Bal-tec™ Home Lapping Outside Diameters
Nearly all of the companies that had supplied specialized services for industry have been forced out of business. As part of our company’s policy, to ingratiate ourselves with our customers, or potential customers, is to help them improve their abilities to better cope in today’s precision and ultra precision market. We are doing this by sharing our special technical knowledge with them.
We are now offering cast iron “ring lapping tool blanks” that we cast in quantity (see sketch #1), based on our many years of in house experience in ultra precise manufacturing. These ring lapping blanks can be easily machined, by any ordinary machine shop, so that they are ready for use, to produce micro inch accurate cylindrical parts.
First the inside cylindrical diameter of the ring lap is drilled and reamed or single point bored, on an engine lathe or milling machine to produce a reasonable round hole that is a snug fit on the part. If available, a light honing operation will further improve the geometric quality of the lapping tool.
After the inside diameter of the ring lap is finished, the lap blank is split longitudinally through the center of the wing to allow for adjustment. This is usually done by sawing, or milling with a slitting saw. One side of the wing is drilled through to the saw cut with a clearance drill and the other side of the wing is drilled with a tap drill and tapped through. Next the lap is slotted part of the way through, from the inside on large laps and from the outside on smaller diameters, ideally at about 120 degrees in two places (see sketch #2, #3, #4, and #5) for acceptable slotting patterns. This is done to increase the flexability of the lapping tool.
For one piece or very small quantity production just a single slot will suffice. On many larger diameter tools you may be able to get by without any extra slots. See Plate A. The lap only has to be flexible enough to be adjusted.
Narrow tools, used for lapping short cylindrical surfaces, can be successfully produced, by counter borring one or both ends of a relatively long Ring Lapping Tool. Counter boring both ends of the lap is preferred, but this doesn’t work when you are lapping up against a shoulder. Having the working surface of the lap out on the end makes it much easier to see where the land that you are lapping is. This counter boring technique is used to keep a narrow lapping tool from twisting sideways, when it is adjusted!
These cast iron lapping tool blanks will allow any machine shop to produce outside, cylindrical diameters, that are accurate within micronches. For production lapping it will be found an advantage to have shallow, narrow serrations in the cylindrical surface to act as a reservoir for the lapping compound between the cylindrical surface of the lap and the part being lapped.
A very shallow very wide, pitch thread that leaves 90 percent cylinder is a good approach. Cylindrical Ring Lapping will generate parts, with an outside diameter which, is almost perfectly round, straight and to any exact size. You just keep adjusting the lap smaller and smaller until you reach the desired cylindrical diameter.
With a somewhat greater effort, parts, may be precision lapped directly from a machined surface, instead of the normal cylindrical ground surface. This process usually requires a rough (500 grit) and then a finer (1000 grit) abrasive to be used with two different ring lapping tools.
A hardened cylindrical part, that isn’t too long (about 2 inches) [50.8mm], or so, can be ring lapped straight, round and to an exact size within about twenty microinches (one half of a micron) [one half of a micrometer] with relative ease. An R.A. surface finish of about two micro inches (50.8micrometers) will come relatively easy.
Any waviness will be, practically nonexistent. When you get deadly serious, the straightness, roundness, and size can be controlled within about two microinches (fifty nanometers) [0.0508 micro meters] with a 0.5 Ra finish. Beyond this quality level, the elasticity of the part and the lapping tools become limiting factors.
This is what we call the bubble gum region. At this low micro inch level everything acts like a wet noodle. This elasticity sets in, at the very low micro inch level and the only way to get beyond this point, is to make the parts out of a much stiffer material, such as tungsten carbide or ceramic.
If you have an academic tendency, there is some material published, Bielby, or Swigara, and in recent technical papers on diamond machining (the ductile region or regimen). Parts with cross holes or under cuts can still be ring lapped. The ring lap must simply be wide enough to span these interruptions in the geometry while still maintaining a purchase on solid material.
The length of the ring lapping tool is important, for controlling the camber or bend of the part. A long lap (the same length as the part or even somewhat longer) will smother the part and tend to correct any bend or camber in the “axis of rotation of the part”, but over stroking the part with this long lap, will make the O.D. of the Cylinder smaller on both ends.
To solve this problem you will need to use two ring lapping tools. A long one, that is used first, to correct the camber and then one that is somewhat shorter than the part, that is used later, to correct the dubbing tendency caused by the long lap, by selectively lapping the center more. This is where the “art” of lapping comes in.
The beauty of the ring lapping process is that the lapping tool, will lap down errors in the part, but at the same time the part will also lap down errors in the lapping tool. It is this averaging phenomenon that makes the ring lapping process, work so exceedingly well.
The lap should “smother” the part so that the prominent points of the lap attack the prominent points of the part and vice versa. In this way both errors are rapidly reduced, to routinely generate a nearly perfect geometry.
The ring lapping tool must be kept snug during use. The fine pitch screw or screws on the wing, of the ring lap can be loosened to take the lap off and to put it on. The operator should feel the torque of the part rotating while lapping.
There is some art involved here, but not as much as might be expected. The final quality of the lapped part is basically determined by the size of the abrasive particles used in the lapping process as well as the stiffness of the part and lapping tool.
This is the reason for making the “Ring Lap” so robust. Obviously the finer the abrasive, the higher will be the quality of the part, but the slower will be the abrasion, so an acceptable balance must be reached, to get work completed economically.
If the quality of the original parts, isn’t very good, there may need to be a rough and a finish, operation. By using two different abrasive grits and two separate lapping tools you will get excellent results, as previously described for lapping unground parts.
The part or the tool is usually rotated at the relatively low speed of 300 to 600 surface feet per minute. Very small diameter parts are usually rotated at a much slower surface speed than this, just from a practical stand point. It isn’t practical to ring lap a small part that is turning a thousand or thousands of revolutions per minute. The surface speed of the lapping process is relatively unimportant to both productivity and quality.
The ring lapping tool is applied to the rotating part with a paste of abrasive powder and a vehicle of oil or grease smeared on the part. This application of abrasive is usually done with the operator’s fingers, or with a soft brush.
Generally speaking, lapping abrasive coarser than 500 grit, isn’t practical for ring lapping and something around 1000 grit is most commonly used.
Steel is usually lapped with aluminum oxide abrasive compound and tungsten carbide or ceramic will require diamond abrasive compound.
Soft materials do not respond well to ring lapping as the abrasive tends to migrate into the part surface.
During the ring lapping process the inside diameter of the ring lapping tool is gradually reduced, by tightening the fine pitch screw or screws on the wing of the lapping tool, to provide a rotational torque that can be judged, by the operator’s feel.
As the part or the tool is rotated by a driving machine of some sort, the lap or the part is reciprocated slowly back and forth to cover the entire part being lapped, with a stroke that has some overlap. The ideal machine for doing cylindrical ring lapping is a manual honing machine, but a metalworking lathe or a homemade, slowly rotating device, will work.
Periodically the lapping tool or the part is reversed 180 degrees. This periodical reversal, works the same way that the rotational high spots on the lap and the part correct each other. This reversing process basically corrects the inaccuracies in the taper of both of the part and the lap.
If there are a number of parts on the order, you would do a little lapping on several parts to break in the ring lapping tool or tools, before trying to finish any of the parts. Our ring lapping tools are made from very fine grained gray lapping iron. This material, will work in almost every lapping application.
This rather soft, gray lapping iron, tends to take on a change of abrasive. This embedded abrasive effectively armor plates, the inside diameter of the lap so that one lapping tool will last for a great number of parts.
Somewhere along the line, we need to define a “practical” cylinder. To do this we need to establish a formal concept of cylindricity. The basis of this concept, we will call the “axis of rotation.” Think of this axis as an infinitely fine diameter wire stretched taunt.
The cylindrical surface of the part is the geometry that is generated by a constant “line radius” that is rotated around this theoretically, axis of perfect rotation. The factors that make a perfect cylinder, are that the line radius remains constant (which is the roundness and consistent diameter of the cylinder) and that the axis of rotation, be perfectly straight (any error in this condition is camber or dog leg) i.e. [bend].
Some of the common errors in cylindicity: are “out of round,” which is simply any variations in the length of the line radius of the cylinder, in one trip around the axis of rotation. The apparent result of this condition, would be a variation in diameter, of the cylinder, but beware, odd numbers of lobes (“polly lobularism”, which is three, five, seven or nine and even more odd lobes) that are commonly generated by processes like centerless grinding. These errors will not be detected at all, by any conventional two point gaging system.
Ring lapping, will quickly correct any polly lobular condition of the parts (see our technical paper on measuring sphericity) to give you a background on the intricies of roundness, and familiarize you with measuring machines such as the Talyrond.
Any variation in the longitudinal radius along the cylindrical axis or axis of rotation, is out of straightness. It is larger and smaller diameters from end to end, and along the way. Camber or “dog leg” is any bend in the axis of rotation. You could have a very round piece of wire, with an exactly constant radius, that is literally tied into a knot.
For a very fine quality cylinder, there is also the surface finish and waviness of the cylindrical surface itself, to consider. The question that is involved here is, where does the geometry of the part end and surface finish and or waviness (surface texture) begin. The real answer to this question is how do these qualities affect, the function of the lapped part. This is the one and the only answer to this controversial question, which has gone on forever, and has literally generated reams of literature.
In extreme situations the orientation of an unsupported horizontal cylinder in relation to the gravitational field, becomes a factor. An unsupported horizontal cylinder will bend or sag due to the force of gravity acting on its mass. This facet of the error equation, is very length dependent.
For cylinders, standing vertical there will be a “girth sag” due to the force of gravity to consider. Don’t po-po this factor, as in very precise situations the mass to size ratio and stiffness are a commanding factors, as is the positions ratio (called the barrel effect) of the material being lapped.
To reiterate some of the common linear or axial errors for cylinders are barrel shape, which is obviously that both ends of the cylinder are smaller in radius than the center. The ends are rolled off or dubbed. Taper is when the radius at one end is gradually or arithmetically smaller than the radius of the other end or other sections.
For pressing bearings on a shaft the direction of any taper, may be a drawing specification. It is advantageous for the leading edge of the shaft to be smaller.
Another more complex but still very common problem is “snaking” which shows up in long I.E. parts with a long length to diameter ratio (aspect ratio). The snake effect can have several variations. There can be several variations in the cylindrical diameter, along the axis or there can be several variations in camber or bend and there is frequently some of both. Think of this factor, in terms of up and down and larger and smaller. This situation can be strongly influenced by any hardness variations along the length of the shaft.
A long ring lapping tool will correct this complex error, totally, but this condition is very hard to measure without using, a sophisticated profile recorder (linear) and a roundness measuring instruments such as the Talyrond system, which actually measures that mystical property that is the “Axis of Rotation.”
The typical, result oriented practice, that is used to cope with this measuring problem, is to lap the part until the errors measured, seem to have been corrected and then to lap the part some more. We offer three standard diameters of eight inch long ring lapping, tool blanks. You simply saw off the desired length of blank and machine it as previously discussed.
There is a rectangular shaped boss or “wing” that runs the length of the outside surface of each of the ring lapping tools. It provides for the clamping screws. This boss is frequently referred to, as the wing. In addition to providing adjustment this wing provides a torque break, which prevents the lap from rotating in the operator’s hands. The rotational force of the lap can be considerable. See plate number 6.
When ring lapping large diameter parts a torque arm can be attached to the sides of the lap, or a “strap wrench” can be used to hold it. The wing on the ring lap can be rested on a torque absorbing bar. For another approach to cylindrical lapping, see our paper on stick lapping.