Grinding discs have a restriction on the amount of revs they are supposed to do. But they say it as 'X Metres/min' ie The outside edge of the disc is not supposed to do more than X amount of metres per second.
On this theory I have always used a 9inch disc on a 9inch grinder and then when the discs worn down to 7inch, I take it off and replace it with a new 9inch disc.
Then, I take the now 7inch disc and put it on a 7inch grinder, then when its worn down to 5inch dia, I take it off and put it aside for the 5inch grinder.
Because the 7inch grinder runs at a higher rev per minute, I expect that its safe to put a worn 9inch disc on it. This will bring up the xmetres/minute back up to the standard metres/minute for that disc.
This all makes proper sense to me, for effiency you should have the grinding edge run as fast as possible but within the capabilitys of the grinder. ie, theres no point in grinding using a 5inch dia disc in a 9inch grinder. It just takes too much time. The reason why you should not run a disc faster than its supposed to be run is centrifical force. If its running too fast for that dia, centrifical force can make the disc explode.
I have always done this. For 30 odd years. Now I pick up a grinder at the Warehouse and the instructions say specificly "Do not put a worn larger dia disc into this grinder"
Ok its damn obvious you don't put a 7inch disc into a 5inch grinder.
But why would they state this? Does that mean as soon as a 9inch disc is worn down to 7inch I should take it off and throw it away?
Now, in three different jobs I have worked on, where theres LOTS of grinding to be done. I have used worn disc on succesivly smaller grinders and someone has stated that "We went thru a lot less grinding discs than we allowed for in the quotes."
Good thing or bad thing?
lonfa,
Aug 17, 1:28pm
I've done the same thing as you,over the years, using quality name brand disks and grinders.
That said, I'd understand why the warning would be on cheap grinder with lower(est) quality disks and possible lesser safe standards. As always, Understand the risks, and buyer beware.
You can't idiot-proof the world but you can stop some Smartarse Lawyer suing you if you put a warning on your cheap equipment.
Not bad in NZ(ACC covers it. ), Worse here in Oz(accident Compo lawyers all over) and Crazy in the US (You'd understand what I mean,- Youtube's a educational place. )
johotech,
Aug 17, 8:11pm
Don't buy a grinder from the Warehouse. Problem solved.
mrfxit,
Aug 17, 8:40pm
The only real complication is with the mounting hole in the disk. The bigger the disk = a larger hole.
Yep done the same thing for many many years, BUT i normally make up a spacer to suit the disk & grinder.
There is a way to calculate for edge speeds that relates to specific disks & speeds with specific sizes of disks including disks worn down to a smaller std size.
The biggest danger for Joe bloggs using worn disks in a smaller grinder is that most don't have the brains to make a spacer to suit the mounting hole difference & the disk ends up mounting off centre along with the resulting bouncing grinder & a face/ hand or leg full of disk
tintop,
Aug 17, 10:03pm
I use new 5" disks in my 4" grinder without exceeding the max revs for the disc.
When they get worn down to the hub, they fit onto a mount that fits my air die grinder.
No waste here!
mm12345,
Aug 17, 10:29pm
Nice work. Do you use a sledgehammer to drive the 5" disk down to fit into the 4" guard? Oh wait - I know what you do.
tintop,
Aug 18, 2:16am
Well - with a a range of welders and workshop tools, a supply of steel sheet - just have a guess at what I did if you are so smart.
mm12345,
Aug 18, 4:07am
Good lord - you welded yourself up a shrapnel resistant steel first aid kit box? Seriously though, I wouldn't have commented except I see what people do when they've bought a 4 1/2" grinder and a cheap pack of 5" discs.
tintop,
Aug 18, 4:10am
Well - just keep in mind that not everyone who posts here is stupid.
Making a larger guard and running a 12,500 rpm rated disc on a 10,000 rpm grinder is not too far into the realm of rocket science.
marte,
Aug 18, 10:12am
mm12345[quoteVery surprised you'd be doing so much cutting that disk use has to be tallied against jobs - but you're looking at grinders in The Warehouse.[mm12345]quote
I was actually trying to find out if I could use a 'Paint and rust stripper' pad on a 5inch grinder when I read this about the revs. The electric drill I have is far too slow and a grinder would work better IF the revs were right. I could also use much more of the face of the pad rather than just the outer edge.
I have used up more grinding discs than I could count. And only one 'accident', gouged a slot in my left knuckle while putting 4 1/2 grinder into its wall holster. damn!
Im a fitter/turner/welder. Me and 4 others spent 4 months down a hole (HydroPower stations) welding (me & 1more) 1.2mm fluxcore, arc airing out excess metal followed by 3 guys on grinders 12 hrs a day. A month at a time. Us welders went onto grinding whenever we could to speed things up. And 9months welding up a 30m steel Cat boat. 11 yrs milling machine/welding up things. 9 months welding Ali for Tiwai
But, cheap (but good) grinders? Get a 'Supercheap' 9inch grinder. Take out the 4 screws holding the reduction head together and locktite them back in. Then weld a strip of metal across the guards face so you can out the grinder on the ground while its still rotating as it slows down. As good as, if not better, than grinders costing twice that much.
I know about the spindle hole size. Normally the nut has a step in it so you can turn it over and put a 7/8th hole disc on it.
marte,
Aug 18, 10:20am
When welding up cavatation I had to take the guard off the 5inch grinder and use it that way. There simply was not enough room. At the same time because of the squeeze, if you let go of it, it just stayed here. (Imagine climbing up your kitchen furniture to lay on the top of the cupboard, while wearing a lether suit, holding a MIG gun, Arc air and stick, air peining gun, 5 inch grinder and helmit and glasses, while theres a hurricaine going on, all above a 8foot dia room thats 6ft high in the center and 4 on the outside. With three other guys in there as well. ) I loved that job.
tintop,
Aug 18, 11:29am
Marte - I have a Ryobi 125" variable speed grinder, I will look up the speed range tomorrow.
Also panel/paint shops use slow revving buffing machines, same as an angle grinder but far slower.
Where do you get the stripper pads from? I have my Hi lux bumper and air dam to prepare for painting.
bill1451,
Aug 18, 7:23pm
In the shed magazine years ago, the article logged the silly accidents people have, removing disc grinder guards, blowing down oily overalls with oxy, using the oxy for spray painting epoxy paint because the compressor was broken, grinder disc explodes flies across workshop hits workmate in neck, he bleeds out and dies,
tintop,
Aug 18, 9:49pm
Marte : The variable speed grinder I have is a Ryobi CGS1250EK. sander / grinder. Speed range 2,800 to 10,000 rpm.
The 125 mm guard I made for my Makita 100mm grinder has a curved edge, and is deep enough to lay the grinder down while it is still spinning. A very handy feature.
Off to look for paint/rust removal pads today. I will try them in the Ryobi.
mm12345,
Aug 19, 7:10am
Problem with 230v variable speed grinders is that as well as rpm drop, there's torque drop. RPM isn't everything. My 18v brushless 125mm grinder is a much better tool to use than my 125mm 230v Makita. The battery tool is only 8,500rpm, but it's got much more torque - the speed stays constant, so for what I use it for, it's much steadier and precise to use. Downside is that it will suck a 5AH battery dry in about 10 minutes or less if its working hard - so useless for big jobs.
mrfxit,
Aug 19, 6:44pm
Yea yea & the guy who knocked over a gas cylinder AND the top snapped off which punched a hole in a brick wall. Or the guy who dropped a spanner across a car battery terminals & the car caught fire.
Whatever . >>
Considering the vast number of power & gas tools being used in this country, extreme accidents like that are pretty rare. If we thought ahead about ALL potentially preventable accidents on a holistic & chain reaction scale, then the likes of marte would never be allowed to work . . . . & the power stations etc would never get maintained, (never mind built in the 1st place)
marte,
Aug 20, 8:51am
In my whole time working 30+ yrs,I have lightly burnt the first 3 fingers on my righthand with the gas torch, hoses caught on a lump of weld on the bench and then flicked back. No scars at all. (did the same burn on my left hand with hot frying oil in the kitchen. No scars. I was very very pleased that my Good Doctor was able to use this as a burns case study for his training assistant. We all got quite involved with the healing process (Indian Doctor in training) Nicked left knuckle with 5inch grinder. Squashed right thumb nail in capstan lathe barfeed. I don't count 'getting metal flakes out of eye at hospital'.
Taking the guard off a angle grinder (except for that one job) is just plain stoopid stupid. And putting a larger dia disc on it afterwards is just (words fail me, the ones that don't I cannot post on trademe. )
We were talking about angle grinder accidents at work and then on the radio theres mention of somebody just being killed, to do with anglegrinder. Turns out the (older) guy was in a car engine bay, using a 4 1/2 inch grinder with a 9inch disc in it, cutting crossmember. It shifted, nipped disc, broke off peice of disc, disc rotated and cut the guy open from his bellybutton upwards and inserted fractured disc into stomach organs. He died in a matter of seconds. --------------------
Supercheap have a 5inch grinder for $50. I have a variable speed device I could use with it to use the paint/rust strippers pads with it. Im not sure on how to know what revs its doing yet/ Will figure that out tonight
bill1451,
Oct 22, 9:07pm
Whatever the speed that the grinder is doing it would be easy to transpolate That into kmh and at that close to the body, it aint gonna slow down much b4 it cuts through your overalls, into your guts and u bleed out, YUP removing that guard/putting bigger disc on is surely Darwin award material
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