Cermet Inserts for Stainless Steel Finishing

Stainless steel is where cermet's chemical stability earns its place. The material work-hardens, welds to the edge and conducts heat poorly, and cermet resists built-up edge better than most tool materials. The trade is that the catalogue sets two firm limits: a depth of cut below 0.1 mm, and no duplex stainless steel.

What the catalogue permits

Cermet is recommended for precision finishing of ferritic, martensitic and austenitic stainless steel with a cutting depth below 0.1 mm. Duplex stainless steel is explicitly not recommended.

The depth limit is the important one. At a cut depth of 0.1 mm or less the positive, sharp-edged inserts cut cleanly at low force and leave a fine finish, which is exactly the regime cermet was designed for. Take a heavier cut and the advantage disappears.

In practice the finishing pass is usually the last operation before the part is complete, so the cut is small by design. Where more material has to be removed, take the allowance off in an earlier operation and keep the cermet pass light, rather than asking a cermet insert to take a heavier cut.

Speed and feed for stainless

Stainless runs slower than carbon steel across the whole programme, and the catalogue is consistent about it. In grooving, stainless steel runs at 70-160 m/min against 80-220 m/min for steel. In turning with the grooving inserts the feed is 0.02-0.05 mm/rev for the turning pass and 0.01-0.07 mm/rev in the groove, depending on insert width.

In threading, stainless runs at 60-80 m/min against 100-150 m/min for carbon, low-alloy and tool steels. In small-bore work it runs at 30-80 m/min against 30-120 m/min for steel.

Recommended geometries

Positive, sharp-edged inserts suit stainless steel particularly well because they lower cutting force and reduce work hardening at the cut line. CCGT, DCGT, VCGT and VBGT cover external finishing and profiling, with TCMT and DCMT for internal and smaller work.

For grooving and internal work, the MGMN grooving inserts and the PSBR, PSGR and PSGFR small-bore bars extend the cermet edge into grooves, bores and face grooves, down to the minimum diameters listed for each bar.

• CCGT and DCGT for external finishing and profiling
• VCGT and VBGT for shoulder work and small profiles
• TCMT and DCMT for internal turning
• MGMN and TGF32 inserts for grooving and parting
• PSBR, PSGR and PSGFR bars for small-diameter bores and grooves

Managing built-up edge and work hardening

Built-up edge is the classic stainless failure, and cermet is chosen partly because its chemical stability suppresses it. Where it still appears, the catalogue's direction is to move to a more welding-resistant grade, raise the cutting speed and keep the edge sharp rather than running it to wear.

Never dwell in the cut. A tool that rubs on stainless work-hardens the surface, and the next pass then meets a harder layer. Keep the feed positive and inside the chipbreaker window, and avoid stopping mid-pass.

A workable starting plan

• Confirm the grade of stainless; duplex is excluded
• Set the finishing allowance so the cut stays below 0.1 mm
• Choose a positive, sharp-edged insert such as CCGT or DCGT
• Set Vc at the low end of the stainless band and tune upward only if the chip is stable
• Run with full flood cooling, or dry if the machine and the operation allow it consistently
• Check the finish across a full edge, since stainless wear can accelerate abruptly

Related

• CCGT Coated Cermet Turning Inserts — G Grade — 27 models
• DCGT Coated Cermet Turning Inserts — G Grade — 17 models
• VCGT Coated Cermet Turning Inserts — G Grade — 7 models
• VBGT Coated Cermet Turning Inserts — G Grade — 20 models
• TCMT Positive Cermet Turning Inserts — M Grade — 25 models
• DCMT Positive Cermet Turning Inserts — M Grade — 16 models
• MGMN Cermet Grooving Inserts — 6 models
• TGF32-R Right-Hand Cermet Grooving Inserts — 11 models
• PSBR Small-Diameter Boring Bars — 8 models
• PSGR Internal Grooving Bars — 18 models

Planning note

Hold the depth of cut below 0.1 mm as stated in the catalogue, and set the speed inside the stainless band rather than the steel band - roughly 70-160 m/min for grooving and 60-80 m/min for threading. Use positive, sharp-edged inserts to limit cutting force and work hardening, and exclude duplex stainless steel.