Slitting saws?

[Andrew TinsleyParticipant @andrewtinsley63637]

I have a job that requires a slitting saw. I have arbours and saws, but I have never used them in anger. What sort of peripheral speed are we looking at and what is the relationship between saw diameter and point where one is cutting?

Several model engineering books mention slitting saws but none of them gives the best way to use them. I need to slit a tube to produce a mandrel handle. Maybe the whole business is non critical, but thought I would ask for some guidance before I make a mess of things!

Regards,

Andrew.

 

[JasonBModerator @jasonb]

Speed will depend on the diameter of the saw blade and also what the material being cut is. Let us know those and then you might get the speed you need, it is usually quite slow.

[Andrew TinsleyParticipant @andrewtinsley63637]

Hello Jason,

The two saws I am considering  are both 3″ diameter and 28 teeth, widths being 1/32″ and 1/64″. I need to cut approx !” long slits, so I would need to use the top or bottom portions of the saw in order to get clearance and cut a 1″ slot. Material is mild steel and I mm wall thickness.

I would normally use a junior hacksaw for a job like this, but I would like to broaden my experience and use a slitting saw this time.

Regards,

Andrew.

[Anonymous]

I’d use a hacksaw, it’ll be a lot quicker. A short length of 1mm wall thickness tube will be difficult to hold firmly. The work will need to be clamped on the ends. It is entirely possible that a coarse tooth slitting saw may pull the work out of the vice. At the very least use a fine tooth slitting saw.

As for cutting speeds, assuming the saws are HSS, the normal value of 100ft/min for mild steel applies. So for a 3″ diameter saw with a circumference of 9.4″ one needs a spindle speed of 127rpm for a peripheral speed of 100ft per minute. I tend to go down a little on calculated spindle speed with slitting saws so I’d be around 90 to 100rpm.

Andrew

[Andrew Johnston]

On Andrew Johnston Said:

I’d use a hacksaw, it’ll be a lot quicker. …

Andrew

I agree.  To my mind, slitting saws are more a precision slot cutter than circular saw.   They cut slowly compared with other saws, so I see their main virtue as being accurate set up of the cut, and there are better ways of doing rough work like slicing tube.

As to rpm, exactly the same rule of thumb as other cutters.   In mild steel, RPM = 10000 / saw diameter in mm.    The adjust RPM up or down depending on the metal being cut, faster for Aluminium, slower for Cast Steel etc.   Copper and Mild-steel are about the same cutting speed.  As always, adjust for best results, RPM probably slower than rule of thumb with a saw.

There’s more science in RPM = 10000 / diameter in mm than might be apparent at first glance.    It’s a reasonable approximation of the cutting speed calculation, not a wild guess!   Depends on the number of teeth, in that a buzz saw makes rapid progress by running many small teeth at high RPM, each taking a rapid succession of tiny cuts.   But the coarse toothed slitting saws I use aren’t run at high-speed, and obey the RPM = 10000 / diameter in mm rule fairly well.

Dave

[BazyleParticipant @bazyle]

You really should have realised that it is the equivalent of turning a piece of 3in dia steel in the lathe using a very thin and delicate parting blade. What speed would you use for that?

If you were cutting this 3in dia piece in the lathe would you feed the tool in gingerly letting it rub more than cut? No? you need to think about feed rate as well as cutting speed.

[JasonBModerator @jasonb]

I did not see mention of short lengths so hopefully you will have plenty of material to hold. I’m assuming the slits are just 1″ in from the end and not right through a 1″ length of tube.

Getting a 1″ depth slot with a 3″ saw may be restricted by the diameter of your arbour which is likely to be more than 1″ over the flange and washer so you may have to come at it from the sides and accept an angled bottom to the cut. Ideally you want at least two teeth in contact and this is one case where the finer toothed blades are the better choice but with careful feeding and the axis lock nipped up a little the 28T should work. Coming in from the sides is also less likely to see the saw wandering.

If it is just a short 1″ length of tube you want to slit along it’s full length then grip by the ends in the vice so only about 2mm is sticking out the side and cut so the saw only clears the back of the cut by 0.5mm

The calculations for speed are mentioned above are OK but if using a small hobby benchtop machine it will be right at the bottom end of the motors speed range and stalling is quite likely so you may have to overspeed it a little to avoid this. If you have hi/low ratio then use low.

Using this bit of tube as a “practice” piece is worthwhile as you will then be better prepared for when you need to use the saws for accurate work and it saves wasting material on practice cuts or worse mucking up an important part if you go in without ever using the saws before.

Feed rate is a bit of a difficult one with slitting saws. If the world were ideal you could decide on a chip load of say 0.001″ per tooth so feed 0.028″ per rev in the case of  a 28T blade which equates to 1.68″ /min feed at 100rpm. However as most slitting saws don’t seem to run true you are likely to only have a few teeth cutting at the desired load and those on the opposite side of the saw may not be cutting at all, those between will have variable load. So use the feed rate calculated as a maximum and go by feel and sound.

[Anonymous]

As a starting point I use a chip load of 4 thou per tooth for slitting saws. It seems to work well and minimises cutting depth variation due to saw eccentricity. For the example above a 28 tooth cutter running at 100rpm would need a feedrate of 11.2in/min. To an extent chip load is independent of saw thickness. After all a 1/16″ thick saw is only 4 off 1/64″ saws running in parallel.

Andrew

[JasonBModerator @jasonb]

Well I did not have any 1mm wall steel tube so turned and bored some ERW tube which is about as nasty a steel to cut as you could want. 80mm x 0.7mm thick 32T saw which is the closest I have to your 28T x 1/32″ which turned out to be a particularly eccentric example at that.

Speed is about as slow as I could go without a stall and feed rate approx 3″/min again any faster and you start stalling. Would be more likely to stall if the blade were thicker on these type of machines which are not at their best at these slow revs.

First blind cuts in the end of a tube, I then moved round the saw round to the front to complete the second cut. If only doing one side then you will have to accept an angle to the bottom of the cut. Secondly a full length slot on a short length.

[Michael GilliganParticipant @michaelgilligan61133]

Excellent video, Jason … and a ghastly noise !!

Sounds like a specialist saw for lovers of Steam Locomotives 🙂

MichaelG.

[Andrew TinsleyParticipant @andrewtinsley63637]

Thanks all,

Bazyle the saw diameter is 3″ not the tube! I am not bothered about an angle at the end of the cut as the cut will finish in a drilled hole.

I could not find any small toothed slitting saws from our usual suppliers, although I am sure that a proper search would find them. The tube is 5/8″ diameter with around 2 mm wall thickness. I have bored out one end to give me a wall thickness of 1mm and I have already drilled the holes where the slots would end. The total length of the tube is 9″.

I really wanted to gain a little more experience and use a slitting saw, although as Andrew pointed out, it is quicker and easier to use a hacksaw.

I will give it a try on some spare tubing, just for the experience. If all goes well then I might try it out on the job. Otherwise a junior hacksaw will be my friend, providing I can find some decent blades.

As an aside, is it just me, or has the supply of junior hacksaw blades become pretty poor quality these days?

Andrew.

[roy entwistleParticipant @royentwistle24699]

If you get proper named junior blades they are usually OK but there’s a lot of c**p out there.

Roy

[Anonymous]

The noise in the video sounds fine to me; what one would expect from a slitting saw.

There is nothing wrong with wanting to gain experience with slitting saws. But it makes sense to gain it on a job where a slitting saw is essential. Experience is not just about feeds and speeds, but also about using the correct tooling for the job in hand.

Andrew

[Andrew Tinsley]

On Andrew Tinsley Said:

Thanks all,

Bazyle the saw diameter is 3″ not the tube!

It does not matter whether it is the work rotating or the cutter.* What matters is the relative surface speed. A 3″ tube and a 3″ slitting saw both have a circumference of 3 x Pi. If you are cutting mild steel with HSS tools (as the slitting saw will be) then a surface speed of 100 feet per minute is typically suggested. So the “ideal” speed for you saw would be:

100 x 12 / (3 x Pi)  ~ 130 RPM

*(Or even if one or the other is moving in a straight line, as in a shaper or planer)

 

[JasonBModerator @jasonb]

I’m all for experience on the job but if you go at a job that matters and not knowing what you are doing you can muck up the job. In this case from another forum not only did they muck up the part that mattered, they took all th eteeth off the saw and by continuing to try and cut with the now blunt saw also cooked the motor on their Warco Mill.

This is what you get for running a 75mm dia HSS saw at 1100rpm with shallow 0.010″ depth of cut and god knows what feed.

Same can happen when milling, I was contacted a couple of weeks ago, someone had a 3mm 2-flute aluminium specific HSS milling cutter and was running it at 20,000rpm and feeding at a snails pace of 20mm/min and wanted to know why the aluminium was welding itself to the cutter.

[BazyleParticipant @bazyle]

post deleted – I see Charles made the point above.

[Andrew TinsleyParticipant @andrewtinsley63637]

Hello and thanks everyone for the help you have given,

I found a packet of Sandvik junior hacksaw blades and using a top quality frame, the cuts were made in a couple of minutes. So Andrew Johnson was spot on.

I then set up my slitting saw as advised by Jason and others and repeated the cuts with another piece of tube. Again a first class finish, with the slots in a dead straight line parallel to the tube axis. Better than my manual cuts!

So I am pleased with the expanding mandrel and have learnt a little more about the arts of engineering.

Thanks again,

Andrew.

 

 

 

[Andrew Tinsley]

On Andrew Tinsley Said:

[…]  have learnt a little more about the arts of engineering.

That’s the really important bit, Andrew

All the technical theory is fine, if you have the parameters fully defined … but in the Real World, it’s an Art.

MichaelG.

[Author]

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