Bal-tec™ Home Report On Ball Sizing
Ball sizing, also known as ball broaching, is the process of forcing an oversize ball through the inside diameter of metal part. It will increase the inside diameter and greatly improve the surface texture of the part. Under ideal conditions, tolerances of one or two ten-thousandths of an inch and a mirror like surface can be achieved. The process is not limited to close tolerances. It is an excellent way of holding ordinary shop limits.
It is the best way to salvage those few under size bores out of a production run. Large increases in production can often be realized by increasing feed and speed rates while holding size towards the low limit, and then finishing the parts by pushing the right size ball through the bores. In order to achieve good results with Ball Sizing, the part must be ductile enough to allow the metal to flow. We usually consider 40 Rockwell C as the upper hardness limit for the process, while 30 Rockwell C and softer is ideal.
We manufacture ball sizing kits that come in a series of basic fractional diameters, between 3/32" and one inch. Each kit provides a series of seven special balls of -.002", ~.001", nominal fraction, +.0002", +.0005", +.001", +.002". Without this selection of balls in kit form, it is difficult to determine the right oversize ball to use. This is because the inside diameter will always finish somewhat smaller than the diameter of the ball used. The metal will recover or spring-back a small amount immediately after the ball passes through it. The amount of spring back is governed by a very complex series of factors. These include the type of metal, its hardness, the wall thickness, the original surface texture and even the lubricant used. Now you simply start with one of the smaller balls and keep increasing until the right size ball is found. The ball sizes included in the kit have been carefully selected to fill most general applications. For those tolerances that fall between the standard increments, we manufacture any size ball that you may need from ten thousandths of an inch to over seventeen inches in diameter. For high production ball sizing, a quantity of the exact size ball needed can be ordered after the right size has been determined through use of the kit.
Ball sizing is a very economical process. The set of balls cost about the same price as a reamer of an equal diameter. Very simple machinery is required and the process can often be combined with other machining operations. The spindle of a drill press or milling machine, the turret on a lathe or an arbor press is often the only machinery that is required. Large diameters, or high production can best be produced on hydraulic or punch presses. The simplicity of the process allows unskilled help to consistently produce the highest quality work. The special balls are manufactured from chrome alloy steel and are hardened to a 62 Rockwell C. They are precision lapped to 25 millionths of an inch roundness and polished to a mirror finish. The dimensional tolerance of each ball is held to less than .0001". The wear life of a complete ball used for sizing is surprisingly long. Each time the ball is pushed through a bore, the probability is that it will go through on a different circumference so that the wear of the ball is distributed over its entire surface.
The enlarging of a bore diameter is a quite logical result of Ball Sizing, but the same process can be used to expand the outside diameter of thin walled parts. A very common application is to enlarge the diameter of copper tubing to physically engage the cooling fins in liquid to air heat exchangers for large refrigeration systems. When Ball Sizing thin walled parts that can not be allowed to increase in size on the outside of the each part must be placed in a very rigid fixture that hugs the outside configuration of the part and prevents it from expanding. The oversize ball should be pushed through the bore at a speed of 40 to 200 inches per minute. Do not eek the ball through the bore, as a low speed will cause ripples in the finished part. If you can think of Ball Sizing as wire drawing in reverse, it may help in visualizing what is happening. After the machining flaws in the surface texture have been completely compressed, there is a wave of metal that actually flows down the bore and out the end of the part. If a large amount of metal is upset by the Ball Sizing process, the flow of metal can actually be extruded out the end of the bore forming a burr. Because metal is literally displaced and not just compressed by this process, multiple progressively larger balls can be used to substantially increase the size of a bore.
Another phenomena that most people find hard to believe is that a bore is tapered over size in one end, can actually be made smaller by flowing metal down the bore from closed end to the open mouth by Ball Sizing. A vivid example of this is a company pressing four progressively larger balls through a sand cast aluminum wheel, flowing the metal enough to eliminate a one and a half degrees draft or taper and holding a close enough tolerance to press fit a ball bearing. This is done without any prior machining of the part. Ball Sizing Blind Holes presents a real challenge. If the ball is mounted on a shaft which is pushed into the blind bore and then pulled out, all of the wear will be concentrated in one line and this ring will soon be worn under size. If the ball can be pushed in and then pushed out from a smaller hole from the opposite end, it will work quite well. In some cases this process can be automated by using a spring-loaded stripper punch embedded in the fixture below the part.
Another clever approach that was used successfully by a manufacturer of small motors was to press the ball into the blind bore and leave it there. In this case, the hard steel ball ended up forming a thrust bearing for the motor shaft. There has been some bad information disseminated about Ball Sizing. It is categorically wrong to think that there must be a minimum up set for Ball Sizing to work. A customer who specializes in Jib Boring gear plates, has tripled throughput by zipping through the bores with the Cutting Tool while shooting for the low limit and then pressing a very slightly lager ball through the holes that are still under size. What you have to realize here, is that the peak to valley height of a 63 Ra surface finish is at least three times that. With .0002 " of surface texture on each side of the bore, you can increase the diameter .0004 " by just squeezing this fuzzy surface down to base metal.
Another myth is that you must have a rough surface finish for Ball Sizing to work. The advocates of this practice believe that Ball Sizing only compresses the surface layer and they do not realize that it actually causes a plastic flow of metal down the bore, actually displacing base metal. Another fairy tale about Ball Sizing is that if the bore is just a little under size after passing an oversize ball through it once, the bore can be opened up some more by making multiple passes. Outside some minute abrading that is possible, a hundred extra passes with the same ball will increase the size exactly zero. A magnetic push rod is clever ploy in horizontal applications. This really solves a dilemma on Turret lathes, chuckers and CNC equipment. The force required to pass a .001 inch larger ball through bores over ¼ inch becomes substantial.
Passing this oversize ball through a ¼ inch steel part using a #2 hand arbor press is getting hard. Ball sizing a ½ inch diameter bore in a steel part can be done on a turret lathe, but doing larger diameters is pushing the envelope. A large hand operated arbor press or much better a vertical hydraulic press will do the job up to several inches diameter. Good safety practices must always be employed particularly as the forces go up. Steel shielding rings can provide protection in case shrapnel is produced by a failed part, punch, or ball. It is very good practice to use a lubricant when ball sizing. Extremely clean thread cutting oil can do a good job when a small upset is involved. For heavy upsets or when a better finish is required, a highly chlorinated oil or wax is the best choice. We produce an extremely chlorinated paraffin wax based oil that has pink tinge of color. It is about 40-weight oil that is easily applied to the bore to be Ball sized. It can be removed with a water-based detergent or mineral based solvents. It should be emphasized that the bore should be lightly coated and that applying the lubricant to the ball is of no use. You will be courting disaster if you use any lubricant with solid particulate such as graphite Teflon or molydisulfide. Under extreme pressure, the plastically deforming metal in these materials will act as an abrasive.