Cermet Tools for Small Parts Machining

Small parts work lives on sharp edges, light cuts and fine finishes, which is cermet's natural territory. Koves supplies a dedicated small-diameter bar programme for internal boring, back turning, grooving, face grooving and threading, with cutting conditions published for bores down to around 1 mm.

The small-diameter programme

The boring family covers five operations, each with a published minimum working diameter. The bars are used for internal features on parts such as medical instruments and implants, electrical connectors, and watch and instrument components, where the bore is small and the finish and tolerance are demanding.

Cutting conditions for small bores

The catalogue publishes speed and feed by bore diameter, which is the right way to think about small-bore work because the bar diameter, not the workpiece, governs rigidity. Speeds run from 30-120 m/min for steel and 30-80 m/min for stainless steel.

Typical parts and requirements

The parts that drive demand for cermet small-bore bars share a common profile. They are small, they carry several internal features, and the bore tolerance and finish are tight relative to the size of the part. Medical instruments and implants, electrical and optical connectors, and watch and instrument components are the usual examples.

Two requirements dominate. The first is bore finish, because a small bore is difficult to inspect and expensive to rework. The second is repeatability over a long run, because a small part is often produced in large numbers and the interval between tool changes sets the cost per part. Cermet helps with both: the sharp edge produces the finish, and the slow, predictable wear keeps the size stable.

• Medical instruments and implants: small bores with tight tolerance and a fine finish
• Electrical and optical connectors: repeated small bores and grooves at high volume
• Watch and instrument components: fine internal features on very small diameters
• Hydraulic and pneumatic micro-fittings: internal grooves and threads in small bores

Choosing the right bar

Start from the operation, not the bar. Boring and back turning use PSBR and PSCR; grooves use PSGR for internal grooves and PSGFR for face grooves, with the groove width chosen from the listed range; threads use PSTR in the 55 or 60 degree profile at the required pitch.

Where the same part has several internal features, the smallest feature sets the bar size, since a bar that fits the smallest bore can usually reach the larger ones as well.

Process rules for small parts

• Watch the rigidity of the bar, not the workpiece - small-diameter bars deflect easily
• Keep the feed inside the diameter-specific window; too low a feed causes rubbing and chatter
• Use the sharpest available edge, since small bores amplify any built-up edge
• Manage cooling as dry or full flood, as with all cermet work
• Keep the bore free of chips, because a small bore clears them poorly
• Check the finish at the bottom of the bore, where the bar is at maximum overhang

Where the external turning fits in

Small parts usually need external turning as well. The positive CCMT, DCMT and VBMT inserts in the M grade, at 0.05-0.35 mm/rev for the JF chipbreaker, cover the external finishing of the same small profiles, so a single part can be finished with cermet on both the outside and inside.

Related

• PSBR Small-Diameter Boring Bars — 8 models
• PSCR Small-Diameter Boring Bars — 6 models
• PSGR Internal Grooving Bars — 18 models
• PSGFR Internal Grooving Bars — 14 models
• PSTR Small-Diameter Threading Bars — 6 models
• CCMT Positive Cermet Turning Inserts — M Grade — 19 models
• DCMT Positive Cermet Turning Inserts — M Grade — 16 models
• VBMT Positive Cermet Turning Inserts — M Grade — 11 models

Planning note

Select the bar from the operation and the smallest internal feature - PSBR/PSCR from 3.2 mm for boring and back turning, PSGR from 3.2 mm for internal grooves, PSGFR from 6.0 mm for face grooves, PSTR from 3.3 mm for threads. Set the feed by bore diameter and keep Vc in the 30-120 m/min range for steel, 30-80 m/min for stainless.