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Optimising the Life of Cut-Off Wheels

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Michael Gilligan24/04/2021 23:48:42
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There has been some interest, on a recent thread, in the performance of thin cut-off wheels at lower speeds.

This is by no means definitive, but it includes some interesting test results: **LINK**

https://multimedia.3m.com/mws/media/1513014O/metalworking-asd-bnl-creatives-044.pdf

N.B. __ they are not investigating very low speeds ... but if these differences are occurring at lower-than-maximum speeds, maybe there will be significant problems at very low speeds.

Any thoughts ?

MichaelG.

Pete.25/04/2021 00:17:17
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Michael, I vaguely recall seeing a cut off wheel being used on a sieg x2 type machine on YouTube a long time ago, not sure I would do it, but it seemed to work, maybe much slower than its original purpose.

Michael Gilligan25/04/2021 06:34:27
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Thanks for that, Pete ... I will have a browse later.

Strangely perhaps, almost everything else I found yesterday was about the dangers of exceeding maximum rated speeds ... no-one seems interested in running discs slowly.

Having ‘slept on it’ ... I’m guessing that running a too low a speed would probably ‘glaze’ ‘load’ the edge of the disc, and soon prevent it cutting properly.

MichaelG.

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Edit: __ Revised my terminology, to accord with this Wikipedia paragraph

https://en.wikipedia.org/wiki/Grinding_wheel_wear#Dressing

Edited By Michael Gilligan on 25/04/2021 06:50:47

JasonB25/04/2021 07:31:58
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There must be quite a bit of flexibility in speeds for cut off disc. We tend to work out the cutting speed for a fresh disc as we would a slitting saw or milling cutter but they still cut until the body of the angle grinder stops them cutting deep enough by which time they may be cutting at half the speed they started at.

Dave S25/04/2021 08:47:38
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There is little difference between a surface grinder wheel and a bench stone.

Think of abrasives as tiny cutting tools, taking a very small DOC on multiple teeth. (Somewhat true)

At that point you can see that to remove much material you have to make a lot of cuts - and to do that in a reasonable time your speed has to increase. Hence the general use of 'high' spindle speeds.

Dave

Clive Foster25/04/2021 09:24:49
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Interesting results that seem to agree with my subjective assessments.

Guard adjustment is important for comfortable work so its pity all the angle grinders I've used make proper adjustment much harder than it need be. One particular horror needed the disk removed to set the guard and, even then couldn't be set decently for pulling.

A point that isn't well covered is optimising the force used to hold the disk into the cut. With any length to do I suspect its common to try and force the disk to cut faster than it wants leading to rapid wear. I'm certainly prone to impatience on a big job! More likely to try to force the tool when pushing. Especially as it wears down. Pulling it seems easier to feel the difference between natural free cutting and forcing. I find that as the disk wears down the natural rate of cut falls off.

Big differences between brands when shopping at the affordable end of the market. The 1.6 mm disks in particular can be very variable. For once LiDL is not a good source. The Parkside 1.6 mm ones I have cut poorly. They last well though. Look to me to be made much more like the ordinary grinding disks than proper cut off ones.

Clive

Michael Gilligan25/04/2021 09:42:21
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Interesting comments so far ... keep ‘em coming please.

MichaelG.

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P.S. __ I should just mention that the ‘lower speeds’ which prompted my sudden surge of interest are perhaps five times lower than the rated speeds for the discs. ... If viable; this would have advantages in terms of quietness of operation, but I am concerned by the possible ‘side-effects’

Zan25/04/2021 10:02:35
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Interesting link. So key points are

pull rather than push

hold close to the grinder as possible to the disc and

regarding low speed. I found this

“Since wear is cumulative over time, at lower speeds contact time during each revolution of the cut-off wheel is longer and thus wear is higher, resulting in shorter life.”
And a more powerful grinder reduces wear probably due to reduced speed during cutting when using the weaker machine

Michael Gilligan25/04/2021 10:20:56
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Posted by Zan on 25/04/2021 10:02:35:

.

regarding low speed. I found this

.

.

Yes, that conclusion is why I posted the link ... But the differences in speed discussed in that note are trivial compared with what John Smith 47 is contemplating.

So ... How far can we extrapolate the findings of the 3M investigation ?

MichaelG.

Dave S25/04/2021 10:52:59
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47 photos

This:

fb77fa72-1fb3-4ed9-b120-5f072707ff67.jpeg

is a cut off wheel for a surface grinder. I’m not going to run it because it has lost its speed markings, but compare with a 41/2” angle grinder wheel:

c439bf97-0a3d-4b88-89a3-d262da945040.jpeg

3d682566-ab57-4ac3-bc17-d06235f65b75.jpeg

364e1cf9-f07c-45ee-81f5-5a7a05787934.jpeg


The surface grinder wheel is 3mm thick. I picked up a grinding rather than cutting disk for the angle grinder as it’s easier to photograph the edge, but essentially grinding and cutting disks for hand held use are pretty much the same (bar thickness).

Notice how the Surface grinder wheel looks the same in grit and such, but is missing the reinforcement mesh. That’s because a surface grinder operator is not likely to bump or twist the wheel in operation. No matter how careful the operator of a hand held grinder is their guiding ability is not the same. That reinforced wheel also has a higher speed capability- precisely because it is reinforced. That in turn means you can take a “bigger” cut with it because the number of teeth going past the cut per second are larger. A massive bonus for hand held work, as no one want to be holding the angle grinder for longer than required.

I have SG wheels which are basically worn (used) to the hub of the grinder so 4” or so. That they are used that far shows they still work ( most of them came second hand with my grinder).

Maybe later I’ll knock up an adapter hub and chuck a 1mm angle grinder disk in the bench grinder, but first I have a fuel pump to fit to my restoration project....

Dave

Dave S25/04/2021 10:59:19
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A look for low speed abrasive cut off saws will also prove interesting.

For instance: **LINK**

I know it’s a 10” unit, but it’s *top* speed is 3000 rpm, and it runs down to 100...

Dave

SillyOldDuffer25/04/2021 11:37:51
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Posted by Zan on 25/04/2021 10:02:35:

Interesting link. So key points are

pull rather than push

 

hold close to the grinder as possible to the disc and

regarding low speed. I found this

“Since wear is cumulative over time, at lower speeds contact time during each revolution of the cut-off wheel is longer and thus wear is higher, resulting in shorter life.”
And a more powerful grinder reduces wear probably due to reduced speed during cutting when using the weaker machine

+1. I saw that too and the full quote may be helpful:

'On average a 1250W tools delivers 30% longer life than an 800W tool. On a 1700W right angled grinder 90% longer life can be expected compared with the same wheel on an 800W tool.The theory explains. The power of a tool is an indication of how well the tool stays on speed under load. Low power tools will turn slower compared to a high power tools under the same conditions. Since wear is cumulative over time, at lower speeds contact time during each revolution of the cut-off wheel is longer and thus wear is higher, resulting in shorter life.'

I think the idea running a tool slower to reduce wear is reasonable but needs looking at in the round. RPM is easy to measure and imagine, so perhaps we focus on it rather than power and torque, when the latter are actually more significant.

I may be misremembering the rule of thumb for steel, but I seem to recall 'one cubic inch of metal removed per minute by one horsepower' as being broadly true of all cutting methods, whether drill, single-point, or grinding. The metal is held together by a certain energy which has to be exceeded to break it apart. RPM is only relevant in so far as energy is delivered effectively: a fine-toothed saw cuts best removing small quantities at high-speed low-torque, whereas a large-toothed saw is happier at slow speed, high-torque. The energy needed is about the same in both cases, and the motor has to deliver it. Unfortunately motors are imperfect, with power, torque and rpm curves giving differently constructed motors of the same 'power' distinctly different characteristics. Thus a single-phase motor has low torque at low speed, while a stepper motor has high-torque at zero rpm. The way my machine delivers a particular rpm may not be the same as yours. Could be all bets are off.

As edge life is determined by how well the cutter material withstands heat, running a tool slowly could extend life by allowing it more time to cool-off. Unfortunately, if slowing down means the tool rubs rather than cuts, then the motor's energy heats the cutter up rather than removing metal, and causes rapid wear. So it's not just about rpm. Bad news if rpm drops because the motor can't deliver enough energy, good news if the rpm is matched to the material and the motor's ability to put energy into cutting. It's the match that matters, a combination of rpm, torque and power balanced between keeping the cutter cool and maximising metal removal, which depends on what it is. Of RPM, it's easy enough to identify too fast and too slow, but hard to decide the optimum. Several other factors have to be considered too. As Michael's link mentions: 'how well the tool stays on speed under load' is one of them.

The paper highlights just how variable the performance of a simple tool can be on the job. Lab results reveal why tools and materials behave as they do giving solid information about starting points and adjustments, but the real world is always more complicated. Theory is far better than guesswork, but it's always worth experimenting for best results in the workshop. Don't draw general conclusions though. Workshops are poor laboratories because results often depend on local conditions. Results are only valid if others can replicate them.

Dave

 

 

Edited By SillyOldDuffer on 25/04/2021 11:40:02

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