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Hubbing, Hobbing

Hubbing, Hobbing.  The process of making a die in steel from a design or pattern in hardened metal with the intent of reproducing all the die cavities of the design. Hubbing is done in a hubbing press and is called diesinking (which formerly included all forms of die making), or more simply, sinking. The process always changes relief – from positive to negative (or negative to positive). A positive hub will sink a negative die; a matrix is sunk from a patrix; a master die (being either positive or negative) would sink a die of the opposite relief. Hubbing will always create dies of the same size, as hubbing cannot enlarge or reduce design and, of course, whatever the design in the original is reproduced in the copy one at a time. Each new die blank obviously must be in softened steel to transfer the design from the harder steel master.

Hubbing effects great savings in time, at a lower cost, and creates dies that are

always identical. Hubbing dies are in contrast to hand cut dies and machined dies

– those cut by reducing pantograph or electrically or chemically etched. A die can be hubbed in 15 minutes which could require days to hand cut, or from 10 to 30 hours to cut by die-engraving pantograph for example.

Also the surface and internal structure of a hubbed die is improved over that of a machined die – with altered grain structure of the steel. Generally, hubbed dies last longer in production than machined dies; average life expectancy of machined dies is, say, 50,000, while some hubbed dies can strike 250,000 pieces or more.

Although hubbing can be employed to make only one striking die (as for an award medal), hubbing is far more ideal for long production runs – as for coinage – where many identical dies are needed for quantity production, and where many dies can be placed in production in many presses.

Cold versus hot hubbing.  Hubbing can be done at room temperature, this is called cold hubbing; its advantages are the exactness in duplicating the original and the ease of accomplishing this in a hubbing press. Most coin and medal dies are hubbed at room temperature to preserve their finite detail.

Hot hubbing requires coating the hardened hub with lampblack, this acts as both an insulator (preventing the hub from being affected by the heat) and as a lubricant. When the hub is ready, the die blank is heated (to 1000 degrees F, a dull red color) then the cold hub coated with lampblack on end is placed on top of the heated dieblock and given a squeeze in the hubbing press. Heating the metal block (to make it easier to diesink) does, however, distort the design somewhat because as the die cools it shrinks slightly (even though this is less than one percent). Thus hot hubbing is only employed where this shrinkage and exact tolerance is not that critical (say for a badge die).

Hubbing's great advantages.  For the most part hubbing is the process of sinking dies from hubs (as for the mass production of dies for large quantity striking as in coin production). It is also an advantage where a large quantity is needed in a very quick time (to meet a very tight deadline, as, say for presidential inaugural medals, where the medals need to be designed, approved, dies made and medals struck between the date of election and the date of the inaugural ceremony). In this case, several dies can be made and these placed in several presses. Also hubbing is a metalworking insurance policy: all dies will break sometime, it is never known when this will occur. Having a hub handy makes it easy to quickly obtain another working die without a great delay in production.

How hubbing is accomplished.  The master die or pattern, in steel (sometimes brass), is hardened by tempering. The steel to become the die is treated, both by heat treating and, where necessary, by machining to prepare the die block in advance of the hubbing. Depending upon the shape of the die stock, the die block can be turned on a lathe to a required diameter for the intended design. The bottom and face will be squared off (it must be parallel!); but also it can be machined with a cone or dome rising from the face to form a cone blank or dome blank.

The hardened hub and the treated die blank are then placed in a  . This is a tube or collar which holds both hub and dieblock in place during hubbing. This assemblage is then placed on the bed of the hubbing press, and the press actuated. Large dies do not need the retainer ring, however, small dies do, particularly those that have a cone or dome to keep them in aligned position during hubbing.

This cone shape will be flattened and pressed into the block beneath the die cavities during hubbing, but is done for several reasons, the most important of which is that the design is not pressed into the dieblock all at once but is done progressively starting in the center and advancing outwards in ever increasing circles. It also aids metal flow, strengthens the die (more so in the center grading outwards toward the edge), and helps, somewhat, to prevent sinking  (with the greater mass in the center of the die, although this is somewhat more compacted with greater density).

The hubbing press (formerly called multiplying press) is usually hydraulic with compressed oil or water in a chamber giving it tremendous power in forcing the two steel blocks into each other, called a squeeze. Often the hubbing press action is less than 1/32-inch per minute (as in hubbing coin dies where the height of relief is not great); higher relief, of course, requires greater power and thrust of the hard steel master into the soft die block.  See hubbing press.

After retracting, the two are examined to see if all relief has been reproduced. A test to prove that all cavities in the die have been created is given below. If another squeeze is required it is softened again. The new die becomes work hardened by the intense metal flow and it is necessary to anneal the die before another squeeze is performed. The hub is again positioned above the annealed die (making sure it is seated correctly – no double hubbing impressions please!) and the press is actuated for a final squeeze. Depending upon size, it may be necessary to machine off the disturbed metal pushed out by hubbing as this will effectively and as was inexperienced in heat treating the dies to prohibit this condition. Thus fresh dies deteriorated rapidly before long runs could be struck.

a dam restricting further metal flow.

Hubbing anomalies.  Among the things that can go wrong during hubbing

include: (1) lack of parallelism: both surfaces, face and bottom of the hub and die blank – in addition to the hubbing press itself – must be absolutely parallel; (2) curing time: the new die must be NORMALIZED properly; (3) improper HEAT TREATING: hub not hard enough or the dieblank not soft enough; (4) improper die stock with, perhaps, internal problems; (5) not fully sinking all the design: if the hub hasn't been squeezed long enough, the HIGH POINT may be lacking; and, of course, (6) not seating the hub and die properly for a second or subsequent squeeze.

Usually such anomalies are not apparent until the die is placed in use when it may

break. The first blow, or subsequent blows, will fracture, or cleave in half like a split log.  See diebreak.

Proving the die sunk from a hubAn operator will usually examine the fresh die under magnification noting particularly the high points; however to insure that all the design has been reproduced in the die it should be proved. This is done by an old diesinking method, with a simple candle. Soot (lampblack) from the burning candle is deposited on the design in the hub. The die is pressed into the hub and retracted. This is done by hand as benchwork, not in a press. The die should remove all the soot from the hub. If it does not the die is not touching that part of the hub and it must be hubbed again.

Touching up the die.  At this stage any touchup of the die can be performed. Lettering can be sharpened, lines in hair deepened for example or design strengthened; also tool marks may be removed if necessary. This is done in the die while still in a soft state by chasing and engraving tools.

Proving may be done after the touchup but only in soft metal – tin or lead – while the steel die is still dead soft (see proving). Once the die is fully complete, and the soft metal proofs approved, the die can be hardened for striking.

Some diesinkers prefer to do smoothing by grinding after the die is hardened, as the removal of raised tool marks. If these are in the field, the grinding could easily remove these and not disturb any relief or lettering.

History of hubbing.  Knowledge of using a hardened iron block to press into a softened iron block had been known for centuries (since early blacksmithing). Coin die designs were first pressed into dies about 550 bc (when we can date the first crude hubbing of coin dies). This was done with the force of a manual sledge blow.

The first screw press for striking coins and medals was developed in the early 16th century and this was, indeed, used for hubbing. In 1789 Matthew Boulton hired Jean-Pierre Droz away from the Paris Mint to gain his knowledge of die making. Boulton had acquired great skill in metalworking and found his methods of hubbing were superior to Droz's.

Boulton continued using the screw press to hub the dies needed for his Soho Mint,

but improved on the technique by using finer tool steels (supplied by Benjamin Huntsman, see IRON). This hubbing was of the device only – to form reduction punches – lettering was still added by hand to each die.

The mints in Europe used their striking presses to do their hubbing, causing a halt in striking while a new die was hubbed. For many mints this continued until the late 19th century. A few mints, particularly in Germany and Sweden, had a separate press set aside for hubbing only. It was not until the mid 19th century that special presses for hubbing – with hydraulic power – were first developed and the first use of the term "hobbing" was in 1851.

The United States Mint used a screw press for hubbing early in the 19th century, with the introduction of the steam press in 1836 this was used for hubbing. The Philadelphia Mint obtained its first hydraulic press (called a multiplying press) devoted strictly for hubbing in 1892.

Improvement of hubbing presses is a 20th century development. Capacity was increased, use of hydraulic power was improved, greater control over the squeeze was increased in both time and pressure. And of course the use of an electric motor over steam power and belting was paramount. With commercial electricity available in the last decade of the 19th century gave rise to an improved hubbing press in the 20th century.

U.S. Mint hubs French medals.  Before the United States Mint could strike large size medals, a number of medals of American national interests were ordered struck by the Paris Mint. These included such medals as the John Paul Jones Medal of 1787, the Washington Before Boston Medal of 1790, and the William Washington Medal of 1789. All were engraved by French medalists.

Numerous requests for the dies by American Mint officials (by Director Pollock and others) toward the Paris Mint were turned down. However the Paris Mint would, of course, strike medals from these dies. In 1862 twenty specimens of these 18th century medals were ordered by the U.S. Mint (and sold to collectors for a few dollars each).

As orders for more medals continued, mint officials decided to replicate the medal within the U.S. Mint. From a specimen of the French striking they hardened the bronze and hubbed the medal into gun metal (90% copper, 10% tin; see bronze table). These are the so-called "gun metal dies" mentioned in Loubat, Julian and by other writers (these "dies" were actually negative master hubs).

Gun metal was chosen because, like brass, it is easy to get a perfect impression in

iron from these nonferrous masters. Thus from the gun metal hubs, positive iron hubs were drawn and these could be used for hubbing negative striking dies.

This process was used for the three medals mentioned above, plus others struck at the Paris Mint. Thus for the next century and a half a collector could obtain a specimen of the same medal from both the Paris Mint and the United States Mint. As dies broke and were replaced, varieties were created – particularly for the Washington Before Boston Medal (and the number of legs beneath the horses) – this added to the collector lore and charm of medal collecting, but only because of hubbing.


C66 {1988} Cooper pp [early] 159-165, [modern] 235.

excerpted with permission from

An Encyclopedia of Coin and Medal Technology

For Artists, Makers, Collectors and Curators


Roger W. Burdette, Editor

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