Title: Printed circuit drill
Pages: 62 - 63
Text: PRINTED CIRCUIT DRILL
Range of numerically controlled printed circuit board drilling machines designed and made by Vero N C Developments Ltd. Southampton.
Last Christmas Vero N C Developments Ltd received an order worth more than £1/2 million from Russia for 22 numerically controlled printed circuit board drilling machines. That order was remarkable, for Vero is a small company which concentrates on designing and making highly-specialised machine tools to meet the particular requirements of individual customers: consequently, production runs rarely total more than three or four for any one machine. The award-winning multi-spindle drilling machines started that way, too, as a one-off for the British Aircraft Corporation. But the combination of exceptional accuracy and quality of drilling, high rates of production possible and reasonable price has attracted attention from many printed circuit board manufacturers.
The development of multi-layer printed circuit boards packed with micro-miniature components requires small holes positioned extremely accurately and finished without burrs, dust, fibres or any smearing of the resin which might inhibit bonding during soldering. The Vero machine will drill within a hole to hole accuracy of 0~003in ptz diameter with repeatabilitv of +0 00025in at a table/ carriage traverse rate of 230 inches per minute. The quality of drilling is ensured by drill speeds being infinitely variable between 30 000 and 70 000 rpm. Strike rate depends on the type of board being drilled, but it can be as high as 90 strikes per minute, which means that a 10 spindle machine working on boards stacked three up can drill some 100000 holes an hour.
A major contribution to this high output is a unique automatic drill changing and depth setting system. It enables the drills to be changed on any number of spindles in about a minute, compared with five minutes required to change the drills manually on only four spindles. Drill changes can be controlled either automatically from prepared tape, processed with the necessary coordinates and code instructions or can be initiated manually. The system enables drill sizes to be mixed, the drill point height being set automatically from the Vero depth setting logic. Spindle speed is predetermined by the setter and the chipload calculations are simplified by the provision of a meter with visual indication of the required feed in inches per revolution. The feed system has been designed so that the drills do not dwell at the bottom of the stroke, so avoiding excessive wear or any chance of resin smear. The drill spindles are carried on Westwind air bearings, the Vero machines being the first of their kind in Britain to use this no-friction system. The spindles are driven by a high frequency electric motor, speeds can be infinitely adjusted within the range indicated and are controlled by a solid state frequency converter. For high production rates the machines have two easily positioned air-cushioned work platens so that, while one is carrying boards under the drill heads, the other can be loaded.
A major part of the success of the Vero machines is due to the fact that the designers have taken full advantage of the company's jig-boring shop which, with an impressive œ4 million worth of equipment and highly-skilled personnel, is capable of some of the most precise work in Britain. This means that all the structural components are machined to extreme accuracy. dimensional stability of the high stiffness steel welded fabrications being ensured by stringent stress relieving techniques. The drill heads themselves are carried on a fixed beam. positioning of the drills being carried out by moving the platens carrying the boards to be drilled.
Accuracy in moving the platens is, of course. vital. The X axis carriage is mounted on eight recirculating split ball bushes running on hardened and ground precision bar ways. with the Y axis carried on recirculating roller units: table travel is square to a nonaccumulative 0 00025in per foot. Both axes are positioned by recirculating ball screws fitted with high stiffness double nut assemblies coupled to rotary resolver units and driven by direct-coupled 3/4hp dc SCR servo motors.
The Vero drilling machines are compatible with most high performance N C control systems. and have the facility of either being controlled automatically from the paper tape or semiautomatically from information fed in manually through thumb-wheel switches. The Vero-designed interface is all solid-state and plug-in modules simplify maintenance.
R H Hartley, Vero's technical director and the man in charge of the design team, says that the drilling machines are simply "good precision engineering". But this is only part of the story, for Vero has a history of innovations and of a breadth of experience in electronics and mechanical design which has undoubtedly contributed considerably to this success. Originally entirely mechanically-oriented, in 1953 Vero became the first British company to introduce numerical control to turret drills and a separate company was later set up to concentrate on the electronics (the nowstandard Veroboard printed circuit board being one result). For the awardwinning drilling machines Vero became the first company in the world to introduce automatic drill changing and the first in Britain to use high frequency bearing drilling spindles.
''Our aim is to keep things simple. Frills are not only redundant but can get in the way of accuracy and reiiability,'' says Mr Hartley. He is the product of the first of the Machine Tool Trade Association's two year postgraduate courses in machine tool design at UMIST under Prof F Koenigsberger. ''Looking back, one of the most useful lectures was metrology,'' he says. ''It's all very well being able to design machines, but when you are working to the accuracies and performances demanded of us now the most difficult part is developing techniques for assessment and testing."
Vero NC s pirated circuit board drilling machine with automatic drill changing and depth setting can drill 100 000 holes per hour with 10 spindles working on Boards stacked three high. The drill heads are on a fixed beam accurate positiomag of the drills Is achieved by moving the platen mounted workpieces. Numerical control instructions can be fed in automatically by punched tape or by thumb wheel switches