Hacksaws: How to choose 'em, how to use 'em, + other "stuff"

[ChaiLatte]

Reason - 9" grinders operate at a lower speed, and the disks are intended for use at that speed (typically around 7000RPM). A 4 1/2" grinder operates at around 11,000RPM so using a worn down 9" disk is obviously expecting the disk to operate faster than it's design speed which could cause it to break up.

Your 'reason' is not correct. Please look at the markings on the disk and think through what they are actually saying.

https://www.kissafety.co.uk/abrasive-wheels-understanding-the-marking-system/

Any maximum rpm given on an abrasive disk relates to its original (unworn) diameter. In the context of the present discussion, a more useful (more critical) figure that is also given is the maximum surface (peripheral) speed. For a bonded wheel (angle grinder disk) it is usually 80m/s.

Using your own numbers, a 9" disk spinning at 7000rpm actually has a higher peripheral speed (82m/s) than a 4 1/2" disk spinning at 11 000rpm (65m/s).

Putting a 9" disk that has worn down to 4 1/2" diameter on a 4 1/2" machine is no different to putting an unworn 4 1/2" disk on the same machine.

In a way, your logic is upside down: to work efficiently, the disks need a cutting speed near 80 m/s. That is why your tiny Dremel wheel goes at >20k rpm, so its perimeter is still moving at near to 80m/s.

Huge wind turbines spin very slowly but the toy windmill you buy at the beach spins very fast. The peripheral speed of the blade tip on each is comparable.

The rpm of the front wheel of your Raleigh Chopper has to be considerably higher than the back wheel in order to maintain the same surface speed along the road.

 

[graduate_owner]

Your 'reason' is not correct. Please look at the markings on the disk and think through what they are actually saying.

https://www.kissafety.co.uk/abrasive-wheels-understanding-the-marking-system/

Any maximum rpm given on an abrasive disk relates to its original (unworn) diameter. In the context of the present discussion, a more useful (more critical) figure that is also given is the maximum surface (peripheral) speed. For a bonded wheel (angle grinder disk) it is usually 80m/s.

Using your own numbers, a 9" disk spinning at 7000rpm actually has a higher peripheral speed (82m/s) than a 4 1/2" disk spinning at 11 000rpm (65m/s).

Putting a 9" disk that has worn down to 4 1/2" diameter on a 4 1/2" machine is no different to putting an unworn 4 1/2" disk on the same machine.

In a way, your logic is upside down: to work efficiently, the disks need a cutting speed near 80 m/s. That is why your tiny Dremel wheel goes at >20k rpm, so its perimeter is still moving at near to 80m/s.

Huge wind turbines spin very slowly but the toy windmill you buy at the beach spins very fast. The peripheral speed of the blade tip on each is comparable.

The rpm of the front wheel of your Raleigh Chopper has to be considerably higher than the back wheel in order to maintain the same surface speed along the road.

Well you may be right regarding peripheral speed, but there is still a maximum speed indicated on 9" disks which would be exceeded by putting on a 5" grinder. I would not want to risk it. myself, having seen the effect of a shattered disk. I think the same applies to abrasive wheels. I would not use a worn down large abrasive wheel on a 5" bench grinder, although this would be a far less likely scenario (and probably more dangerous too).

 

[Spectric]

Using the wrong disc on any grinder is a very dangerous practice and can lead to some very nasty injuries, in a place of work all abrasive machinery such as grinders come under the PUWER regulations and fitting the wrong disc to a machine could lead to dismisal.

 

[throbscottle]

Thanks for writing such a great article. Nice to learn some things and get some affirmation of my own methods

I'll add that the absolute game changer for me was getting a decent solid hacksaw frame (Facom, seemed the best in my budget). Gone are the wobbly cuts from my old cheap tubular frame!

Using Bahco blades. Not quite as good as the gold coloured (expensive!) Draper ones I was using before, but cheaper, and better than any others I've tried.

Going to read your filing article next, I'm sure it will be well worth it.