Turning cast iron parts with ceramic

Modern Machine Shop, Feb, 1990 by Henry Stier

The following is an attempt to specify the necessary steps for the manufacturing engineers involved in planning the production of high volume cast iron parts to be rough turned with ceramic cutting tools.

The CNC lathe should be designed from the base up for higher cutting forces and speeds up to 5,000 SFPM. The frame must be more rigid and the size of the ways enlarged. A base filled with epoxy cement or similar material will increase the vibration dampening when compared with cast iron bases. Increasing the spindle bearing diameter will also help.

The power transmission from motor to spindle requires more attention for ceramic application than for a lathe with carbide. Cycle times of less than 30 seconds are common when machining even bigger cast iron parts with ceramic. The power consumption can become much higher and 30 to 50 Kw motors are necessary in most cases.

With ceramic cutting tools the material removal rate is higher than with coated carbide. Special attention has to be paid to the chip disposal. Each tool should be equipped with a chip deflector to avoid chips settling on machine parts where they can cause production interruption like on automatic loaders. All ways should be protected by covers to avoid excessive wear.

Coolant is not used when machining cast iron with cutting tools, even so the machining compartment has to be enclosed. A vacuum system should also be used to remove fine dust.

Special attention has to be given to the chuck design. Parts have to be held securely at much higher speeds, up to five times that when turning with carbide. Self balancing chucks may be necessary when machining heavier unbalanced parts. The often used carbide wafer type insert should be replaced with small 120 degree cone pointed, round carbide inserts brazed in multiples to the top jaws. The points penetrate the casting skin and hold the part more firmly.

The tooling has to be very rigid to avoid vibration. Tool blocks should be a minimum of two inches thick if they overhang the machine slide. The tool block should support the toolholder up to the cutting edge. Use toolholders with 1-1/2 to 2 inches shank size in turrets.

Machining with ceramics is more than just substituting one insert type for another. To effectively use all that ceramic inserts have to offer, it is necessary to rethink the entire machining system, this includes the inserts themselves, the machine, chuck, chip-removal resources, toolholders workpiece processing and even overall programming scheduling, and shop floor control.