If the speed of the grinder is measured from the spindle then the speed of the wheel should remain constant as the wheel size decreases. If you draw a line from the centre of a disc to the edge it will always rotate the given number of times per minute no matter what size, the line will always pass a zero mark at the same time.
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Pete.
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That's true, but the forces it's subjected to decrease in line with the decreasing diameter. RPM alone is meaningless, it's peripheral speed that counts. This is why many grinder wheels have (or had) M/S on them - meters per second of peripheral speed.
brightspark
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,,,ive got disks and abrasive bets ive had for 40 years . none have disintegrated or the abrasive fall of the belts . manafacturers date them and advise to cover there own a,,,se . ive still got loads and will use them till i snuff it and it wont be with using out of date disks 
evobilly666
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Disk speeds are usually measured in rpm. This is the same when measured from the center or the outside and regardless of disk size. 100 turns a minute is 100 turns. Anyone thats seen a disk blow will be more cautious of them. Its not just the speed of the disks surface against the surface being cut. There are huge centrifugal forces inside. The same forces that make a car tyre grow taller during high speed driving.
Again, just an opinion, but disks are cheap, even branded ones. Businesses should pass the cost on to the customer and hobby users dont get through that many. I used 30 1mm stainless cutting disks to tray back my discovery, less than 30 quid to cut a car in half and rebuild it and i wasted a lot.
The cheap ebay ones, i was using three 1mm 4.5 inch to 1 equivalent dronco so is false economy.
Is the cost really worth the potential implications?
Also don't want to advise potentially fatal practices to people that may read this and have no idea of the dangers involved.
Again, just an opinion, but disks are cheap, even branded ones. Businesses should pass the cost on to the customer and hobby users dont get through that many. I used 30 1mm stainless cutting disks to tray back my discovery, less than 30 quid to cut a car in half and rebuild it and i wasted a lot.
The cheap ebay ones, i was using three 1mm 4.5 inch to 1 equivalent dronco so is false economy.
Is the cost really worth the potential implications?
Also don't want to advise potentially fatal practices to people that may read this and have no idea of the dangers involved.
Tiggy_dorset
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I would agree, the safety benefits of wearing a full shield outweighs the benefits of just goggles, in my opinion. I havn't had any discs explode, but my face shield has proved its moneys worth several times from a couple of kickbacks, and one occasion when a job was ripped out of the vice and launched directly at my face.
Type of face shield might make the difference. I had a cheapo Sealey one that was rather flimsy and kind of opened out/splayed out at the bottom. The one I use now fits and hugs nicely just like a welding mask. The downside is that I have been caught out thinking I was wearing my welding mask, flipped it down and arc'd up
Pete.
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Well, exactly. Centrifugal forces are a function of rpm AND diameter AND mass. If you reduce the diameter and the mass ( like wearing down a 225mm blade to 125mm) the forces reduce enough to allow the speed to increase.
evobilly666
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Yet this practice is still frowned upon, and regardless, stood in the dock, being questioned by an HSE officer, the manufacturers instructions being ignored will be the end of anybodies defence. I couldn't live with myself if someone got hurt due to my silly actions or advice such as this. I'm no health and safety lover to the extent it can be preached but being careful and a candidate for a darwin award can be a fine line
Pete.
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I'm not promoting the practice at all, I'm just making comment on the misconception the RPM is the only thing you have to consider when mounting a cutting disc. The RPM is put on the disc to advise people of the maximum rpm it's safe to run at the diameter it's provided at. If you take the guard off a smaller grinder and fit a bigger disc well then you're just plain reckless and playing russian roulette with your well-being but if you do actually understand why that limit's printed on there and how the forces change when the size diminishes then you can make an informed decision for yourself about fitting a worn-down 9" disc to a guarded 5" grinder. I've done it out of necessity but not had to do it often mainly because I get all my discs for free.
Here's some interesting data for you anyhow.
A 9" disc has 3.24 times the mass at 9" diameter than it has at 5" diameter and that means it's subjected to 5.8 times the centrifugal force at 9" than it does at 5" running at the same speed. Increasing the speed from 6600 to 10000rpm you multiply by 1.53 which you have to square to to calculate the extra centrifugal force so 1.53 x 1.53 = 2.34. so by taking a 9 inch disc and wearing it down to 5 inches you reduce the force by a factor of 5.8 and speeding it up to 10k you increase it again but by only 2.34 - less than half. (Ball-park figure using an online calculator - the actual maths is phd stuff).
Here's some interesting data for you anyhow.
A 9" disc has 3.24 times the mass at 9" diameter than it has at 5" diameter and that means it's subjected to 5.8 times the centrifugal force at 9" than it does at 5" running at the same speed. Increasing the speed from 6600 to 10000rpm you multiply by 1.53 which you have to square to to calculate the extra centrifugal force so 1.53 x 1.53 = 2.34. so by taking a 9 inch disc and wearing it down to 5 inches you reduce the force by a factor of 5.8 and speeding it up to 10k you increase it again but by only 2.34 - less than half. (Ball-park figure using an online calculator - the actual maths is phd stuff).
evobilly666
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I'm not trying to question your intelligence, and i know you are a well respected member on this forum especially on your knowledge of lathes and the like. (I even wanted to buy your viceroy when you had it last year). I have read thousands of posts over the last four years and comment very rarely since actually signing up.
I totally understand what you are saying in theory although you can baffle me with the actual math and have done the same in the past myself. Indeed, most accidents if not 99% are caused buy using damaged discs, over tightening discs or cutting material without considering tension or weight. I dont want to fight or get on a high horse, my knowledge is nothing compared to most seen on here, I just cannot advise these practices to people even less informed than myself.
Pete.
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Quite right, and you'll notice I haven't advise it either. Don't take my response aimed at you personally, please, it was meant to increase understanding not make an argument. Disgareeing without getting personal makes for healthy discussion and great things can come of it.
brightspark
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all i can say is make your own choice .the pros and cons have been debated so the information is there 
it isnt undustry its private workshops . if i feel somethings not rite i wont use it
it isnt undustry its private workshops . if i feel somethings not rite i wont use it
Pete.
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I have to say, die-grinders scare me much more than angle grinders. you can literally mount anything that will fit in the collet and people are more complacent because of the small sizes of the wheels. I recall someone a few years back suffering a shattered eye socket from a die grinder wheel bursting.
Flat-sided cutting discs are among the safest abrasive wheels out there, even in failure mode IMO.
Flat-sided cutting discs are among the safest abrasive wheels out there, even in failure mode IMO.
evobilly666
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I haven't even considered the implications of them yet lol. I've just brought my first one, a 110v makita, something like 26k rpm. Haven't brought any attachments yet but will consider what you say when I do
Wow , that made me cringe and look away !!
HMB
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Hardly due to the disk then?
marlinspike
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Someone once posted on here a photo of the resulting injury when a 9" grinder kicked back into their face. There was also a photo of how it looked when stitched up - surgeon made a fantastically neat job. I often think of that picture when using power tools and it certainly focuses the mind on safety gear and procedures.
A stone disc is actually quite good on grp, we used to use them a lot as diamond cutting wheels were expensive, as the price of them dropped we went the diamond cutter route. The stone discs dont load up much at all. Dont try it with a steel disc though they load up straight away!
Note the borrowed grinder i was using ran fast, Looking back i suspect it was a 4" jobby so my own fault really.
Note the borrowed grinder i was using ran fast, Looking back i suspect it was a 4" jobby so my own fault really.
Gritineye
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A industrially uninformed lady relative heard my grinder had packed it in so kindly bought me a cheapo 115mm angle grinder from a tool dealer at a vintage rally, the first time I switched it on the disc shattered and flew all round the w/shop. It hadn't even touched any metal and inspecting the bits showed there was no reinforcing in it at all!
I put a decent disc in and then felt bits of the internal fan hitting my fingers, within a few minutes the bearings had collapsed and it was reduced to scrap, which it was all along really!
