(click on images for larger pictures)
Finishing Metal Surfaces

Fig 1.
Machined bearing surface.


Fig 2.
Surface lapped after machining.


Fig 3.
Cast-iron lap and its holder.


Fig 4.
Lap holder.


Fig 5.
Laps made from castings.


Fig 6.
Lap in a lathe carrier.


IN the workshop, lapping usually consists in fitting bearing bushes and their spindles to a higher degree of accuracy and finish than can be obtained by ordinary machining methods. It is carried out by charging a metal arbor or sleeve with an abrasive and working it, in turn, over the mating surfaces of a bush and its spindle respectively.

When highly magnified, the surface of work turned in the lathe will show a continuous series of ridges, faintly resembling a fine-pitch screw thread. Moreover, the lathe is a copying machine and, as such, reproduces on the work any inaccuracies present in its mandrel and bearings, as well as those occuring in the sliding members. Because of this, from a geometric standpoint, the ordinary lathe can hardly be expected to turn exactly parallel or to produce truly circular work.

Nevertheless, these errors can be corrected by hand lapping and, in addition, the operation will produce a finish superior to that obtained by other methods. Where, as represented in Fig. 1, the bearing components are merely machine-finished, contact is made between the crests of the ridges left on the work surfaces; these may penetrate the oil film, causing metal to metal contact. The attrition of these high spots may result in scoring of the bearing surfaces and possibly seizure.

Evidence that this process is taking place is shown by blackened oil exuding from the bearing, due to the presence of suspended metal particles. Clearly, a bearing fitted in this way will soon develop looseness thus defeating its primary object of maintaining exact alignment combined with quiet running even at high speeds. On the other hand, a correctly-fitted bearing with lapped surfaces, as represented diagrammatically in Fig. 2, will run quietly and with a minimum of friction. The oil will remain clean and need only occasional replenishment.

A typical example of lapping operations carried out in the workshop is the finishing of a steel spindle and its cast-iron bushes to provide an accurate, close running fit, leaving only sufficient clearance for the oil film to prevent metal to metal contact.

The shaft is first turned in the lathe to a diameter 1 or 2 thou greater than the finished size, depending on the surface finish obtained; this is to allow for lapping to size.

A useful and easily-made type of external lap is illustrated in Fig. 3. It consists of an inner cast-iron sleeve mounted in a simple form of holder. After the sleeve has been turned all over, drilled and bored slightly under size, the bore is rendered smooth and parallel with a hand reamer. The lap is finally slit obliquely through to the bore, and only partly slit in the opposite direction on the other side of the bore. This allows the lap to be adjusted by the pressure screws.

The holder is made from a mildsteel ring and carries two long screws which compress the die and also serve as handles. The upper, short screw has a coned point to engage in the cross slot. Further support is given to the die by the pointed grubscrew, which seats in a dimple drilled in the opposite surface. Constructional details of the lap and its holder are shown in Fig. 4; but these dimensions can be altered as required.

When in operation, adjustments to the lap are made by tightening the upper coned screw to expand the die; tightening the long screw closes the die and secures it firmly in the holder. All four screws should be tightened when the final adjustment is made.

The two improvised laps, Fig. 5, were made from parts sawn off discarded iron castings and, although seemingly somewhat primitive, they proved fully effective. The larger lap, having a bore of 1-1/4 in., was used for lapping the spindle of a vertical milling machine that was made in the workshop. As shown in Fig. 6, a lathe carrier can on occasion be used for holding a lap, but there is no provision for expanding the die.

Lapping compounds in wide variety and suitable for all lapping operations are manufactured by the Carborundum Co. of Trafford Park, Manchester. The carbon silicate compounds, denoted by the letter C, are fast-cutting and suitable for finishing hardened metals. The H series of Aloxite products are slower in action and are intended for lapping softer metals to a high finish.

The grit sizes of these compounds range from 60 to 700 in the C series and from 240 to 700 in the H series. The carrier medium for suspending the abrasive grains may be either fluid oil, designated OF, or water; but oil is preferable for ordinary workshop use, since the consistency remains constant and the fluid does not tend to dry out.

For most lapping operations a small stock of compounds will be sufficient, including C240-OF and C500-OF, with H700-OF for giving a polished surface finish. After the spindle has been turned to a good surface finish and slightly in excess of the nominal diameter, the lapping operations can be begun.

The lap is adjusted to slide freely along the spindle after its screws have been tightened. To hasten the. operation, coarse-grained lapping compound is first applied thinly to the spindle. With the work rotating at a moderate speed, the lap is guided by hand over the whole length of the spindle, and the measure of resistance felt will indicate any high spots due to lack of parallelism. These areas must be mainly worked on until the lap moves smoothly from end to end of the spindle. In time, the abrasive grains of the lapping compound will become blunted and will have to be replaced by fresh paste.

Use the micrometer to measure the diameter of the work and check that all toolmarks have been removed; this should only be done after thorough cleaning with paraffin. Where necessary, lapping is continued with fresh compound, after the lap has been adjusted to take up wear.

Fig 7.
Set of taper gauges.


Fig 8.
Micrometer caliper.


Fig 9.
Telescoping gauges.


Fig 10.
Small hole gauges.


Fig 11.
Sheet metal taper gauge.


Fig 12.
Expanding lap.


When an even surface has been obtained and parallelism established, finegrain compound can be used to give the work a final finish. During the lapping operations in the lathe, care must be taken to keep the abrasive from reaching the tailstock centre.

Small spindles can be lapped more conveniently when they are rotated in the drilling machine. The lap is then controlled by hand with an up and down movement extending over the full length of the work. Proficiency in lapping comes with practice; it is at best a rather slow process and one that should not be hurried.

At the end of the operation, the spindles must be thoroughly cleaned with paraffin to remove all trace of abrasive, and finally its diameter is measured with the micrometer and recorded.

The bearing bushes are turned all over and made a light interference fit in their housings, so as not to contract the bore of the finished bush when they are pressed into place. The bore is first drilled under-size and then machined with a small boring tool to from 1 to 2 thou under the finished size, according to the surface finish obtained. A tool with a narrow cutting edge should be used to avoid chatter marks, which cannot easily be removed by lapping.

At this stage, there is the need of some method to accurately measure the bore diameter. A direct measurement can be made with an inside micrometer or with the taper gauge, Fig. 7, which is graduated in thousandths of an inch.

An indirect measurement by transference to an outside micrometer can be taken with the adjustable, springcontrolled micrometer caliper, Fig. 8. This instrument was made in the workshop and fitted with a scale also graduated in thous. A standard taper mandrel can also be used for this purpose by marking the limit of its entry into the bore and measuring the indicated diameter with the micrometer. The ordinary inside calliper can be used for making transfer measurements, but some skill is required for this operation.

Telescopic gauges, Fig. 9, are made in sizes for taking internal measurements from 1/2 in. upwards. The gauge in the closed position is entered in the bore and the spring-controlled contact is released by turning the finger nut at the end of the handle.

After again locking the contact pad, a check is made to ensure that the gauge is a correct fit in the bore. Finally, the gauge is measured with the micrometer to obtain the exact internal diameter of the bore.

The small hole gauges shown in Fig. 10 are designed for measuring internal diameters from 1/8 in. to 1/2 in. They consist of two hemispherical, springmounted contacts which are moved apart by an internal, conical wedge. When the gauge is entered in the bore, the finger grip at the end of the handle is turned until contact is established.

As before, the distance over the contact points is measured with an external micrometer.

The sheet-metal taper gauge shown in Fig. 11 is easily made and enables accurate measurements to be taken by transference to an outside micrometer. As explained in a previous article, the two edges of the strip must be filed truly straight.

If the gauge is made to the dimensions given in the drawing, 1/8 in. along the gauge will represent one thou. When the breadth of the gauge is measured with the micrometer across a pencil mark at the face of the bush, the diameter of the bore is accurately recorded.

Internal laps of various kinds are used in the workshop. One of the oldest is illustrated in Fig. 12 and consists of a tapered arbor carrying a castiron sleeve, internally bored with a corresponding taper. The sleeve is slit either for its full length or with several saw cuts, extending for some distance only from either end, to allow of more uniform expansion.

The overall diameter of the lap is controlled by the adjusting nut which forces the sleeve along the tapered arbor. One advantage of this device is that sleeves of different sizes can be used with a single arbor.

Fig 13.
Modern Boyar-Schulz internal lap.


Fig 14.
Coned expanding lap, also by Boyaz-Schulz.


Fig 15.
1-1/4 dia. lap machined in the workshop for a specialised application.


Fig 16.
Details of the 1-1/4 in. lap shown in Fig. 15. Mild steel for the arbor is necessary only for heavy or repeated duties.


Fig 17.
The lathe setup for lapping a ball-thrust bearing race. Note that the lap is in the hand but steadies by and supported by the tool rest.


A more modern form of internal lap is the Boyar-Schultz shown in Fig. 13. Here, an arbor which is slit axially is fitted with a setting screw to provide the necessary expansion.

The copper sleeve is fenestrated to retain the abrasive compound, and at its outer end a tongue fits into the arbor slot to prevent rotation. This make of lap in the larger sizes is fitted with an internally coned sleeve carried on a splined taper arbor. As in the previous example, the lap sheath consists of a copper sleeve.

To provide an acurate means of adjustment, the arbor is fitted with an AlIen pressure screw and a threaded locking ring.

For lapping the cast-iron bushes carrying the spindle of a vertical milling machine, the 1-1/4 in. dia. lap shown in Fig. 15 was machined in the workshop. The arbor was made from a length of 1-1/8 in. dia. mild steel rod, reduced at its outer end to 1.120 in. dia. to carry the sleeve. The shank was cross-drilled and slotted and, after the arbor had been slit axially, the four screws shown in the drawing, Fig. 16, were fitted.

The off-cut of 16 gauge copper tube, used for making the sleeve, was slit along its length and then sprung on to the arbor, where it is retained by the two keep screws. The diameter and parallelism of the lap can be set by adjusting the two AlIen screws. Smaller laps have also been made in the workshop with sleeves of sheet copper. Laps made in this way can be finished to size by pressing them into a part bored to the nominal diameter. Where a lap is needed for only a limited number of operations, it can quite well be made from copper or aluminium rod, which is turned to size, slit lengthwise and fitted with an adjusting screw to expand the lap and take up wear.

Lapping the bores of the bushes is carried out in much the same way as lapping the spindle, but it is advisable to rotate the lap in the lathe or drilling machine and work the bush to and fro when held in the hand. This usually gives a more sensitive feel and prevents the abrasive reaching the chuck parts. Start with the coarser abrasive to remove all tool marks and ensure parallelism of the bore; then change to the fine-grain compound to obtain a high surface finish. It will be appreciated that the lapping process is a fitting operation to gain free rotation of the spindle with a minimum of working clearance.

The diameter of the spindle has already been checked with the micrometer, and the bore of the bush is measured by the gauge and micrometer as previously described. When the bush is lapped nearly to size, it should be thoroughly cleaned with paraffin and then well oiled before being tried on the spindle,

Continue the lapping operation until a satisfactory running fit is obtained. It will be found that for a 1/2 in. dia. shaft about 1/4 thou is correct, and for a 1 in. spindle this should be increased to some 1/2 thou.

When the parts are dry, the spindle will rotate almost without friction; but when oiled a slight drag will be felt from the shearing of the oil film. Before putting the parts to work, make sure that they have been thoroughly cleaned and that no trace. of abrasive remains. The bearings of some work shop machines fitted in the way described show no evidence of wear after being in use for some 20 years, and even the fine surface tracings left by the lap are still present.

When a thrust bearing was required to support a heavy vertically-mounted belt pulley, incorporated in the changespeed drive of a vertical milling machine, two ball races of the type fitted to a cycle steering head were used for this purpose.

The rather rough ball tracks were lapped to a good finish to reduce friction and promote quiet running. The races were, in turn, accurately centred in the lathe 4-jaw chuck and run at high speed. As shown in Fig. 17, a lap, held in the hand and supported on the tool rest, was pressed against the race.

The lap should be made of soft material, such as a piece of aluminium rod with a curved end which, when charged with abrasive, is worked over the ball track until an even finish is obtained. A final ploish can be given with fine abrasive carried on a wooden lap which is shaped to the curve of the race. A process akin to lapping can be applied to screw threads to improve the surface finish left by a lathe tool or threading die.

The work is rotated in the lathe at a moderate speed, and the hand rest is set up to support and guide the improvised lap.

The lap consists of a strip of soft wood with its end grain pressed against the work. Abrasive is applied to the lap and it is traversed along the work by the screw thread.

The grain of the wood, by conforming to the contour of the thread, polishes it on all surfaces alike. The examples of lapping described represent only a few of those that, from time to time, have been carried out to obtain close working fits and resistance to wear.

Other applications of the lapping process include finishing cast-iron jockey pulleys and their spindles, fitting the headstock and tailstock bearings and spindles of a pair of dividing heads made for the milling machine, and sizing machine spindles for mounting ball bearings to a close sliding fit.


Other articles by Geometer (half of DUPLEX).

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Ref: ME Vol 131, Numbers 3285, 3287, 3288.