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Excel Model "0" Jig Boring Machine. The "Ultra" and B.C.A. Mk. 2 versions were almost identical with many interchangeable parts
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Although all early examples of the machine appeared superficially identical, various improvements were made as UK production got underway, with some machines having 6-inches of clearance beneath the spindle nose and others 7; speeds also varied with those on the "Ultra", for example, becoming 365 to 1045 r.p.m whilst the Excel had a range that extended from 565 to 1560 rpm. Another version was also built, presumably for some special-purpose task, with the motor spindle carrying a reduction gearbox that gave a maximum speed of only 500 r.p.m. - a figure so low as to render it completely unsuitable for use with small-diameter cutters. However, probably the most significant improvement was to the head where, instead of being limited to 40-degrees of inclination each side of vertical, an increase was made to a more useful 45-degrees - with precise location back to zero assured by the use of a ground dowel pin. Although the changes to the head angle were welcome, the machine was still stuck with a drive system that mitigated against further improvement - a fractional H.P. motor bolted to the back of the column that drove the spindle with a round (originally composite-construction leather and fabric) "rope" via a series of jockey pulleys. After leaving the motor pulley the rope ran first over a jockey pulley mounted on a bar pivoting concentrically with the motor's spindle and tensioned by a heavy tapered cast-iron weight suspended within the main column. The drive then passed round a pair of pulleys carried on an adjustable (spring-loaded) arm mounted on a bracket formed as part of the head casting. Although the various guide pulleys ensured the belt was always wrapped snugly around the spindle pulley, as the head reached its maximum inclination the rest of the assembly struggled to keep the belt in place - and anyway, the boss carrying the jockey pulleys eventually fouled the side of the column. To be blunt, the drive is a thorough nuisance and on one example of the "Ultra" owned by the writer a simple yet ingenious solution had been found - the motor was fixed to a plate carried on a boss that fitted into a hole on the back of th column an arrangement allowed the motor to be swung in sympathy with the head and the belts to run almost in line, no matter how far over the head was inclined. Unfortunately this system was not further developed - indeed, later versions of the machine, built by both B.C.A. and R.E.Godfrey, had heads restricted to just 30 degrees of tilt. A solution seen some years ago on a friend's machine was to remove the jockey pulleys and use their mounting bracket to fit a small but powerful DC motor taken from the Winchester disc drive from an old main-frame computer. This particular set-up gave, in conjunction with a variable-speed controller and the 3-step pulley (using a modern poly-cord belt), a speed range that ran from 100 to over 4000 r.p.m. Today a small 3-phase motor under the control of a phase inverter would be the way to go.
On the original Wolf Jahn "WJA" the head would have been based on one taken from the company's smaller precision bench lathes and held a spindle and bearings made from glass-hard ground steel lapped to a perfect finish and advertised as being capable of "up to 5000 rpm.". On the Boley and subsequent versions the head was a specially-designed unit and contained a 7/16-inch bore spindle made from high-quality steel hardened, ground and running in hand-finished parallel-bore (slotted) phosphor-bronze bushes, threaded at both ends, and set in tapered housings. Serrated ring nuts, that drew the bearings down into their seats, were used for adjustment; however, to limit the extent to which they could be closed, each had a thin bronze insert (that could be thinned if necessary) slipped into their "compression slot". The upper and lower adjustment rings for the top bearing (the smaller of the two), were each extended to hold a large self-aligning SKF ball race designed to absorb the considerable spindle end thrust experienced during boring operations in harder materials. The 3-step cast-iron drive pulley was keyed to the spindle and secured by a single slotted-head screw that bore against the key. On the left-hand side of the head was a screw-adjustable depth stop and most machines also had a simple clamp (to the right of the spindle) to lock the head when taking cuts with the compound or rotary table. The spindle nose was 5.5-inches above the table (minimum distance 2-inches) with cutters and boring heads held in rather small collets of the Lorch "long-series" type. Some of these collets have been found with a thread inside, ideal for holding cutters that would otherwise tend to "walk out" of a plain collet if used to cut sideways. The feed screw, in hardened steel, ran though a long bronze nut slotted vertically halfway along its length (but without an adjustment screw); play in slide was adjusted out by either an ordinary "push-screw" gib strip or a thick, tapered gib block that, like the slideways, was hand scraped to a perfect flatness. Although the head carried an engraved ruler scale extending to 5 inches, only 4.5-inches of travel was available.
While one range of five speeds of 565, 700, 935, 1160 and 1560 r.p.m. had been available on the "Excel" the B.C.A. Mk. 2 offered the choice of two: 350, 470, 590 and 1000 rpm as standard or, at extra cost 700, 940, 1180 and 2000 r.p.m. Although useful for larger-diameter boring work neither range was really fast enough to get the best out of very small-diameter cutters and it is not unusual to find machines where owners have fitted a 2800 r.p.m motor, or a larger drive pulley, to obtain significantly higher revolutions.
Using scraped ways, the compound-slide rest had a longitudinal travel of 7 inches and across of 5 inches - though the Elliot sales literature mistakenly claimed 8 inches for each axis of both the "Ultra" and the "Excel". The feed screws were ground from hardened stock and ran though bronze nuts with the simplest form of backlash compensation - a slot cut through the nut with a screw to nip up the clearance. Although the micrometer dials were rather small, and lacked vernier scales, they were calibrated in 0.001" increments and each axis was also provided with a ruler fitted into a clamp such that it could be adjusted quickly and easily. Attachment points to hold dial-indicators were also fitted, as were screw-adjustable stops - though, unaccountably, some machines have been found without either of the latter two refinements. Instead of a conventional T-slotted rectangular table the top section of the compound slide was formed as a permanent 8-inch diameter rotary table, engraved at 2-minute intervals and with 3 (later 4) T-slots. The table worm-drive gear was contained in a rectangular housing at the front (on the German versions this was rounded off) that could be disengaged by slackening two cross-pin-headed screws, so allowing it to be spun by hand, Occasionally machines are found with a rectangular T-slotted slide on top of the rotary table (which severely restricts the height of jobs that can be accommodated) or (even more unusually) a rectangular table in place of the round.
The final development of the borer, which attempted to address the remaining shortcomings of the Mk. 2 and make it into a truly useful machine, was the B.C.A. Mk. 3. Of significantly more massive construction, with a much deeper and wider main column, this version was arranged to carry the head higher and so give both an extra 2 inches of clearance between spindle nose and table and a slide travel of a little over 6 inches. The head castings were heavier, with slightly wider slides (though the spindle and bearings remained unaltered) and on most machines both a direct-acting screw and a side-mounted handwheel were fitted, the latter driving (at 90-degrees) through worm-and-wheel gearing to give an extra-fine feed. Unfortunately these welcome modifications resulted in the head's maximum inclination being reduced from 40° or 45° to only 30°. Both elements of compound table were lengthened to give an extra two inches of travel on each axis and the feed screws equipped with ball thrust races and larger micrometer dials. The number of slots in the 8-inch rotaty table was increased to four and, by using a 2-speed 3-phase motor, 11 spindle speeds were available spanning 300 to 3250 rpm, a significant improvement on the limited and comparatively slow ranges available previously. The drive system wwas also modified and, instead of a tensioning weight within the column, an extra pair of jockey pulleys was used, mounted on a swinging bracket pivoting on a bar socketed into the left-hand side of the column.
Installed in a modeller's or experimenter's workshop any type of B.C.A. is probably the ultimate in a compact, versatile precision milling machine; the built-in 8" rotary table means that complex jobs can be set up and left in place through multiple machining operations, whilst the ruler and dial-indicator mounts mean a quick and easy way of checking lengths and depths of cut. The continuing usefulness of the machine is demonstrated by the fact that, in the 21st century, the Mk. 3 remains in production (late examples are finished in a rather pleasing smooth black finish) with the price of over £10,000 (in 2000) reflecting both the great care taken in its construction and the use of the good quality materials.
Tony Griffiths..
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