Arbors and work holding devices

The methods of holding and clamping rough castings for the first or chucking "operation" are so diversified that the subject must, necessarily, be treated by means of examples representing different varieties of work. Nearly all of the examples shown are more or less cylindrical in shape, for the reason that elliptical, rectangular, or odd-shaped parts require special treatment and, therefore, can only be touched upon in an article of this kind. In the general course of manufacturing, there are occasionally pieces of peculiar shape which require chucking fixtures, but as this work is of such great variety, it is difficult to give much information regarding its handling except in a general way. Any piece of work of peculiar shape requires a thorough knowledge of the conditions governing its use, in order that it may be chucked properly and located from the surfaces which are of the greatest importance.

In the design and construction of chucking devices, there are a number of points to which the most careful consideration must be given. In some cases, the work must be held by the cored interior, as, for example, an automobile piston, or, in fact, any other work in which it is necessary to have an equal division of metal throughout the cylindrical walls. In other instances, however, some method of exterior holding may be perfectly satisfactory. The term "exterior holding" does not necessarily mean that chuck-jaws are referred to, for various devices other than jaws will be cited during the following discussion of holding methods.

Having determined whether the work is to be held externally or internally, let us take up the important points in the design of holding devices.

First: The important locating surfaces should be carefully considered, always having in mind the future handling of the piece in its various operations. Great care should be taken that no locating points are so placed that they will come in contact with the work in places where the pattern is gated, or where numbers or letters may appear.

Second: In setting up a rough casting there should never be more than three fixed supporting points; any others which may be necessary for the proper support of the work must be made adjustable, with some approved method of clamping securely after adjustment.

Third: The work must be firmly secured so that no distortion can take place under the strain of clamping.

Fourth: When the work is of such a nature that difficulty is experienced in obtaining proper clamping surfaces, it is sometimes advisable to consult with the patternmaker in regard to the addition of clamping lugs to the pattern. In cases of this sort, these lugs should be so applied that their subsequent removal can be effected readily.

Fifth: In designing a chucking fixture the safety of the operator should be considered carefully, and by that is meant that protruding heads of screws, bolts, clamps and similar parts should be avoided as much as possible. A little forethought in this regard may be the means of saving an operator from mutilation or death.

Sixth: Convenience and accessibility in setting, locating and clamping the work, are also of primary importance.


ARBORS FOR SECOND OPERATION WORK

Cylindrical work which cannot be completely machined in one setting and which requires concentricity of the various surfaces obviously makes necessary some method of holding it for the second operation which utilizes a previously machined surface for securing the proper location. When this surface is external, the use of soft jaws, a stepchuck or collet jaws is feasible, but when an internal surface is the locating point the most efficient method is conceded to be some form of arbor. This arbor may be either a plain stud made to fit the hole in question, or it may be so designed as to be susceptible of a certain amount of expansion and contraction in order to take care of slight variations in the finished hole. The degree of accuracy required in the finished product determines the form of arbor which should be used. If a variation of 0.002 to 0.003 inch in concentricity is permissible, a plain arbor with some method of driving the work will answer the purpose very well. When very accurate work is required, however, greater care must be used in the design, and the expanding type of arbor is commonly used.


Important Points in Design of Arbors

The fundamental features which tend to make an arbor thoroughly efficient are as follows: Expansion must be uniform along the entire periphery; release must be quick and easy; ample driving facilities must be provided; clamping the work must be effected without chance of distortion. As an additional refinement, provision may be made for truing up the arbor so that it will run accurately with the center line of the spindle.


Lathe Arbors

Let us first consider the arbors designed for use in the engine lathe, adapted to be held between centers and driven by means of a dog on one end. The arbor shown in the upper part of Fig. 1 is the simplest of all those which have a split sleeve or bushing capable of expansion or contraction. The mandrel D is slightly tapered and is flattened on each end to receive the dog for driving. The sleeve G is correspondingly tapered and is drilled entirely through at A, after which it is saw-cut at B is allow for expansion. This arbor is the poorest of all the expanding types in that the expansion is not uniform, being in two directions only, and it cannot be depended upon to give results which are absolutely accurate.


WORK-HOLDING ARBORS AND METHODS FOB TURNING OPERATIONS

The developments in the design of machine tools during the last ten or fifteen years have brought these machines to a high degree of perfection. Many are provided with features which make great precision possible, and a workman who understands how to get the most out of one of these modern machine tools can produce very accurate work. It should be remembered, however, that no matter how accurate and how well adapted to rapid production the machine may be, if the methods of holding the work are not equally well thought out there is comparatively little gained. As a matter of fact, this point is neglected in a great many machine shops. In a few instances, we find planning departments and efficient tool-designing departments where the methods and appliances to be used in manufacturing are carefully considered. In the majority of shops, however, the workmen, or at least the foremen, are left to devise for themselves the methods by which the work is to be held in the machines. In the few cases where the workman is unusually ingenious, this may be of advantage, but it is seldom possible for the man at the machine to consider both the accuracy required and the rapidity of production with anything like the care that can be done by a designer especially detailed to do this work.

Therefore, it is becoming generally recognized that in order to take advantage of the full capacity and adaptability of modern machine tools, it is necessary that the work-holding and machining methods be worked out by designers of equal ability to those who actually design the machine. In the following a few methods will be shown for holding different classes of work for turning and facing operations in the lathe. The arbors and devices shown were designed at the Jones & Lamson Machine Co., Springfield, Vt., for use in the Fay automatic lathe; but as far as the methods for holding the work are concerned, they may be employed with equal advantage in any engine lathe, and are therefore capable of wide application. The tooling arrangements shown in each case are, of course, especially adapted to the Fay automatic lathe with its front and rear tool-holders, but by means of a special tool-block many engine lathes could be rigged up to perform the work in a similar manner. These tooling arrange- ments will probably suggest other machining methods.