The introduction of a Zenith microprocessor-controlled (CNC) drill grinding machine, from Z. Brierley Ltd., Ferry Farm Road, Llandudno Junction, N. Wales LL31 9SF, at the Ford Engine plant in Dagenham, just east of London, England, has improved tool tolerances and reduced the time required to regrind a wide range of pilot subland drills. Drills ground on the Zenith and used in production of the new DOHC engine as well as 1.8 liter and 2.5 liter diesel engines, were found to last longer between regrinds than manually ground tools.

CNC pilot drill grinding was introduced in the precision grinding department to meet a requirement for improved accuracy of the tool pilot length for the new DOHC engine. Tolerances previously were held to [+ or -] 0.025 mm, but the company now requires tolerances to be one quarter of that range.

Ford's established method of regrinding pilot drills required a minimum of two setups on different machines. The first operation was to grind the shoulder using a special fixture and then hand grind the front end of the pilot. As the pilot length is crucial, this operation required considerable manual skill and dedication.

Additionally, it was impossible to manually grind the shoulder and drill land independently, so the shoulder was increased more than necessary to accommodate an undercut. This meant that the drill had to be cut back and the entire pilot section reformed from time to time.

The operators found that with the Zenith machine they could ensure that the undercut is minimal and this more than doubles the useful working life of the drill. Because drills can now be reground in one continuous operation, accuracy is enhanced giving more confidence on the production line. The overall regrind time has been greatly reduced from hours to minutes after program prove out.

The "teach and repeat programming" development time for a "new" drill takes about two hours overall, though the joystick-style input method will allow an operator to input a basic part program in about ten minutes.

The physical setup of a drill in the machine is aided by a simple mechanical fixture. The pilot drills ground on the machine are all high speed steel ranging from three to 25 mm (about one-inch) diameter. An early change to a harder grade of wheel means that dressing is required only once in six months. Ford has found that at the end of the small end of the drill range, the automatic cycle makes regrinding much easier.

The company's engineers planned the workload on the machine by color coding the boxes used to transport the drills. The color coded boxes correspond with a color number code on disk on which the program is stored.

When compared with the previous manual operation, the company has found that they can now regrind in drill batches. By regrinding in drill batches, the system introduces further economies and simplifies product control.