Geoff]

No illusions about safety here, I know they're called Death Wheels for a reason!

 

[03protege]

While I own a couple of $100+ grinders, I don't discount the orange/red HF grinders.

For sanding discs (flap discs) and strip discs they have plenty of power. The added benefit is they keep the wear and tear off your more powerful ($$) grinder.

I'm down to 4 4.5" units now (one HF broke) and I have a couple 7" grinders as well. All fitted with a different types of discs. BIG time saver vs swapping back and forth.

My safety gear includes Z87.1 safety glasses, face shield, my welding gloves, and before my next major project I will be investing in a leather smock.

Also check out lee valley abrasives for some of the highest quality and most affordable discs out there.

 

[Monte]

There is a quadratic correlation between the tool speed and air flow. If you reduce the speed of your power tool by 50% the air flow through the tool only will be as high as 25%. So if you use your angle grinder as a polisher it might die the heat death.

 

[General Geoff]

They actually specify in the manual:

CAUTION:
If the tool is operated continuously at low speeds for a long time, the motor will get overloaded and heated up.

Not to worry, I'm very conscious of these kinds of tools overheating, from prior experience with POS Dremels.


Also, I did find a suitable polishing disc (thanks for the tip 03protege!)
http://www.lehighvalleyabrasives.com/abrasives-felt-polishing-flap-disc-4-1-2-x-7-8-sku

 

[General Geoff]

Well I got the tool in, and picked up a few Diablo flap discs as well. Tested it by cutting through an old padlock with 1/2" hardened shackle. Ate through without drama in about a minute, without applying any pressure above the tool's own weight onto the shackle. I also used it to strip the old paint and rust off a steel railing on my porch, and I gotta say, this thing is nice. Very smooth, relatively quiet (I still use hearing protection), and doesn't get very hot, even on lower settings.

 

[BK13]

but upon reading the instruction manual,

That's it, turn over the man card! LOL

Glad you like your new toy. I've pretty much decided any new grinders I get will be Metabo or Fein, but I do like the Makita 4" I have.

 

[General Geoff]

Also of note, is that the label on the tool states "Assembled in USA." I realize that probably means the housing, gears, and motor assembly were made in China and they're just put together here in the US, but hey, it's something.