Small-Scale Setup for Small PartsSmall-Scale Setup for Small Parts

When the required tolerance is .01 mm on a part 5 mm wide, you've gotto work at it. U.K. trade moulder Nordell, in West Sussex, produces a polystyrenespool that forms the core of a reel of ribbon used in machines that imprintaccount numbers and other information on bank checks. Nordell's customerreceives tape 10 mm wide, splits it longitudinally, and then winds it ontothe spool from a pair of mandrels. Since the spool forms the tape's onlysupport, its dimensions must be precise enough to ensure there is no irregularoverlap during winding and that it despools exactly during printing.

A 16-cavity tool would have ensured a high output rate for Nordell, butto achieve the necessary precision, the cost would have been inordinate--fiftytimes the cost of the single impression tool subsequently chosen, witha further requirement to use a new machine and hot runner specified bythe tool supplier. As it is, the present hardened-steel tool, running ina Model 12/90H machine from Butler Designs, cycles at a satisfactory 12seconds, producing 50,000 spools a month. Machine and tool cost 9,500 and2,000 (US$ 16,000 and US$ 3,400) respectively, and both proved adaptablewhen it was necessary to improve the part tolerances at the user's request.

Initially the spool was moulded to .025-mm tolerance, but by the time thecustomer had ground his tape spacer to its target precision, the tolerancesfor the spool had to be improved to ensure consistent tracking during winding.Dimensional variations were attributable to the effect of changes in ambienttemperature on the water temperature of the moulding machine; this wasstabilized with the combination of a thermostatic control and a 20-minutewarm-up during which all production is discarded. However, once running,the machine maintains continuous production throughout a five-day weekand the scrap rate reduces to .25 percent.

Ambient temperature changes can also cause the machine's air pressure supplyto cycle between 85 and 100 psi. Nordell's solution was to add a boosterfrom SMC Pneumatics, which maintains pressure at a constant 100 psi. Fourstabilizer pads--in effect, hardened steel, precision ground shouldersmatching the height the cavity stands away from the tool face--have beenbolted to the face to eliminate any micromovement between the two halvesof the tool, a modification that cost about 100 (US$ 170).

An SPC regime has also been introduced. Every half-hour, three or fourspools are taken from the machine and allowed to cool for 15 minutes. Theyare then inspected with a digital micrometer and compared with figureson a control chart. These inspections, carried out by workshop personnel,are independent of those performed every 2 hours by the patrol inspector.As a result, the more demanding tolerances are now routinely held all day.Could the inspection frequency be cut to once an hour? No, says NordellMD, Ray Harrison, the half-hourly rate provides "peace of mind."

The Butler Designs Model 12/90H was acquired specifically for this workbecause it was thought that Nordell's two existing Model 10/60s would not(at 6 tons) have had sufficient lock. As things turned out, boosters havealso been added to these machines, which can now also produce spools tothe required tolerances.

Contact Information:
Nordell
Mr. Ray Harrison, Managing Director
King Edward Close
King Edward Avenue
Worthing, West Sussex BN14 8DJ
UK
Tel/Fax: +44 (1903) 212751

Butler Designs
Mr. Nigel Rust
83-84 Empire Industrial Park
Brickyard Road, Aldridge
West Midlands WS9 8UO
UK
Tel: +44 (1922) 745744
Fax: +44 (1922) 745733