Author Topic: Slitting saws  (Read 9333 times)

Offline AdeV

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Slitting saws
« on: March 21, 2012, 09:29:35 AM »
Whilst attempting to carve a big chunk of aluminium into some smaller chunks, I figured the best way to do it with minimal wastage was to use a slitting saw. Since both my commercial saw arbors are trashed (can't undo the allen bolt due to the flats becoming rounds...), I made myself a shiny  new one. Very shiny, actually, it was made from some rock-hard steel I had lying about, I think it used to be a pin in a hydraulic machine.

Anyhoo, the pin is around 30mm fat, so I cut around 1/2mm of the end down to 1", so the saw had something to sit on & run more or less true. In fact, the fit was beautiful & the saw pretty much clicked into place. The bottom was then screwed on & tightned to murder tight, the whole lot then mounted in a 3/4" collet.

So... 1st saw: 1/32" thick, 6" diameter, quite coarse teeth. Speed 550rpm, DOC varied up to 4". It worked great for a while, then there were a couple of nasty cracking noises - the saw has essentially cracked from the centre towards the edges.

2nd saw: 1/16" thick, 6" diameter, lots of smaller teeth. Speed down to 275rpm, DOC varied again, feeding very slowly by hand. Exactly the same story as above.

3rd saw: 3/32" thick, otherwise similar to saw 2. Speeds/feeds the same. Starting to run out of saws now... this one also cracked.

Eventually, I finished off using my trusty Evolution Rage Pro carbide tipped saw @ 2300rpm, but that's quite a thick saw & takes 2-3mm of metal away.

Clearly, I was doing something wrong with the slitting saws; either running them too fast, too slow, too deep or something else - any ideas?

I'll post some pics tonight.
Location: Wallasey, Merseyside. A long way from anywhere.
Occasionally: Zhengzhou, China. An even longer way from anywhere...

Offline Lew_Merrick_PE

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Re: Slitting saws
« Reply #1 on: March 21, 2012, 11:12:31 AM »

The thing about slitting saws is that they are not designed to do really deep cuts.  Way back when they were called screw slitting saws.

They are sensitive to heat build up and they (especially in thinner sections of larger diameters) are rarely relieved for side clearance or thermal expansion (that's why larger table saw blades have those funny holes and slots near the periphery).  Spindle speeds pay a role in the generation of heat.  Lots of lubricant fed into the cutting side with a cooling blast of air focused on the coming out of the cut side can help, but this is only a palliative.  Slow, wet, and cold help.

I more normally use (in order of preference) my table saw (11 inch X .095 kerf, specially ground for aluminum), horizontal/drop bandsaw, or vertical bandsaw before even thinking about a slitting saw.  Hopefully, something here will help you.

 ... Lew

Offline hopefuldave

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Re: Slitting saws
« Reply #2 on: March 24, 2012, 10:13:23 AM »
Ah, slitting saws...

Lets look at some figures:

6" diameter, so call it a foot and a half circumference (near enough)...

550 RPM - multiply that by a foot and a half, you have a surface speed of around 800 Foot/Min!

Would you turn a 6" aluminium piece on the lathe at 550 RPM with an HSS lathe bit?

Carbide in Aluminium, max (industrial cutting speeds, with flood coolant and a *rigid* machine, cutters considered as Expendable, nice if they last a shift before changing) - Machinery's says 500 - 1000 Ft/min, so in the ballpark for *carbide*...

HSS in aluminium (same conditions) 200 - 300 fpm - which explains why your slitting saws weren't happy!

For steel these figures woiuld be 50 - 75 Ft/min for HSS, 150 - 200 using carbide tools...

Allowing for *hobby* conditions (no flood coolant, less rigid machine, caring about wear on the cutter more than production rate) you'd want to drop the speed to around a third of the industrial rates *at the most*, so 70 to 100 Ft/min, which gives a spindle speed of 50 - 75 RPM. If it were steel, you'd be looking at "slower than the spindle goes"...

Next we come to feed, I take it that these were fine-tooth saws? For aluminium you really need quite a coarse saw (maximum of 8 tpi in my experience) and a feed of about 3 to 5 thou" per tooth *if there's enough space for the chips*, too much feed and the teeth will clog, too little and they'll skid and wear badly (even in aluminium), so say your cutter has 160 teeth, you want a feed of (f'rinstance) 3 thou" x 160 per revolution, so approx 1/2" - at 50 RPM this gives a feed of 25"/minute, so cutting your aluminium blocks will want a slow spindle speed and a surprisingly fast crank on the handle :) Lsten to the cut, it'll sound good at the right feed rate :)

WD40, although a crap general lubricant, makes a great cutting oil for ally, so keep the saw wet, it helps prevent a built-up edge from the ally melting on the saw teeth.

YMMV, not warranty expressed or implied etc. etc.

Dave H. (the other one)
Rules are for the obedience of fools, and the guidance of wise men.

Offline awemawson

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Re: Slitting saws
« Reply #3 on: March 24, 2012, 05:38:30 PM »
When you turned down the diameter to fit the saws, did you undercut the shoulder? If there is the slightest radius at the shoulder the saw will be stressed at the edge of its hole. I think this is where you said the cracks started. Run a parting tool into the 1" diameter at the shoulder for 20 thou or so to eliminate any radius.
Andrew Mawson
East Sussex

Offline fixerup

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Re: Slitting saws
« Reply #4 on: March 24, 2012, 07:08:37 PM »
 This is why I like this site so much.  :mmr:
I learn so much from others knowledge.
Last time I used a slitting saw, I tried to do a deep cut, even though the lube was flowing I was having major heat generated due side friction, I didn't now the saw had no side relief. but I knew I had a problem.  To get the job done, I moved the saw up a couple thou  then slowly slowly fed the saw in, can't go fast or else the saw will flex and fall back in the original have to let the saw cut its own groove. I then moved the saw down a couple of thou and repeated the above. This is definetely not production work but it got the job done when I need it right away.