The 8 inch Diameter Flat Lap — Micrometer Lapping Plate — Flat Lapping Plate — Cast Iron Lapping Plate — Gray Cast Iron Lapping Plate

Gray cast iron is a superior material for use as a lapping tool. A high quality gray cast iron flat lapping plate will find wide use for reconditioning measuring tools. Our extremely rugged family of eight inch (8.0”) [203mm] diameter, gray cast iron lapping plates, are our basic Part Number LAP-8 are all one and one half inch (1.5”) [38.1mm] thick. This plate weighs 21 pounds ( 45.4 KG).

Major error sources for depth micrometers are worn measuring rod ends and worn bases. When relapping the depth micrometer base, simply place the micrometer base down on the lapping plate covered with a very, very thin layer of three micron diamond compound. Stroke the base forward and back with long straight line motion while applying a light downward force. These strokes should be in line with the long axis of the micrometer base. The strokes should be spread out to cover the entire face of the lapping plate.

Never stroke the micrometer across the short axis of the micrometer base. If the base is badly worn, the pattern of the stroke can be constantly changed from one slight oblique to the other. If you use an angle greater than 15°, you risk throwing the base out of square with the thread axis. Periodically index the lapping plate one quarter turn.

To correct wear and squareness errors of the measuring rod end, use the depth micrometer itself as a lapping jig. Place the measuring rod through the micrometer with the micrometer set at zero or slightly below. Do not lock down the measuring rod. Place the base of the micrometer on the lapping plate. Use the same long strokes while putting a slight pressure down on the upper end of the measuring rod. Continue lapping until the end of the measuring rod is cleaned up 100%.

Badly worn outside measuring micrometer spindles, may need to have their flat ends relapped square to their cylindrical diameter. This is done by placing the micrometer spindle in a squareness lapping fixture that matches the cylindrical diameter of the micrometer spindle, see our catalog of “micrometer Spindle Squaring Fixtures.” Use the same basic technique as used for lapping the depth micrometer. Our 8 inch diameter gray cast iron lapping plates are finished on both sides so you get twice the utility, at one half the cost. Be sure that you place the lapping tool on a soft backing to protect the bottom surface from damage during use.

The simplest and least expensive version of the 8 inch ( 203 mm) diameter gray cast iron lapping plate, is Part Number LAP-8-GR. This plate is precision, surface ground, on both sides, flat within 0.0003 inch ( 0.0076 mm ). The serrated version of the 8 inch (203 mm ) diameter gray cast iron lapping plate, is Part number LAP-8-SGR. This plate is precision surface ground on both sides flat within 0.0003 inch ( 0.0076 mm ). One side is divided into one quarter inch (0.250”) [6.35 mm ] squares, by 0.030 inch deep serrations. These serrations form a reservoir for lapping compound which dramatically accelerates the lapping action.

These serrations also break up the hydro dynamic wedge of fluid that is generated by the motion of the part. This broken pattern of serrations makes it easier to hold good flatness. We also offer the serrated version of the 8 inch (203 mm) diameter gray cast iron lapping plate with the serrated side of the lapping plate precision lapped flat within 0.0002 inch (0.00508 mm). The plain unserrated side, is precision ground flat within 0.0003 inch (0.0076 mm ).

The part number for this 8 inch (203 mm ) diameter gray cast iron lapping plate with the serrated surface lapped is LAP-8-SLP. We offer a very rugged plywood box to protect the lapping tools during use and to provide safe long term storage. The unique design of this box consists of two twelve inch (12”) by twelve inch (12”) pieces of three quarters of an inch (3/4”) thick plywood with a through hole in the center that is slightly larger than eight inches (8”) [203mm] diameter laps. Each of these two 12” by 12” pieces are covered with a one quarter inch (1/4”) piece of plywood that is glued and screwed together.

Part Number	Major Diameter	Approximate Thickness	Serrations	Finish	Price
LAP-8-GR	Approx. 8 inch 203 mm	1.5 inch 38.1 mm	None	Ground 0.0003” 0.0076 mm	$124.00
LAP-8-SGR	Approx. 8 inch 203 mm	1.5 inch 38.1 mm	Serrated 1/4”x 1/4"	Ground 0.0003” 0.0076 mm	$168.00
LAP-8-SLP	Approx. 8 inch 203 mm	1.5 inch 38.1 mm	Serrated 1/4” x 1/4"	Lapped 0.0002” 0.00508 mm	$248.00
LAP-12-GR	Approx. 12 inch 305 mm	1.5 inch 38.1 mm	None	Ground 0.0003” 0.0076 mm	$280.00
LAP-12-SGR	Approx. 12 inch 305 mm	1.5 inch 38.1 mm	Serrated 1/4” x 1/4"	Ground 0.0003” 0.0076 mm	$368.00
LAP-12-SLP	Approx. 12 inch 305 mm	1.5 inch 38.1 mm	Serrated 1/4” x 1/4"	Lapped 0.0002” 0.00508 mm	$518.00
PLB-12	14 inch × 14 inch	1.5 inch 38.1 mm			$96.00
LAP-14-GR	Approx. 14 inch 356 mm	1.5 inch 38.1 mm	None	Ground 0.0003” 0.0076 mm	$280.00
LAP-14-SGR	Approx. 14 inch 356 mm	1.5 inch 38.1 mm	Serrated 1/4” x 1/4"	Ground 0.0003” 0.0076 mm	$368.00
LAP-14-SLP	Approx. 14 inch 356 mm	1.5 inch 38.1 mm	Serrated 1/4” x 1/4"	Lapped 0.0002” 0.00508 mm	$518.00
PLB-14	16 inch × 16 inch	1.5 inch 38.1 mm			$96.00
PLB-8	12 inch × 12 inch 304 mm	2 inch 50.8 mm		Oiled Plywood	$48.00
DC-3	3 Micron Diamond Lapping Compound	6 grams			$45.00

If you are going to use the laps continuously, you will occasionally need to recondition them. The standard procedure to hand recondition lapping plates is to systematically lap three identical laps against each other. This process only requires two of our plates as both sides of our tools are precision finished. First, lap the first lap against the second lap with the first lap down on the table.

Next, lap the first lap against the third lap with the third lap down on the table. Then, lap the second lap against the third lap with the second lap down on the table. Then, reverse the starting position of the laps and lap them again. Continue this until all three laps are all cleaned up.

The upper lap will tend to go concave and the lower lap will go convex. The third lap helps correct waviness and the reversals averages out so they all three come out very flat. This procedure was developed in the nineteenth century by “Lord Witworth.” This process can generate accuracies within microinches very systematically.

Diamond Compound

Hand lapping with cast iron tools requires a highly concentrated compound of diamond in a carrier that has serious extreme pressure qualities without any sulfur or chlorine additives. An organic material is highly desirable, both environmentally and for easy of clean up. Our DC-3 compound meets all for these requirements.

Only one grit of abrasive compound is usually required for reconditioning measuring instruments. This compound uses 3 micron diamond abrasive.

Stick Lapping — Cylindrical Lapping — Lapping Cylinders

Lapping is the most accurate, material removal processes. It can also produce the highest quality surface finishes. There is a secret process widely used by old-time gage makers to remove very small amounts of material from a cylindrical surface while maintaining almost perfect geometry. This technique is referred to as “stick lapping.”

During this process the cylinder to be lapped is rotated slowly between centers or held in a rotating chuck. A surface speed somewhere between 200 and 400 surface feet per minute (which is quite slow) is the proper speed for this process. E.g.: A 3/8” inch diameter cylinder should be rotated about 70 R.P.M. (revolutions per minute). The very flat, abrasive-covered surface of the stick lap is simply stroked against the rotating surface of the cylinder to be lapped. The lap is lifted up and stroked again and again until the cylinder reaches the desired size and shape. Each stroke will remove a few microinches, depending on the size and nature of the abrasive grit being used, and the roundness of the part will not be affected at all.

A typical diamond grit could run between one micron on the fine end, up to fifteen micron at the coarse end. One of the beauties of this process is that the diameter of the cylinder can be contoured by selectively stroking the stick lap more frequently in one area of the cylinder than in the others. This process can be used to produce an extremely straight cylinder or it can be used to selectively reduce the diameter toward one end or the other.

For press fitting bearing on a shaft it is very desirable to have the leading end of the shaft slightly smaller than the back end. When pressing a dowel pin into its mating hole, it is very desirable to start the pin standing up in the hole. You do this by selectively stroking on one end of the dowel pin while it is rotating. When the stick lapping tool is brought into contact with the rotating cylinder the cushy nature of the operator’s fleshy hands will bring the stick lap into perfect contact with the rotating surface of the cylinder, literally within micro-inches.

The standard stick laps are made of lapping-grade cast iron. The smallest version is our part number 34-10-CI it is three-eighths of an inch in thickness, three-quarters of an inch wide, and ten inches long. The largest version is our part number 15-10-CI it is one-and-one-half inch wide, three-quarters of an inch thick, and ten inches long. One of the most valuable features of this tool is the ability to mount very thin Mylar tape coated with diamond over the stick lap and use it this way. This procedure can produce a surface finish of micro inches or even sub-micro inches.

A very effective tool for use with the stick lap is a diamond-charging roller. This tool can be used to embed abrasive-grains deep into the surface of the lapping tool. The bonded-in abrasive will increase the stock removal by the stick lap and it will armor plate the tool to reduce wear. See Diamond-Charging Roller or this website.

Diamond-Charging Roller — Abrasive-Charging Roller - $135.17

Lapping is the most accurate material-removal process. It can also produce the highest-quality surface finishes. One of the features of the lapping process is that it can be done using the simplest machinery, sometimes entirely by hand. One frequently used trick to increase stock removal and armor plate the lapping tool, is to embed--or charge--the diamond abrasive or other lapping compound directly into the surface of the lapping tool.

A handy tool to facilitate the charging of the abrasive into the surface of the lapping tool is a handheld, hard-steel roller, of very rugged construction. The standard hard-steel roller is our part number DR2. It is one inch in diameter and two inches long. It is made of extremely hard steel that is 62 on the Rockwell “C” hardness scale. It has a ten inch long handle that allows for exact positioning as it is rolled over the surface of the lapping tool to be charged.

There is a large diameter ball immediately above the roller that allows the maximum downward force to be applied directly on the roller by hand pressure. These roller charging tools are offered in three colors: green, blue, and purple. Color coding is important to keep entirely separate the different abrasives sizes you are using. The diamond charging roller is very effective for charging stick-lapping tools as well as conventional flat-lapping plates. This tool can double or triple the stock removal rate as well as armor plate the lapping tools to maximize the length of time the lapping tool can be used without reconditioning.

Price is $135.17

See Stick-Lapping tools and flat lapping plates on this website.