[How-To] Choosing the Right carbide inserts (Beginner here)

[HillbillyDeluxe39]

I been trying to decode all the insert tool holders that came with my lathe. ENCO 14x40. So far I have learned that there is a sea of inserts to choose from and depending on material to be turned, the insert can make or break the operation.

A list of holders are as follows, MTANR 12-3, MTGNR 12-3, MCLNL 12-4, NKLCL 1005b, MSDNN 12-3, MCLNR 12-4b along with some others.

Starting with the MTANR and MTGNR I am trying to find the correct insert for turning 6061 aluminum. I know these holders have a built in negative rake and aluminum likes a positive rake. With that information I determined that a TNMx 332 insert would give a total positive rake. This is where things get fuzzy. A TNMS (according to a chart) has a chip breaker with a 20 degree positive rake and should result in 13 degree total positive rake.

Does this sound right or am I totally lost? Ive also looked at the TNMP and TNMG (which seems most common) however the TNMG has a landing before the positive chip breaker rake starts and would require a minimum depth of cut to turn aluminum.

It is also worth noting that these tool holders will need to be machined down on the bottom to lower the tool tip to center on my lathe. In a previous thread I asked about this and it seems the BXA tool post is on the tall side for my lathe.

Im also open to other suggestions such as a 1/2" shank holder set to get me started, specifically in aluminum (if it matters).

Thanks

 

[rabler]

I been trying to decode all the insert tool holders that came with my lathe. ENCO 14x40. So far I have learned that there is a sea of inserts to choose from and depending on material to be turned, the insert can make or break the operation.

While the insert can be a deal breaker, there is more flexibility in using inserts than the manufacturers specifications might imply. You loose some optimization, but much of the insert specs are designed for production use in CNC machines. Learning which specs matter and which can flex is a ongoing challenge for me. I think fully explaining it is well beyond one reply, although perhaps others will weigh in with some hints/highlights

@davidpbest on this site has written a book, I'd call it a starters guide to lathe tooling. I highly recommend it.

Introduction to Indexable Tooling for the Metal Lathe: A User Guide: Best, David P.: 9798753874993: Amazon.com: Books ​

A list of holders are as follows, MTANR 12-3, MTGNR 12-3, MCLNL 12-4, NKLCL 1005b, MSDNN 12-3, MCLNR 12-4b along with some others.

Starting with the MTANR and MTGNR I am trying to find the correct insert for turning 6061 aluminum. I know these holders have a built in negative rake and aluminum likes a positive rake. With that information I determined that a TNMx 332 insert would give a total positive rake. This is where things get fuzzy. A TNMS (according to a chart) has a chip breaker with a 20 degree positive rake and should result in 13 degree total positive rake.

Does this sound right or am I totally lost? Ive also looked at the TNMP and TNMG (which seems most common) however the TNMG has a landing before the positive chip breaker rake starts and would require a minimum depth of cut to turn aluminum.

It is also worth noting that these tool holders will need to be machined down on the bottom to lower the tool tip to center on my lathe. In a previous thread I asked about this and it seems the BXA tool post is on the tall side for my lathe.

Im also open to other suggestions such as a 1/2" shank holder set to get me started, specifically in aluminum (if it matters).

Thanks

 

[Janderso]

I'm making some automotive lift extensions as we speak. I bought 3.5" OD 6061 round bar.
I'm regrouping as I could not get the chips to break. Those long curly sharp-hot strings just aint' going to work.

 

[rabler]

Also, never hurts to learn to use HSS. It tends to be a bit more forgiving for hobbyist use especially while coming up to speed.

 

[davidpbest]

I'll second @rabler - obviously. I'm not a fan of the triangle-style inserts and prefer the rhombus (CCMT) type instead because the nose angle is greater which makes the inserts less prone to fracture. The 2nd digit in the insert specification ("N" in your case) designates the clearance angle built into the insert. N = 0°, thus the insert must be mounted with a negative rake toolholder to provide clearance under the cutting edge, and indeed your tool holders all have negative rake.

Unfortunately, everything about a TNMG is badly suited for use with aluminum. The periphery is not ground, the top cutting surface is rough molded, and the cutting edge will most likely be dull. Also the holder will have much more negative rake than you want for Aluminum. TNMG inserts (and associated holders) are really intended for use on ferrous materials and aggressive DOC's.

You'd be better off switching to a different toolholder style with a neutral or positive rake than trying to find an insert for your existing toolholders if you want to fine finish aluminum. If you do switch, there are a variety of insert shapes that you could choose from including the triangle, rhombus (or others like the D or V acute angle type). The same triangle insert shape is available with a hooked-shaped chipbreaker, a 7° relief angle for use with neutral rake tool holders - it's designation is TCGT but they do not fit your existing tool holders.

You are correct that for best results with aluminum you want a sharp (rather than molded) cutting edge. You also want an uncoated insert. If you want to ensure a sharp leading edge, pick an insert with a honed or ground edge - those will be designated with a "G" in the 3rd digit rather than an "M". The "G" in this case is the tolerance specification for the insert dimensions, and "G" is sufficiently tight that it requires the insert be ground or honed to meet that tolerance spec. Also, keep in mind that that insert nose radius has a direct impact on appropriate depth of cut - smaller nose = shallower DOC. This is all discussed in detail in the aforementioned book along with specific recommendations for toolholders.

Hope this helps.

 

[compact8]

I am just a beginner and my very limited experience is that the sharpness of the insert matters the most in cutting 6061 or the finish will be poor. That said, I have got a hobby lathe ( EMCO Compact 8 ) only so it may be different for bigger machines. Those types of inserts recommended by vendors for cutting aluminium are usually very sharp, have no coating and exhibit large positive rake. I have tried using these inserts on brass and they fractured very easily so they are good for aluminium and plastic only.

 

[Fermic]

I have tried using these inserts on brass and they fractured very easily so they are good for aluminium and plastic only.

Brass is very grabby for any cutter that's not a negative rake or 0°, it can get very dangerous and can really fuċk up your part or break your tool.

That happens if I used positive rake cutter on this.

 

[davidpbest]

Brass is very grabby for any cutter that's not a negative rake or 0°, it can get very dangerous and can really fuċk up your part or break your tool.

That happens if I used positive rake cutter on this.

View attachment 386572

For brass, I recommend the A.R. Warner indexable HHS inserts with no chipbreaker and a cutting edge that will not dig in and grab the material. Check the following link:

Standard and Specialty Inserts

 

[HillbillyDeluxe39]

You'd be better off switching to a different toolholder style with a neutral or positive rake than trying to find an insert for your existing toolholders if you want to fine finish aluminum.

Your book is on order from Amazon

And I agree, aluminum specific tools are what I need. I milled down one of my 3/4" CNMG negative holders to fit a BXA and while it faces well it gave a horrible wavy finish when turning. Note that while this could be tool related its also a high probability the operator (Me - a novice) has the feed or speed wrong, 1150 rpm @ .004 feed.
Insert is a CNMG 432 GM 324 if that matters (using these because I have them)

I checked and tightened the gibs but there was no change.

 

[davidpbest]

Without being at your lathe and observing, it's difficult to give you a precise answer, but based on what you have already said I can make a few reasonable comments/suggestions. You didn't say what your depth of cut is, nor the manufacturer of the insert, so I'll speculate a bit. A CNMG 432 GM 324 insert would be mounted in the tool holder at something like a 5 or 6 degree negative rake. The "GM" indicates the insert is intended for general machining (that's somewhere between fine finishing and roughing), and since it has an "M" tolerance, I can guess without seeing it that it's molded with a blunt cutting edge. The 324 is the manufacturers grade, so without knowing who made the insert, it's difficult to know what that grading designation means. But with those aspects alone, that insert is best suited for use on steel and cast iron. It has a 1/32" nose radius, which suggests that the ideal depth of cut would be >0.015" at a feed rate of <0.015 when used on steel or cast iron.

When that type of insert is used on aluminum, given the negative rake and blunt cutting edge, relatively high tool pressures are being generated since the insert is not free cutting. When facing, the tool pressures are axial - toward the chuck/spindle with considerable rigidity. When turning, the tool pressures are radial - across the axis of rotation and against material being cut which has less rigidity pushing against the chuck/spindle. So when turning, the material is likely to start flexing during the cut, and this can set up a vacillation that shows up in the surface finish as waviness or chatter. In a turning operation, you can alter the direction of the tool pressures some by increasing the feed rate. Increasing the feed rate will shift the tool pressure more towards the chuck rather than mostly against the material being turned. This might help smooth out the surface finish, although with that insert, the aluminum is being torn, not cut. Increasing the depth of cut will also produce more consistent "cutting" action when turning with this specific type of insert on aluminum and could improve surface finish, but it will also increase the radial tool pressure suggesting a higher feed rate as well. The best thing you could do is switch to a neutral or positive toolholder with an insert for aluminum. Hope this helps.