Slitting saw - how to choose

[mikey]

I'm trying to visualize how you're going to use at least a 3" saw to cut the end of a shaft held in a milling attachment sitting on the cross slide of a lathe. Do you even have room to do it?

Have you considered using an end mill?

 

[Weldo]

I'm not even sure either! I should mock it up and see...

I guess I could use an endmill? It would have to be less than 1/8" and I'd have to take really light cuts but I would have total control over the slot width.

 

[mikey]

Most importantly, it would be doable. You could use a 1/8" end mill to cut out the bulk from the center, then use an indicator to raise and lower the milling attachment to bring the slot to width. You would need to do each step in stages. I mean, cut the center out until you reach final depth, then set up to cut one side and cut to depth is steps, then repeat for the other side.

 

[Weldo]

Ok, so I mocked up and it looks like I do have room for up to a 4" saw blade. I will have to start above the saw and feed downwards.

In the following pic I have mocked up a 4" saw blade and a piece similar to what I need to cut. The piece is projected out form the face of the collet block 1" and my slot will be to depth of 0.875" so there's a wee bit of safety room.



This picture demonstrates how I would need to feed, vertically downwards. I'm guessing I'd want the lathe turning in reverse to make this cut? My lathe does not have reverse but I may be able to feed downward from the backside of the spindle. I actually didn't check that...



The main downside to this set up is that it moves the cross slide a bit off of the dovetail. I still have about 3 turns of the handle before it disengages the cross slide nut.



To make this situation better I could try a 3.5" saw or even a 3". The spacer rings on my arbor (3/4") are 1.125" so I'd lose half of that from the diameter of the saw, plus my depth of cut (0.875"). So I should be at 1.4375", right? Then a 3" saw would be juuust big enough?

 

[Weldo]

I just checked and feeding from behind the spindle is impossible. So that leads to my next question.

Does it matter how you feed a slitting saw? If I can't do it as pictured above I'm gonna have to add reverse to my machine. Which is something I've wanted to do for a while anyway.

 

[mikey]

The way you've mocked it up would work with a slitting saw. The diameter of the saw you get must account for the depth of cut and the OD of the saw arbor/holder. You have to have enough blade exposed to get your depth of cut plus a little clearance. They sell saws with the width of cut you need but you'll have to search for the right saw diameter.

Keep in mind that saws have different hole diameters. The reason this matters is because the slitting saw arbor you use must fit that saw hole fairly precisely. The most common holes are 1/2", 5/8" and 1". 3/4" is not a common arbor size, although the multi-step ones will work or you can make your own. The bigger the hole in the saw, the bigger the arbor will be and therefore, the bigger your saw OD must be in order to have enough blade area for your depth of cut. Typically, a the arbor for a 1" saw hole will require an arbor with about a 1.5" OD. Add in your 0.875" depth of cut and you're looking at a 3.5" - 4" saw diameter.

You may not be able to find a blade of the right diameter, arbor hole and thickness you need. It is possible to stack two thinner saws on the same arbor to get the width of cut you need.

Also, the rule of thumb for the number of teeth required is to have at least 3 teeth engaged in the cut at all times. For aluminum, you want a coarser saw to enable chip clearance when possible. This is not a hard and fast rule. If you have more teeth, just feed slower; the saw will still cut fine.

 

[Weldo]

Thanks for the advice! I have a 3/4" and a 1" arbor so hopefully I can find something that works for me!

Do you think it would work Ok to feed downward like in the second picture above? It seems likely to grab or chatter as the teeth engage the end of the stock.

Also should I be attempting to set the full depth of cut and just feed slowly or make multiple passes while incrementally increasing the depth of cut?

 

[mikey]

Do you think it would work Ok to feed downward like in the second picture above? It seems likely to grab or chatter as the teeth engage the end of the stock.

I sure hope so because there isn't any other way to feed into the stock with your set up. I would try it and see. It's going to be fun trying to feed smoothly while also lubricating the cut. Slitting saws need a lot of lube to reduce friction and help chips flow out of the cut so your can feed hand over hand to provide a smooth feed, while also lubing the cut with your third hand!

Also should I be attempting to set the full depth of cut and just feed slowly or make multiple passes while incrementally increasing the depth of cut?

Slitting saws are intended to take full depth cuts so set your full 0.875" depth of cut and use your feed to control the cut. You want to feed fast enough to feel a slight positive resistance to the feed and then maintain that feed. This keeps the saw cutting continuously but without forcing the cut. Works good this way. I've been doing it for a very long time using manual feed and have yet to see it fail. Steel, stainless, aluminum, brass and one time, wood. The technique is the same, the saw is the same.

 

[Weldo]

Awesome, thank you!

One last thing, what about these fancy "side milling" saws...



Do they offer any significant advantage over a simpler blade like the one below?

 

[mikey]

Yeah, those big saws can take off a lot of material at a time but you need a lot of power and rigidity. Your set up is nowhere near rigid enough for that kind of saw. I would stay with the slitting/slotting saws if I were you.