Carbide tip choice advice info

[JohnFParticipant @johnf59703]

I came upon this in another forum but thought the advice was clear and concise so may be useful !

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A lot of guys stick exclusively to positive inserts (like CCMT or DCMT) when they start out because they take light cuts easily and don’t load up smaller lathes.

While positive inserts are great for low-rigidity setups, sticking only to positive tooling means you’re leaving a lot of edge life—and money—on the table when turning solid steel or cast iron.

Here is a quick refresher on when to run positive vs. negative geometries and why it matters:

Positive Inserts (CCMT, DCMT, TCMT)

Geometry: Built with a built-in clearance angle (usually 7 degrees) underneath the cutting edge.

Best for: Small benchtop lathes, light finishing passes, internal boring bars, and soft materials like aluminum or brass.

Trade-off: You only get half the edges per insert (a CCMT only gives you 2 cutting corners). Because the edge is sharper and thinner, it chips more easily under heavy interrupted cuts.

Negative Inserts (CNMG, TNMG, WNMG)

Geometry: The cutting edge is perpendicular (90 degrees) to the top and bottom faces, relying on the tool holder’s pocket angle to provide clearance.

Best for: Heavy roughing, interrupted cuts, cast iron, and medium-to-large CNC or rigid manual lathes.

Trade-off: Requires more machine horsepower and rigid setups because of higher cutting resistance.

The Big Advantage: Double the cutting edges! A CNMG insert gives you 4 usable corners, and a WNMG gives you 6. That instantly cuts your consumable costs in half on heavy stock removal.

When should you switch?
If your lathe has enough rigidity and motor power to take at least a 1.5mm to 2mm (0.060″+) depth of cut in steel, switching your main roughing holder to a negative style like CNMG or WNMG will save you a ton on insert costs over time. But if you’re turning thin-walled tubing or slender shafts without a tailstock, stick with positive CCMT to prevent chatter.

What is your default setup for general steel roughing—do you stick with 2-corner positive inserts for convenience, or do you run 6-corner WNMGs to keep running costs down?

[HollowpointParticipant @hollowpoint]

I tried some negative inserts on my Boxford. They were terrible.

[Howard LewisParticipant @howardlewis46836]

There used to be a holder that allowed the 100 degree corners of CCMT tips to be used. (I use one for roughing) I think that mine came from Greenwood, but see if JB Cutting tools can supply.

Howard

[Howard Lewis]

On Howard Lewis Said:

There used to be a holder that allowed the 100 degree corners of CCMT tips to be used…

I’ve got one and use it mainly for roughing.

Depth of cut is not the only pertinent parameter when looking at removal rates. Professionals use much higher feedrates than the home workshop. Out of interest I looked up some Korloy negative rate inserts. Recommended feedrates are 0.4mm to 1.5mm per rev. Rather higher than common in the home workshop I would guess. I would expect negative rake inserts to be poor when used at low feedrates, which may explain Hollowpoint’s experience.

Julie

[JasonBModerator @jasonb]

I would say **MT inserts are not the best for small hobby use. Your text does not make mention of the “sharper” **GT inserts which work very well on the less rigid and less powerful hobby machines. These inserts being sharper and uncoated are also better for non ferrous then **MT.

 

As mentioned by Howard you can get holders to use the two obtuse corners of the typical CCMT and CCGT inserts so you get 4 usable corners from them too.

No “usually” about it, if you understand the codes then all those inserts with *C** are 7deg, change that letter and then you get other angles.

Not really had problems with interrupted cuts either.

As well as feed that Julie mentions the diameter of the work will also come into it particularly on non belt or gear head machines as they may be able to take a 6mm DOC on 25mm material but will stop dead if you try anything like that on 250mm material.

This is my roughing setup using the “spare” corners of CCGT inserts

[old martParticipant @oldmart]

Negative inserts in the general size of TNMG11, WNMG08 and CNMG12 are best left to users who have a minimum of 16mm square toolholders. The WNMG06 could be ok for 12mm square and all of these shapes are available in aluminium grades, WNGT06 and 08, TNGT11 and CNGT12, which have the advantage of 6 or 4 cutting edges. There are toolholders for the CNMG/CNGT inserts which use the obtuse edges which are otherwise wasted and do have limited applications.

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