Parting is such sweet sorrow

[PeterT]

I don't think so. Because the relief on the tool shank is typically based on superimposing the maximum parting OD of circular stock.
So if max stock diameter for parting was 1" the circular relief would be something like orange circle - offset a bit more. But cutting into wall thickness of a larger (example 4") diameter would look like this. It interferes with the 1" diameter relief pocket long before the tool can penetrate the same 0.5" At least that's how my parting toolholders look. You would be better off using a straight blade parting tool within its stick-out limits.

 

[Susquatch]

It interferes with the 1" diameter relief pocket long before the tool can penetrate the same 0.5"

Good catch. You are right. Mine actually has roughly a 1/2 inch radius is the tool support web. Just on a WAG, it might be able to cut a 1/4 inch wall on a big pipe. The blade type parting tools would be much better.

I'm surprised at how much deeper we can assess such a thing just discussing it.

Truth is that I prolly wouldn't part something huge anyway. I'd cut it on the bandsaw or chop saw or even an OA Torch with lots of margin for error and then clean up the backside on the lathe.

It would be nice to be able to part something like say a 3" in diameter bar though. I have a 1" tall blade that takes these same GTN3 Inserts. But I don't have a holder for it. A plinth for that 1" blade was going to be the outcome of this thread. Another project I need to move up the priority list lest it get forgotten totally.

 

[CWret]

The only drawback is that this particular tool is limited to 1" OD.

1- Thanks Susq, that tool answer one of my questions “how to hold a parting tool is my small qctp with max height capacity of 1/2?” (Myford has a big heart but it is pretty small).

2 - I noticed that Clough42 has removed his compound. That’s an idea that @gerritv suggested is pretty helpful for a smaller lathe. Gonna try that!

3 - I thought using cutting oil with carbide was not recommended?

 

[Susquatch]

So, I usually start medium feed then switch to slow feed, then sometimes go manual for the last little bit. And the deeper the parting, sometimes you have to peck at it and throw in more lube/coolant.

I was just reviewing the history of this post and noticed this.

I would have thought faster was better as the cutting diameter gets smaller.....

I think pecking and the need to get the hacksaw out was what got me looking for better ways to part in the first place. Pecking is one thing, but I flat out hate the hacksaw.

I was surprised at how well the GTN inserts work if you "Give er".

Ya, time to rethink a dedicated parting plinth.

 

[trlvn]

@CWret Mark Presling did a video series on a rear mounted parting tool holder. I think he makes a good case that mounting the blade on the rear has a lot of benefits. The T-slots in your cross-slide would make it pretty simple to add such a system.

In fact, you could also follow the Clough42 approach and exchange the compound for a solid piece, as well. Then compare and contrast the two for parting ops!

Craig

 

[Susquatch]

1- Thanks Susq, that tool answer one of my questions “how to hold a parting tool is my small qctp with max height capacity of 1/2?” (Myford has a big heart but it is pretty small).

You can get a larger tool and mill the top or bottom of the tool bar a bit to fit. The one I have in the photo was originally 1" and I milled it to 3/4 to fit a tall tool holder. It was originally purchased by another member for a smaller lathe but even 3/4 would have been way too big. He gave it to me cuz he figured I could make it work. And I did.

2 - I noticed that Clough42 has removed his compound. That’s an idea that @gerritv suggested is pretty helpful for a smaller lathe. Gonna try that!

The whole point of a plinth is to improve rigidity for tough jobs - like parting. They say that there are three secrets to good successful parting - rigidity, rigidity, and rigidity. Parting is all about rigidity. So ya, your thinking about making a plinth for your lathe to improve its parting performance is in the right direction.

But I suspect that you will have to either make a custom plinth (like the subject of this thread) or at least a custom tool holder. Provided that the tool stick out isn't too great, rigidity can be significantly improved just by making the tool holder the perfect height so its bottom rests on your compound. The only thing better would be such that it sits directly on the cross slide. Your cross-slide has T-Slots. So you have lots of opportunities to have the best of both worlds. A true quick change tool post AND a removable Plinth that fits the appropriate T-slot.

Of course, none of this is really required if you part upside down. In this case, rigidity can actually work against you cuz you actually want the tool to let go. Your threaded spindle is the problem there.

Even so, with T-Slots you have the option to part upside down on the backside! A perfect solution provided your cross-slide slots will reach.

3 - I thought using cutting oil with carbide was not recommended?

Who told you that? I use it all the time and it helps significantly. You might have heard that about aluminium or brass. I do cut brass dry but I seldom use brass anyway - it's too expensive. For aluminium, I sometimes cut dry just because I can but a little WD40 will reduce build up on the tool and improved cutting surface finish too.

Edit - I see @trlvn beat me to the punch. Nothing he said conflicts with my advice.

 

[thestelster]

3 - I thought using cutting oil with carbide was not recommended?

You don't want to cause thermal shock to the insert. So, start with cutting fluid, and continue applying until you're finished. You shouldn't apply cutting fluid half way through a cut.

 

[Susquatch]

@CWret - one thing that hasn't been mentioned is the advantages of a powered parting cut.

Clearly, a powered cross-slide is the way to go if you have one. And parting advice often includes a rather healthy feed rate. But you don't have a powered cross-slide. So you will have to learn how to produce a steady feed rate in the right range manually.

Fortunately, you have a mill that requires you to do that already. So I'm pretty sure your technique is already very good. My advice here is to unlearn the "feel" - but only for parting. What feels like a good rate to your hands holding the wheel probably isn't enough. In my experience, you really do need to "GIVE ER". This gets scary. Every fiber of your body will be screaming at you to back off. Even worse, you might break a few tools because you did, but your mind will tell you that they broke because you were pushing too hard. Your mind will be wrong. You will just need to grow a pair, and develop a good firm steady hand. I'd suggest using some calibrated facing cuts, a known surface speed, and a stop watch to measure your manual feed rate. Then trust yourself and ignore that voice in the back of your head.

 

[MrWhoopee]

I use inserted carbide tooling for everything on my Heavy 10, everything EXCEPT parting. I don't use power feed, I like to be able to feel the cut. I long ago learned what @Susquatch stated to eloquently, if it starts to chatter, feed it harder. Parting is no more stressful than turning.

Back when I had employees, we got several of the GTN style parting blades. Every time someone had an oops, it wiped out the insert AND the blade. It didn't take long before we were back to HSS.

 

[CWret]

Mark Presling did a video series on a rear mounted parting tool holder. I think he makes a good case that mounting the blade on the rear has a lot of benefits.

Thanks for posting that. Really liked his schematic explanation why a rear mounted parting tool has geometric advantages (seemed really convincing). I agree that the Myford cross slide T-slots would simplify this build (but the screw-on chuck is an issue, as Susq pointed out below).

The whole point of a plinth is to improve rigidity for tough jobs -

Plinth? Architecturally a plinth is a large solid rectangular foundation. So exactly what is a plinth on a lathe? - Is that a tool holder sitting on top of the cross slide without a compound sandwiched in between?

Your threaded spindle is the problem there.

I’m guessing this is more than JUST a problem - if parting loads are heavy or abrupt or chattering- then the chuck will just let loose, so this then rules out parting upside down from the back with a threaded spindle - right?

Re: my comment / question about not using lubricant on carbide:

You don't want to cause thermal shock to the insert. So, start with cutting fluid, and continue applying until you're finished.

Susq also said “where did you get that idea”
Guess I got this idea (advice) somewhere from YouTube. I recall being told “When milling with indexable EMs, coolant/lubricant causes thermal shock unless it is a flood coolant/lubricant” so i thought the same applied to the lathe. My understanding was that with carbide use either flood coolant/lubricant or nothing.

 

[Susquatch]

Plinth? Architecturally a plinth is a large solid rectangular foundation.

A large solid rectangular foundation.

So exactly what is a plinth on a lathe? - Is that a tool holder sitting on top of the cross slide without a compound sandwiched in between?

Doesn't matter where you put it. As long as it's a large solid rectangular foundation, it's a plinth. But I'd see it going on your cross slide.

I’m guessing this is more than JUST a problem - if parting loads are heavy or abrupt or chattering- then the chuck will just let loose, so this then rules out parting upside down from the back with a threaded spindle - right?

Nope - the opposite. Upside down on the back is the same as right-side up on the front. The chuck will not unscrew.

Guess I got this idea (advice) somewhere from YouTube. I recall being told “When milling with indexable EMs, coolant/lubricant causes thermal shock unless it is a flood coolant/lubricant” so i thought the same applied to the lathe. My understanding was that with carbide use either flood coolant/lubricant or nothing.

I don't agree. Shock can be a problem but as long as there is some there, you won't get shock.

Except for brass and bronze, I use cutting oils with almost everything I do carbide or not.

I have no flood or mist. It's not an issue. I just use a drip for parting or a film for turning. There is no thermal shock.