Author Topic: Say it ain't so Joe, say it aint so.  (Read 1846 times)

Offline DavidA

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Say it ain't so Joe, say it aint so.
« on: November 12, 2016, 04:37:11 PM »
I'm sat here contemplating buying a metal cutting band saw from Machine Mart tomorrow to take advantage of their no VAT offer.  It will save me about 50.

Then I got around to thinking 'Why not just put a metal cutting blade on my small bench top wood cutting band saw?'  And of course I thought that the blade speed would be too fast.

So I did the maths; and the answer is something of a surprise.

The wheel on the motor (blade driving wheel) is about 6" diameter. The motor, according to the plate turns at 1450 rpm.

From this I get 6*Pi = 18.744 inch circumference.

At 1450 rpm this gives me 18.744 * 1450 = 27178.8 inch per minute.

and 27178.8/12 gives 2264.9 feet per minute.

That's a lot considering that a metal cutting band saw runs at around 80 fpm for mild steel.

Can someone tell me where I've got my calculations screwed up ?

Dave.

Edit.  The drive wheel connects direct to the motor shaft; no reduction pulleys.

Offline John Rudd

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Re: Say it ain't so Joe, say it aint so.
« Reply #1 on: November 12, 2016, 04:56:23 PM »
Motor RPM multiplied by the Motor pulley diameter divided by the Driven pulley diameter times the Bandsaw wheel diameter times pi (3.1416) divided by 12 = S.F.P.M.

Motor RPM X Motor pulley diameter / Driven pulley diameter X Bandsaw wheel diameter x 3.1416 / 12 = S.F.P.M.

So from the above I get:
Motor speed= 1425 ( nominal speed for a 4 pole motor)
Driver /driven = 1 because its direct drive
Bandsaw wheel dia = 6"
Plug in the numbers turn the handle....
(6* 3.142)/12*1425= 2238 f/min



Hmmm....too fast enough....needs gearing down...

Joe.... :)
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Offline sparky961

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Re: Say it ain't so Joe, say it aint so.
« Reply #2 on: November 12, 2016, 05:02:35 PM »
I'm sat here contemplating buying a metal cutting band saw from Machine Mart tomorrow to take advantage of their no VAT offer.  It will save me about 50.

For what it's worth, whatever way you decide to go with to introduce a metal cutting band saw into your shop, you will absolutely love it.

I think the horizontal bandsaw is #1 the most important tool in a metal shop.  Vertical bandsaw is ok but not near as versatile.  And of course most of the small 4x6's and such can be used as a vertical in a pinch.

I've found at least a few of these in the used market.  Good restoration project if you're on a tight budget.

Offline DavidA

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Re: Say it ain't so Joe, say it aint so.
« Reply #3 on: November 12, 2016, 05:34:39 PM »
..Hmmm....too fast enough....needs gearing down...


About 28:1

Maybe there is an interesting project in there somewhere. With pictures; naturally.

Anyway, thanks chaps for confirming my suspicions.

Oh God, it's decision time.

Can I really justify spending the money or should I concentrate on converting my existing saw.

I hate these moments.  Need a strong coffee.

Who's that looking over my shoulder. The guy with the horns and the pointy tail ?

Dave

Offline AdeV

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Re: Say it ain't so Joe, say it aint so.
« Reply #4 on: November 12, 2016, 07:58:09 PM »
We have a saying in our office....

"Get it spent!"

 :lol:

2000+ fpm sounds way too fast, even for a wood saw. Are you sure the motor isn't internally geared?

Edit to add: Here's a way to check... paint a white mark on the outside of the blade (Tippex would do), a nice broad one. Switch saw on. As the white mark passes (and even at a slow speed it'll be whizzing by) start a stopwatch. Count 10 (or some convenient number) of rotations. Or, count until 1 minute has passed. In the latter case, multiply the number you get by the length of the blade in feet to give your your real FPS. The maths for the former case is currently beyond my ken, it involves division and I don't do division after midnight, in case the ghoulies come out and get me...

Edit #2: According to this website, 3000FPS is normal for wood  :bugeye:
Cheers!
Ade.
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Offline Chilliphil

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Re: Say it ain't so Joe, say it aint so.
« Reply #5 on: November 13, 2016, 04:16:43 AM »
Spookily, this subject or something very similar, has just come up on another well known forum. I asked if I could cut metal on my vertical, one speed wood bandsaw and the answer seems to be a qualified yes, within certain boundaries of course. http://www.model-engineer.co.uk/forums/postings.asp?th=122226&p=1#2090336

Offline John Rudd

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Re: Say it ain't so Joe, say it aint so.
« Reply #6 on: November 13, 2016, 04:34:54 AM »
Irrespective of what others say and do wherever in the world, try it and see what happens with your chosen weapon of choice and whatever material you want to cut.... :dremel:
But, do be careful.....and if it works for you then good.... :beer:
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Offline DavidA

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Re: Say it ain't so Joe, say it aint so.
« Reply #7 on: November 13, 2016, 03:41:49 PM »
I should have added that I have cut aluminum alloy on the wood saw. usually up to 10 mm thick.

I have to feed very carefully and the shrieking noise is a bit off-putting.

I did fit a new Starrett blade that is designed for metal, but the first attempt to cut a piece of 1/8" mild steel took the set off the blade. It is much too fast.

I have decided to drop the new saw option. Can't really justify it when I can get any large pieces cut up where I used to work.  The convenience of my own saw would have been nice, but I think that I am going to have to consider some kind of reduction for the table saw. I have a few ideas.

Dave.

Yes, there will be pictures.

 :proj:


Offline howsitwork?

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Re: Say it ain't so Joe, say it aint so.
« Reply #8 on: November 14, 2016, 05:16:12 PM »
Dave

wood saws do run at much higher speeds and have deeper gullets to the teeth to clear the saw dust. You can get away with aluminium if you're careful as you found.

One of the guys in the York club is converting his by putting on a smaller pulley and then using a specific metal cutting blade ( much higher tpi and smaller gullets / teeth).  Hamilton edge tools will be able to advise on blade selection. :thumbup:

Must admit I got a metal saw ( plus 2 band saws so far) and have no regrets ( and fewer blisters )