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7600 RPM (Metabo)

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Steve_P

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Sep 15, 2010
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I have two of the 9600 RPM 5" grinders and I almost always operate them at a lower speed than max. They work great at lower speeds as they're high torque motors that don't need the higher RPMs to prevent from bogging down under load.
To compare:
9600*5=48,000
7600*6=45,600
So, the 5" and 6" will have similar linear travel speeds at the outside of the wheel. Yes, I know that I left out Pi, but it's irrelevant for a comparison.

If your goal is max material removal in the shortest time, brute force, working on heavy equipment, then probably go for a 9600 RPM 6" model. If you like options, the variable speed is unbelievably nice; I find that it gives a lot more control and I essentially never use my other non VS angle grinders anymore.
 
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neophyte

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Some German manufacturers produce grinders intended for use working with Stainless Steel, and those grinders generally have a lower top speed.
I presume Metabo does this, but it’s been a while since I looked thru their tool offerings.
Flex and Fein definitely offer grinders specifically for stainless processing, although the grinders usually have some specific indicator such as “Inox” in the model name/number, to indicate the grinder is made for stainless processing.

I checked thru an older tool catalog from maybe 20 years ago, and it looks like Fein grinders used to top out at 7,000rpm for a 150mm (ie. 6”) grinder.
There may be specialty accessories in 6” or close size, requiring a 6” sized angle grinder, that cannot be safely run at above a 7,000-8,000rpm speed.
In this case, a company with proper safety protocols in place would not issue an angle grinder with a higher rpm speed that the accessory/s are rated for, since speed dials can accidentally get adjusted higher, and since a failed electrical module in a variable speed tool, can cause that tool to increase to the maximum possible rpm speed.
I believe this is actually an OSHA safety regulation.
Most decent industrial power tool manufacturers offer models in a wide variety of maximum speeds etc., to tailor the tool yo the task is routinely bring done.
 
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Wamsutta

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Wamsutta

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Steve_P said:
I have two of the 9600 RPM 5" grinders and I almost always operate them at a lower speed than max. They work great at lower speeds as they're high torque motors that don't need the higher RPMs to prevent from bogging down under load.
To compare:
9600*5=48,000
7600*6=45,600
So, the 5" and 6" will have similar linear travel speeds at the outside of the wheel. Yes, I know that I left out Pi, but it's irrelevant for a comparison.

If your goal is max material removal in the shortest time, brute force, working on heavy equipment, then probably go for a 9600 RPM 6" model. If you like options, the variable speed is unbelievably nice; I find that it gives a lot more control and I essentially never use my other non VS angle grinders anymore.
Click to expand...

I don't think 7600 is going to be fast enough. Today I spent two hours grinding down a weld with my grinder spinning at 11,000.
 
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neophyte

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Wamsutta said:
I don't think 7600 is going to be fast enough. Today I spent two hours grinding down a weld with my grinder spinning at 11,000.
Click to expand...
7,600rpm+/- used to be the standard speed for a 6” angle grinder.
It was also the standard speed for grinders meant for Stainless Steel processing.
Some steel alloys, like stainless alloys, work harden quickly, so a slower speed is better, because once the metal work hardens, the metal becomes significantly mire resistant to grinding and cutting.
11,000rpm used to be reserved for 4-1/2” grinder or smaller.
If it’s taking 2 hours to grind a weld, maybe purchase a variety of different grinding discs and flap discs, and see if any of those work better on whatever alloy you are grinding, because that sounds way too long, unless the weld was significantly long or large.
 
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DerekV

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The variable speed model maxes out at 7600 rpms and the non-vs model is 9600. The motors are identical, so the question is why are max rpms limited on the variable speed model? My theory has always been due to the need to keep the motor cooler at the lower speeds enabled by the vs electronics for reliability/tool longevity. On the vs model, max spindle rpm is limited via gear reduction, so at full tilt both vs and non-vs motors are spinning at the same speed. This means the motor fan is blowing the same amount of air through the motors despite the different output rpms.

With that out of the way…the vs Metabos have electronics to maintain output speeds under load. That means a ton of current (heat) can be introduced through the motor at sub-optimal cooling fan speeds. Motor speed = cooling fan speed, so if you need more cooling for reliability reasons, you get higher motor speeds for a given output spindle rpm via gear reduction.

My anecdotal experience to back this up: with just 3 years of home hobbyist use, I’ve overheated a commutator on a WEV 15-125 HT (aka 5” vs model with same motor but 9600 rpm output) that I converted for 6” use. The low end rpms (which I used a ton) were just too slow for a 6” wheel, the thing got HOT even with light use. And I mean little AND light use. Higher motor rpms probably would've saved the motor via the additional cooling. I did buy another one and still use it with 6” wheels, I just make sure to set the vs dial no lower than ~”3”.
 
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Wamsutta

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neophyte said:
7,600rpm+/- used to be the standard speed for a 6” angle grinder.
It was also the standard speed for grinders meant for Stainless Steel processing.
Some steel alloys, like stainless alloys, work harden quickly, so a slower speed is better, because once the metal work hardens, the metal becomes significantly mire resistant to grinding and cutting.
11,000rpm used to be reserved for 4-1/2” grinder or smaller.
If it’s taking 2 hours to grind a weld, maybe purchase a variety of different grinding discs and flap discs, and see if any of those work better on whatever alloy you are grinding, because that sounds way too long, unless the weld was significantly long or large.
Click to expand...
That's very interesting what you say about stainless steel work hardening when it gets too hot. I didn't know that.

The weld I had to grind was 8 inches long, an inch wide and about an inch thick. A very big weld because I was practicing vertical up MIG with ER70S-6. I don't know what the weld metal is made out of exactly, but what I do know that it's a lot harder to grind than the base metal itself. I've decided to keep my welds at a single pass; that way they will be easier and quicker to grind down and remove.
 
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Wamsutta

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DerekV said:
The variable speed model maxes out at 7600 rpms and the non-vs model is 9600. The motors are identical, so the question is why are max rpms limited on the variable speed model? My theory has always been due to the need to keep the motor cooler at the lower speeds enabled by the vs electronics for reliability/tool longevity. On the vs model, max spindle rpm is limited via gear reduction, so at full tilt both vs and non-vs motors are spinning at the same speed. This means the motor fan is blowing the same amount of air through the motors despite the different output rpms.

With that out of the way…the vs Metabos have electronics to maintain output speeds under load. That means a ton of current (heat) can be introduced through the motor at sub-optimal cooling fan speeds. Motor speed = cooling fan speed, so if you need more cooling for reliability reasons, you get higher motor speeds for a given output spindle rpm via gear reduction.

My anecdotal experience to back this up: with just 3 years of home hobbyist use, I’ve overheated a commutator on a WEV 15-125 HT (aka 5” vs model with same motor but 9600 rpm output) that I converted for 6” use. The low end rpms (which I used a ton) were just too slow for a 6” wheel, the thing got HOT even with light use. And I mean little AND light use. Higher motor rpms probably would've saved the motor via the additional cooling. I did buy another one and still use it with 6” wheels, I just make sure to set the vs dial no lower than ~”3”.
Click to expand...

So you're saying at the low RPM the motor was turning at with the heavier load of a 6-inch wheel mounted on there - wasn't getting enough airflow from the armature mounted fan; correct?

I did the same thing about 20 years ago. I put a 6-inch disc on my 4-1/2" grinder that spins at 11,000 RPM. The motor was getting so hot that the epoxy on the windings were giving off a sweet sugary smell. And that was with the grinder free spinning with no load on it.
 
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scooby074

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Oct 26, 2008
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Its a concrete / stone grinder

Optimum processing of stone: robust, powerful angle grinder with gear reduction and reduced, adjustable speed for material-friendly working
 
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Wamsutta

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scooby074 said:
Its a concrete / stone grinder

Optimum processing of stone: robust, powerful angle grinder with gear reduction and reduced, adjustable speed for material-friendly working
Click to expand...
That's what Amazon says as well, but Metabo never mentions it being a concrete grinder.

Amazon.png
 
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Lasu

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Oct 20, 2010
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Finland
High torque (gear reduction) 2000-7600 rpm rpm range. I have used the 125 mm "inox" version, 2000-7600 rpm, 5Nm of torque.
 
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