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Impetus-Metalmaster
Universal Machine Tool
With thanks to Mike Collins - who made available much of the material for these pages.

Impetus-Metalmaster Page 2

Not be confused with the Metal Master light hobby lathe by the Metal Master Tool Company of Mt. Ephraim, New Jersey, USA; the Atlas "Metalcraft" or the Bradford Metalmaster also built in America, the UK-built Impetus was a general-purpose "combination" metalworking tool designed by W. D. Urwick C.Eng., M.I.Mech.E.  and manufactured in the early 1950s by Samuel Jenkins of Rougham Hill, Bury St. Edmunds.  Mr. Urwich was the owner of a small foundry and devised the machine to both exploit his patented triangular gib key and to demonstrate many of his other unique and innovative ideas; the aim was to get the maximum versatility and accuracy out of a small, affordable machine tool. 
Unfortunately, the project ran into what must have been production or costing difficulties and only a dozen examples were manufactured - though, fortunately, several seem to have survived, including one with a set of original advertising and technical literature. By further good fortune Mr. Urwick wrote about his design, and explained the theory behind it; not being able to improve upon this comprehensive description, it is reproduced in full here.
Another "Combination Machine", the more robust Labormil, was eventually to be produced based closely on Mr. Urwick's design, even to the use (which was unacknowledged) of the patented Triangular Radial Gib Key. The key was at the heart of the design's success for, when moved up or down the main column (by a jacking screw) and re-clamped, the far end of the bed did not deviate by more than .001" to .002" in radial alignment - a remarkable achievement for the key was operating (at that radius) at a 'disadvantage' of about 20 to 1. Not only was the clamp highly effective but, when released, the two components were able to slide with perfect smoothness.
Continued below:

Continued:
Notes from the original Instruction Book:

General Description:
This machine should not be regarded as a special form of lathe. It represents a new conception of a general purpose machine tool made possible by a simple patented invention. This invention consists of a triangular gib key (British Patent No. 696773) which enables any member enclosing and sliding upon a column to be maintained in strict radial relationship to the axis of the column.
Within its capacity this machine will perform the duties of a milling machine, a horizontal borer, and a lathe with no special adaptation. The attachment of a hand lever also enables light shaping work, such as keyway or spline cutting, to be carried out without the usual tedious handwheel operation.
The backbone of the machine is a massive column on which the main bed, complete with cross-slide, leadscrew, and changewheel arm, can slide under the control of a vertical feedscrew. This main bed is keyed to the column by means of the gib key already mentioned and its radial relationship to the column is thereby maintained. It may be clamped by means of the key at any desired height with complete confidence in the accuracy of its alignment.
The headstock is seated on the head of the column and means are provided for rotating it within fine limits so that the mandrel may be lined up accurately with the main bed and locked in this position.
At the back of the headstock two brackets carry an auxiliary Tee section bed on which is mounted the tailstock. Again, fine adjustment is provided for setting the centres in accurate alignment.
It will be appreciated that the parallel alignment of the mandrel, main bed and auxiliary bed in the vertical plane is taken care of in the design of the machine, but that in the horizontal plane there is a radial adjustment under the control of the operator. With the aid of a test bar, adjustment is not difficult and once set, the accuracy of the machine should be retained over long periods. The ability to check over the alignment in a few moments gives added confidence where particular accuracy is required.
The drive from the motor is transmitted to the mandrel by means of a single belt driving a large four-step pulley weighing approximately 12 lbs. This pulley serves a number of purposes: a) it serves as a dividing head
b) it takes the place of a back gear
c) the momentum stored in it greatly helps to eliminate chatter, particularly in parting off operations
d) it is very convenient as a handwheel when using taps and dies in the tailstock, the weight giving good control as the threads are cut
e) it can be used freely as a break to stop rotation after switching off the motor, and generally for turning chucks, etc., mounted on the mandrel.
The headstock mandrel is bored to take 3/4" dial. stock and the nose is bored No. 3 Morse taper for suitable collets.
For crating and transport, the entire machine can be rapidly dismantled into a number of pieces, each of which is readily handled and packed. Re-assembly is an equally straightforward procedure.
2. CROSSFEED AND LEADSCREW ARRANGEMENT
Both longitudinal traverse (approx 12") and cross traverse (7") of the boring table are accomplished by solid turned handwheels attached to the ends of the operating screws, the rims of these wheels form index dials 3 1/2" and 3" diameter. respectively.
The leadscrew nut is not split, nor is a rack and pinion traverse provided. For normal operation the four way toolpost is mounted direct on the boring table and the feed applied by means of the leadscrew and crossfeed screws. The absence of a topslide is no inconvenience on a small machine such as this and there is greater rigidity without it. A small topslide forms part of the equipment but its use should be confined to short tapers and for screwcutting.
The leadscrew nut consists of a shouldered bush which is an easy fit in a suitable hole bored through a lug on the underside of the saddle. The end of this nut protruding from the casting is threaded and a steel tube, enclosing the leadscrew, screws on to this threaded portion and locks the nut in position. When it is desired to rack the saddle with a hand lever, the leadscrew nut can be released and spun down to the remote end of the leadscrew. The steel tube referred to above also provides some protection to the leadscrew where it emerges from the saddle.
Automatic feed is applied to the leadscrew through a dog clutch keyed to the headstock end of the screw. A second dog clutch is also provided in the mandrel and its use is described in the section on screwcutting.
The use of solid nuts for both feedscrew and leadscrew means that they are floating in so far as they are supported only by their respective nuts and the thrust bearing. This arrangement gives a very smooth action and tends towards greater accuracy.
3. SCREWCUTTING
Though not original, the method of screwcutting on this machine is unorthodox, but very convenient. It may not be familiar.
A sleeve, permanently fitted on the mandrel, carries a 24-tooth wheel, which drives the change wheel 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 change wheel train, with the result that it is impossible to pick up the wrong thread. It will be found that the changewheels spin quite easily as the saddle is traversed back to the start of the thread when this clutch is disengaged.
With one exception, the number of teeth on the changewheels are all multiples of three. The calculation necessary to determine suitable changewheels for any particular number of TPI is extremely simple, since

No. of Driven Teeth    = TPI x 3
No. of Driving Teeth

For example, to cut 6 TPI:
No. of Driven Teeth   = 18
No. of Driving Teeth
i.e. one 18 tooth wheel on leadscrew together with suitable idlers to give desired rotation.

To cut 52 teeth:
TPI x 3 = 156 = 39 x 4 = 39 x 72/18
To cut 56 teeth:
TPI x 3 = 168 = 21 x 4 x 2 = 21 x 72/18  x  18/27
Numerous metric threads can be cut by the introduction of the 38 tooth wheel included in the set. It will be appreciated that the 38 tooth wheel fitted tot he leadscrew with two suitable idlers provides a 2 mm pitch and that other metric threads can readily be calculated as multiples up and down from this basis. The 38 tooth wheel also enables 19 TPI (i.e. 1/4" and 3/8" gas thread) to be cut, with a simple train.
4. MILLING
Apart from the removal of the toolpost or topslide, the machine requires no modification whatever to become a small milling machine.
A cross-feed of 7" and the large slotted table 10" x 4 1/2" enable quite large workpieces to be machined, but it must be borne in mind that this is a light machine and excessive cuts should be avoided.
A machine vice can conveniently be mounted on the slotted table and end mills, slitting saws and fly cutters can be used for a variety of work.
If the gib key screws are correctly adjusted, it will sometimes be found that the vertical feed can be used with advantage in certain milling operations.
A dividing head or rotary table, of course, greatly extends the range of work which can be accomplished.
5. TAPER TURNING
When turning up a mandrel for holding work to which it is desired to give a very slight taper, it will be found sufficient to slacken the adjusting screws of the main bed gib key. The bed than can be slewed to a small angle of taper (which can be accurately clocked) and then locked in position by means of the clamping bolt.
For such things as Morse tapers it is necessary to remove the gib key altogether. Raise the bed to full height, lock in position and lower feedscrew until the upper end emerges. Insert a piece of wood between the end of the feedscrew and the underside of the bed. Slacken off the adjusting screws until the gib key falls down the keyway in the column, when it can be removed. The bed can no be lowered and rotated to any desired degree of taper. If the taper is not too great, the automatic feed can be used an tapers turned for the full travel of the saddle. This procedure will be found to be simple and extremely accurate.
6. DIVIDING
The main driving pulley is provided with 60 equally spaced and numbered holes in the rim. Any of these holes can be picked up and the mandrel locked by means of a link rod, one end of which is attached to one of the slots in the changewheel carrier arm. Movement of the carrier arm allows fine adjustment of work held in the chuck before locking the mandrel in any fixed position. The link rod itself has a subdividing arrangement enabling the 60 divisions each to be divided into 6, thus giving total divisions of 360 degrees.
7. KEYWAY CUTTING
The machine is well adapted for small shaping operations such as keyway and spline cutting. For this purpose a hand lever can quickly be attached for racking the saddle along the bed and making the operation less tedious than is usually the case when lathes are used for this purpose. The leadscrew nut can be released from the saddle (as described in section 3) so that the latter is free to slide and a fulcrum plate can be attached at any convenient position along the bed to give the desired movement by means of a hand lever and link.
8. BORING HEAD
A small boring head with radial feed, operated by star wheel and trip pin, at once converts the machine into a miniature horizontal borer. This addition greatly extends the scope of the machine and makes it particularly useful for facing and boring linker ends and other awkwardly placed bosses.
The slide of the boring head is provided with five positions for the boring tool. This is to enable both internal and external work to be done wit the same direction of rotation of the feedscrew and also so that the most appropriate position can be used to avoid excessive out of balance of the slide.
9. LARGE WORK
With the Tee bed in position work up to 8" diameter. can be accepted between centres and, with care, it is possible to machine work of this size, particularly in lighter materials such as wood or aluminium.
The Tee bed can, if necessary, be removed when work 14" diameter can be swung over the bed. A suitable block or machine vice can be used to raise the tool post to centre height and pulleys or other awkward jobs can be successfully drilled and bored in an emergency. Alternatively large pieces of material or castings can be mounted on the boring table for drilling and facing operations.
10. VERTICAL MILLING
A vertical drill mounted on the Tee bed makes the machine into a small radial drill. The drill head may be lowered on its column until the work can e drilled on the cross-slide table and traversed under the drill as desired. If the drill is taken well out to the end of the Tee bed, quite large sheets or other work can be passed beneath this bed for drilling. Alternatively , when positioned at the inner end of the Tee bed, the radial drilling of work held in the chuck may be carried out, in conjunction with the indexing arrangements on the driving pulley. A small milling head could easily be fitted and would be a useful adjunct.
11. GEARCUTTING
With the dividing head fitted, gears up to 18 or 20 D.P. can be cut with blanks up to 5" diameter. The cutter may be mounted on an arbor between centres and the blank traversed beneath it and indexed as necessary. Small worm wheels may be hobbed in a similar manner.
12. TOPSLIDE
For normal lathe work it will be found most convenient to use the 4-way toolpost attached direct to the cross-slide. A small topslide is, however, supplied and is intended mainly for turning short tapers and for screwcutting, if it is desired to feed the tool in at the thread angle. The topslide is rectangular in form, which will be found convenient when setting with a protractor.
13. THE TAILSTOCK
The tailstock designed for the machine is fitted with a rack feed to the barrel. A graduated dial, with zero setting arrangement, is fitted, and this will be found useful for depthing accurately when drilling. It may also be used when boring blind holes, as a depth gauge, if it is made to follow the saddle as the tool progresses.
The barrel is locked by means of a gib key similar to that used on the column. This key can be adjusted so that there is no rotary shake in the barrel and therefore less chance of drill snatching. Only finger tight screwing down of the locking screw is necessary to hold the barrel, owning to the wedge action of the key. It is also instantly released.
Being independent of the main bed, it is seldom necessary to hold work with the barrel fully extended, since the tailstock may be passed right over the saddle and brought up close to the work.
14. SETTING UP
A ground test bar is required with the machine and one end of this bar should be reduced to No. 3 Morse taper to fit the mandrel nose.
Insert the test bar in the mandrel nose. Ease off the nut on the central stud passing through the headstock casting. By means of the adjusting screws, rotated the head on the column in the appropriate direction until no movement is recorded on a clock fitted in the toolpost as the saddle is traversed up and down the bed. Tighten down the nut on the central stud when this is so.
Insert centres in headstock and tailstock, bring up the latter until the two centres are touching. By means of the adjusting screws BBBB bring the centres into true alignment, at the same time keeping the Tee bed parallel with the main bed, sighting by eye.
Slide back the tailstock and introduce the test bar between centres, test with the clock in the toolpost, and correct by means of the screws BB in the outer bracket only. Tighten up the locking screws underneath each bracket.
Once set, the accuracy of the machine will be retained for long periods, but the above procedure should be used for resetting after dismantling or for checking if particularly accurate long turning work is to be done.
In any case, only a few minutes are required to check the adjustment and the operator can feel that at all times he has the accuracy of the machine under his control.
Vertical Feed Adjustment: Slacken all gib key adjusting and locking screws.
Tighten clamp bolt at rear of main bed casting until lifting screw can be lowered and bet held suspended. Slacken clamp bolt until bed just falls under its own weight.
Tighten gib key adjusting screws individually, again adjusting until the bed can just fall under its own weight.
15. MOTOR DRIVE
The most convenient drive for this machine is a 1/2 hp geared motor, having a final shaft speed of about 300 rpm. If this shaft is provided with two pulley 1 3/4" and 8" diameter., a satisfactory range of eight speeds can be made available with suitable belts. The motor should be mounted on a hinged plate with take up adjustments. .

The Metalmaster in use as a simple horizontal milling machine.
The generously-proportioned 10" x 4.5" T-slotted table allowed a decently strong  vice - to be mounted and, with a cross-feed travel of 7" - equal to many medium-sized milling machines, a lot could be accomplished in a small space.
Note the edge of the cross-feed handwheel doubling as a micrometer dial - an easy and effective way to enjoy a large, clear graduations at minimum expense.
Another very unusual English lathe which happened to use this feature was the Edgar.

A small boring head - with automatic radial feed, operated by a star wheel and trip pin - was available as an extra and converted the machine into a small but useful horizontal borer.
A similar design of boring head was produced at one time by a third-party accessory manufacturer for the Myford 7 Series lathes.

By clamping a fulcrum point and lever assembly to the bed, and picking up a mounting point on the cross slide, it was possible to use the lathe as a shaper.
The lathe was fitted, as standard, with a four-way tool post mounted directly onto the cross slide - an excellent way of achieving extra rigidity. A small top slide was offered as an optional extra for cutting short tapers and assisting with screwcutting.
A very clever arrangement was fitted to exploit the 60 "division" holes which ringed the front face of the large drive pulley - the lever carrying the engagement pin (it can be seen fastened to the banjo carrying the changewheels) was provided with 6 incremental positions which extended the total number of divisions that could be created to 360.

The drilling head (which was omitted from the small batch of machines made) shown in its operating position with the simple but clever drive system of a bar and pulley driven between centres.
Because its support ran the length of the bed, the drill became, in effect, a miniature radial-arm type.

The Designer Writes
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                          Additional Metalmaster photographs here

The designer's own lathe - now in the sympathetic hands of Mick Collins
The Metalmaster - a modern review:
At some point in the early 1970s David became interested in model Stirling engines and it was as a result of an article that he wrote on his moving regenerator type in Model Engineer that I first wrote to him.
A long correspondence and friendship followed and when David returned to England from his retirement home in Malta I was able to visit him at his new home in Somerset. Naturally we gravitated to his newly-built workshop where I saw not only his collection of Stirling engines but also his Metalmaster, a machine that had fascinated me ever since reading about it in the Model Engineer back in the fifties.
Unfortunately it wasn't working as, during the journey back from Malta, the cross-slide feed-screw nut had been lost. However, within a couple of days I had made and posted a replacement to him and the next time I visited I was able to play with the machine.
Sadly, a few years later, poor David became a victim of Parkinson's Disease and, anxious that his machine should go to an appreciative owner, he offered it to me. A fair price was agreed and I became its proud owner.
Seen here with the machine are some of the accessories, including the boring head and topslide mentioned in the brochure, together with the vice designed to fit the cross-slide and a matching dividing head. The saddle fitted on the parallel auxiliary bed is my own addition and can be slid along by hand before locking in position. It was originally made to hold a fixed steady but, in addition, proves very useful for holding the lamp, the D.T.I. and the home-made tool height setter, a device similar to the old 'Unique' indicator, which can be swung out from under the saddle to sit on the tool tip.
There is little I can add to David's article re the versatility of the machine, it is certainly all that he claimed for it and can be converted to any of its functions within a couple of minutes. Its accuracy was all that he claimed for it although now, after nearly fifty years of 'amateur' use, is slightly impaired by wear at the headstock end of the bed. (In retrospect, David thought that since the tailstock was no obstacle, the saddle could have been made rather longer with some advantage) I can't really comment on the machine's rigidity since I normally use it for building small model Stirling engines and handle it like cut glass - nevertheless, recent jobs have included skimming the brake drums of my wife's Morris Minor and re-machining the base of an Indian cylinder barrel, requiring the full fourteen inch swing to accommodate the finning around the valve seats. Sumitomo tips are very satisfactory for all these straightforward turning and boring jobs.
David's geared-motor drive was becoming very noisy and has been replaced with a Sinclair C5 motor with infinitely-variable speed control since when parting off inch and a quarter steel has become a real pleasure - albeit a slow one!
Finally, I must agree with his remarks regarding the usefulness of the vertical feed - handling a conventional lathe now feels like working with one hand tied behind my back. And the sheer versatility of the machine has given me a new hobby - that of finding new jobs for it to do. Mitering picture frames took longer than expected though, due to the need for spotless cleanliness ...

Milling machine mount

Headstock cover

Headstock-mounted dog clutch

Spindle pulley-cum-flywheel-cum dividing ring

Complete headstock assembly

Spindle nose

Leadscrew dog-clutch

Graduated handle on leadscrew

E-Mail Tony@lathes.co.uk 
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