Machine stops, trips and locking devices

In recent years, stops, trips and reversing mechanisms have been applied to a vast number of machine tools. The stops employed vary from the simple adjustable stop, tappet or dog, to the mechanisms in which these are combined with cushion devices, means for reversing feed movements, etc.

It may be advisable at the outset to call attention to the difference between a "self-acting" and an "automatic" movement. Many machines which are not wholly automatic contain self-acting movements. A slide-rest is self-acting, though the lathe is not automatic, because the movements of the slides have to be thrown in and out by the operator. The greater number of turret lathes are semi-automatic or self-acting, as distinct from the automatic or "full automatic" screw machine. A number of gear-cutters and grinders, in which all the movements proceed without intervention from the attendant, are also in the class with the fully automatic machines. It is in these classes of machines that the highest developments of the mechanisms to be considered are found.

There are two kinds of stops: "Dead" stops are those which positively arrest a movement, and gage a length or diameter in repetitive work; "trip" stops or "trips" throw out a movement, reverse it, or throw it in again. Dead stops alone are not sufficient to check a power feed or self-acting movement; some means must also be provided to throw out the feed. Then a dead stop may or may not be incorporated to form a positive check. In many cases the tools themselves, as in some turret-lathe work, constitute dead stops, and render the provision of additional stops unnecessary. A dead stop is used in hand operated mechanisms to prevent the operator from moving the slide or other portion further than the predetermined limit, thus guarding against error, and insuring a duplication of dimensions without the need for measurement or gaging. Again, it is often possible to throw out a dead stop temporarily, and go past it for certain purposes, such as inspection, and again throw it in at the same setting as before. A number of dead stops may be located close together, to enable selection to be made at will or in regular rotation, as in the case of a turret one for each tool-hole. Frequently duplex stops are arranged, to enable the choice of two distances for a single slide, one stop being thrown out of the way. Fine adjustment is in some cases provided for a dead stop, so that a very precise setting can be obtained.

An important point in the design of dead stops is that of rigidity; a solid abutment should always be used, and any excessive overhang tending to cause springing must be avoided; otherwise accurate results are impossible. In the operation of a hand turret lathe or other machine tool there is necessarily a great deal of banging and rough treatment, especially in the hands of a careless operator, and weak and badly-supported stops will cause unsatisfactory work. The binding arrangement for a stop must also be efficient, so that it will not slip and cause a batch of work to be turned out to wrong dimensions. The hardening of contact surfaces is also advisable for preventing wear and bruising that would affect the dimensions of work produced.

The position and method of attachment of a dead stop depends on the class of machine and the design. Where a sliding table has to be stopped, it is in many cases possible to attach the stops or dogs by means of a bolt and T-slot in the edge of the table, this being a very simple method and permitting easy adjustment; or a round rod may be held in bearings on the edge of the table, and adjustable dogs be clamped to the rod by set-screws, or by split ears or lugs. Another method is to have a fixed stop bolted, to the table edge, and adjustable dogs attached to a rod in front, these being struck by the stop according to the movements of the table. A favorite device for short slides, such as the cross-slides of turret lathes, is to attach the stops to a rod or screw passing through a hole in the slide, the faces of the latter coming into contact on each side alternately with the stops. Plain cylindrical parts, if of small diameter, are often controlled by a collar or lug, clamped to them by means of a set-screw, and arranged to encounter the face of the bearing through which the part moves.

It is evidently impossible to show all the different kinds of stops which are in use on various machine tools, but the following selection of typical examples embodies the principles involved in the design of all stops. Slight modifications are made in different machines.

CLAMPING AND LOCKING DEVICES APPLIED TO MACHINE TOOLS

Devices for clamping and locking various parts are found on practically all machine tools, and the different methods used afford a very interesting study. In considering this subject we disregard permanent fastenings that is those which are not released and tightened as part of the operation of the machine and take into account only those devices which are expressly designed to permit of more or less rapid loosening and tightening, to allow adjustments. There are a great many conditions under which these devices are required, and the particular type adopted may vary widely in character; a design that is exactly suited to one case may be utterly unsuitable for another. For instance, the pressure from the point or end of a screw is sufficient for holding some parts, but in other cases this would be an unsatisfactory method to adopt. Again, friction may be ample to hold a certain part, while in another case a positive device is necessary.

The distinction between clamping and locking which will be made in the following is this: Clamping produces a decided pressure, sufficient to enable a part of a machine to resist the shocks or vibration tending to shift it, while locking is only a method of temporarily holding a piece in position, by means of a plunger or other medium, sufficient to retain it, but without giving a powerful clamping or squeezing action. A locking device, therefore, might not be powerful enough to act as a clamping device, so that these functions must be regarded as distinct from each other. As a matter of course we say that a slide is locked, when we ought to say that it is clamped, because the parts are drawn together powerfully, and not merely prevented from shifting by a pin or other means. As a general rule it may be said that locking holds a machine part in a definite position, or in one of a series of positions previously known, by means of holes, slots, or grooves, which determine these positions; but a part may be clamped at any location, with or without the use of graduations or other means to determine the setting. In some cases, although these are not very common, locking and clamping are combined, the latter supplementing and assisting the former.


Clamping Devices

Dealing first with clamping, the simplest example is a set-screw pressing upon the portion that has to be secured. This is a cheap device, but is open to objections. On a flat surface it is efficient, but the pressure is too local, and this construction is not adapted to withstand heavy strains without slipping. Moreover it has the bad effect of forcing the parts away from each other when screwed up, so that a fruitful source of vibration is introduced, whereas other and better methods of clamping pull the parts together and act as clamps in the true sense of the word. Usually the pressure of a set-screw point is objectionable, and a soft pad or shoe is employed to avoid the marring effect otherwise met with. This pad or shoe may be shaped to correspond with the form of the surface against which it bears. Fig. 32 is an example of a set-screw in an awkward situation, this example being taken from one of the Seller's tool-grinders; the screw passes through a bushing, and presses upon a pad shaped to fit the outside of the cylindrical sleeve.


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