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I Forge Iron

Old stone grinding wheel Safety

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I almost died today.

I took my old pedal powered stone sharpening wheel and hooked it up roughly to my belt grinder motor, set on the middle speed pulley configuration, it was running smooth and true. I put a piece of steel to the wheel while it was running, I was off to the side not sitting on the seat. If I had been sitting in the seat I would either be in the hospital in a vegetative state from a crushed in skull or laying in a bed with massive blunt trauma, or dead laying in my shop until someone found me. The stone exploded, and when I say exploded I don't mean it came apart I mean it exploded with a bang, broke into 4 main pieces and in line with how it was spinning. 2 small chunks went through my roof, 1 chunk went through my small ryobi wood bandsaw and embedded chunks of stone and the bandsaw itself into the 2x4 shelf beam behind it, another main chunk flew into my KMG motor which was powering the grinder and put a 1/2" deep 2 1/4" round dent into it and broke off the magnet inside and pulverized the wireing box on the front, the other 2 main chunks and small bits went into the wall and pulverized. A split second after it exploded I went running out of the shop, seeing the directions and sizes of the pieces after the aftermath I was lucky I was not hit by a ricocheting piece that could have seriously damaged my body or killed me. In hindsight it did not seem that stupid an idea, but thinking deeper a natural sandstone(?) wheel as compared to the vitrified wheels that run just fine 3 times as fast on my bench grinder was not a smart idea. If you own one of these grinders, do not attempt to motorize it. I thankfully made it through this event unscathed, but it could have been worse in more ways than it could not have.

I am talking about this type of grinder


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Glad your still with us!!! What kind of rpm was the stone at as i have a number of stones like that which i was going to put power to???The stones would be going on a setup i bought at auction that is powered and has seen a lot of use with this type of stone already in place.Do you think it was due to the speed you were at that it blew???

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I have to say NEVER power up a stone intended for hand use. the maths is rather complicated for me, but maximum rpm anything should spin at has alot to do with ft/min or surface speed. An increase of an inch in diameter has big changes on centrifugial forces!
If a grinding stone has no 'safe rpm' label best to use it by hand only. Every time my maintenance fitter mate sees me buy an old wheel (originally for a bench grinder) he shakes his head, even when i stress i'm using it to hand rub scale off when tempering.
Even the 'rings like a bell' test isn't as good as buying a new wheel.

I'm very glad to hear your ok ApprenticeMan; we want to see what neat stuff you'll churn out from your Anyang!

keep safe,

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A-MAN, glad to hear you were safely out of the way, we are still praying for Frosty, and would not wish you to be on that list, just a good reminder to all of us, danger is lurking just around the corner. Like I tell my 16 year old son who is starting to drive. Every time you see someone involved in a car accident, remind yourself, they did not plan on being in a wreck today, as we look into our shops, we should start and finish with the thought, how can I be safe today!!

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Heres the result of some simple math. Speed at the rim of a 6" diameter wheel at 1850 rpm = 48 feet/second. Speed of a 24" wheel at 1850 rpm = 194 feet/second. Speed of 24" wheel at 100 rpm = 10.5 feet/second.

Sam if your bench grinder wheel size and rpm and your pedal grinder wheel size and rpm are even remotely similar to these then this was always going to be a recipe for disaster.

Your observation of the difference in the composition of the original sandstone and the modern composite wheels is very valid but even if you tried to spin a modern wheel up around 20 times its design speed I suspect you would get a similar result.

Really glad you survived the experience unscathed.

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Glad you weren't hurt! I had a "new" grinding wheel years ago, and since then, whenever changing wheels on ANYTHING, 4", 6", 8", bench grinders, angle grinders, cut-off saws, get out of the line of the wheel and run them free for ONE FULL MINUTE, and this does not guarantee anything, but it helps. People also have a tendancy to pick up tools, drop them, not say a word and put them back as found, I have came back from restroom, etc., picked up an angle grinder, ( that I was using a minute ago ) switched it on and had the wheel break immediatly.

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Yeah, that could have been real bad. We all have accidents from time to time and it's most fortunate when nobody gets hurt and all that's damaged is machinery and your pride.

Although the natural sandstone may have been cracked, I believe this simply resulted from too much speed. Natural wheels have been powered in knife factories and other applications since the 1800's so artificial power is not the problem - it's that the edge of the wheel was moving way too fast. Mick provided the math formula - and for comparison, a 14" chop saw wheel is only rated at 4,300 rpm, which amounts to 262 ft/s at the periphery. However, those wheels are quite thin and reinforced for that type of work plus they have much less mass. I've had chop saw wheels break and bang around the shop (which scared me half to death) but the difference in impact is like comparing getting hit with a ping pong ball and a cannon ball - moving at the same speed. You didn't say what rpm the motor was running but you may have been turning that stone wheel closer to the equivalent of a 4-1/2" angle grinder.

Stay safe and count your blessings...

Edited by HWooldridge
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I would expect the proper rpm to be in the range of 15 - maybe 30 rpm. Excessive speed is not from using a motor, it's from turning it too fast. I'd almost bet you could blow one up pedaling. I would have no problem with powering one. Yeah, I'd run some numbers first. Glad you made it out alive, son!

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Exploding wheels are not fun. I built a 2 x 48 belt grinder somewhere around 15 years ago and have not had a rock grinder in the shop since. Yes, good quality grinder with a good toolroom quality rock is very handy for sharpening bits. I keep one 4 1/2" side grinder with a rock and one with a flap wheel (and should have another with a cutter). I sharpen with all 3 (depending on the need). Large stock removal is with the 7" and a Norton (screw on) rock. I have die grinders that serve well too. Glad no injuries.

As a post script. I do have a water sumped rock grinder that has not seen use for probobly 10 years. It has a cast sump base and is used for some sharpening issues. It is powered and the wheel is very slow (and not completely round). It does not go fast enough to be dressed with a standard commercial wheel dresser. I have in the past dressed it somewhat with a commercial reciprocating hacksaw blade to clean it up some and restore the cutting surface.

Edited by Ten Hammers
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Here are some SPEED guidlines for grinding wheels.
Bear in mind that these are for BALANCED wheels, of MODERN construction, mounted with SAFETY FLANGES and GUARDS, on machines of suficient RIGIDITY< and with PROPER BEARINGS. All of the capitalized words are contributors to Sams near death experience.

Sam, you don't know how lucky you were to survive this incident, not just alive, but unmaimed. Had a segegment of that wheel struck you, you may well have been better off dead than alive.

For vitrified wheels, an operating speed of 5500 surface feet per minute is recomended and in no case should exceed 6500 s.f.m.

Table of R.P.M. to give 5500 S.F.M.

Wheel Diameter R.P.M.
6 " 3500
7" 3000
8" 2865
10" 2100
12" 1750
14" 1500
16" 1310
18" 1167
20" 1050

Inspect grinding wheels carefully before mounting. Do NOT overtighten mounting nut.

As noted by a previous poster, after mounting a new wheel, allow the wheel to run for one minute while you stand off to the side.

Always use a well adjusted and tightly bolted work rest on grinding machines. This cannot be overemphasized.

Use great care in handling unmounted wheels.

Start your work slowly into the machine. Give the wheel time to heat up.

Ensure all guards are in place before using a grinding machine.

It should go without saying, do no grinding of any type whatsoever, without safety glasses.

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Considering how many dumb things I have done, I know a few things about power tools.

They eat flesh with no remorse.

How can guys I work with not know to let a new stone wheel run for a bit (I let them run 2 to 5 minutes.. got some brilliant monkeys digging through the stock room) with all hands clear of the area? My boss gave me xxxx for leaving the bench grinder running with no one in the shop. For just that reason, in case they let go no one wears it for a fancy hat.

they don't know that they don't know

but then again I didn't have a face shield .. till the day after I almost ate something off my wire wheel....

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I probably should add that I have natural stone wheel, about 20 inches across that is powered, to about 20 to 25 rpms... that is just enough to do what I want to do with it. more than that and I get extra wet from the water slinging off the wheel.

I had never really thought about running it to 1750 or more rpms... just didn't need it to go that fast.. thanks for the thought provoker...

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