Thread: Grinding/cutting disc shelf life?

Originally Posted by steveG

All my grinding discs are over 20 years old.
I was given my 5" Makita as a 21st present, and my uncle supplied a box of used discs that were cast-offs from the 9" grinders at his workshop that had got too small to be useful.
Twenty-something years later I'm still working through the box

I've had no issues at all with using them, but it would be interesting to hear the official response from from the manufacturer (eg Norton in Sprint's case).

Steve

Steve,

be aware that although both discs (5" & 9") have a 22mm bore and the worn 9" discs will fit the 5', They are rated at significantly different revs. The 5" probably spins around 9000rpm while the 9" only does 3500 or so. The worn 9" discs are not rated for the sort of revs a 5" will do. Particularly old used ones.

Paul

so long as they haven't been exposed to oil, water or solvents they will be fine.

but play it safe cause you never know.

Originally Posted by pop058

Steve,

be aware that although both discs (5" & 9") have a 22mm bore and the worn 9" discs will fit the 5', They are rated at significantly different revs. The 5" probably spins around 9000rpm while the 9" only does 3500 or so. The worn 9" discs are not rated for the sort of revs a 5" will do. Particularly old used ones.

Paul

Interesting thoughts Paul - I've never seen a rating for a worn 9" disc. Do you have a reference for that?

If anything, I would think that a 9" disc would have a slightly heavier construction near the mounting flange to resist the higher bending loads generated by its larger diameter when full size, and therefore would actually be stronger at a worn diameter of 5" than a new 5" disc.

Steve

I hadn't considered loads and construction, only revs. A worn 9" would be spinning at nearly 3 times it's max recommended revs if on a 5". The disc flying apart at those revs would certainately end in tears.

Paul

Originally Posted by pop058

The disc flying apart at those revs would certainately end in tears.

Both of the crying and bleeding type!

Originally Posted by pop058

I hadn't considered loads and construction, only revs. A worn 9" would be spinning at nearly 3 times it's max recommended revs if on a 5". The disc flying apart at those revs would certainately end in tears.

Paul

While your approach is safe and conservative, to me its very important to understand that its not purely the RPM that causes dangerous forces - its the combination of diameter and RPM that generates the force.

For a given material - whether its a grinding disc or even pizza dough - the smaller the diameter is the faster you can rotate it before it will disintegrate.

Therefore if you take a grinding disc and reduce its diameter you can SAFELY use it at an increased RPM.
A 9" disc that has been reduced to a diameter of 5" has the same forces on on it at a given RPM as a new 5" disc so can be used at the rated speed of a 5" disc.

Steve