Parting Trick

[mikey]

I just bought this but not sure about one detail.

Cut Off Parting Blade High Speed Steel 3/32 Inch x 1/2 Inch x 4-1/2 Inch | eBay

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It has been my understanding that the front edge needs some relief, L to R and it's not clear if this has any or just assumes the user will grind in the relief.

Which side the relief is on depends on which side is the keeper part.

If all this is not a figment of my imagination, one would have do something other than just turn the tool upside down, no?

I really wanted a 1/6" tool but the price was too good to quibble.

js

You bought a P2 blade and it will work. All of these blades need to be sharpened and I recommend that you grind it flat across the front instead of at an angle. The reason for this is so you can use it to part, cut grooves with actual flat bottoms and demarcate cuts with it.

I also recommend a 7 degree relief angle under the cutting edge. This leaves the cutting tip in front, with the area below it angling back at a 7 degree angle. This is a compromise that allows free cutting with good support under the edge for long life.

In order to use a parting tool upside down, you need a tool holder that will allow you to get the tip of the blade at the center height of your lathe. You can make one or buy this one. All of this may seem like a hassle but I assure you that a good parting set up is worth the work.

 

[pontiac428]

Mikey,
I also grind a 7 degree relief in P-type blades, because it is a good universal clearance angle (5 to 10 degrees, depending on material). I've experimented with different relief angles, and it makes no difference to the cut as long as there is chip clearance (which on round material should exist regardless). Not much difference there. My tool holder is raked up 5 degrees (fixed) so the back rake is always the same. Do you have any recommendations for ideal back rake angle for parting blades?

 

[mikey]

I think you're referring to the ubiquitous tool holder that holds a parting tool at an upward angle that are used on many Aloris-type tool posts. I have one of those, too, and I don't really like it because I feel it puts an excessive load on the cutting edge in an area where it has little support. A P-type blade is built like a girder and it relies on that construction for its load bearing abilities. By angling the blade upwards, we eliminate much of the support the body of the blade supplies and I think this adds to the potential for chatter. Moreover, if you do not get the cutting edge dead on center the tool will not work well and I think a lot of the problems folks have with parting is due to the tool holder (and the assumption that close to center is good enough).

I believe a horizontal blade configuration is much stronger and makes better use of the support the body of the blade provides. For example, the rear mounted tool holder I made for my Sherline allows me to use a P1-N blade that is only 0.040" thick to cut material that is double the size that is recommended for this tool. Ordinarily, a P1-N is limited to roughly a 3/4" work piece; I have cut a 2" OD work piece with that blade with no issues at all. I honestly feel this is because the tool holder holds it dead vertical and provides a lot of stiffness to back the cutting edge. My tool holder gets the tip of the blade on the exact centerline of my spindle; it is machined to put the tip there and only there. As a consequence, it works.

My suggestion is to make a tool holder for your parting blade that holds it dead horizontal and dead vertical. It should allow the body of the blade, the part under the wider top edge, full contact with the tool holder along its length. You can make it from steel or aluminum, both will work. I need to run out for a bit but if you want to discuss this tool holder further, we can.

 

[pontiac428]

Mikey,

Thank you. Yes, I am talking about the ChinAloris-type tool holder. I measured the back rake at 5 degrees because I cut the P-type blade groove in the holder over the weekend. The blade cut about the same as usual afterward, maybe slightly better, but that may have just been from the fresh hone. I've been parting with a 0.062 blade lately because the P1-N 0.040 blades have been exploding in my face. I don't like the lack of full support on that tool holder. It doesn't hold my tool true against a precision straight edge, and it allows for the parting blade to wag, leading to harmonics that combine with the QCTP's vibes.

Unless I go through with the idea of making a plinth cross slide for my Atlas, I'm stuck using the QCTP. There is no rigid way to do an inverted parting tool mount. I could make a base to replace the compound slide using a spare pintle top plate and mount the inverted tool to that. On the QCTP, I have a 9/16 stud and cone flange nut holding the tool post in place. I get most of my noise/vibes from the tool post through to the carriage; it's not a very rigid setup. Oddly, I have the best results parting stainless because of the need to maintain heavy infeed. With brass, aluminum, and steel, it just leaves room for improvement.

I don't want to crank down my gibs. I'd rather leave them in adjustment. I would consider replacing one center-located gib screw per gib (carriage, cross, top) with a thumb screw for quick lockouts. I need to get some small stock to work from for that. All in all, my machine is rather tight but it's a (minor) compromise to work around wear areas in the ways. My babbitt head H-48/10F was one of the first off the line for 1937, so it's a little older than most. My spindle bearings are adjusted fantastically. Just background.

So while I consider my mounting options for an inverted tool holder, I am interested in being able to adjust the back rake angle on the parting tool. I could machine the 5 degree shelf to another angle easily (permanent). I could shim or angle-block the tool. I should also double-triple check my center. I use a planer gauge that serves me well for that. Aaand, I could buy another lathe... like a 14" machine from the '70s from Japan or Europe. Yeah, I'll do that later. But for now, I'd like to get parting to be something I'm successful at in 2 out of 3 attempts, at least.

 

[WarrenP]

Only thing I think of is, if your doing your testing with PVC I dont know that you can really tell a difference. PVC is very easy to cut , should be able to cut it easy both ways. Try your testing with steel to see if whatever your trying really does make a difference....

 

[Illinoyance]

Your animation shows very clearly why parting off running forward tends to jam the tool. Running in reverse tends to allow the tool to move away from the work, preventing jams. Well done.

 

[jschmidling]

You bought a P2 blade and it will work.

I spent the last 20 mins trying to find out what (P2) means or even just the P. I see that T type is a cross section but can't find any pic of the cross section of P.

>In order to use a parting tool upside down, you need a tool holder that will allow you to get the tip of the blade at the center height of your >lathe. You can make one or buy this one.

I recently bought a low end QCTP and it looks like the half inch blade can be centered either way.

One other thing occurred to me and that is, would not this be a better way to cut threads for all the same reasons?

Thanks,

js

 

[NortonDommi]

All of these blades need to be sharpened and I recommend that you grind it flat across the front instead of at an angle. The reason for this is so you can use it to part, cut grooves with actual flat bottoms and demarcate cuts with it.

Agree also an angle on the front face introduces a side load which is definitely unwanted on smaller machines.

 

[mikey]

I spent the last 20 mins trying to find out what (P2) means or even just the P. I see that T type is a cross section but can't find any pic of the cross section of P.

>In order to use a parting tool upside down, you need a tool holder that will allow you to get the tip of the blade at the center height of your >lathe. You can make one or buy this one.

I recently bought a low end QCTP and it looks like the half inch blade can be centered either way.

One other thing occurred to me and that is, would not this be a better way to cut threads for all the same reasons?

Thanks,

js

JS, here are the sizes:



The critical thing is that the cutting edge is at the exact centerline of the lathe. If your holder can get it there then you're all good. If not, either buy or make one like the Eccentric Engineering FoR tool holder.

Threading is not usually an issue. However, you can turn the tool upside down and feed toward the tailstock if you prefer. I have done this and it works quite well. I just prefer to feed towards the headstock; been doing it for several decades so it isn't an issue for me. Neither is parting for that matter.

 

[mikey]

@pontiac428, I would take a good look at the Eccentric Engineering FoR tool holder or make one to fit your lathe and tool post that will allow you to mount your parting tool upside down and still get it on the lathe centerline. That way, no need to mess with your lathe or anything else. You do have to be able to run in reverse, though.

I did want to mention the getting on center thing. In my opinion, this is one of the most important things to get right when using lathe tools and parting tool in particular. I suggest making a tool height gauge or you can use your planer gauge if it works for you but you need to measure the exact spindle centerline height with a cut. I'm sure you're capable of it already but for those who may someday read this, let me detail what I'm talking about.

Determining the lathe's center height is a really important parameter to know. I know many recommend putting a gauge pin in a collet and measure it that way but since we're talking about tenths here, I think the stacking tolerances with this method is excessive. A much more accurate way is to put a piece of rod in your chuck; doesn't matter if it is a 3 jaw, collet chuck or whatever because this is a first order operation so whatever you use will be accurate as long as you don't take it out of the chuck. Take a 0.010" depth of cut, then a 0.002-0.003" deep skim cut to produce a nice finish. Now use a height gauge to measure from the bed or cross slide or back side of a flat compound and measure to the top of the pin you just cut. Then measure the OD of the pin you just cut, subtract 1/2 of that from the height gauge measurement and you have you lathe's centerline height. This is specific to your lathe and very accurate; make sure to write it down where you won't lose it.

Use this to make a tool height gauge and it will serve you well.

Parting tools need to be on center height. They do not work well when they are above or below center height. Eyeballing it doesn't work all that well, nor does trying to use a tailstock center to align it. Use a tool height gauge or something like it and you will see a difference when you part.