1236T CCMT vs CNMG?

[ClayPort]

Quick question on the whether or not the 1236T has enough power to get a good finish using CNMG or if I need to stay with CCMT. After recently switching from a 10" to the 1236T, I am taking my first venture into insert boring bars and see more insert tooling in my future. I would prefer to stay with as few insert styles and sizes as possible. I have looked through about 20 threads and can't find a minimum HP for a smooth surface finish for both steel and AL. I've only come up with a SB 10" is too small and 16" 5HP works great. Thanks in advance for the newbie question.

Ccmt vs cnmg

I was going to purchase some boring bars to be used on a CNC Gang lathe and was set on buying 3/4 shank bars that would use the CCMT insert. After considering it, I am now thinking that the CNMG might be better as you get four cutting edges vs 2 for the CCMT. What would be a benefit of one...

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Thread 'Are CNMG inserts actually negative rake?'

Dec 16, 2021

Just ordered a new multifix tool post and am looking at getting some indexible tooling for my South Bend Heavy 10 lathe. I think I'm leaning towards CCMT or CNMG so I can use one insert for external and boring operations. I know CCMT is often recommended for smaller lathes due to it's positive rake, however the SCLC holders you use for them are actually flat with no positive rake at all. It seems all of the positive rake is built into the insert it self. On a CNMG the tool holder is tilted down for negative rake so you can run a zero clearance insert, but the inserts seem to have a similar...

• Lucas E
• Replies: 20
• Forum: GENERAL HOME MACHINE SHOP DISCUSSIONS

 

[DavidR8]

Generally speaking negative rake inserts take a lot of horsepower to run effectively. I personally use CCMT or WCMT (my preferred) inserts on my 1237G.

 

[mksj]

Does not really have the Hp and rigidity for negative rake insert holders, I would stay with CCMT/CCGT. I have used WNMG inserts when I had my 1340GT, but was mostly setup for aluminum, and worked well. Still I stay with neutral rake insert holders for that lathe. Same goes for boring bars, either neutral or positive rake insert pockets. The inserts also have their own rake at the cutting edge, so a ground edge small nose radius insert will take less Hp, vs a molded edge, but takes lighter cuts.

 

[G-ManBart]

I've had lathes as small/light as a SB Light 10 and as big/heavy as my current Victor 1640 (5hp) and quite a few 12-14" lathes in between. Even my Clausing 14" and Logan 14" lathes don't really have enough power/rigidity to take advantage of CNMG and the Victor sort of does, but I still get a better finish with CCMT tools/inserts. For the bigger machines I like the larger 400 series CCMT tools, but also have the 32 series for certain things like boring bars. I don't believe a 12" PM really makes sense with CNMG.

 

[davidpbest]

I think a lot depends on what material you will be turning. IMO, the rigidity of the compound on the 1236T will limit your ability to run negative rake inserts as much as the HP of the machine. I have a PM-1340GT, but have replaced the motor with a vector-drive Baldor 2HP driven by a VFD. I also use a sold tool post that significantly improves the rigidity. With this setup I can run WNMG tooling (negative rake) for roughing 4140 and 316 stainless. I don't think I would attempt that on a stock 1236T and expect good results. Aluminum, is probablyy no problem, but harder materials you would be pushing the limits of the machine. I recommend you stick with neutral and positive rake indexable tooling for most work.

 

[coolidge]

Generally speaking negative rake inserts take a lot of horsepower to run effectively. I personally use CCMT or WCMT (my preferred) inserts on my 1237G.

David when you have time can you elaborate on the WCMT inserts, why you like them? Thanks! I wish there was a long running inserts thread.

 

[coolidge]

I think a lot depends on what material you will be turning. IMO, the rigidity of the compound on the 1236T will limit your ability to run negative rake inserts as much as the HP of the machine. I have a PM-1340GT, but have replaced the motor with a vector-drive Baldor 2HP driven by a VFD. I also use a sold tool post that significantly improves the rigidity. With this setup I can run WNMG tooling (negative rake) for roughing 4140 and 316 stainless. I don't think I would attempt that on a stock 1236T and expect good results. Aluminum, is probablyy no problem, but harder materials you would be pushing the limits of the machine. I recommend you stick with neutral and positive rake indexable tooling for most work.

For David (or anyone) what's the preferred insert for avoiding work hardening stainless? If one exists.

 

[davidpbest]

David when you have time can you elaborate on the WCMT inserts, why you like them? Thanks! I wish there was a long running inserts thread.

I don't have or know about WCMT inserts. Assuming you meant WNMG, I will quote from page 38 in my book Introduction to Indexable Tooling for the Metal Lathe: A User Guide:

"This [WNMG] insert is called “trigon” because of its triangular shape, but the insert actually has six cutting faces – three on each side of the insert. Thus, the insert is double-sided with a zero clearance angle. The rake angle is negative (meaning the insert tilts down slightly when mounted in the toolholder), facilitating aggressive material removal rates – provided the lathe has enough horsepower and rigidity to support these operations. . . . . . The insert cutting face has an 80° included angle, so the toolholder can be positioned for both turning and facing operations without having to rotate the tool post between operations."​

Additionally, since the face has an 80° included angle, it is equivalent to the CNMT/CCGT style insert in terms of edge strength and durability. In terms of strength and durability (increasing insert life), it is second only to a round or square insert shapes.

 

[davidpbest]

For David (or anyone) what's the preferred insert for avoiding work hardening stainless? If one exists.

In my experience, controlling stainless work hardening has more to do with with factors other than picking a specific insert. To be sure, there are inserts specifically graded for use on stainless, but those gradings have more to do with insert durability and cutting edge strength than whether they lower work hardening. I use a couple of strategies to reduce or eliminate work hardening of stainless - you probably know all this already. The first is to take off enough material during the cutting operation that the heat ends up in the chip/swarf rather than being retained in the part being machined. Factors such as RPM, SFM, feed rate and depth of cut all influence where the heat goes during the cutting operation. I tend toward the more aggressive end of the SFM for speed/feed calculations. This is one reason I use WNGT insert tooling for roughing stainless. Certainly adding a cooling strategy to the operation also helps. For instance I use a Fogbuster MQL system with a water-based coolant and the air turned up to 70 PSI.

Having said that, an insert with a sharper cutting edge will produce less heat than one with a molded edge. The durability is less than an insert with a moulded cutting edge, but less heat is generated, workhardening is better controlled, and surface finish is a better. Stainless is gummy and responds better to being cut than torn. The key to getting an insert that has a ground/honed cutting edge is to pick one with the "G" tolerance spec, such as CCGT, WNGG, DCGT etc. In situations where I'm forced to work with shallow depths of cut, I wil often pick an insert with a small nose radius (1/64th) that has a honed or ground edge and is designated for use with aluminum - something like this.

Hope this helps.

 

[Karl_T]

CNMG inserts for AL actually have a positive rake and good chip breaker. They work fine with most steels. i use these on my 10EE and Leblond