Plan View showing Method of driving Steering Knuckle and Arrangement of ToolsFig. 34. Plan View showing Method of driving Steering Knuckle and Arrangement of Tools
Plan View showing Method of driving Steering Knuckle and Arrangement of Tools
Fig. 34. Plan View showing Method of driving Steering Knuckle and Arrangement of Tools
These pieces, which are rough drop forgings, are first reduced to the approximate size. When it becomes necessary to grind the tools, they are reset and those parts which have been roughed out are turned to the finished size. The average time for the first operation, which includes starting, stopping, turning and replacing the piece, is one minute, while for the second operation with the finer feed, an average time of two minutes is required. The work is driven by sleeveS, which fits over the spindle and is held in position by the regular driver, as shown. This sleeve is notched to fit the knuckle, so that the latter can easily and quickly be replaced when finished.
One of the interesting features of this job lies in the method of locating the shoulders on each knuckle, at the same distance from the holeHwhich is drilled previously, and which receives the bolt on which the knuckle swivels when assembled in a car. As soon as the knuckle has been placed between the centers, a close-fitting plugP(Fig. 33) is inserted in this hole and the indicator arm with its attached gage or caliperGis swung up to the position shown. The stop-rod on which the stops have been previously set for the correct distance between the shoulders is next adjusted axially until the gageGjust touches the plugP. The indicator is then swung out of the way, and the piece turned. If the next knuckle were centered, say, deeper than the previous one which would, of course, cause it to be located nearer the headstock, obviously all the shoulders would be located farther from the finished hole, provided the position of the stops remained the same asbefore. In such a case their position would, however, be changed by shifting the stop-rod until the gageGagain touched the plug thus locating all the stops with reference to the hole. As the adjustment of the stop-rod changes the position of the taper templet as well as the stops, it is evident that both the shoulders and the taper are finished the same distance from the hole in each case. The connection of the bracket (to which the templet arm is attached) with the stop-rod is clearly shown inFig. 33. This bracket can either be locked to the ways or adjusted to slide when the stop-rod is moved.
First and Second Operations on Automobile Transmission Shaft—Lo-swing LatheFig. 35. First and Second Operations on Automobile Transmission Shaft—Lo-swing Lathe
First and Second Operations on Automobile Transmission Shaft—Lo-swing Lathe
Fig. 35. First and Second Operations on Automobile Transmission Shaft—Lo-swing Lathe
The part illustrated inFig. 35is an automobile transmission shaft. In this particular case, cylindrical, tapering and spherical surfaces are turned. The upper view shows, diagrammatically, the arrangement of the tools and work for the first operation. After the shaft is “spotted” atAfor the steadyrest, thestraight partCand the collarBare sized with toolsSandRwhich are mounted on the left-hand carriage. A concave groove is then cut in collarBby toolR, after which spherical endDis formed by a special attachment mounted on the right-hand carriage. This attachment is the same, in principle, as the regular taper-turning attachment, the substitution of a circular templetTfor the straight kind used on taper work being the only practical difference.
After the surfaces mentioned have been finished on a number of pieces, the work is reversed and the tools changed as shown by the lower view. The first step in the second operation is to turn the bodyEof the shaft with the toolTon the left-hand carriage. The taperFand the straight partGare then finished, which completes the turning. It will be noted that in setting up the machine for this second operation, it is arranged for taper turning by simply replacing the circular templet with the straight one shown. When this taper attachment is not in use, the swiveling armM, which is attached to a bracket, is swung out of the way.
The method of driving this shaft is worthy of note. A dog having two driving arms each of which bears against a pinNthat passes through a hole in the spindle is used. As the ends of this pin, against which the dog bears, are beveled in opposite directions, the pin turns in its hole when the dog makes contact with it and automatically adjusts itself against the two driving members of the dog. The advantage of driving by a two-tailed dog, as most mechanics know, is in equalizing the tendency to spring slender parts while they are being turned.
Axle End turned in One Traverse of the Five Tools shownFig. 36. Axle End turned in One Traverse of the Five Tools shown
Axle End turned in One Traverse of the Five Tools shown
Fig. 36. Axle End turned in One Traverse of the Five Tools shown
InFig. 36another turning operation on a lathe of this type is shown, the work in this case being a rear axle for a motor truck. The turning of this part is a good example of that class of work where the rapid removal of metal is the important feature. As the engraving shows, the stock, prior to turning, is 31/2inches in diameter and it is reduced to a minimum diameter of 11/16inch. This metal is turned off with one traverse of the carriage or by one passage of the five tools, and the weight of the chips removed from each end of the axle is approximately 12 pounds. The time required for the actual turning is about 9 minutes, while the total time for the operation, which includes placing the heavy piece in the machine, turning, andremoving the work from the lathe, is 12 minutes. The axle revolves, while being turned, at 110 revolutions per minute and a feed equivalent to 1 inch of tool travel to 60 revolutions of the work is used. It will be noticed that the taper attachment is also employed on this part, the taper being turned by the second tool from the left. As the axle is equipped with roller bearings, it was found desirable to finish the bearing part by a separate operation; therefore, in the operation shown the axle is simply roughed down rather close to the finished dimensions, leaving enough material for a light finishing cut.
Lathe Knurling Tool having Three Pairs of Knurls—Coarse, Medium and FineFig. 37. Lathe Knurling Tool having Three Pairs of Knurls—Coarse, Medium and Fine
Lathe Knurling Tool having Three Pairs of Knurls—Coarse, Medium and Fine
Fig. 37. Lathe Knurling Tool having Three Pairs of Knurls—Coarse, Medium and Fine
Knurling in the Lathe.—Knurling is done either to provide a rough surface which can be firmly gripped by the hand or for producing an ornamental effect. The handles of gages and other tools are often knurled, and the thumb-screws used on instruments, etc., usually have knurled edges. A knurled surface consists of a series of small ridges or diamond-shaped projections, and is produced in the lathe by the use of a tool similar to the one shown inFig. 37, this being one of several different designs in common use. The knurling is done by two knurlsAandBhaving teeth or ridges which incline to the right on one knurl and to the left on the opposite knurl, as shown by the end view. When these two knurls are pressed against the work as the latter revolves, one knurl forms a series of left-hand ridges and the other knurl right-hand ridges, which cross and form the diamond-shaped knurling which is generally used.
If the surface to be knurled is wider than the knurls, the power feed of the lathe should be engaged and the knurling tool be traversed back and forth until the diamond-shaped projections are well formed. To prevent forming a double set of projections, feed the knurl in with considerable pressure at the start, then partially relieve the pressure before engaging the power feed. Use oil when knurling.
The knurls commonly used for lathe work have spiral teeth and ordinarily there are three classes, known as coarse, medium and fine. The medium pitch is generally used. The teeth of coarse knurls have a spiral angle of 36 degrees and the pitch of the knurled cut (measured parallel to the axis of the work)should be about 8 per inch. For medium knurls, the spiral angle is 291/2degrees and the pitch, measured as before, is 12 per inch. For fine knurls, the spiral angle is 253/4degrees and the pitch 20 per inch. The knurls should be about3/4inch in diameter and3/8inch wide. When made to these dimensions, coarse knurls have 34 teeth; medium, 50 teeth; and fine knurls, 80 teeth.
The particular tool illustrated inFig. 37has three pairs of knurls of coarse, medium and fine pitch. These are mounted in a revolving holder which not only serves to locate the required set of knurls in the working position, but enables each knurl to bear against the surface with equal pressure. Concave knurls are sometimes used for knurling rounded edges on screw heads, etc.
Relieving Attachment.—Some lathes, particularly those used in toolrooms, are provided with relieving attachments which are used for “backing off” the teeth of milling cutters, taps, hobs, etc. If a milling cutter of special shape is to be made, the cutter blank is first turned to the required form with a special tool having a cutting edge that corresponds with the shape or profile of the cutter to be made. The blank is then fluted or gashed to form the teeth, after which the tops of the teeth arerelieved or backed off to provide clearance for the cutting edges. The forming tool used for turning the blank is set to match the turned surface, and the teeth are backed off as the result of a reciprocating action imparted to the toolslide by the relieving attachment. The motion of the toolslide is so adjusted that the tool will meet the front of each tooth and the return movement begin promptly after the tool leaves the back end of the tooth.
Hendey Relieving Attachment applied to a LatheFig. 38. Hendey Relieving Attachment applied to a Lathe
Fig. 38. Hendey Relieving Attachment applied to a Lathe
These attachments differ somewhat in their construction and arrangement but the principle of their operation is similar.Fig. 38shows a Hendey relieving attachment applied to a lathe. A bracket carrying the gearingAthrough which the attachment is driven is mounted upon the main gear box of the lathe, and the special slideB, which is used when relieving, is placed on the cross-slide after removing the regular compound rest. The gears atAare changed to suit the number of flutes or gashes in the cutter, tap or whatever is to be relieved. If we assume that the work is a formed milling cutter having nine teeth, then with this particular attachment, a gear having 90 teeth would be placed on the “stud” and a 40-tooth gear on the cam-shaft, the two gears being connected by a 60-tooth intermediate gear.With this combination of gearing, the toolslide would move in and out nine times for each revolution of the work, so that the tool could back off the top of each tooth. (The gearing to use for various numbers of flutes is shown by an index plate on the attachment.) The amount of relief is varied to suit the work being done, by means of a toothed coupling which makes it possible to change the relative position between the eccentric which actuates the toolslide and the cam lever, thereby lengthening or shortening the reciprocating travel of the tool.
Relieving a Formed CutterFig. 39. Relieving a Formed Cutter
Fig. 39. Relieving a Formed Cutter
Application of Relieving Attachment.—Some typical examples of the kind of work for which the relieving attachment is used are shown inFigs. 39to42, inclusive.Fig. 39shows how a formed milling cutter is relieved. The toolslide is set at right angles to the axis of the work, and the tool moves in as each tooth passes, and out while crossing the spaces or flutes between the teeth. As the result of this movement, the tops of the teeth are backed off eccentrically but the form or shape is the same from the front to the back of the tooth; hence, a cutter that has been relieved in this way can be ground repeatedly without changing the profile of the teeth, provided the faces are ground so as to lie in a radial plane.
When relieving, the cutting speed should be much less than when turning in order to give the toolslide time to operate properly. A maximum of 180 teeth per minute is recommended, and, if wide forming tools are used, it might be advisable to reduce the speed so low that only 8 teeth per minute would be relieved. It is also essential to use a tool having a keen edge, and the toolslide should work freely but be closely adjusted to the dovetail of the lower slide. Before beginning to back off the teeth, it is a good plan to color the work either by heating it or dipping into a strong solution of copper sulphate. This will enable one to see plainly the cutting action of the tool in order to stop relieving at the proper time.
Relieving Side of Angular Milling CutterFig. 40. Relieving Side of Angular Milling Cutter
Fig. 40. Relieving Side of Angular Milling Cutter
Fig. 40shows a method of relieving the teeth of an angular cutter. For an operation of this kind the toolslide is swiveled around at right angles to the side that is to be relieved. By the use of an additional universal joint and bearing to permit the toolslide to be swung to a 90-degree angle, the teeth of counterbores, etc., can be relieved on the ends. When the attachment is used for relieving inside work, such as hollow mills and threading dies, the eccentric which controls the travel of the toolslide is set so that the relieving movement is away fromthe axis of the cutter instead of toward it. This change is made by the toothed coupling previously referred to, which connects the cam lever and oscillating shaft, the latter being turned beyond the zero mark in a clockwise direction as far as is necessary to obtain the desired amount of travel. For internal work it is also necessary to change the position of the opposing spring of the toolslide, so that it will press against the end of the slide and prevent the tool from jumping into the work.
Relieving a Right-hand TapFig. 41. Relieving a Right-hand Tap
Fig. 41. Relieving a Right-hand Tap
Fig. 41shows how a right-hand tap is relieved. The ordinary practice is to first set the tool the same as for cutting a thread. The motion of the toolslide is then adjusted so that the tool on the forward stroke will meet the front of each tooth, and start back as soon as the tool leaves the end of the land or top of the tooth. Taps having a left-hand thread can be relieved by two different methods. With the first method the cut starts at the cutting edge of each tooth, and ends at the “heel,” the tool moving in toward the center of the work. With the second method, the cut begins at the heel and discontinues at the cutting edge, the tool being drawn away from the work during thecut. When using the first method the tap must be placed with the point toward the headstock, the shank end being supported by the tailstock center. This is done by providing an extension or blank end at the point of the tap long enough to hold the driving dog. With the second method, the tap is held between centers the same as one having a right-hand thread, but the travel of the toolslide is set the same as for inside relief.
Relieving a Hob having Spiral FlutesFig. 42. Relieving a Hob having Spiral Flutes
Fig. 42. Relieving a Hob having Spiral Flutes
Relieving Hobs or Taps Having Spiral Flutes.—With this attachment, taps or hobs having “spiral” or helical flutes can also be relieved. (A spiral flute is preferable to one that is parallel to the axis, because with the former the tool has cutting edges which are square with the teeth; this is of especial importance when the lead of the hob or tap thread is considerable.) When relieving work having spiral flutes (as illustrated inFig. 42), the lead of the spiral and the gears necessary to drive the attachment are first determined. After the attachment is geared for the number of flutes and to compensate for the spiral, the lead-screw is engaged and the backing-off operation is performed the same as though the flutes were straight. The carriage should not be disengaged from the lead-screw after starting the cut, the tool being returned by reversing the lathe.
When gearing the attachment for relieving a tap or hob having spiral flutes, the gears are not selected for the actual number of flutes around the circumference but for a somewhat larger number which depends upon the lead of the hob thread and the lead of the spiral flutes. Let us assume that a hob has 6 spiral flutes and that the attachment is geared for that number. The result would be that as the tool advanced along the thread, it would not keep “in step” with the teeth because the faces of the teeth lie along a spiral (or helix which is the correct name for this curve); in other words, the tool would soon be moving in too late to begin cutting at the proper time, and to compensate for this, the attachment is geared so that the tool will make a greater number of strokes per revolution of the work than the actual number of flutes around the circumference.
With this attachment, the two gears listed on the index plate for the actual number of flutes are selected, and then two compensatinggears are added, thus forming a compound train of gearing. The ratioRof these compensating gears is determined as follows:
in which
For example, if a hob has a pitch circumference of 3.25, a single thread of 0.75 inch lead, and 6 spiral flutes, what compensating gears would be required?
The leadLof the spiral flutes is first determined by dividing the square of the circumferenceCof the hob at the pitch line by the leadlof the hob thread. Thus leadL = C2÷ l, or, in this case,L= 3.252÷ 0.75 = 14 inches, approximately. Thenr= 14 ÷ 0.75 = 182/3. Inserting these values in the formula for ratio R,
Hence, the compensating gears will have 56 and 59 teeth, respectively, the latter being the driver. As the gears for 6 flutes listed on the regular index plate are, stud-gear 60 teeth, cam-shaft gear 40 teeth, the entire train of gears would be as follows: Gear on stud, 60;drivenintermediate gear, 56;drivingintermediate gear, 59; cam-shaft gear, 40. It will be understood that the position of the driving gears or the driven gears can be transposed without affecting the ratio.
Classes of Fits Used in Machine Construction.—In assembling machine parts it is necessary to have some members fit together tightly, whereas other parts such as shafts, etc., must be free to move or revolve with relation to each other. The accuracy required for a fitting varies for different classes of work. A shaft that revolves in its bearing must be slightly smaller than the bearing so that there will be room for a film of lubricant. A crank-pin that must be forced into the crank-disk ismade a little larger in diameter than the hole, to secure a tight fit. When a very accurate fitting between two cylindrical parts that must be assembled without pressure is required, the diameter of the inner member is made as close to the diameter of the outer member as is possible. In ordinary machine construction, five classes of fits are used,viz; running fit, push fit, driving fit, forced fit and shrinkage fit. The running fit, as the name implies, is employed when parts must rotate; the push fit is not sufficiently free to rotate; the other classes referred to are used for assembling parts that must be held in fixed positions.
Forced Fits.—This is the term used when a pin, shaft or other cylindrical part is forced into a hole of slightly smaller diameter, by the use of a hydraulic press or other means. As a rule, forced fits are restricted to parts of small and medium size, while shrinkage fits have no such limitations and are especially applicable when a maximum “grip” is desired, or when (as in the construction of ordnance) accurate results as to the intensity of stresses produced in the parts united are required. The proper allowance for a forced fit depends upon the mass of metal surrounding the hole, the size of the work, the kind and quality of the material of which the parts are composed and the smoothness and accuracy of the pin and bore. When a pin or other part is pressed into a hole a second time, the allowance for a given tonnage should be diminished somewhat because the surface of the bore is smoother and the metal more compact. The pressure required in assembling a forced fit will also vary for cast hubs of the same size, if they are not uniform in hardness. Then there is the personal factor which is much in evidence in work of this kind; hence, data and formulas for forced fit allowances must be general in their application.
(Newall Engineering Co.)
[1]Tolerance is provided for holes, which ordinary standard reamers can produce, in two grades, Classes A and B, the selection of which is a question for the user's decision and dependent upon the quality of the work required; some prefer to use Class A as working limits and Class B as inspection limits.
[2]Running fits, which are the most commonly required, are divided into three grades: Class X for engine and other work where easy fits are wanted; Class Y for high speeds and good average machine work; Class Z for fine tool work.
Allowance for Forced Fits.—The allowance per inch of diameter usually ranges from 0.001 inch to 0.0025 inch, 0.0015 being a fair average. Ordinarily, the allowance per inch decreases as the diameter increases; thus the total allowance for a diameter of 2 inches might be 0.004 inch, whereas for a diameter of 8 inches the total allowance might not be over 0.009 or0.010 inch. In some shops the allowance is made practically the same for all diameters, the increased surface area of the larger sizes giving sufficient increase in pressure. The parts to be assembled by forced fits are usually made cylindrical, although sometimes they are slightly tapered. The advantages of the taper form are that the possibility of abrasion of the fitted surfaces is reduced; that less pressure is required in assembling; and that the parts are more readily separated when renewal is required. On the other hand, the taper fit is less reliable, because if it loosens, the entire fit is free with but little axial movement. Some lubricant, such as white lead and lard oil mixed to the consistency of paint, should be applied to the pin and bore before assembling, to reduce the tendency of abrasion.
Pressure for Forced Fits.—The pressure required for assembling cylindrical parts depends not only upon the allowance for the fit, but also upon the area of the fitted surfaces, the pressure increasing in proportion to the distance that the inner member is forced in. The approximate ultimate pressure in pounds can be determined by the use of the following formula in conjunction with the accompanying table of “Pressure Factors.”
Assuming thatA= area of fitted surface;a= total allowance in inches;P= ultimate pressure required, in tons;F= pressure factor based upon assumption that the diameter of thehub is twice the diameter of the bore, that the shaft is of machine steel, and the hub of cast iron, then,
Example:—What will be the approximate pressure required for forcing a 4-inch machine steel shaft having an allowance of 0.0085 inch into a cast-iron hub 6 inches long?
A= 4 × 3.1416 × 6 = 75.39 square inches;
F, for a diameter of 4 inches, = 115 (see table of “Pressure Factors”). Then,
P= (75.39 × 0.0085 × 115)/2 = 37 tons, approximately.
Allowance for Given Pressure.—By transposing the preceding formula, the approximate allowance for a required ultimate tonnage can be determined. Thus,a= 2P÷AF. The average ultimate pressure in tons commonly used ranges from 7 to 10 times the diameter in inches. Assuming that the diameter of a machine steel shaft is 4 inches and an ultimate pressure of about 30 tons is desired for forcing it into a cast-iron hub having a length of 51/2inches, what should be the allowance?
A= 4 × 3.1416 × 51/2= 69 square inches,
F, for a diameter of 4 inches, = 115. Then,