It is possible to machine a great variety of shapes and sizes of work on planer. This has been possible because of availability of wide variety of holding and clamping devices. Various holding devices are plane vise, planer centres, etc.
Various clamping accessories are:
i. Bolts,
ii. Clamps,
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iii. Clamping blocks,
iv. Shims,
v. Planer jacks,
vi. Braces,
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vii. Planer poppets,
viii. Stops,
ix. Toe-dogs,
xi. Planer strips,
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xii. V-blocks,
xiii. Angle plates, etc.
In any planer set up it should be ensured that the work is clamped neatly and securely. If possible, a stop should be used. Particular care is required to avoid springing of any part of the work. Bolts which are too short or too long should be avoided. The surfaces of all strips, parallels, and fixtures must be clean and free from burrs.
Following points may be remembered in this regard:
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i. The vise jaws, parallels, angle irons used should be accurate and their accuracy tested.
ii. The finished surface should be protected from the roughness of a clamp.
iii. While handling rough casting, the surface of table, vise jaws, angle irons, etc. should be protected by using protecting pieces of card board, etc. All chips and dirt should be removed beforehand.
iv. After a piece is clamped in a vise, tap lightly with a babbitt hammer to seat it. Once work is seated, do not tighten vise again as same would only lift the work from its seat.
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v. Use a stop against work wherever convenient.
vi. Avoid unnecessary clamping.
vii. It should be ensured that work is clear under cross rail and along the sides next to the housings.
viii. Proper-size wrench should be used to avoid rounding of corners of the nuts.
ix. A washer should be placed between the nut and the clamp.
x. Plan for planing several pieces at a time.
xi. Work should be shimmed or blocks, particularly under the clamps, used to avoid springing and ensure proper seating.
xii. Rough castings should be supported at the proper points by blocks, shims or braces so that it will not buckle or spring under the clamping pressure.
xiii. Casting for planing must be levelled with surface gauge in such a way as to average the corners for height, with due consideration for later planing of the other side.
xiv. Remember that the screw pressure of bolt and nut will sprang even a heavy casting unless it is solidly supposed under the clamp.
xv. Dogs should be set for proper length of stroke.
xvi. Proper tool should be selected and set.
xvii. The edge of the work may be levelled with a chisel or an old file to prevent a ragged corner.
The planing operations are quite simple, once the parts to be planed are properly set. The output of planer is greatly influenced by proper setting up of work which is said to be more difficult than any other machine. Often considerable skill and ingenuity is required in setting parts on a planer and clamping them in the best manner, more so, with small and medium sized pieces of irregular shape.
It should be remembered that planing is high-power cutting operation and workpiece is subjected to extremely high cutting forces. The rigidity of work set up is therefore very important. For finish cuts, proper shimming must be done to avoid bending or warping strains due to clamping since such strains would distort the workpiece upon release of clamping pressure.
Three important points requiring the consideration in respect of rigidity or work set up are:
(i) The casting or forging must be held securely to prevent its being shifted by the thrust of the cut. Shim jacking is used to avoid any wobble. For large workpiece with relatively slender sections, jacks or blocks should be used to damp vibrations. A strap or pin clamp should be used over each jack or block to ensure that the workpiece rests solidly on the table. The work can be fastened down with T-bolts, straps, step blocks and other anchoring devices.
(ii) The work should not be sprung out of the shape by the clamping pressure.
(iii) The work must be held in such a position that it will be possible to finish the surfaces that require planing, in the right relation with one another.
Sometime set up plates having T-slots for mounting work are used to increase planing efficiency. Work is secured to plate away from planer.
Much or the work done on a planer is usually clamped directly to the platen; in some cases planer chucks, magnetic chucks or special fixtures have to be used. A number of clamps, stops, angle brackets and other anchoring devices are used for setting up the work properly.
Fig. 15.6 shows the common accessories for work set up and clamping on planer table.
These include:
(a) Plain clamp,
(b) Offset clamp,
(c) U-clamp with pin end,
(d) Pin clamp,
(e) T-slot stop bracket,
(f) T-slot bolt,
(g) Stud,
(h) T- slot removable nut,
(i) Stop pin or planer stop,
(j) Chisel point,
(k) T-slot stop block
(l) Jack-screw.
Jack screw is used to support the overhanging part of the workpiece. These devices are generally used where several different kinds of jobs are machined on the planer.
Causes of Errors and Their Prevention in Clamping Parts:
When any part as cast or forged is to be set up on a planer, it is possible that it may not touch the platen at the ends where clamping is to be done because of its rough and uneven surfaces. If the clamps are tightened without supporting the work at the unsupported ends, the entire casting is likely to be sprung out of shape more or less, depending on its rigidity; consequently the planed surface will not be true after the clamps are released, because the casting then resumes its natural shape.
In such cases, there should always be good bearing just beneath the clamps, which can be obtained by inserting pieces of sheet metal. Thin copper or iron wedges are also used for ‘packing’ under the clamps. This practice allows the part to spring back to its normal shape and the finished surface remains true after clamps are removed.
Very long and heavy castings if supported only from ends, may bend by their own weight or may spring out of shape by the pressure of the planing tool, unless supported at the weak points. When setting such castings on the planer, small jacks form a very convenient means of support.
Distortion of Castings by Internal Stresses after Planing:
Castings, even if properly clamped, may spring out of shape by the internal stresses existing in the casting itself—which may be due to the unequal cooling of the casting in the foundry. For great precision in such cases castings should be first rough planed and then allowed to “season” for several weeks before taking the finishing cuts.
Work Holding Fixtures for Planing:
Some parts are so shaped that a great deal of time is required for clamping them with ordinary means, and for such work, (particularly if large number of pieces are to be planed), special fixtures must be used. Such fixtures are designed to support the casting in the right position for planing and they often have clamps for holding the workpiece in place.
Use of Stop Pins and Braces in Conjunction with Clamps and Bolts:
Many parts cannot be simply held securely by clamps and bolts, because the pressure of the clamps is in a vertical direction, whereas the thrust of the cut is in a horizontal direction, which tends to shift the work along the platen.
To prevent such a movement, practically all work that is clamped to the platen is further secured by one or more stop-pins. These stop-pins ordinarily are placed at one end of the work to take the thrust of the cut, but sometimes they are also needed along the sides to prevent lateral movement.
Some castings have surfaces to be planed that are at considerable distance above the platen and the end resting on the platen is comparatively small. If such a casting be simply supported by clamps at the lower end, it would tend to topple over when being planed, because the thrust of the tool is so far above the point of support.
To prevent any such movement, braces shown in Fig. 15.7 are used. These serve practically the same purpose as the stop pins. The location of all braces and clamping appliances should be determined by considering the thrusts to which the part will be subjected during planing operation.
Use of Angle Plates for Holding Work:
Angle plates may be used for holding parts while planing them, especially if a previously planed surface must be held in a vertical position while planing some other surface. Angle-plates are generally used for holding pieces, which because of their odd shape, cannot very well be clamped directly to the platen. In certain cases, angle-plates can be used in conjunction with clamps for holding castings.
Holding Cylindrical Parts for Planing:
Planer is sometimes used for cutting keyways or splines in shafts. In order to hold and at the same time, align round work with the platen, V-blocks are used. These V-blocks should have a tongue at the bottom to fit into a T-slot in the platen. Then angular groove in the V-block should be parallel with the tongue so that it can hold a round shaft in alignment with the travel of the platen.
Work Clamping Methods:
The planer table is made with evenly spaced T-slots and holes. Use of the T- slot is the most common method of holding work against the surface of the machine. In Fig. 15.8 are shown the various methods for clamping by using T-slot clamps. Blocking materials (like scrap materials or step blocks or screw jacks etc.) are used to support the heel of the clamp at the proper height.
Flat work can be held on the table by means of toe dogs, chisel points and poppets. Fig. 15.8 (right side) shows the clamping of thin plates by chisel points. In order to exert sufficient downward pressure, the chisel points are set at an angle of 8 to 12°. Methods employing electromagnetism or hydraulic and vacuum clamping may also be used when aided by mechanical blocking methods.
