Question: Parting Blade Thickness

[Glenn Brooks]

Revisiting my previous comment... today experienced a lot of chatter when parting. Checked the blade and it had gotten ever so slightly roughed up on the cutting edge, from the previous part. “Chattered” to beat the band... as it were.

So touched up the cutting edge and lost most of the chatter. Not all... checked again and the blade has a slight bevel and cant to the cutting edge - still causing some chatter. Tomorrow will regrind in better light, attempting to make a flat, straight and horizontal cutting edge. Which is what I started with before it wore down...and started making noise.

So for me, still, a high grade steel blade first. Then grind a nice square sharp cutting edge seems to be very important.

Glenn

[RMinMN]

Regrind, then hone the cutting edge. I've been able to do a reasonable job of parting on my mini-lathe by keeping that edge very sharp and everyone says that a mini-lathe is as rigid as a wet noodle.

[John Hasler]

Parting without tears

I give my parting blades both side and back relief. I grind them so that all of the tool that will ever be in a cut tapers slightly from top to bottom so that the upper face is widest. This reduces the risk of chips getting wedged in. I also add a slight front to back taper so that the cutting edge is slightly wider than the body. This is needed because the groove is no wider than the cutting edge. As nothing is ever perfectly flat, perfectly straight, or perfectly square leaving the body the full width of the edge is certain to result in rubbing. Top rake can be slightly positive but must never be negative. Everyone knows that the tool must be accurately on center, of course. A small amount of front relief is needed. I've never tried to go past about 8:1 on length to thickness. Usually less.

Use lubricant and keep the chips cleared out. They can pack into the groove above the tool. Some people put a chip breaker in the center of the edge but I've not found this to be necessary. Lock down everything and minimize overhang. Rigidity, rigidity, rigidity.

In difficult conditions it can help to stagger the cut so that the groove ends up wider than the cutting edge.

I still break one every once in a while, though. I've only been at this a few years: I'm sure Harold knows a thousand times as much about the subject as I do.

[BadDog]

That's why I hate grinding parting tools. Without using something like a fixtured grinder, it's hard to keep it square to the shank (otherwise harder to align), and tapering for clearance all around as described.

Blades don't taper along the sides, but make a decent compromise in my expended effort, though for years now I more often grind them for grooving rather than parting (saves a lot of grinding from square). I have ground the side clearance into a blade, but of course that presents it's own problems for general use (different depths). The latter can be mitigated by having multiple blades. Just make sure you have the right holder. Not all blades work in all blade holders. And again, not the cheap garbage.

If you can manage it (budget/machine), for my preference, using an insert parting blade makes life so much easier and less stressful. They just work so much better and the inserts have silly longevity. The top profile sort of bows the chip to provide clearance exiting the slot, and it rolls up these cool little spirals that look like clock springs. I started using them on my 11" Rockwell, but bigger is better. My 17" lathe parts large steel shafts with no drama or stress at all (theoretical up to 2", though I've probably not done above 1.5" or so)

[John Hasler]

BadDog writes:
I have ground the side clearance into a blade, but of course that presents it's own problems for general use (different depths).

The taper doesn't need to continue all the way back: just far enough to make the tip the widest point.

[Harold_V]

It isn't common for the cutting edge to degrade from use, especially with one cut, unless you're running unreasonably fast. You can get away to some degree with excessive speed if you're using Stellite, which tolerates heat far better than HSS. It isn't commonly on the market these days, however, although still available. TanTung is one of the options. Cutting tools of this nature are NOT magnetic, which may help a person identify them when markings may be absent.

Fast is a recipe for chatter. Nothing cures chatter when parting, assuming you have the necessary rigidity, than slowing the spindle and increasing feed rate.

I do NOT own a parting tool, one that is made commercially, anyway. I hand grind all of mine, starting with ½" blanks. They are ground as described by John, with slight taper in all directions, so only the cutting edge contacts the part. I use the periphery of the wheel to create a slight positive rake. Tools so ground will often create a coiled chip that accumulates in the grooved. Lubrication is a must, but it need not be flood. I generally use an acid brush dipped in the lubricant of choice for the material I'm parting.

H

[John Hasler]

BTW an angle grinder works fine for roughing out a parting tool. No need to remove all that material with the pedestal grinder. I do all the roughing from one side so that when I start the finish work on the pedestal grinder the other side is still flat and square.

[BadDog]

The taper doesn't need to continue all the way back: just far enough to make the tip the widest point.

Absolutely. But if you want the clearance, you have to grind sufficient for depth of cut. But then if you lose the end (fast, chatter, inclusion, etc), you now have to remove the ground area, or if acceptable, take that as the new "wide" area, and taper from there, repeat until not acceptable. If you are grinding for 1" DOC, and don't want to start from narrowed area, you loose 1" each time you need to do more than hone the edge. Of course, as long as you minimally clear, it won't matter too much the first few times, until it does...

For parting lube (and others actually), I've found nothing better than a steady drip from a "lab washdown" container. No leaks, no inverting, very controlled. And works great even when DEEP into a parting pass.

Also, as Harold noted, feed more than you think you need to. At least that was true for me. It's scary how fast (feed, still relatively low surface speed) you can part with those insert parting blades, and never any chatter. I grind a lot of my lathe tools, but having these on hand, I'm just not motivated enough to do it for parting tools...

[John Hasler]

When I break a parting tool it becomes a grooving tool. Doesn't happen often any more.

Took me a while to understand that more feed solves chatter. That only works up to the limit of the rigidity of your lathe, though.

I'm getting bored with grinding and honing HSS (not just for parting tools), though. I've got some positive rake carbide inserts that look pretty sharp. After I get time to make some holders and experiment with them I might go over to the dark side.

[mcostello]

There is a chart in The Machinist Handbook giving cut off blade thicknesses recommended for commercial uses when saving material matters.

[BadDog]

When I break a parting tool it becomes a grooving tool. Doesn't happen often any more.

That's about all I use HSS parting blades for anymore. Fast and easy to make a snap or o-ring groove tool.

I'm getting bored with grinding and honing HSS (not just for parting tools), though. I've got some positive rake carbide inserts that look pretty sharp. After I get time to make some holders and experiment with them I might go over to the dark side.

If you can, and have a lathe to run it, I highly recommend the sandvik blades. I got comfortable enough with HSS parting way back when, but it was always a bit stressful even though it mostly just worked. First of these I used was at a friend's shop running his Mori Seiki, and it blew me a away. He had some extras (he buys/sells tooling), and I bought a small one with a pack of inserts from him that day. Since I started using these blades, particularly with my current 17x60 lathe, parting doesn't give me any more pause than turning OD. And it's so fast, plus produces those cool little heavy clock springs that don't bind in a cut even when down nearly 1" deep.

[John Hasler]

$190 for just the blade? I can grind a lot of HSS for that. I don't think I can afford anything Sandvik sells.