Thread: Sanding disc construction

Originally Posted by Black Bear

I reckon I can come in well below the 4 or 5 hundred dollars asked for the simplest of these gadgets from Carbatec or other places.

Thanks BB

But will yours be cheaper than $ 165.00 being the cost of one from H & F. Btw I've had one for about 5 years and never had any problems with it.


Peter.

Sturdee, yeah but I'll bet yours doesn't have an electric motor with a speed of about 2800rev at the centre and much faster at the outer edge. That's where the money is

Oh You Guysyippy:

Originally Posted by Ian Smith

Sturdee, yeah but I'll bet yours doesn't have an electric motor with a speed of about 2800rev at the centre and much faster at the outer edge. That's where the money is

No, unfortunately the speed is only 1440 RPM to prevent burning of timber.


Peter.

Originally Posted by Ian Smith

Sturdee, yeah but I'll bet yours doesn't have an electric motor with a speed of about 2800rev at the centre and much faster at the outer edge. That's where the money is

Centrifugal force?

Originally Posted by Ian Smith

a speed of about 2800rev at the centre and much faster at the outer edge.

As a matter of academic interest wouldn't the revs remain the same and only the distance on the outer edge travel further then the centre bits.

If so is not the centre of the disc sanding faster than the outside area?


Peter.

Oh You Guys:<!-- google_ad_section_end -->
I don't believe this.I can hardly stand up from laughing so much

Originally Posted by Sturdee

As a matter of academic interest wouldn't the revs remain the same and only the distance on the outer edge travel further then the centre bits.

If so is not the centre of the disc sanding faster than the outside area?


Peter.

Yer absolutely right!!.
That's why, as well as all those pulleys, he's going to need a differential... to avoid transmission wind up. It's going to be hard to keep the costs down

Originally Posted by Ian Smith

Yer absolutely right!!.
That's why, as well as all those pulleys, he's going to need a differential... to avoid transmission wind up. It's going to be hard to keep the costs down

Thanks fellas, I didn't realise that I would be involved with rocket scientists.

Forget it.!!!!!

C,mon Black Bear - Lighten up.
Sturdee gave you a good tip, and if you don't like H&F then Carba-tec have a similar beast for $179. Carba-Tec 12" Economy Disc Sander : CARBA-TEC

Just think about what sort of Heath Robinson construction you'll end up with messing about with all those pulleys.

I ain't no Rocket Scientist but I'm here to tell you that if a disc is spinning at a given RPM it doesn't matter how big or how small it is, all points on the disc will travel at that same RPM. However , for any given RPM the SPEED at which a point on the disc is travelling will increase the further it is from the centre, but the RPM won't change.
I'm sure that's what you meant, but that's not what you said, and the devil made me do it

Ian

Originally Posted by Ian Smith

C,mon Black Bear - Lighten up.
Sturdee gave you a good tip, and if you don't like H&F then Carba-tec have a similar beast for $179. Carba-Tec 12" Economy Disc Sander : CARBA-TEC

Just think about what sort of Heath Robinson construction you'll end up with messing about with all those pulleys.

I ain't no Rocket Scientist but I'm here to tell you that if a disc is spinning at a given RPM it doesn't matter how big or how small it is, all points on the disc will travel at that same RPM. However , for any given RPM the SPEED at which a point on the disc is travelling will increase the further it is from the centre, but the RPM won't change.
I'm sure that's what you meant, but that's not what you said, and the devil made me do it

Ian

Okay, I know I am well out of my league here, as far as the genius department is concerned...but let me get this clear.

It spins in the middle at a certain speed and it spins at the the same revolutions at the outside of a disc, but is not quicker or faster but the speed increases for any point out side the inner circle that is travelling at a slower speed but not slower in revolutions but slower in speed, So speed does not mean the rate of fastness but another scientific term used to tell someone they are an idiot, as the speed does not get faster as it only increases, so increased speed means that it is not faster but the same.

I will use that the next time I get pulled over for Speeding. "I wasn't going too fast officer, all that happened is that my speed increased".

"Oh! Okay" he will say, You must be one of those Woodies forum geniuses....Off you go then"

Black Bear, you are simply confusing 2 different things.

You were talking about revs, or RPM, revolutions per minute, ie 2850 vs 1440 RPM. As Ian pointed out, it doesn't matter which point on the disk you measure, it still revolves at the same revolutions per minute.

What doesn't stay the same is the Peripheral Speed, measured in either feet per minute or metres per second. The further out on the disk you go, the more the peripheral speed increases. So the peripheral speed of a 6" grinder is lower than an 8" grinder, even though they both revolve at the same revolutions per minute.

To calculate peripheral speed you can use this method

SO, is my 300mm disc travelling "much faster" on the outer edge or not. We could have avoided all this if someone had said "How do you mean faster?"

I still would like to have a couple of different RPM, being as facilitating v pulleys would give. One cannot connect a single phase motor to a speed/ RPM controller under about $1400 so a sparky told me.

I think I will go it alone, and if it doesn't work I will be subject to experience, which gives the test first and the lesson afterwards.

Thank you, Big Shed. I was half way through picking out my response when yours rolled in but I'll give it a go anyway

Black Bear,
On any spinning disc, the further out a point moves from the centre the faster will be its speed past a fixed point. Do not confuse revolutions per minute (RPM) with feet per second (FPS) or kilometres per hour(KPH).

For a disc with a diameter of 300mm (radius 150mm) the circumference (that is the distance around the circle) is about 942mm. So for every revolution of the disc a point on the outer edge of that disc will travel 942mm.
If the disc is rotating at 1,000 RPM then that point will travel 942mm x 1000 every minute, or about 56 KPH

On that very same disc, at the very same time, a point which is only, say 50 mm out from the centre, where the circumference is about 314mm, will travel only 314mm x 1000 every minute, or about 19 KPH.

Ian