Thread: Lathe carbide tool geometry

Most of my carbide holders, which are cheap Chinese copies of industry standards, have negative rake. e.g.:
Screen Shot 2021-04-18 at 2.50.34 pm.jpg
That one is designed for TNMG inserts, which are reversible so have no relief. The 6° down rake is to stop the side of the insert from rubbing under the cut - basically 6° relief.


Unless I get very well moulded inserts which have ridiculous top angles:
Screen Shot 2021-04-18 at 2.58.33 pm.jpg
then this tool will never cut well - it will always roughly tear chips off instead of machining nice spirals.


Boring bars are a more interesting example:

s-l1600.jpg

• The original Iscar S08K SCLCR-06 seems to have 11° downward rake: https://www.iscar.com/eCatalog/Item....FSTYP=M&srch=1
• I think my Chinese copy seems to be at least 15°!

So, given that this one takes CCMT inserts, which have 7° relief, why the hell do they grind the flats on the thing to have an additional 10+° of negative rake??????



(Threading tools have flats similarly ground, but their inserts are always aggressively raked, so they seem to zero out. The better shaped inserts even have nice shapes to carve metal nicely:
Screen Shot 2021-04-18 at 3.20.41 pm.jpg
)




Anyone worked out how to grind/pack them into a toolholder so that they are closer to zero?

(I was thinking of just grinding the top flat around, and mounting it in the V groove in the bottom of a "boring bar QCTP holder", but thinking further, that wouldn't really help.)

Hi Nigel, Guys,

I thought that was what the seat shims were for !

The boring bar for inserts will always look negative, the turning tool change the for positive rake holders, get ones that take the CCMT.
The threading tools have a shim under the insert, as with the boring bar they will be negative in appearance , I would suggest that you don't grind the top of the insert, only the sides if you need to reduce the nose radius.

modern carbide inserts are completely designed around tearing the material into small chips, long spiral chips and cnc lathes aren't a good combination.

Originally Posted by nigelpearson

So, given that this one takes CCMT inserts, which have 7° relief, why the hell do they grind the flats on the thing to have an additional 10+° of negative rake??????

Simple: if the insert pocket was neutral, the bottom corner of the insert would rub on the inside of the bore.

The bar you linked is 8mm for a minimum bore of 10.5mm; with the extra 11 degrees, you have 0.2mm of clearance from the bottom corner of the insert to the bore, it can't physically be anything less than that. Without the extra negative rake, It. Will. Not. Fit.

EDIT

Anyone worked out how to grind/pack them into a toolholder so that they are closer to zero?

Don't. I promise you the manufacturers know what they're doing better than we do.

Originally Posted by BaronJ

Hi Nigel, Guys,

I thought that was what the seat shims were for !

Heh. Maybe if the shims were ground as wedges


I stopped using the Hercus' lantern holder years ago.
Only way I can think of tilting/twisting in a 4way or QCTP is with wedges -
either under the boring bar or insert holder, or under the toolpost block?

Originally Posted by nigelpearson

Most of my carbide holders, which are cheap Chinese copies of industry standards, have negative rake. e.g.:
...
That one is designed for TNMG inserts, which are reversible so have no relief. The 6° down rake is to stop the side of the insert from rubbing under the cut - basically 6° relief.


Unless I get very well moulded inserts which have ridiculous top angles:
...
then this tool will never cut well - it will always roughly tear chips off instead of machining nice spirals.

• The original Iscar S08K SCLCR-06 seems to have 11° downward rake: https://www.iscar.com/eCatalog/Item....FSTYP=M&srch=1
• I think my Chinese copy seems to be at least 15°!

So, given that this one takes CCMT inserts, which have 7° relief, why the hell do they grind the flats on the thing to have an additional 10+° of negative rake??????

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).

I doubt that rotating it in the holder will make any difference to its performance, with the possible exception being that you might be able to make a deeper cut - given you generally set the tip on center, the deeper DOC you set, the more of the insert is 'off centerline'. That's purely theoretical though, it may not have any real effect. Interestingly though, the Graziano came with a couple of Sandvik boring bars that use TPMR inserts - they're presented dead flat in the direction yours are tilted (perpendicular to the spindle), but parallel to the spindle they're canted back on quite a serious angle to generate positive rake at the cutting edge. Might lend some credibility to the theory - or not, as the TPMRs present a square edge to the bottom of the bore, CCMTs have a bit of a lead angle thing going on which likely comes into play.

Likewise, by putting those TNMG inserts in that you've pictured, in the turning direction the tool has quite serious positive rake. In the facing direction they'll act more like a shear tool.

With the other inserts you have, it's going to depend on what lathe you're running them on. I don't really think my Graziano at 1300kg and 5hp can necessarily make proper full use of them, anything less than that I think is going to be a struggle. I need to play around with the CNMG holder I have at some stage, and see what I can get out of it, but 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.

Just my thoughts, I could be way off base though. A few of the guys who've already replied here could probably give some general parameters for TNMG or CNMG inserts that they use at work that might help determine whether you're just way outside the envelope, or have something else going on. I know a lot of guys also like WNMGs, but on both the AL335 and the Graziano I've just been unimpressed. Could be the quality of the Chinese inserts, and I haven't thrown anything of decent diameter in the Graziano to try it on, so not the fairest comparison, but I far prefer CCMTs so far. The Sandvik KNUX based tool I have also seems to work quite well despite having a 1.5mm or 2mm nose radius on the 100 odd inserts that came with it - the geometry it presents is going to be pretty similar to those deeply grooved TNMGs you've found to work well.

The first insert holder I ever owned was a TNMG. I saw it in a tool shop in Hong Kong and bought it because the price was right and I liked the look of it. I was clueless. I had no idea it had a negative rake angle. Anyway it works surprisingly well on my little Hercus 260. It's a long way from being the first tool I select these days, but if I want to tear off metal in a hurry, I'll set the drive belt for max torque and go for it. I can make very nasty long blue coils of hot swarf with it. Probably not what it's designed to do, but it makes me feel like a real machinist.

Originally Posted by nigelpearson

Screen Shot 2021-04-18 at 2.58.33 pm.jpg

BTW, what is the designator of these inserts? I've seen a few YouTubers using them and they seem to work really well.

Originally Posted by Jekyll and Hyde

Negative rake is not necessarily negative rake, it all depends on the top of the insert...

Exactly. We've got some Iscar CNGG inserts at work for soft materials that have insane positive rake (something like 20 degrees), so the negative holder still ends up with highly positive cutting geometry.

Originally Posted by elanjacobs

Simple: if the insert pocket was neutral, the bottom corner of the insert would rub on the inside of the bore.

The bar you linked is 8mm for a minimum bore of 10.5mm; with the extra 11 degrees, you have 0.2mm of clearance from the bottom corner of the insert to the bore, it can't physically be anything less than that. Without the extra negative rake, It. Will. Not. Fit.

True, but I'm cheap, so that is my only boring bar

1. For machining larger IDs, there is a little more clearance. I could either twist the bar around, or drop the cutting tool height. But your explanation helped me realise something - the triangle of top and bottom edge of the insert will still only slightly clear under the cut. Even if I grind a little more clearance on the bottom edge of the insert and holder.
2. My curiosity was about the seeming extra relief or negative rake on this Chinese implementation.
   Mine is actually a S12M-SCLCR06, so min. dia. is 12.?mm, but it has maybe 5 degrees extra, which should allow a smaller min. dia. than the Iscar?

Anyway, I do realise most of my insert cutters will only be for "roughing" and I'm OK with that. I bought cheap inserts that I will treat badly, and as long as it doesn't work harden the job too much, we will all survive.


For finishing cuts when turning, I usually use a round insert holder (SRACR1616H08) because it is my only tool holder with zero rake (and inserts have a reasonable top rake).

For finishing cuts when boring, have actually started thinking of using threading inserts!

Originally Posted by nigelpearson

For finishing cuts when boring, have actually started thinking of using threading inserts!

When finish is critical I use a HSS boring tool.

Originally Posted by jack620

BTW, what is the designator of these inserts? I've seen a few YouTubers using them and they seem to work really well.

Here is a store that has them: https://www.aliexpress.com/item/4001130678321.html

The standard identification codes mainly describe the envelope of the shape. There are some codes that describe the chip breaker aspect, but these weird and wonderful die shapes are new inventions?

(well, some are copying Sandvik designs, but you know what I mean)


I just search through the pictures until I find one that looks like it will carve instead of grind
but many of the pictures seem to be renderings, not actual photos

Originally Posted by nigelpearson

Mine is actually a S12M-SCLCR06, so min. dia. is 12.?mm, but it has maybe 5 degrees extra, which should allow a smaller min. dia. than the Iscar?

No. S12M = 12mm shank, so you're looking at 15-16mm minimum bore at a guess. Put an insert in and measure from tip to the back of the bar, that will give you the absolute minimum it will fit in, then add about 0.5mm to get the smallest bore you could feasibly attempt (it still might not quite fit, but you definitely won't get anything less than that)

For finishing cuts when boring, have actually started thinking of using threading inserts!

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.

Originally Posted by elanjacobs

Try some CCMT060202 inserts ...

I’m pretty sure that’s what I have. I’ll try them with your DoC and feed numbers.