Help! > Mistakes, muckups, and dangerous behaviour

Slitting saw and destroying my mill

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jadge:
There are two sorts of slitting saw, fine tooth and coarse tooth. The saw used by the OP is fine tooth. They are intended for cutting shallow slots as on screw heads. For deep slotting a coarse tooth saw is needed, like the one in the video by Jason. A coarse tooth saw has a large gullet so the swarf can be cleared without jamming.

The saw looks very blunt; HSS steel tools should be happy running at several hundred degrees Centigrade, so it if has become blunt during the cut I'd question it's parentage. There's zero point in buying cheap cutters.

For deep slotting with a 3" diameter saw, in low carbon steel, I'd be running at about 80rpm. Like any machining operation depth of cut and feedrate are important, so I don't know why there has been a suggestion to ignore them. For slotting I will take up to 1/2" DOC per pass. Like all tools slitting saws need to cut, not rub, so a sensible feedrate is essential. I normally use around 4 thou per tooth chip load. For a saw with 30 teeth running at 80rpm that gives a feedrate of 9.6" per minute.

Irrespective of the cutting problems I am amazed that the motor gave up the ghost and released the magic smoke.

Andrew

uuu:
I have found that sometimes, when taking a deep cut, the blade bends and I end up with a banana shape.  The blades are quite flexible.  I expect my blade must be worn more on one side than the other.  So doing the cut in more that one pass works for me - the already-cut slot helps guide the blade straight.

Wilf 

jadge:

--- Quote from: uuu on September 19, 2023, 11:45:45 AM ---I have found that sometimes, when taking a deep cut, the blade bends and I end up with a banana shape....

--- End quote ---

I've noticed that too when using a slitting saw on the vertical mill, the blade goes walkies. I've never seen the issue when using the horizontal mill with the arbor supported at both ends, even with 2" depth of cut. So I suspect the problem is due to the arbor support.

Andrew

Charles Lamont:
Andrew, we will have to agree to differ. I would say that for a thin slitting saw in a light milling machine such as many on this forum will have, a feed rate of 4 thou per tooth is, putting it politely, not a good idea. I am perfectly happy to accept that you may be able to do that, but in my opinion 0.0004" per tooth would be more like it for most of us.

Jasonb:
Question is do you apply that 4thou to all the teeth or just the ones making contact each time round  :thinking:

The heavy feed that Andrew is able to take goes a good way to eliminating the usual eccentric running of most slitting saws as all teeth should be cutting something if not the whole 4 thou. On the benchtop hobby machines we probably can't feed fast enough to get all teeth cutting let alone a decent amount.

On the Small SX2.7 in that video any faster feeding would have been too heavy a load for the machine and it would have stalled agaian, I think you can see it at one point where I turn the handwheel a bit too rapidly even with the faster spindle speed. When I originally took that video it was to show that a "high torque" brushless mill may still not be able to turn a large diameter cutter at slow speed. Interestingly my usual X3 which has an older brushed motor like the OP and a high/low gearbox will not be stalled when in low at 100rpm.

I've not tried a carbide saw in the benchtop machines but in theory the ability to run them faster should get the motor running in a sweeter spot and even if the same low chip load at least it is easier to steadily hand feed as the pace gets faster than a snail. Downsid ewould be the saws are a bit more fragile and expensive too.

One other thing to consider as it looks to be the same part that was giving you stringy swarf is that you may not be using a particularly free cutting steel do you know it's parentage or was it just sold as "mild steel"

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