Slitting saw and destroying my mill

[redhouseluv]

And another topic from me ......... this time using a slitting saw and destroying my mill  Everything was going well making a mandrel until I had to cut the length of it.

I was taking .010" cuts at a time/ speed 1100 rpm and used oil; I thought this was okay, clearly it was get very hot from the colour of the mandrel and the saw blade and the amount of smoke as I applied more oil. What I didn't realise was some of the smoke was coming the motor and it was not until I heard a strange noise that it dawned on me.

I now have a broken mill .......hopefully Warco will have a replacement motor in stock and I haven't damaged anything else!

What did I do wrong; I don't want a repeat performance, an expensive one ........

[Dave Otto]

Way too high surface footage for that cutter, with out knowing the exact diameter I would guess two or three hundred RPM would be more appropriate.

Dave

[Vixen]

Geeez

You did a pretty comprehensive job on the slitting saw, workpiece and milling machine.

Next time, try a lower speed around 200 RPM or less. The replacement cutter should remain silver coloured, the steel workpiece should remain silver coloured and the chips should not be coloured either. A flow of suds coolant will help tremendously.  If anything starts to change colour, or starts to smoke, then the cutter surface speed is far too high and you are driving everything way too hard.
A happy slitting saw goes zick zick zick.

Mike

[Jasonb]

You need to look back at the other recent advice you have been given about speeds, doe snot matter if the work or the tool is rotating the speed where the two meet is what matters.

Assumimng an HSS saw blade of 100mm dia then the surface speed at the tips is 3.142 x 0.1m = 0.314m/rev the cutting speed for HSS should not be more than 30m/min.

30/0.314 = 95rpm you you were about 12 times too fast and also taking much too shallow a cut.

75mm saw would be 30 / (0.075 x 3.142) = 127rpm

BASIC Rule as diameter of tool or workpiece increases rpm decreases

[Jasonb]

Your arbor is also not shaped like the drawing showed. You simply need a cut that goes right through and splits the arbor in half.

A simple hacksaw cut is all that is needed and is a lot less costly

[redhouseluv]

Oh dear, oh dear ......... yeah, I've done a proper job this time; the saw was 3" (75mm) in diameter BTW as per Jason's calculation. What sort of depth of cut should I go for at the speed specified?

I've just spent the last 1/2 hour dismantling the mill top end and it looks like (and smells like) its just the motor that's cooked (see pics); the gearing all seems to be fine. Looks like the motor is single unit so will have to do straight replacement - fingers crossed they have one otherwise I'll be out of action for a while.

Your arbor is also not shaped like the drawing showed. You simply need a cut that goes right through and splits the arbor in half.

A simple hacksaw cut is all that is needed and is a lot less costly

hmmm, so I can use a hacksaw, that's interesting, the original article is what led me down this path. It suggested a 1/16" slitting saw as far down the length of the shaft as possible. Which bit of the arbour is incorrect, it may look odd as I turned the piece around halfway through the cutting fiasco, so I haven't cut the arbour in half, but cut a little out of each side hence its appearance

A lesson learned, a costly one; I won't be doing that again ........

[john mills]

as said the dia of this slitting saw should be run  slow   often the trouble is the machine has not got a slow enough speed so that has to be
it can be run slower that Jasons 95 rpm wont damage the cutter and make it last longer.Usually the slitting saw should do the full
depth of cut in one go.   a proper cutting oil can be ok but a modern soluble cutting would be better.
John 

[Jasonb]

It was the part cut down the opposite sid ethat made me say it was incorrect. You could have cut right through that in one steady pass though a saw with less teeth of similar diameter would be better a sthe bigger gullet between the teeth won't clog with swarf, keep the finer toothed saws for things like slotted screw heads.

As John says some smaller hobby machines can suffer with stalling due to  acombination of slow spindle speed and the large dia of the cutter. If you mill has high/low speed range use the slow range and you may have to overspeed a little to stop stalling and/or cut say half depth at a time.

Looking at that motor I wonder if it is just the brushes that have overheated as that is really the only place it looks cooked

Here is a video that first shows a smaller mill stalling (no hi/lo option) when trying to cut at calculated speed, Speed then increased to when it will cut. The feed is not very even as I was doing it one handed due to holding the camera you should have both hands on the handwheel rim to keep a constant feed  and would also have added a bit of cutting fluid with a brush is doing it for real.

<a href="https://www.youtube.com/watch?v=_nOcZWfiI2A" target="_blank">http://www.youtube.com/watch?v=_nOcZWfiI2A</a>

[redhouseluv]

Thanks a lot for the advice and I will definitely proceed with a more cautious approach once the mill is back in action. So slower speed and one pass is the correct way, I was trying to do the opposite i.e. a little at a time along the length, then come back to the beginning and start over

I have now seen a few mentions of issues with the motor units on these type of mills (notwithstanding operator misuse ). I too wondered if it could be just the brushes and will see if that maybe a cheaper option. I have found the same motors on eBay so it'll be interesting to see the price Warco quote me!

[Charles Lamont]

What sort of depth of cut should I go for at the speed specified?

Don't think about the depth of cut. Feed a slitting saw very gingerly by hand. Mike said how a slitting saw should sound when cutting. It will not be running dead true and if you are using it within its comfort zone it will only cut on part of the turn. Use the table slide lock to give a little bit of resistance so that it cannot grab. Do not climb cut, but if you have to, you need the slide pretty stiff.

It can be a good idea to run clockwise when looking at the outboard end of the arbor so that if the saw wants to jam it just unscrews the clamping screw.

[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]

I have found that sometimes, when taking a deep cut, the blade bends and I end up with a banana shape....

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 

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"