How to make precision steel balls
The ball grinding and lapping machines were invented in Schweinfurt Bavaria, Germany by Friedrich Fisher in the late 19th Century. Schweinfurt was the target of some large bombing raids in 1943 designed to disrupt the production of ball bearings during WW II. Fisher designed this machine to make high precision balls of uniform size. His invention ushered in the worldwide ball bearing industry. FAG, the company he formed is still in business and is one of the world leaders in manufacturing ball bearings.
Precision steel balls come in many sizes depending on their use. Miniature Steel Balls that this discourse is mainly about are considered those steel balls ranging from 0.010″ to 0.625.” Other sizes can range above 0.625″ to several inches in size depending upon their use. The largest steel balls were used in battleship turrets to make the turret turn smoothly. Other super-sized balls were used in swing bridges to make the deck of the bridge rotate. In either case the balls were placed in a horizontal grooved race holding several balls in tandem. The rough balls are made as forgings usually from 52100 type hi-carbon/chrome or similar steel that is then ground and lapped to finished size. The super-sized balls were made individually by grinding and lapping the rough balls one at a time in a special grinding machine having two grinding/lapping heads set at a 60 angle from each other. The heads were each equipped with a cup shaped grinding wheel, and soft metal cup shaped lappers. It takes several hundred hours to grind and lap a ball of this size. Another way of grinding and lapping these super-sized balls was by placing them between two ring shaped plates with one plate capable of rotating, and the other plate being stationary. It is possible to exert great pressure on the balls by tightening the top plate onto the lower plate. In practice the device looks a lot like a wine press with a rotating plate on the bottom, and a movable stationary plate on top. This is basically the form that all grinding and lapping machines take.
Other then these special super-sized balls the smaller balls are made by other processes, depending on their size. Generally they are all made by variations of the same process. The different kinds of balls are segregated by size into different groups. Each size range of balls presents the manufacturer with a different set of problems that are subdivided by size range. The largest balls will often develop a blemish that appears to look like railroad tracks that appear as the balls are receiving their finish lap. There is nothing that can be done except to grind these balls down to the next lower size, and hope the railroad tracks go away.
Most of the problems that arise seem to happen in the finish lap operation. Many of them are caused by hardening problems. The author once had a whole batch of 1 inch balls that broke up into pieces in the lapping machine. A batch of balls that size weighs around 3 tons. Another common problem involved balls that actually grew in size. This was also a hardening problem brought about by a thin skin of hardened steel that was under pressure against a slightly softer metal in the interior of the ball. As this was ground away it relieved the pressure on the interior of the ball allowing it to expand.
The rough balls are made in a header, a special kind of machine that takes short pieces of metal usually of wire or rod stock that are carried between a pair of opposing dies that are shaped like a concave hemisphere with a hole at the bottom of each hemisphere to accommodate a device called a knockout pin. This moves up and down in the die assuring that the headed ball is knocked out of the die as it is finished. The knock-out pin is actuated by a cam located in the header. When the dies close the pin is located below the surface of the hemisphere and when the dies open the pin comes above the bottom of the hemisphere knocking the headed ball out of the dies making the machine ready to form the next ball. The balls fall into a bin and held for further processing.
There are two different types of ball grinding machines either horizontal or vertical. Essentially both machines perform the same job; grinding and lapping balls. They consist of one non-movable plate with one or more annular grooves turned onto the side close to the rotating plate that has a matching set of annular grooves on the side towards the non-movable plate. There is a gap cut into the non-movable plate equipped with a shredder to strip the balls off as they come through the grinding cycle so that they can be returned to the grinding cycle again and again. The grinding machines are used to perform several different tasks on a batch of steel balls.
The first of these tasks is called flashing. The headed balls come out of the header with a certain amount of metal in the form of an equator and poles. These have to be removed before the balls can undergo further processing. This is accomplished in a grinding machine equipped with two plates both of which are made out of a special kind of cast iron called Meehanite,’ a sort of metal that seems to be a cross between cast iron and steel. This metal is very hard and tough. The plates for the grinding machine are made from this metal. In use the rough balls are fed between these two plates under pressure. The grinding cycle in the flasher knocks off the poles and equator of the headed ball. At this point the balls are flashed down to about .020″ of finished size.
Then the balls are transferred to another grinding machine, after being thoroughly cleaned and inspected. The next operation is rough grinding that is accomplished using a machine equipped with a rotating grinding wheel of #70 mesh grit. This is placed against a fixed cast iron wheel and the grinding cycle goes on until the rough ground balls are about 0.003″ larger then the finished balls.
The rough ground balls are then cleaned, inspected and moved on to the hardening process. This can be accomplished in several different- types of hardening furnace depending on the size of the ball. One thing most of these furnaces have in common though is that they operate in an atmosphere of either helium of nitrogen so that the balls will not oxidize in the process. After they are hardened they are cleaned and inspected once again. They are now ready for the finish grinding process.
This takes place in the same type of machine, but it is separate from the others to keep contamination from affecting the balls. In this case a finer grade of grinding wheel is used usually a #220 mesh grit grinding wheel. The balls are ground down to about 0.002 larger then finished size. They are then removed from the machine; cleaned and inspected, and made ready for finish lapping.
The balls are then transported to a clean-room with a temperature controlled atmosphere where the temperature never varies more then a half of a degree Fahrenheit. The lapping is accomplished by another ball grinding machine equipped with soft cast iron wheels capable of being charged with lapping compound. For the first part of this operation the lapping compound is a product called levigated alumina.’ This product is used to lap the balls down to about 0.000040″ 0f finished size. The final metal is removed by switching the lapping compound to red rouge’ a form of finely divided iron oxide. This takes off the final 0.000040″ leaving you with a finished batch of balls. Usually this final lapping finishes the balls, but some special types of ball undergoes a further finishing step by tumbling in a tumbling barrel with more red rouge. The balls are measured for size by using a comparator gage that compares them with a bureau of standards certified ball. Under normal circumstances this ball is kept in the inspection department, and the ball the operators work with is derived from it by measurement.
They are then further tumbled in a product called corn maize’ that is made from ground up corn cobs. This last step takes off the lapping or tumbling compound then the balls are ready for finish inspection and packing. All of the balls once they are finish lapped and/or tumbled are kept in a super clean and controlled temperature room until shipping out of the factory.
In each step the balls are individually visually inspected, and some balls out of each batch are further inspected for sphericity and size. It is impossible to have all the balls in a batch come out a uniform size. The answer to this problem is to pass all the balls in a batch over two revolving rollers that can be adjusted. These particular machines come in different sizes depending on the size range of the balls being sized. The two rollers are made so that they can slightly diverge. As the balls roll down the rollers the different sized balls drop through the roller dropping into different hoppers. Each ball falls into a hopper along with other balls of the same size.
The balls are finally inspected under a microscope to find any rejects. Rejects commonly are hardening defects that can cause a ball to be discolored or cracked. Sometimes you can find balls that have been deformed in the grinding operations. The final packing of the balls once again depends on the size. Most of the balls are packed in boxes. The miniature balls however are packed into glass bottles with super refined oil. They are now ready for shipping to the customer.
It takes several hundred hours to grind and lap a batch of balls from start to finish. Undoubtedly, manufacturing precision balls is one of the most difficult processes in modern manufacturing. The procedures used look simple enough, but in practice the processes used are more akin to witchcraft rather then manufacturing. An example of this hocus-pocus is what do you do with a flick of cigarette ash into a batch of balls or of what use is a small piece of lemon peel?
