Modern Machine-Shop Practice Part 138

PROFILING MACHINE.--The profiling machine is employed mainly to cut the edges of work, and to sink recesses or grooves in the upper surface of the same to correspond to a pattern. A provisional template of the form of the work is fastened on the bed of the machine, and from this is cut in the machine a thicker one termed the "former," which is then used to copy the work from.

Fig. 1991 represents Pratt & Whitney's profiling machine. On the cross slide are two separate sliding heads, each of which carries a live spindle for the cutting tool, and beside it a spindle to receive a pin, which by being kept against the pattern or _former_ causes the work to be cut to the same shape as the former.

The work is fastened to the table, which is operated upon the raised [V]s shown by the handle on the left, which operates a pinion geared to a rack on the underneath side of the table. The handle on the right operates the heads along the cross slide also by a rack and pinion motion. The gearing and racks in both cases are double, so that by two independent adjusting screws the wear of the teeth may be taken up and lost motion prevented. By means of these two handles the work may be moved about the cutter with a motion governed by the form or shape of the _former_, of which the work is thus made a perfect pattern both in size and shape. The tool used is a shank or end mill, such as was shown in Fig. 1928. In some profiling machines the spindle carrying the guide or former pin is stationary, in which case the provisional template is put beneath it and the _former_ is cut by the live spindle, and for use must be moved from the position in which it was cut and reset beneath the _former_ spindle. This machine, however, is provided with Parkhurst's improvement, in which the _former_ spindle is provided with a gear-wheel, by which it may be revolved from the live spindle, hence the provisional template may be set beneath the live spindle in which the guide pin is then placed. The cutter is then placed in the _former_ spindle, and the _former_ cut to shape from the provisional template while in the actual position it will occupy when used.

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Fig. 1992 represents Brainard's machine for grinding milling cutters. It consists of a threaded column A to which is fitted the knee B, which as it fits the top of the threads on the column may be swung or revolved about the column without being altered in its height upon the same except by means of the threaded ring C. At D is a lever for clamping the knee B to the column after adjustment; W represents the emery wheel mounted on the end of the horizontal spindle having journal bearing at the top of the column. The face of the knee B has a slideway _d_ for the fixtures, &c., which hold the cutters to be ground, and at E is a lug pierced to receive an arbor whereon to place cutters to be ground, the lug being split and having a binding screw to lock the arbor firmly in place. F is a slide for receiving the grinding attachments, one of which is shown at K carrying a milling cutter in position to be ground on the face.

Fig. 1993 shows the fixture employed to grind parallel cutters, S representing a stand upon slide F (which corresponds to slide F in the general view of the machine in Fig. 1992) in which is fixed the arbor H.

The cutter C is slid by hand along arbor H and beneath the emery wheel, the method of guiding the cutter to the wheel being shown in Fig. 1994, which represents a front view of the machine. At E is the lug (shown also at E in the general view) which has a hole to receive a rod P, and is split through at S, so that operating binding screw L locks rod P in E. At R is a rod secured to the rod P, and G is a gauge capable of swivelling in the end of R and of being secured in its adjusted position. The end of this gauge is adjusted to touch the front face of the tooth to be ground on the cutter C, which must be held close against the end of the gauge in order to grind the cutting edge to a straight line parallel to its axis.

A not uncommon error is to place the gauge G against the tooth in front of that which is being ground, as in Fig. 1995, the gauge being against tooth C while tooth B is the one being ground. In this case the truth of the grinding depends upon the accuracy of the tooth s.p.a.cing. Suppose, for example, that teeth B and C are too widely s.p.a.ced, tooth C being too far ahead, and this error of s.p.a.cing would cause tooth B to be too near the centre of the emery wheel and its cutting edge to be ground too low.

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The object of feeding the cutter by hand along the arbor H is twofold: first, the amount of cut must be very light and the feed very delicate, for if the grinding proceeds too fast the cutting edge will be what is termed burned, that is to say, enough heat will be generated to soften the extreme cutting edge, which may be discovered by holding the front face of the tooth to the light, when a fine blue tint will be found along the cutting edge, showing that it has been softened in the grinding, and this will cause it to dull very rapidly.

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The second object is to insure parallelism in the cutter. Suppose, for example, that the cutter C was fast upon arbor H and was fed to the wheel by moving slide F, and if the arbor H stood at an angle, as in Fig. 1996, to the slide upon which F moved, the cutter would be ground taper, whereas if the cutter is fed along the arbor it will be ground parallel whether the arbor is true or not with the slideway of F, the only essential being that the arbor H be parallel and straight, which is much easier to test and to maintain than it is in the slideway (D, Fig.

1992). Here it may be noted that oil should not be applied either to arbor H or to the cutter bore or slideway D, as lubrication only increases the wear of the parts, causing the fine emery particles that inevitably fall upon them to cut more freely.

As thin cutters would not have sufficient length of bore to steady them upon the arbor and insure parallelism, the cutter sleeve shown in Fig.

1997, which is from _The American Machinist_, is employed to hold them.

It is provided with a collar, is threaded at T for the nut N to hold the cutter against collar C, and is bored to fit the cutter arbor H, which corresponds to H in Fig. 1993.

This device also affords an excellent means of holding two or more thin cutters requiring to be ground of exactly equal diameters.

It follows from what has been said that taper tools, such as taper reamers, must be held with their upper face parallel to the line of their motion in being fed to the wheel, as in Fig. 1998, in which line M represents this line of motion, line N the axis of the reamer, and line O the line on which the fixture that holds the reamer must move, O being parallel to M.

Fig. 1999 represents Slate's fixture for this cla.s.s of work. A is a stand that bolts upon the slideway _d_ in Fig. 1992. Upon A is fixed a rectangular bar B, upon which (a sliding fit) is the shoe C. Upon C fits the piece D, which is pivoted to shoe C by the pin at E. At the other end of D is a lug, against which abuts the end of screw G, which is threaded through the end of C, so that by operating the screw G, D may be set to any required angle upon C, and at F is a set-screw threaded through D and ab.u.t.ting against C, so as to lock D in its adjusted position. At P is a pointer for the graduations on C, which are marked to correspond with the graduations upon the taper turning attachment of a lathe.

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The work is held between centres, the head H fitting to a slideway on the top of D, and being secured in its adjusted position by the screw I.

The work should obviously be set so that its upper face lies horizontal, and is fed to the wheel by moving shoe C by hand along bar B, the long bearing keeping C steady, and the lightness of the moving parts making the feeding more sensitive than it would be were it required to move bar B.

The tooth being ground is held by hand against the gauge G in Fig. 1994, as was described with reference to that figure, and the reamer, therefore, in the case of having spiral grooves, revolves upon its centre while being fed to the emery wheel.

For tapers that are beyond the capacity of this device, and also for holding cutters to have their face teeth ground, the device shown in Fig. 2000 is employed. Upon the slide F is fixed knee K (the corresponding parts to which are seen in the general view, Fig. 1992), whose disk face at R is graduated as shown. Piece S is pivoted by a pin pa.s.sing through the hub of K and having a nut T to secure it in its adjusted position. S is bored to receive the cutter arbor H, and is split through so that by means of the screw at V the arbor may be gripped and locked in S. The stud W for holding the gauge G pa.s.ses into a bore in the bracket X, and is secured therein by the screw at Y, the lugs through which Y pa.s.ses being split through into the bore for W. As shown in the figure, the arbor H is set for grinding the side teeth of the cutter, but it is obvious that S being pivoted to K may be swung out of the vertical and to any required angle, so as to bring the face of the tooth that is to be ground horizontally beneath the emery wheel, as shown in Fig. 2001, which represents an angular cutter in position. We have now to consider the adjustment of the cutter to the emery wheel, necessary in order that the cutting edges may be given the necessary clearance.

First, then, suppose in Fig. 2002 that the line A A represents the line of centres of the emery-wheel spindle and the cutter arbor, and if the front face B of the tooth be set coincident with this line, as in the figure, then the top of the tooth partaking of the curvature of the wheel that grinds it would have its heel C the highest; hence the edge at B could not cut.

If, however, the line A A in Fig. 2003, still representing the line of centres, we so set the gauge (G, Fig. 1994) that the heel C of the tooth comes up to line A A, then the curvature of the emery wheel would give clearance to the heel C, and therefore a cutting edge to face B of the tooth.

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The amount of clearance that may be given in this way is limited by the s.p.a.cing of the teeth and the diameter of the emery wheel, as is seen from Fig. 2004, it being obvious that when tooth A is being ground the emery wheel must clear the rear tooth B or it will grind its edge off, and it is obvious that the smaller the emery-wheel diameter the more the tooth to be ground may be set in advance of the line of centres of the wheel and spindle. It may be pointed out, however, that there are two methods of adjusting the cutter to the wheel.

In Fig. 2005, for example, let A A represent the line of centres of the cutter and the wheel, and line B the plane of the front face of the tooth being ground; and in Fig. 2006 let line A represent a vertical line from the axis of the wheel, and B a vertical line pa.s.sing through the axis of the cutter, the tooth edge C occupying the same position in both figures. Now suppose we employ cutting edge C as a centre and swing the cutter until its axis or centre moves along the arc D to the dot E, and it is evident that during this motion the heel of the tooth will have approached the axis of the emery wheel and that more clearance will therefore have been given to the cutting edge C.

The actual curve of the top face, as C, Fig. 2007, of the tooth T will remain the same in either case, but its position with relation to the front face will be altered. As this curve is greater in proportion as the diameter of the emery wheel is diminished, and as the curvature weakens the cutting edge of the tooth, it is obviously desirable to employ a wheel of as large a diameter as possible.

To eliminate this curvature it would appear that the position of the emery wheel might be reversed, as in Fig. 2008, but as the emery wheel would wear only where in contact with the tooth, it would gradually a.s.sume the shape in Fig. 2009, there being a shoulder at S that would destroy the cutting edge of the tooth.

This may to a great extent be remedied by presenting the cutter diagonally to the wheel, as in Fig. 2010, employing a wheel so thin that the whole of its face will cross the tooth top during a revolution. Or if the side faces of the wheel be recessed, leaving only a narrow annular grinding ring at the circ.u.mference, the wheel might be mounted as in Fig. 2011, thus making the top of the tooth quite flat. It may be observed, however, that the usual plan is to revolve the wheel at a right angle to the work axis, as was shown in Fig. 1994.

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In grinding cutters having their teeth a right-hand spiral, care must be taken that in grinding one tooth the emery wheel does not touch the cutting edge of the next tooth.

Thus in Fig. 2013 it is seen that the corner C of the emery wheel is closer than corner D, and being at the back of the wheel and out of sight it is apt to touch at C unless a thin emery wheel be used.

In a left-hand spiral, Fig. 2012, it is the corner D that is apt to touch the next tooth, the liability obviously being greatest in cutters of large diameter.