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EXPANDING LAPPING TOOLS use a taper to expand a cylinder shaped lap. The inside diameter of the Lap and the outside diameter of the Arbor have a mating locking taper. The outside diameter of the Lap is ground straight and is split via a thin slot. The slot allows the Lap to expand by moving the Lap up the taper of the Arbor. Tools that are fully supported by the taper, and lock on it, produce the best geometry. There are mechanical tools that use fingers to hold the lap available. These tools allow the lap to be expanded while the lapping process is in motion so the spindle need not be stopped. Hydraulic expansion is available for high production use in a lapping machine. Some of these tools have cycled over 100,000 times

SPEEDS Lapping rotation speeds fall in the range of 750 RPM for 1/4 diameter bores and shafts and 300 RPM for a 3/4" diameter work part. Some materials can be lapped faster and slower than the above. Experimenting with the fastest speed possible is up to each individual job.

TOLERANCES Most tool work can be lapped straight and round to 25 millionths of an inch. (.000025"). Gage work type parts can be held to 2 - 3 millionths of an inch if the material is stable enough.

FINISHES The best finish possible is determined by the abrasive compounds used. Finishes to less than 1 micro inch are easily obtained if the material lapped is dense enough. Lapping is usually specified when 2 - 8 micro inch surface is needed.

FLAT LAPPING A flat surface lapping plate or rotation table is used to lap flat parts. Most production parts are lapped on rotary table machines. Hand lapping can be done on a flat lapping plate. These plates usually have slots or grooves machined in the surface to assist in compound and lubricant distribution. Plates without grooves lap much slower than plates with grooves.


Internal Lapping Mount the Lap Arbor onto or into a spindle that rotates. Prime spindle examples are; lathe, hone, mill, drill press or even a hand held drill.

The next step is to slide the Lap over the Arbor so the tapers are mated. The large inside diameter end of the Lap goes on the Arbor first. Once the Lap is on the Arbor a Lap Expander is then used with a hammer to drive the Lap up the taper of the Arbor. When the end of the Lap and the end of the Arbor are flush the diameter of the lap will be at the size that it was originally ground to.

The technique for sizing the lap to the work part is as follows. With the Lap mounted on the Arbor apply a thin layer of lapping compound with a brush or other means. Move work-part onto Lap and start to expand the Lap with the Lap Expander until an interference fit is achieved.

Once the fit is achieved turn the spindle on and move the work-part back and forth over the Lap. The desired stroke length is to go one third the length of the work-part off of each end of the Lap. Once the abrasive is spent new abrasive needs to be applied and the Lap needs to be expanded, usually every 5 - 15 seconds of lapping.

While the spindle is under rotation move the work-part off of the Lap. Using a shop towel wipe the spent compound off of the Lap and apply new lapping compound. Then using the Lap Expander tap the Lap a few times to expand the Lap to compensate for wear and to increase the work-part's size. The desired amount of lapping compound to apply is as follows. Use the minimum amount of lapping compound as possible. Enough to keep the Lap moist and so the compound does not collect at the ends of the lap. Using to much compound usually results in a bellmouth condition in the work-part.

Repeat the above steps until the desired diameter and finish are achieved. To start another work part pull the Lap back down the taper using a Lap Puller. This is done as follows. Insert the tip of the Lap Puller screw into the center hole of the Arbor. Adjust screw so the Lap Puller Disc is past the back end of the Lap. While holding the Lap Puller so the disk has as muck bearing surface against the Lap as possible turn the T-Handle screw clock wise to move the Lap down the taper of the Arbor.

After a few work-parts are lapped the operator will achieve a feel for how far to expand the Lap to achieve the desired diameter.

External Lapping  For external lapping the work part is usually put under rotation. Using some kind of chuck or work-part holding fixture mount a work-part into the spindle. Now take an External Lap and slide it inside an External Lap Holder. Turn the adjusting screw of the Holder so the Lap will no longer move in the Holder. Next the Lap needs to locked into the holder to prevent it from rotating. Insert a work-part into the Lap and tighten the Holder Adjusting Screw until the work-part will no longer move inside the Lap. With work-part inside the Lap and the Lap inside the Holder tighten the locking set screw into the Lap. The set screw has a cup point and will leave an indentation in the Lap. Loosen and tighten the screw a few times to leave an indent for the set screw to hold the lap from rotation. After the final tightening of the set screw back the set screw off so it is still making contact with Lap but is not distorting the Lap, about 1/16 of a turn.

To begin the lapping process spread a small amount of Lapping Compound on the work-part while it is under rotation. With the spindle stopped slide the Lap and Holder over the work-part and turn the adjusting screw clock wise until an interference fit is achieved. Now turn spindle on and stroke the Lap and Holder back and forth. The ideal stroke length is to go one third the length of the Lap off of each end of the work-part.

Once the interference fit has been diminished it is time to apply new compound and adjust the Lap. The regular cycle time for External Lapping is between 10 and 20 seconds. With the spindle still under rotation slide the Lap and Holder off of the work-part. Using a shop towel wipe off swarf and re-apply compound. Adjust the Lap by turning the adjusting screw in the holder and slide Lap and Holder over work-part and complete another cycle.

The ideal amount of Lapping compound being applied is so that the Lap is moist and there is no excess compound on the ends of the work-part. Continue the process until the desired size and surface finish are achieved.

                                                                            LAP MATERIALS

IRON - The most popular material for I.D., O.D., and flat laps is cast iron. The metallurgical structure provides for soft areas for the lapping compound to embed. The compound then is backed up by harder areas to hold it in place so it will abrade harder materials. This is the basic reason a softer lap will cut and wear hardened parts faster than the lap wears. The iron lap is also hard enough to maintain shape which is essential to producing good geometry while lapping. It can produce very good finishes.

BRASS - Brass is also a popular lap material. It holds shape well but does not have a structure to hold the abrasive in place when embedded. It allows an abrasive grain to be torn out as fast as it is embedded. This provides slow cutting, but in some materials it can produce a very good finish.

COPPER - Copper is softer than brass and holds it's shape well. The softness allows it to be used for lapping soft materials without embedding abrasive into the work-part. It can produce very good finishes.

LEAD - Lead was a popular choice many years ago. Lead laps are easy to produce, but will not hold abrasive well. Abrasive grains tear out as fast as they are embedded. Lead is used very little today.

DIAMOND PLATED - Diamond plated lap tools eliminate the need for loose abrasive to be applied continually. They will produce astounding stock removal speed and excellent geometry. These type tools are the star of the future , as one tool will produce over 10,000 parts.

NYLON - The softness and density allow nylon laps to produce lustrous finishes; but at the cost of good geometry. It has some advantages when used on soft materials.

BAKELITE RESIN FIBER - Bakelite resin fiber has a good material structure that allows laps to be embedded with abrasive easily. The fiber helps hold the abrasive from tearing out. It can produce very good finishes, but geometry can be poor.

FILLED PLASTICS - Filled plastics can have specialty uses on ceramics and carbides. Usually the filler material is one of the above metals. Filled plastics require engineering to each individual job.

WOOD FIBER Wood fiber is usually used to polish and improve brilliance and finish. Wood will not produce good geometry.


SILICON CARBIDE - Silicon carbide is a sharp, fast cutting abrasive with long sliver like pieces. The sharp grains produce deep scratches and cut marks. Finishes obtained are average and can be dull. Silicon carbide is best used when stock removal is paramount.

ALUMINUM OXIDE - Aluminum oxide is cubic in grain size and is mostly used for hardened steel work-pants. It can produce excellent finishes and geometry. It is preferred for valves and gage type work.

DIAMOND - Diamond is the fastest cutting abrasive. It will produce excellent finishes and geometry. It is one of the few abrasives that will cut ceramics and carbides efficiently.

BORON CARBIDE - Boron carbide is very hard and is a substitute for diamond, but does not cut like diamond. Its uses are limited.


KEROSENE JELL - Kerosene jell and kerosene are very good lubricants and give the best results on steel and iron.

MACHINE OIL - Machine oil works almost as well as kerosene and is a good substitute when kerosene jell is not available.

MINERAL OIL - Mineral oil works well and has some advantages in special uses. It keeps the abrasive free cutting longer.

LARD OIL - Lard oil is as good as the above for lapping uses. It does have shelf life and sanitary considerations. Vegetable shortening is used when a thick water soluble lubricant is needed.


SOLVENT BRUSH - Solvent brush cleaning is the easiest. Kerosene and nylon brushes work well to loosen the swarf and wash it away. Sometimes a clean rinse follows the brushing.

SOLVENT VIBRATION - Solvent vibration is also available especially if diamond abrasive is used. The vibration will loosen embedded abrasive grains.

SOLVENT SWAB - Solvent swab is used with Q-tips in small bores and cotton balls in medium size bores. Parts are usually blown dry with compressed air.

DIPPING - Dipping and soaking in solvent works for large volumes of parts. Sometimes vibration is added to the dipping

Diamond plated tools cuts fast and last a long time. Diamond Plated Tools do not require abrasive compound to cut. The plating can last for 10,000 or more bores and cuts quickly because the abrasive grains are held in place so they cannot move to the ends of the tool and be lost with lubricant wash. Roundness and straightness are very good and bell mouth ends, caused by compound collection on the lap ends, is nonexistent. Some borazon plating is used in special situations.


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