Thread: Lathe carbide tool geometry

If it doesn't work for you, CCGT might give a better finish. The inserts we use at work are Iscar with geometry and coating designed specifically for softer materials like mild steel. The CCGT inserts are generally polished on top and ground on the edges, so they're super sharp, but are often uncoated so they are best in aluminium; they'll still cut mild steel, but won't last as long.

Yeah, I use CCGT all the time for turning operations. Love them. I’ll try and track down a boring bar for them.

Originally Posted by elanjacobs

Try some CCMT060202 inserts and about 0.1-0.2mm depth of cut. That's how I did this. 1045 mild steel, 12mm bar, 16mm bore, 0.15mm DOC, about 5 thou feed, 1000 RPM.

Elan has hit on another key ingredient of insert selection - nose radius. Mostly we tend to just use what we can get, but if you want to take light cuts with good surface finish, the 02 nose radius he's specified here often works brilliantly. I've also found on super gummy stuff sometimes that the 02 nose radius gives a far better finish than an 04 even on deeper cuts.

This thread actually got me off my bum and out to the lathe with some bits of 32mm bar to try out the CNMG tool, which has been a good little learning experience. Two bits of bar, one is something like 4140, the other probably 12L14 (both scrapbinium). First up I went for a 2mm DOC (4mm off the diameter) on the 4140 at 1150rpm with a 0.0125"/rev feed. First time I've ever broken a chip in this stuff (I have a fair bit of it), and they were showering off hard and fast. Could hear the lathe was working, although definitely not struggling, and I'm pretty sure my cross slide got pushed back a touch. Further experimentation with 1 and 2mm cuts showed that RPM didn't seem to matter much on this material re surface finish (at least down to 480rpm), but it needed at least 0.007"/rev feed to break the chip. The 12L14 however, had a slight decrease in surface finish from 1150rpm to 750rpm, and a noticeable dropoff at 480rpm. It would break a chip with slightly less feed. Again varying between 1 and 2mm DOC (off the radius) didn't seem to make much difference.

Of course, at those feedrates you get a 'ringed' effect, and the bar was deflecting (or moving in the bellmouthed chuck jaws), so returning the tool without retracting leaves a groove, and a spring pass leaves predictably unpleasant surface finish. I have the luxury of likely being able to push the DOC up towards 4-5mm and dropping the feedrate down to potentially get a shiny finish without the rings. I didn't bother playing with that at the moment, not knowing what size machine you're running - I figured 1-2mm DOC was likely to be within the realms of the average hobby sized machine, no point me figuring out the limits of my machine for this exercise.

I also threw in the typical SCLCR CCMT tool, and ran a pass on the 4140 at the same 1mm DOC, 1150 RPM and the 0.007"/rev feed, and it probably yielded a slight better surface finish with less load on the machine. Effect would likely have been far greater if I did that test on the AL335.

I have pretty good luck getting decent finishes on light passes with CCMT09T304 inserts as a general rule, but one thing I want to experiment with now I have a big tub of good quality HSS is grinding up a vertical shear tool - could be something worth looking in to? Supposed to be able to take very light cuts (only, no good for any depth at all), and leave a very good finish. Doesn't help with the bores, but if you build yourself a little slowspeed carbide grinder, you can hone your insert edgess to be razor sharp for that purpose - or try the CCGT as Elan mentions....

*edit* Forgot to mention what the CNMG insert was - no idea of brand, I'm guessing probably Seco as most of the random inserts that came with the machine are (no pack of CNMGS). It has a small chip breaker groove, but it doesn't extend to the edge of the insert, so there is a flat land between the side of the insert and the chipbreaker, making it a true negative rake at the cutting edge. Between squinting at the verniers, and the required feed rate, I'd assume the land is about 0.007" wide...

Thanks for your detailed response!

Originally Posted by Jekyll and Hyde

Negative rake is not necessarily negative rake, it all depends on the top of the insert... In the case of your CCMT boring bars, that's not really negative rake in my book. I'd call that extra side relief for getting into smaller bores - but you're not cutting in that direction (unless you're facing the bottom of a bore). If you're cutting on the end as you normally would, the insert is presented flat in that direction, and the grind on the top of the insert makes it a positive rake tool (with a bit of extra shear action going on due to the tilt perpendicular to the spindle).

Oops. I only just realised that. Thanks. It is the angle of the insert base relative to the bar length that matters.

So, what I need to do is heat up the end of the bar and twist it up at the end !!!!

the Graziano came with a couple of Sandvik boring bars that use TPMR inserts

Yes, I have a tin of those inserts with broken edges. They have quite large relief on them - at least 10°?

I believe that true negative tooling needs to be run pretty hard to turn 'cleanly'. High RPM, decent DOC and high feed to keep it from rubbing and smearing, where more positive geometry seems to be more forgiving of a lighter touch (hence why CCMTs work so well on hobby machines). The material you're turning is going to make a difference too, some will work better with positive than negative, and vice versa. Likewise, material diameter will play a big part, as the cutting forces on negative tooling are significantly higher, so material deflection is more of a problem, so I'm not real sure that negative rake is the go for much under 2" workpieces for example. Slop in the machine probably comes into play more with negative tooling as well.

This is the stuff that I'm finding harder to learn;

• My Hercus has a lot of slop, so unless I'm machining brass, everything looks like a roughing cut
• The Hafco AL336 isn't hugely powerful, but tolerance-wise, should be capable enough. I have taken 1.5mm cuts in many random materials, at a variety of rotation and feed speeds, but haven't gotten "wow" results yet.
  Until I get coolant setup, I'll blame the cheap inserts? (most of mine are a dollar or two each)

I know a lot of guys also like WNMGs

I was gifted a real Iscar WNMG06 holder. Haven't been impressed with using it, but am using scavenged old inserts. There are quite impressive insert die shapes available now:

Screen Shot 2021-04-18 at 5.56.05 pm.jpg https://www.aliexpress.com/item/4000703967323.html

Originally Posted by elanjacobs

No. S12M = 12mm shank, so you're looking at 15-16mm minimum bore at a guess

Of course. It would need grinding down in diameter. It is the insert clearance I was thinking of. More angle on the insert means more relief/clearance.

I don't have access to a shadow thingy, but will try to measure the angles and clearances, to see what diameter the insert should clear, as compared to the bar diameter.

Try some CCMT060202 inserts and about 0.1-0.2mm depth of cut. That's how I did this. 1045 mild steel, 12mm bar, 16mm bore, 0.15mm DOC, about 5 thou feed, 1000 RPM.

Thanks for the pic. I currently have CCMT 060204. Will try those on one end, and borrow a sharper/pointier insert to bore the other end

Originally Posted by nigelpearson

...but haven't gotten "wow" results yet.

I use this style of insert when I want ‘wow’ results in ally and MS. The polished edges and large nose radius seem to be the secret.

https://www.aliexpress.com/item/4000859649103.html

Originally Posted by nigelpearson

Yes, I have a tin of those inserts with broken edges. They have quite large relief on them - at least 10°?

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This is the stuff that I'm finding harder to learn;

• My Hercus has a lot of slop, so unless I'm machining brass, everything looks like a roughing cut
• The Hafco AL336 isn't hugely powerful, but tolerance-wise, should be capable enough. I have taken 1.5mm cuts in many random materials, at a variety of rotation and feed speeds, but haven't gotten "wow" results yet.
  Until I get coolant setup, I'll blame the cheap inserts? (most of mine are a dollar or two each)

11 degrees on the TPMR. If you have the Engineers Black Book, pages 128 on cover insert designations, I refer to it often. I've actually got some boring specific inserts coming that look very much like your deep groove TNMGs, will be interesting to see if I have TOO much postive rake...

On those two machines, I'd personally not bother with any negative rake holders. Grab a couple of CCMT tools, and enjoy life! My two favorite tools on the AL335 were an SCLCR16H09, and a SCBCR1616H09 (until I made my own 45 degree lead angle CCMT tool). The SCLCR for general facing and turning, the SCBCR for using the 'spare' corners of a CCMT (and it works brilliantly for heavy roughing cuts).

DCMT and TCMT also would work, and probably have almost the range of inserts available as for CCMT.

Coolant - don't you have a shower in your house? The only time I've ever found it useful was hogging out that 4140 type stuff I mentioned earlier, when it's not breaking chips it makes horrible blue death ribbons that are tough as buggery, and heats the workpiece enough to kind of work harden it, meaning the next pass had to be done at far lower DOC (or wait for it to cool down). Coolant resulted in much softer silver death ribbons, and ability to maintain a 1.5 - 2 mm DOC, but surface finish actually went downhill, and I had to spend an hour mopping the floor, the walls and the ceiling. Just a product of my inexperience, seems like I can break the chip no worries with a CCMT tool now, just wasn't feeding hard enough. Much better on a manual machine to run carbide dry and work out how to break the chip IMO, and you don't have to deal with the rust issue then either.

Originally Posted by jack620

I use this style of insert when I want ‘wow’ results in ally and MS. The polished edges and large nose radius seem to be the secret.

https://www.aliexpress.com/item/4000859649103.html

I've found it can work both ways, I've had some steels respond really well to a bigger nose radius, and others that turned into a horrible mess, but a smaller nose radius worked beautifully...

The AL335 is still here (was supposed to get picked up this weekend by a mate, but life got in the way), so I threw in a bit of that 12L14 or whatever it is. Horrible soft stuff, either way. Larger diameter is 1.5mm DOC (off the radius), smaller diameter is 0.1mm DOC. Both at 1050 RPM, and 0.092mm/rev feed. Photo makes it look worse than it is, the 1.5mm DOC is actually very shiny and smooth, the 0.1mm looks a tiny bit smeared, but would very quickly polish up with a fine grit sandpaper.

Keeping in mind this is horrible soft stuff, and as you can see the insert (knockoff Mitsubishi CCMT09t304) is not exactly fresh either. Just for an idea of how even cheap CCMTs can perform. Surface finish goes way up on better grades of steel.

IMG_1548_1400x1050.JPG

Originally Posted by Jekyll and Hyde

Keeping in mind this is horrible soft stuff...

12L14 shouldn’t be horrible. Maybe it’s 10xx?

Originally Posted by jack620

12L14 shouldn’t be horrible. Maybe it’s 10xx?

Could be anything really, but it seems to be softer and gummier than some small diameter 1020 I have... Could be 1004 or 1010 I guess. Was a bunch of bar ends I got from the scrap yard with serious surface rust on them. Either way, it gives pretty much the worst surface finish of anything I have kicking around here, which is why I grabbed it for this.

I was rummaging around in the shed, and remembered I'd bought some inserts for the CNMG holder - but not CNMGs. I picked these Kennametal CNMP 120408 inserts up for about $40 the pack of 10, but I hadn't actually even taken the box out of the bag yet, so I had a look.

2020_0101_001355_003_1400x1050.jpg
2020_0101_001543_005_1400x1050.jpg

These things are pretty cool - the 'arc' on the cutting edges completely cancels out the negative rake when you sit them in the holder, and presents the cutting edges as slightly positive rake, in addition to the deep chipbreaker that extends all the way to the edge creating more rake. Effectively a completely positive rake insert, but without losing the extra corners you get from a negative rake insert. Be interesting to see how they go.

Looks like TNMPs exist, and from the couple of photos I just looked at they have the same arc on the cutting edge and deep chipbreaker. Perhaps an viable alternative to CCMT/TCMT? The Chinese don't really seem to be making cheap knockoffs (although I did find Lamina brand CNMP and TNMP on Aliexpress for $65), so if you just want cheap inserts CCMT/TCMT holders and inserts would be better. But if you're looking out for name brand inserts on Ebay, looks like CNMP/TNMP are roughly the same sort of price as CCMT/TCMT - but you get double the corners. Just another potential option for those who've already got negative rake holders they'd like to use.

And for those with WNMG holders, seems you can get WNMP inserts as well. Of course, check your clamping method with whatever holder you're using to make sure it will retain it.

OK. My assumption about my cheap negatively-raked tooling was wrong.


I finally did some turning of a nice steel - a tool steel (2373?) - and the results were beautiful in comparison.


First, facing off the roughly bandsawed end of the slug with WNMG 06 holder at a less than ideal angle:
IMG_1418.jpgIMG_1419.jpg
Tried a variety of RPM (from 82 up to 500) and cut depth, but for the first few mm off the face, it always tore/chattered in the middle. It seemed like there was a hard spot in the middle, but maybe the edge on the insert wore out as I went?

(i.e. the shiniest result is probably from a tool slightly polishing as it cuts)


Next, TNMG 16. Acceptable, but I finished with the WNMG:
IMG_1420.jpgIMG_1417.jpgIMG_1421.jpgIMG_1422.jpg
Note the pile of long stringy swarf. My earlier statement that negative rake = tearing only seems true for crappy steel.





All of that turning was on a much larger Hafco with a larger QCTP - the 42 position "Swiss" ones - using my 16mm carbide holders grabbed by 2 of the 3 screws (because my holders are all too short to fit under the 3 screws!). Even with this bigger setup, the tool post was still visibly deflecting on some of the deeper cuts.


Anyway, just to prove it wasn't a fluke on a bigger lathe, today I went back to the AL-336:
IMG_1428.jpgIMG_1429.jpg
This was mainly finishing cuts, so not as deep, and not peeling off metres long strings of swarf, but still spiralling off little blue/purple coloured springs (and sometimes straw or light blue if I pushed it too far).

P.S. This is a die, for punching the recessed hole in the bottom of stainless steel sinks.
Its owner split the original. This will still need hardening, and then final machining with ceramics

Good to see you're making progress. Amazing what a difference the base steel can make, and as you say the harder it is the better your luck will likely be with negative rake.

The bad finish in the centre will most likely be speed related. Facing is often a bit of a struggle on bigger parts, as the rpm that gives good SFM at the outer edge ends up being way too slow in the middle - if you listen to CNC lathes facing, you'll hear the spindle speed increasing as they approach the centre to keep in the sweet spot. To some extent you can cancel it out by feeding by hand, and increasing feed speed as you approach the centre. If the max speed you ran was 500, I'd pretty much guarantee that was the source of the issue - stopping a bit before it went to crap and bumping the speed up to 800-1000 rpm would likely have sorted it.

I actually experimented the other day with the CNMP inserts, and honing the absolutely crap WNMG inserts I have. Honing the WNMG was interesting, although finish only improved a bit on any decent depth of cut, what did happen was I managed to take a 0.04mm DOC (radius) pass and get a pretty damn good finish. Of course, doing the same to a CCMT insert would likely give even better results. I freehanded the nose radius and didn't do a great job, so setting up a rest to do a nice clean nose radius would likely help with deeper cuts, as well as taking the edge back a bit further to clean up the crappy rounded edge these cheap Chinese inserts have.

The CNMPs - they were okay. Not bad by any means, but the CCMT tool still gave a better finish. Not entirely fair, as the grade of these CNMPs is intended for stuff like Inconel and titanium - but they are also intended for stainless, so I tried on some of that as well. Still undecided on whether the CNMP or the CCMT gave a better finish on that, I initially thought the CCMT, but it depends on how the light hits it.

Have to say, the more I play with different inserts, the more I think that if you want to stick with just one insert, CCMT is absolutely the go. They tend to just work for most things, with a big sweet spot...

Originally Posted by nigelpearson

Tried a variety of RPM (from 82 up to 500) and cut depth, but for the first few mm off the face, it always tore/chattered in the middle. It seemed like there was a hard spot in the middle, but maybe the edge on the insert wore out as I went?

Not a hard spot, not wear, just physics. This is due to the surface speed decreasing as you get closer to the middle. All materials and all inserts will have a "sweet spot" for machining speed, once you get below that speed, the tool can no longer form a chip as intended and you get a dull finish, it's particularly noticeable in steels. This is unavoidable in manual machines (unless you have infinitely variable speed, either mechanically or by VFD), but CNC machines can get around it to an extent by ramping up the spindle speed as the tool gets closer to the centre until they hit the maximum speed set in the program (or overload the spindle drive if you forget to add a G50 ); our 20mm swiss lathe tops out at 10k rpm and we still get the dull middle bit, but it's only once you get to about 5mm diameter, depending on the material.

My earlier statement that negative rake = tearing only seems true for crappy steel.

Not crappy, soft. As mentioned earlier, it's all about the insert; the softer the material, the sharper the insert needs to be. General purpose inserts don't come to a sharp point, so they're never going to work very well on soft steels. Have a look through the 16 different WNMG edge geometries HERE, particularly how much sharper the edges on the finishing and soft material inserts are compared to the general purpose ones; if you swap your insert for a different one, you'll get a different result. It also looks like the nose radius on your T insert is much smaller than the W insert, so that will affect the surface finish as well.

EDIT: Looks like J&H was typing at the same time. CCxx is the go for pretty much everything, there's so many different geometries that you're bound to find one that does exactly what you want.

I also did some test cutting of a variety of other materials from the scrap pile. Prepare for copious images...


1) Some Ally at 500rpm with WNMG:
IMG_1403.jpgIMG_1404.jpgIMG_1405.jpg


2) Random steel(mild?) at 500rpm with TCMT 110204:
IMG_1406.jpgIMG_1407.jpg

and the other end of it with the neg. raked TNMG 160402:
IMG_1408.jpg

and again at 1400rpm!:
IMG_1409.jpg

Try a fresh (corner of the) tip at 800rpm:
IMG_1410.jpgIMG_1411.jpg


and as an experiment, RPMT 08 (positive rake round) at 500rpm:
IMG_1412.jpg
which used to be my best finishing tool, but here was worse!


3) Different bit of random steel. Start with TCMT again:
IMG_1413.jpgIMG_1414.jpg
It took me a few minutes to realise this is cast iron


4) Another bit of steel. 500rpm at 30 thou. This is the first one whose swarf didn't fall apart
IMG_1415.jpgIMG_1416.jpg



5) Lastly, a torture test. 6" dia. disc with a hacksaw slit across on the face. Start with TNMG at an odd angle:

IMG_1423.jpgIMG_1424.jpgIMG_1425.jpgIMG_1426.jpg

(which was a little burnt/worn by the end). Finish off with shallow cut by the TCMT:

IMG_1427.jpg


Both tools seemed to cope OK with the bounce over the slit, but this final cut (which actually had the triangle tip at an appropriate angle to the face) was actually vibrating at high frequency over a lot of the surface.




I might retry this last one with the WNMG. Still yet to try the boring bar as per Elan's directions