Cutting disk sizes

[mrjynx]

I have a dual purpose polisher and grinder, it says to put disks on it no smaller than 115mm 4.5"

now Ive looked at angle grinders, and seen disks that are 4"

and Ive seen air compressor cutting tools that take a 3" 75mm disk.

so my question would be, why does it say not to put smaller disks on, and why cant you get 3" cut off disks for normal angle grinders?

 

[shenion]

the arbor sizes are different. 3" has 3/8" hole. Most angle grinders have 5/8" or lager.

Smaller will work but the case of the grinder gets in the way. I have a 4" angle grinder and really wish i had a 4.5". Real hard to get the disk in tight places.

 

[hotrodder]

4" disc = 16mm bore, 4.5" = 22.2mm bore so while a 4" disc will physically fit over the M14 spindle of a 4.5" grinder it won't fit the arbor and so won't be located properly. A 4.5" disc quickly becomes 4" (and then 3" etc) as it wears- doesn't take long to end up with a collection of smaller discs for those times when a fresh disc is too bulky but there's enough access to reach without needing a smaller grinder if a part worn is used...

 

[mrjynx]

Oh wow, you cant even put a 4 on a 4.5" ? that sucks.

Air grinders are obviously very light and easy to manipulate but, it means having a compressor, which doesnt make it very mobile.

I didnt know the disks physically got smaller, thanks for the replies.

Another question, why do some have 3 small holes, and some have 4?

the manual was pretty poor and didnt explain much.

 

[rory1]

i think that the 'small holes' are not holes, just marks where they indent the metal ring to hold it on to the disk, different manufacturers must use a different amount of indents.

edit: they could be holes to give a bit of purchase when the ring is set in the resin around the disk

 

[mrjynx]

edit: they could be holes to give a bit of purchase when the ring is set in the resin around the disk

that makes sense.. thanks for the reply.

 

[dannyweld]

different size discs are manufactured to run at different speeds, although only this week i started with a 9" disc in a 9" grinder, then put it into a 7" grinder which runs 2000rpm faster, then into a 5" grinder which runs faster still. i think 115mm grinder runs even faster again. could'nt bother with doing the maths, but i presume the outside of the disc was running at a similar speed most of the time.

 

[madkayaker]

that's a bad idea as the 4.5 grider runs twice the speed of the 9 so the centre of the disk is under more speed than designed

 

[dannyweld]

that's a bad idea as the 4.5 grider runs twice the speed of the 9 so the centre of the disk is under more speed than designed

off course, the disc just disintergrated by the time i was finished.

 

[Pete.]

that's a bad idea as the 4.5 grider runs twice the speed of the 9 so the centre of the disk is under more speed than designed

Can't say as I agree with that. The RPM limit is to keep tip-speed down so you don't put a 9" disk on a 5" grinder when it's new but if it's worn down to 5" the tip speed is the same as a new 5" disk except the worn-down 9" one is stronger. Since the center is strong enough to support a 9" disk on a 2000w grinder why would it not be strong enough to support it on a much less powerful 5" grinder once it's worn down small enough to fit in the guard?

 

[dannyweld]

I've never had a problem with the centres, they have got some 9" grinding discs on site though, there so bad everybody would want to put on goggles when i,m grinding. its no joke in short sleeves.
i have a 7" makita which i always put the Inox cutting discs into when the get down to size.