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	Home Machine Tool Archive Machine Tools For Sale & Wanted E-MAIL tony@lathes.co.uk Impetus-Metalmaster The Designer Writes Metalmaster Home Page The Metalmaster in Use Photographs A Modern Review With thanks to Mike Collins - who made available the material for these pages. By W.D.URWICK. C.Eng., M.I.Mech.E. First Published in the SIMEC Newsletter in September 1973 Some twenty years ago I designed and built for my workshop the small general-purpose tool here described and it has proved such an unqualified success that I think the ideas behind it must interest readers of your Newsletter. I felt that for too long we have been constrained by the principles of Maudsley and that the model engineer particularly, requires a small machine with the greatest possible scope and capacity, which the orthodox bench lathe in miniature is far from providing. A centre lathe makes a poor milling machine for lack of a vertical feed to the cross-slide or work table. A milling machine makes a poor lathe because of the difficulty of maintaining accuracy for turning between centres with a rise and fall cantilevered bed. A horizontal boring machine is a fine maid-of-all-work but is not much use as a lathe and is not, in any case, available in a small workshop size. All these machines, nevertheless, have a headstock, a tailstock and a work table or cross-slide and, after a great deal of thought, I managed to rearrange these components to give me the flexibility I sought. The headstock of my machine is mounted on top of a massive column, with a cantilevered bed carried on the column beneath it and raised and lowered by a jacking screw at its point of balance, An auxiliary Vee bed carries the tailstock and is mounted in brackets behind the headstock. The top surfaces of these brackets are ground true with the mandrel and the auxiliary bed is held by screws forcing it upwards against straps seated on the ground surfaces of the brackets. Thus, the working surfaces of the two beds, the headstock mandrel and the tailstock barrel are all strictly parallel in the vertical plane. However, when looking down on the machine from above, alignment of all these parts can be achieved by radial adjustment, with the column as centre, and this adjustment is under the control of the operator. It will be clear that, for the success of this arrangement, one component, the cantilevered bed, must retain its radial relationship with the column with extreme accuracy and this I achieved with a special triangular gib key, which I invented and patented at the time. A sketch of part of the original Patent Drawing is shown here, though the patent itself has, of course, expired..

The startling performance of this key in maintaining accuracy has to be seen to be believed. Its very simplicity is deceptive. When raised or lowered on the jacking screw and re-clamped, the outer end of the bed can be relied upon not to deviate by more than .001" to .002" in radial alignment. One must appreciate that at this radius the key is operating at a disadvantage of about 20 to 1! An error of this order over a length of 18" is probably as good as one would expect from any orthodox light machine of this class.
The Triangular Gib Key has three great points in its favour for this particular application:
(1) The alignment of the bed with the mandrel can be maintained to very close limits indeed, when it is raised or lowered, on account of the keyway faces themselves being radial.
(2) When locked, the key acts as an extremely rigid clamp and being of wedge shape, can be adjusted to a very close sliding fit, like a gib strip, and yet it clears instantly on release. The sliding clearance at the key faces can be so small that, with normal work anywhere near the chuck, it is unnecessary to apply the locking screw and clamp the bed. The vertical feed can be used for milling or other purposes as freely as the cross-slide or leadscrew feeds.
(3) If the key is removed, the whole bed can be swung radially and locked, for turning work to any desired taper. In fact, the arrangement can be summed up by saying that parallel turning is merely a case of "zero taper."
Adjusting screws are provided for truing up the mandrel, and the auxiliary bed and tailstock to fine limits and with the aid of a test bar, this radial lining up of the machine can be achieved in a matter of minutes.
In considering the general details of this machine many 3 1/2" centre lathes were considered and an attempt was made to adopt the best features that could be found amongst them. Of these lathes, undoubtedly the most original and useful of all was the 3 1/2" Exe machine, on the market in the 1930's. The headstock arrangements and screwcutting details of this beautiful little machine were adopted and no better choice could have been made. The mandrel carries at its outboard end a 12" driving pulley weighing about 12 lbs. The power unit used is a Higgs 1/2 h.p. geared motor, the output shaft running at approximately 300 r.p.m. This shaft carries a 2-step driving pulley, thus giving eight mandrel speeds varying from 45 to 900 r.p.m., using a single Vee belt which can be freely thrown on and off the pulleys. However the main function of the heavy pulley on the mandrel is to give "flywheel effect" and provide momentum between workpiece and tool as the cutting operation takes place. With a light machine this momentum makes all the difference in eliminating chatter and I can turn without difficulty 8" dia. C.I. traction engine wheels or a 12" dia. aluminium pulley. The auxiliary bed is removed to accommodate large diameters above 8". I was recently able to put a taper bore in the boss of a 10" dia. marine propeller. It is very useful to be able to swing such an article, the operation itself being well within the capacity of the machine. For this job the main bed was rotated on the column to the correct taper, after the key had been removed and, at this slight angle, automatic feed could still be used.
The screwcutting arrangement too is most ingenious and possibly unfamiliar. A sleeve on the mandrel carries a 24-tooth wheel, which drives the changewheel train, and also incorporates a single-dog clutch by which it takes its drive from the mandrel. In screwcutting, this clutch is used to engage the changewheel train, with the result that it is impossible to pick up the wrong thread!
In addition to this, the number of teeth on the changewheels are all multiples of the number 3 instead of the usual 5. These numbers 18, 21, 24, 27, etc., break down into simple fractions far better than the usual ones and the formula with an 8 T.P.I. lead-screw is simply:

No. of driven teeth
---------------------- = 3 x T.P.I.
No. of driving teeth

Metric threads can also be cut with the addition of one 38-tooth wheel. The number 3 and its multiples are so convenient mathematically that it is difficult to understand why the awkward system using multiples of 5 has persisted for so long.
A boring head with cross feed, operated by star wheel and trip pin, was designed to fit the mandrel nose and with this in action, the machine can, with justification, be described as a miniature horizontal borer. The boring head is in use almost as much as the chuck, for facing, boring and even internal screwcutting awkward shaped castings.
I should feel completely lost without my vertical feed. There must be a saving in time alone of the order of 20% in never having to pack tools and boring bars. When parting off, I find that I can raise or lower the tool to the best cutting height whilst actually making the cut.
After machining many hundreds of parts for models and other things, I feel that I have still not explored all the possibilities opened up by the facility of being able to feed the worktable freely in three dimensions in front of the mandrel nose.
W. D. Urwick
Malta


	Home Machine Tool Archive Machine Tools For Sale & Wanted E-MAIL tony@lathes.co.uk Impetus-Metalmaster The Designer Writes Metalmaster Home Page The Metalmaster in Use Photographs