6 carbide insert grades every Indian job-shop should stock (and what each one is actually for)

Walk through a typical MSME machine shop in central India (Jabalpur, Indore, Pithampur, Nagpur, Raipur) and you will find a tool drawer with thirty different inserts in twenty different boxes, most of them part-used, none of them clearly catalogued, and at least half of them used on materials they were never designed for. The shop owner usually knows that "the gold ones are good" and "the black ones are for steel". The cycle time, the surface finish, and the insert spend all suffer.

The carbide grade chart is not the most exciting topic in a fab-and-machine shop. It is, however, one of the few topics where a well-stocked shop directly outperforms a similarly equipped competitor by 20 to 40 % on tooling cost and 10 to 25 % on cycle time. The grade choice matters as much as the geometry, and far more than the brand.

This guide is for purchase managers and proprietors of Indian MSME machine shops, ancillary turning units serving auto, agri, pump and valve OEMs, and captive maintenance machine shops in cement, sugar and process plants. The six grades below cover roughly 95 % of the common workpiece materials we see in the central Indian belt: mild steel, low-alloy steel, stainless 304/316, cast iron, aluminium, and the occasional hardened steel job.

We give grades in the ISO P/M/K/N/S/H letter system because it is the cross-supplier standard. Most major insert manufacturers selling into the Indian MSME channel publish a grade-cross-reference chart against the ISO code; ask your supplier for theirs in writing before stocking any new family.

1. P25 to P30, your default grade for mild steel and general carbon steel

The P-series covers steels. P25 to P30 is the broad-purpose mid-range: tough enough to take an interrupted cut on a roughing pass, hard enough to hold a respectable edge on finishing. For 90 % of the EN8, EN9, IS 2062, mild steel and free-cutting steel that lands on an Indian MSME shop's lathe, this is the right starting point.

What to ask your supplier for: an ISO P25 to P30 substrate with a multi-layer CVD coating (typically TiCN plus Al2O3 plus TiN), in the chip-breaker geometry coded "M" or "MM" for medium-duty turning.

Where it shines: roughing and medium-finish turning on carbon steels at 150 to 280 m/min, feed 0.15 to 0.4 mm/rev, depth of cut 1 to 4 mm. Coolant or dry, both work; CVD-coated grades in this band are forgiving of inconsistent coolant supply, which matters in shops with old centrifugal pumps and clogged manifolds.

Where it does not: chatter-prone setups, very low-rigidity machines, hardened steels above 45 HRC, and stainless steels. Pushing P25 onto stainless will glaze and crater the rake face within a single shift.

What it costs to substitute: a shop using a single "general purpose" grade across mild steel and stainless typically gets 60 to 80 % of the tool life it could on the right grade pair. On a job-shop running ₹40,000 a month in inserts, that is ₹8,000 to ₹16,000 a month in avoidable spend.

2. P10 to P20, your finishing grade for steels

When the dimensional tolerance is tight (IT7 or better) and the surface finish requirement is below Ra 1.6 µm, P25 is too tough. The cutting edge needs to be harder, sharper, and run faster. P10 to P20 is the finishing grade: harder substrate, thinner coating, sharper edge prep.

What to ask your supplier for: an ISO P10 to P20 substrate with a thin CVD or thin PVD coating, ground edge prep, and a finishing chip-breaker (codes "F" or "MF").

Where it shines: finishing passes on carbon and low-alloy steels at 250 to 400 m/min, feed 0.05 to 0.2 mm/rev, depth of cut 0.2 to 1 mm. With a wiper geometry, surface finishes of Ra 0.4 to 0.8 µm are routine. Coolant flood preferred for thermal stability.

Where it does not: roughing, interrupted cuts, casting skin, and any setup with workpiece deflection. P10 chips at the edge under shock load.

A practical tip: stock one finishing geometry (CCMT, DCMT or VBMT in 06 or 08 sizes) in P10 to P20, dedicated to the last pass. The cycle-time saving on a typical shaft job is 15 to 25 % versus running the rougher grade through the finish pass.

3. M25 to M35, the right grade for stainless 304 and 316

This is the grade most Indian job-shops get wrong, and the wrong choice is unforgiving. Stainless steel work-hardens, gums onto the edge, and generates sustained heat that destroys a P-series insert in minutes. The M-series substrate has the right balance of toughness and heat resistance for stainless and duplex grades.

What to ask your supplier for: an ISO M25 to M35 substrate with a PVD TiAlN or AlTiN coating, polished rake face, sharp positive edge prep, and a chip-breaker coded "MM" or "SM" for stainless work.

Where it shines: turning, facing, and grooving on 304, 316, 410 and duplex stainless at 120 to 220 m/min, feed 0.15 to 0.35 mm/rev, depth of cut 1 to 3 mm. Coolant flood mandatory, ideally with a 7 to 10 % emulsion. Sharp edge geometry (positive rake, polished rake face, "MM" or "SM" chip-breakers) reduces work-hardening on the next pass.

Where it does not: hardened stainless above 35 HRC; for those, drop to M40 or step into the H-series.

What it costs to use a P-grade on stainless: insert life drops from 60 to 90 minutes per edge to 8 to 15 minutes per edge. A shop running 100 stainless components a day on a P25 insert burns through ₹4,000 to ₹8,000 of inserts daily; the same job on M25 to M35 settles to ₹1,500 to ₹2,500.

A separate point Indian shops often miss: stainless is not steel. The M-series has its own coatings (often PVD TiAlN or AlTiN rather than the multi-layer CVD that dominates the P-series) and its own geometries. Running the same insert holder with two grades and changing the chip-breaker between steel and stainless work is not a substitute for buying the right grade.

4. K10 to K20, the grade for cast iron and abrasive grey iron

Cast iron is the easy material on the chart, in the sense that the right grade has been clear for forty years. K10 to K20 is the answer: a hard substrate with a wear-resistant coating, designed for the short, brittle chip and abrasive scale of grey and ductile iron.

What to ask your supplier for: an ISO K10 to K20 substrate with a wear-resistant CVD coating (typically TiCN plus Al2O3), in a roughing or "T" heavy-duty geometry for cast-iron work with hard scale.

Where it shines: turning and milling on grey cast iron and ductile iron at 200 to 350 m/min, feed 0.2 to 0.5 mm/rev, depth of cut 1 to 5 mm. Dry cutting is preferred; cast iron does not need coolant, and coolant on cast iron creates a slurry that clogs swarf conveyors and damages way covers.

Where it does not: ferritic ductile iron with a tough matrix, where the K-grade can chip under interrupted cuts. Step up to K30 with a slightly tougher substrate for those jobs.

A note on Indian foundries: castings from local pattern-and-foundry units routinely have hard scale, sand inclusions, and porosity. A K-grade with a tougher edge prep handles these realities better than a "European catalogue" K-grade with a sharp finishing edge. Ask your supplier for the heavy-duty or "T" geometry variant.

5. N10 to N20, the grade for aluminium and non-ferrous

Aluminium machining is unforgiving in a different way: the chip is long, sticky, and welds to the edge of any insert that is not specifically designed for it. The right answer is uncoated, polished, sharp-edged carbide, often with a positive geometry and a ground-and-polished chip-breaker.

What to ask your supplier for: an ISO N10 to N20 uncoated, polished, fine-grain carbide substrate with a positive ground geometry and a sharp aluminium-specific chip-breaker (codes "AK" or "ALU").

Where it shines: turning, facing and grooving on aluminium 6061, 6063, 7075, copper, brass and bronze at 400 to 1,500 m/min, feed 0.05 to 0.4 mm/rev, depth of cut 0.5 to 4 mm. Coolant or air-blast strongly recommended for chip evacuation; aluminium chips that recirculate through the cut destroy the surface finish.

Where it does not: any ferrous job. An uncoated polished N-grade run on steel will fail in seconds.

A separate stocking note: aluminium-specialist inserts are usually under-stocked in Indian MSME shops because the volume is lower. Shops that machine occasional aluminium jobs on their default P-grade get acceptable but slow results, and ruin their surface finish targets. For any shop running more than 50 aluminium components a week, a dedicated N-grade insert family is worth the small additional inventory.

6. H10 to H20 (or CBN), for hardened steel above 45 HRC

The sixth grade is the one most MSME shops do not stock at all, and consequently send out for grinding instead of turning. H10 to H20 (sometimes written as the "H" series for hard-part machining) is a fine-grain carbide with a TiAlN or AlCrN coating, designed for hardened steels in the 45 to 62 HRC range.

For materials above 55 HRC and for the highest-quality hard turning, ceramic and CBN inserts are the better answer. CBN inserts cost ₹600 to ₹2,000 each (against ₹150 to ₹400 for carbide), but they out-perform carbide on hardened steel by an order of magnitude.

What to ask your supplier for: a fine-grain ISO H10 to H20 carbide with a TiAlN or AlCrN PVD coating, or a CBN-tipped insert in CNGA / DNGA geometry for the highest-tolerance hard-turning work.

Where it shines: turning hardened gear blanks, shafts, and dies at 80 to 200 m/min on hardened steel, feed 0.05 to 0.15 mm/rev, depth of cut 0.1 to 0.5 mm. Dry or with minimum-quantity lubrication. Replaces grinding for many tolerance bands above IT7.

Where it does not: any non-hardened material; H-grades are too brittle to take an interrupted cut on soft steel.

A practical upgrade path for MSME shops: keep two H-grade carbide inserts and one CBN insert in the tool drawer. The next time a hardened-steel job comes through (a regrind shaft, a hardened pulley, a worn spline), turn it instead of grinding it. The cycle-time saving is typically 50 to 80 %, the surface finish is comparable on a rigid lathe, and the customer pays the same.

A starter stocking list for an MSME machine shop

The following stocking depth covers most of what an Indian MSME job-shop will see in a year:

CNMG 120408 in P25 (steel roughing), 10 inserts.

CNMG 120404 in P15 (steel finishing), 10 inserts.

CNMG 120408 in M25 (stainless), 10 inserts.

CNMG 120408 in K15 (cast iron), 10 inserts.

DCMT 11T308 in P15 (small-diameter steel finishing), 10 inserts.

DCGT 11T304 in N10 (aluminium finishing), 5 inserts.

WNMG 080408 in P25 (rigid steel work), 10 inserts.

TNMG 160408 in P25 and M25 (medium-rigidity setups), 10 each.

A single CBN insert in CNGA 120408 (hardened steel), 1 insert as a capability-extender.

Total inventory cost: roughly ₹15,000 to ₹25,000 depending on brand, with about ₹12,000 of working capital tied up at any one time once consumption stabilises. The value-recovery is fast: a shop that has the right grade for every job typically reduces its insert spend by 25 to 40 % within three months and reduces the rework rate (out-of-tolerance, surface-finish rejects) by 5 to 15 %.

A pre-purchase checklist for any new insert

Match the workpiece material to the ISO grade family (P / M / K / N / S / H), not to the brand colour or the operator's habit.

Match the chip-breaker geometry to the operation: M for medium, R for roughing, F for finishing, MM or SM for stainless.

Match the coating to the cutting environment: CVD for steady cuts and consistent coolant, PVD for interrupted or thermally-cycled cuts and inconsistent coolant.

Match the size and nose radius to the depth of cut and the rigidity of the setup. A 0.8 mm radius is a reasonable default; 0.4 mm for finishing on slim shafts; 1.2 mm or 1.6 mm for heavy roughing on rigid lathes.

Confirm the insert holder geometry (negative or positive rake, lead angle) is correct for the substrate. A positive holder with a P25 negative-geometry insert is a chronic chipping setup.

Confirm BIS or equivalent traceability on the box, with the manufacturer's grade code printed on the insert. Repackaged or unbranded carbide is endemic in the Indian market and is the most common cause of grade-related failure complaints.

Where Tejwala fits

We carry a curated insert range across the grades and geometries above, sourced from major insert manufacturers with full IS / ISO traceability, with single-piece sales for shops that want to test a grade before committing to a sleeve, and bulk-pack pricing for production users. Replacement holders, boring bars, parting tools and threading kits in IS-traceable inventory.

For an insert-stocking audit on your shop floor, a grade-by-grade recommendation against your typical job mix, or pricing on a starter set, send us a list of the three most common workpiece materials and a photo of the lathe spindle and turret. We will come back with a stocking proposal within two working days and dispatch same-day from Jabalpur to most central India destinations.

WhatsApp the Tejwala sales team at +91 98270 87528 or email sales@tejwala.com.