Slitting Saws

[David Hill 5Participant @davidhill5]

Hi, I am having problems with using a slitting saw to cut some cast iron eccentric straps. The teeth on the saw simply blunt after a cut of about 2mm. The straps are 10mm wide so it is not doing much cutting before becoming blunt! Has anyone any ideas of what is going wrong? The saws I am using are 0.040" thick. Thanks.

[David Hill 5Participant @davidhill5]

[roy entwistleParticipant @royentwistle24699]

David What speed are you cutting at ?

Roy

[David Hill 5Participant @davidhill5]

About 700rpm. The slitting saw is 2.75" dia.

[Brian WoodParticipant @brianwood45127]

Hello David,

I can offer a couple of explanations.

1. The castings are hard, or you are cutting through the hard outer skin

2. The saw speed is too high or you are not feeding the saw and letting it rub

Suggestions

I would really do that sort of cutting with a hacksaw and then mill the cut surfaces afterwards, if the castings are hard hacksaw blades are a lot cheaper. Hard castings can sometimes have their grain structure refined by heating to red heat, soaking at that temperature for a few minutes and then cooled slowly in really dry ashes. If you have an open fire then put them in at bedtime, get them hot, pull them out of the fire and leave to cool down with the dying fire at the side of the grate overnight.

DON'T quench them, that was what caused the hard spots in the first place

If they are sand castings, then remove the hard sandy skin with an old file, or better still an angle grinder to get down to the metal below

Regards Brian

 

Edited By Brian Wood on 14/01/2018 10:29:42

[not done it yetParticipant @notdoneityet]

Roy’s reply was was well founded. I might suggest that around 500 surface feet per minute (150m/min) is about 5 times too much. And the lower value would likely be requiring cooling. The feed rate per tooth needs taking into account, too.

Edited to add that Brian would have been more direct re the speed , had he not been typing at the time of your reply.

Edited By not done it yet on 14/01/2018 10:42:16

[Chris Evans 6Participant @chrisevans6]

It does sound like a combination of chilled (hard) casting and to quick a spindle speed. I would try nearer 400 RPM

[HopperParticipant @hopper]

Waaay too fast. Let's get back to basics. You calculate the rpm the same way as you would if you were turning a job of 2.75" diameter with a HSS tool.

The easy formula is 4 X cutting speed (in Feet/min) divided by the diameter (in inches).

Cutting speed for cast iron is about 70 feet per minute.

So 4 x70 = 280

Now divided by the diameter in inches

280/2.75 = 101.8 rpm.

So try cutting at 100rpm and see how you go.

(If the outer skin is very hard,  you might even give 50rpm a go.)

Edited By Hopper on 14/01/2018 12:00:28

[JourneymanParticipant @journeyman]

I would be inclined to use a much lower speed. A mid-range cutting speed for cast iron is 70 feet/min. A quick and simple approximation can be made using the formula: RPM=4xFPM/Dia for the given 2.75" saw this would work out as: 4×70/2.75 = 101RPM (The formula gets rid of Pi and inch to feet conversions). As my maths is usually fairly ropey I stand to be corrected

John

Hopper posted whilst I was typing, we seem to agree on the speed

Edited By Journeyman on 14/01/2018 12:02:29

[HOWARDTParticipant @howardt]

I have sliced up a piece of 1"x 2" cast bar with a slitting saw, approximately 100 inches of cut with a 4' slitting saw at around 100rpm. The biggest problem i found was the cutter jamming so requiring frequent backing out. But had no cutter life problem and is was all hand feed on a Sieg SX2P.

[Paul LousickParticipant @paullousick59116]

I recently cut a set of eccentric castings in half, They are for a 2.5" dia eccentric and 5/8" wide.

Firstly, I machined the sides and bore of the castings to remove the hard outer layer, leaving material for final machining to size. Then used a slitting saw at slow speed to slice them in 2 (can't remember speed). The saw had no problem cutting thru the casting by starting the cut on a machined surface first.

Paul.

[David Hill 5Participant @davidhill5]

Well, thanks everyone. That was great to have so many responses. I did resort to using the hacksaw and milled the hacksawed surfaces flat. I take everyone's point about cutting speeds and will try my next job at about 100 rpm. I must pay more attention to cutting speeds! Thanks chaps, really appreciate your replies.

David

[Neil WyattModerator @neilwyatt]

What hopper says.

In broad terms anything above 100rpm will ruin a slitting saw in short order.

Neil

[David Hill 5Participant @davidhill5]

I can confirm that!

David

[not done it yetParticipant @notdoneityet]

Feed rate is also important. Cut per tooth needs to exceed a value at which ‘rubbing’ can occur.

[Author]

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