Thread: 4' or 5' angle grinder

Originally Posted by BobL

I think you may want to review that formula and reconsider?

ta - how's the rework Sir?

Gents All,
For what it's worth please ensure you ALWAYS use the guard on your grinder,
whether cutting or grinding as with those tip speeds the broken discs/wheels become lethal.
Also please don't use "cutting wheel" to grind; the cutting wheel only has 2 reinforcing layers of fibre-glass [one east-west & one north-south]
while the grinding disc has a third or sacrificial reinforcement layer of fibre-glass.
Personally I'll only buy and use the correct size cutting wheel or grinding disc for the machine;
it's just not worth the risk.
Cheers, Crowie

Originally Posted by bonox

ta - how's the rework Sir?

I think you'll find a(r) = w^2 * r

Originally Posted by BobL

I think you'll find a(r) = w^2 * r

and again i ask "are you happy with the rework"?

For what it's worth, my 180mm grinder drives 115mm disks without any problem or lack of performance, and has enough clearance to be able to comfortably make use of most of the disk.

Originally Posted by bonox

and again i ask "are you happy with the rework"?

Sorry Bonox, I'm half asleep here - all good - well done.
- the angular accel is a measure of how well the disc has to hold together while spinning around
- the tangential accel is a measure of the force the pieces of disc flying off will hit the operator by, BTW that's v^2/r, where "v" is the linear tangential speed.

Cheers

cheers

Originally Posted by BobL

the tangential accel is a measure of the force the pieces of disc

correct me if wrong here though, but you won't get a tangential acceleration unless the disc is changing angular speed. (ie spinning up/down). (excepting the groundward tropism component of course! )

You are interested in the impulse for the bits hitting you, which is a change in momentum over time, not an acceleration expression.

Originally Posted by bonox

cheers

correct me if wrong here though, but you won't get a tangential acceleration unless the disc is changing angular speed. (ie spinning up/down). (excepting the groundward tropism component of course! )

You are interested in the impulse for the bits hitting you, which is a change in momentum over time, not an acceleration expression.

Acceleration is the rate of change of velocity, and don't forget that both are vectors.
An object rotating at a constant angular velocity has a tangential velocity (v) that is constantly changing direction which means there is an constant tangential acceleration = v^2/r . This is used to determine centrifugal force (F = m x v^2/r)

Neglecting losses due to air resistance this is also the Force with which an operator is hit by the bits (of mass m) of a grinding wheel with tangential velocity v. Impact Force together with contact area of skin can then used calculate to calculate pressure which determines if a puncture will occur.

Impulse is just another way of looking at the same thing. The bits coming off the grinding wheel have no inherent impulse (rate of change in momentum) until they strike something. This is determined by the type of material being struck, the contact area (or size of the bits) and the initial momentum of the bits. The difficult thing to measure is the time taken for the bits to stop moving so that is why the force - pressure approach is usually used even though this is also not a perfect method.

Using Google you can easily look up "how much pressure is needed to break skin" but I could not see one reference to "how much impulse is needed to break skin". BTW Google also does not show the scientific literature on this topic which is quite extensive since the puncture pressure for various skins are reasonably well known for medical purposes, and studies of compressed air puncture wounds of high pressure compressor operators and the . BTW these are horrible wounds and can lead to a loss of limbs and even death.

All of this is highly simplified because contact areas are not constant and bits are usually irregularly shaped, and points and sharp edges make a big difference. Spinning pieces also add to the complexity of the situation.

So what you are both trying to say is that flying pieces of grinding/cutting disks are potentially dangerous. Got that! The impact is an energy factor as in impact energy of a firearm projectile. I am not going there either!

Had a piece of 1mm cutting disk impact on my cheek about two weeks ago. I definitely felt a serious impact, but the damage was pretty minor. Everyone at work accepted that SWMBO hit me.

Just for comparison
Brand new disk, 125mm diameter, 11,000rpm. 72 m/sec or 259 km/hr.
Muzzle velocity from REM 223 Centrefire Rifle 1005 m/sec or 3621 km/hr (Load picked at random, 60cm barrel, 1/12 twist).

I remember reading about a work accident recently where leg artery was severed by exploding disk from angle grinder and I think the victim died.

Dean

Originally Posted by Oldneweng

So what you are both trying to say is that flying pieces of grinding/cutting disks are potentially dangerous. Got that!

Not quite the original question was is it OK to use a 9" grinding disc on a higher revving 5" grinder once the disc has worn down. The answer is probably not.

Smith and Arrow sell Inox grinding disks for a good price. 125mm x 1.1mm cutting disks for $1.10 each if you buy 25, plus freight of course. why bother taking the risk. How often do you want 3mm thick disks on a 5 inch grinder anyway.

Dean

Originally Posted by Oldneweng

Smith and Arrow sell Inox grinding disks for a good price. 125mm x 1.1mm cutting disks for $1.10 each if you buy 25, plus freight of course. why bother taking the risk. How often do you want 3mm thick disks on a 5 inch grinder anyway.

Dean

How often do I want a 3mm disc on a 5" grinder?
Answer - every day. 1mm discs have their place but are potentially more dangerous in several ways. 1/ They do not handle side loading particularly well. 2/ They disintegrate fron being bumped far more easily than conventional 3mm discs.
For fine work on Aluminium or Stainless the 1mm discs are excellent but for most general cutting the 3mm will outwear and outperform them hands down.

Answer - every day. 1mm discs have their place but are potentially more dangerous in several ways. 1/ They do not handle side loading particularly well. 2/ They disintegrate fron being bumped far more easily than conventional 3mm discs.

Totally agree with both points. I have 3mm disks to use when required but cannot remember the last time I used them. Maybe due to different work uses. I have a cut off saw and a horizontal band saw. Angle grinder is only used if these cannot do the job. I guess this mostly leaves fine work and sometimes cutting up recycled materials which is shaped as to prevent fitting into vise of band/cut off saw.

Two main reasons for side loading a disk is tension in the metal causing pinching which I know all about, and lack of control of grinder. You need to look in line with the cut to keep the grinder square to cut, in all dimensions so I avoid thick disks as they throw far more material towards your face.

I know this is going to produce comments about safety but it is my experience that damaged disks are generally caused by factors that can be avoided by observation and experience. A lot of people are scared of grinders. This can cause situations that are more dangerous due to cautious actions as opposed to positive. I spent many years operating centreless grinders with wheels up to 600 mm diam and 300 mm wide. I lost any fear during that period.

Understand them, treat them with respect but use them for your purpose. My worst injury in about 25 years with angle grinders was as mentioned in my previous post. Not worth worrying about.

Dean

I wonder just how dangerous the 1 mm thin discs really are? I must have broken ~50 1 mm thin discs in my time using them. Almost everytime I was putting uncalled for side pressure on the these things when I should have known better. All bar a couple of times they always broke near or at the arbor coupling. The discs generally didn't came off the grinder - they slowed down very quickly and sort of just flopped around a few times till the arbor stopped spinning - no great drama really.

The only time that a piece actually flew off and hit me from a 1 mm disc was when was when I slipped and the ginder hit my machinists vice and a 10 mm size chunk flew off and hit my arm with about the same force as a wet tissue - being so light it seems like air resistance slows them down very quickly.

Has anyone seen a documented report about any injuries with these things?