[1]Cutting speeds for tools of a good grade of high-speed steel, properly ground and heat-treated.—FromMachinery's Handbook.
Average Cutting Speeds for Turning.—The cutting speed is governed principally by the hardness of the metal to be turned; the kind of steel of which the turning tool is made; the shape of the tool and its heat-treatment; the feed and depth of cut; whether or not a cooling lubricant is used on the tool; the power of the lathe and also its construction; hence it is impossible to give any definite rule for determining either the speed, feed, or depth of cut, because these must be varied to suit existing conditions. A general idea of the speeds used in ordinary machine shop practice may be obtained from the following figures:
Ordinary machine steel is generally turned at a speed varying between 45 and 65 feet per minute. For ordinary gray cast iron, the speed usually varies from 40 to 50 feet per minute; for annealed tool steel, from 25 to 35 feet per minute; for soft yellow brass, from 150 to 200 feet per minute; for hard bronze, from 35 to 80 feet per minute, the speed depending upon the composition of the alloy. While these speeds correspond closely to general practice, they can be exceeded for many machining operations.
The most economical speeds for a given feed and depth of cut, as determined by the experiments conducted by Mr. F. W. Taylor, are given in the table, “Cutting Speeds and Feeds for Turning Tools.” The speeds given in this table represent results obtained with tools made of a good grade of high-speed steel properly heat-treated and correctly ground. It will be noted that the cutting speed is much slower for cast iron than for steel. Cast iron is cut with less pressure or resistance than soft steel, but the slower speed required for cast iron is probably due to the fact that the pressure of the chip is concentrated closer to the cutting edge, combined with the fact that cast iron wears the tool faster than steel. The speeds given are higher than those ordinarily used, and, in many cases, a slower rate would be necessary to prevent chattering or because of some other limiting condition.
Factors which limit the Cutting Speed.—It is the durability of the turning tool or the length of time that it will turn effectivelywithout grinding, that limits the cutting speed; and the hardness of the metal being turned combined with the quality of the tool are the two factors which largely govern the time that a tool can be used before grinding is necessary. The cutting speed for very soft steel or cast iron can be three or four times faster than the speed for hard steel or hard castings, but whether the material is hard or soft, the kind and quality of the tool used must also be considered, as the speed for a tool made of ordinary carbon steel will have to be much slower than for a tool made of modern “high-speed” steel.
When the cutting speed is too high, even though high-speed steel is used, the point of the tool is softened to such an extent by the heat resulting from the pressure and friction of the chip, that the cutting edge is ruined in too short a time. On the other hand, when the speed is too slow, the heat generated is so slight as to have little effect and the tool point is dulled by being slowly worn or ground away by the action of the chip. While a tool operating at such a low speed can be used a comparatively long time without re-sharpening, this advantage is more than offset by the fact that too much time is required for removing a given amount of metal when the work is revolving so slowly.
Generally speaking, the speed should be such that a fair amount of work can be done before the tool requires re-grinding. Evidently, it would not pay to grind a tool every few minutes in order to maintain a high cutting speed; neither would it be economical to use a very slow speed and waste considerable time in turning, just to save the few minutes required for grinding. For example, if a number of roughing cuts had to be taken over a heavy rod or shaft, time might be saved by running at such a speed that the tool would have to be sharpened (or be replaced by a tool previously sharpened) when it had traversed half-way across the work; that is, the time required for sharpening or changing the tool would be short as compared with the gain effected by the higher work speed. On the other hand, it might be more economical to run a little slower and take a continuous cut across the work with one tool.
The experiments of Mr. Taylor led to the conclusion that, as a rule, it is not economical to use roughing tools at a speed so slow as to cause them to last more than 11/2hour without being re-ground; hence the speeds given in thetablepreviously referred to are based upon this length of time between grindings. Sometimes the work speed cannot be as high as the tool will permit, because of the chattering that often results when the lathe is old and not massive enough to absorb the vibrations, or when there is unnecessary play in the working parts. The shape of the tool used also affects the work speed, and as there are so many things to be considered, the proper cutting speed is best determined by experiment.
Rules for Calculating Cutting Speeds.—The number of revolutions required to give any desired cutting speed can be found by multiplying the cutting speed, in feet per minute, by 12 and dividing the product by the circumference of the work in inches. Expressing this as a formula we have
C× 12R=———πd
in which
R=revolutions per minute;C=the cutting speed in feet per minute;π=3.1416;d=the diameter in inches.
For example if a cutting speed of 60 feet per minute is wanted and the diameter of the work is 5 inches, the required speed would be found as follows:
60 × 12R=—————=46 revolutions per minute.3.1416 × 5
If the diameter is simply multiplied by 3 and the fractional part is omitted, the calculation can easily be made, and the result will be close enough for practical purposes. In case the cutting speed, for a given number of revolutions and diameter, is wanted, the following formula can be used:
RπdC=——12
Machinists who operate lathes do not know, ordinarily, what cutting speeds, in feet per minute, are used for different classes of work, but are guided entirely by past experience.
Feed of Tool and Depth of Cut.—The amount of feed and depth of cut also vary like the cutting speed, for different conditions. When turning soft machine steel the feed under ordinary conditions would vary between1/32and1/16inch per revolution. For turning soft cast iron the feed might be increased to from1/16to1/8inch per revolution. These feeds apply to fairly deep roughing cuts. Coarser feeds might be used in many cases especially when turning large rigid parts in a powerful lathe. The depth of a roughing cut in machine steel might vary from1/8to3/8inch, and in cast iron from3/16to1/2inch. These figures are intended simply to give the reader a general idea of feeds and cuts that are feasible under average conditions.
Ordinarily coarser feeds and a greater depth of cut can be used for cast iron than for soft steel, because cast iron offers less resistance to turning, but in any case, with a given depth of cut, metal can be removed more quickly by using a coarse feed and the necessary slower speed, than by using a fine feed and the higher speed which is possible when the feed is reduced. When the turning operation is simply to remove metal, the feed should be coarse, and the cut as deep as practicable. Sometimes the cut must be comparatively light, either because the work is too fragile and springy to withstand the strain of a heavy cut, or the lathe has not sufficient pulling power. The difficulty with light slender work is that a heavy cut may cause the part being turned to bend under the strain, thus causing the tool to gouge in, which would probably result in spoiling the work. Steadyrests can often be used to prevent flexible parts from springing, as previously explained, but there are many kinds of light work to which the steadyrest cannot be applied to advantage.
Roughing Cut—Light Finishing Cut and Coarse FeedFig. 15. Roughing Cut—Light Finishing Cut and Coarse Feed
Fig. 15. Roughing Cut—Light Finishing Cut and Coarse Feed
The amount of feed to use for a finishing cut might, properly, be either fine or coarse. Ordinarily, fine feeds are used for finishing steel, especially if the work is at all flexible, whereas finishingcuts in cast iron are often accompanied by a coarse feed.Fig. 15illustrates the feeds that are often used when turning cast iron. The view to the left shows a deep roughing cut and the one to the right, a finishing cut. By using a broad flat cutting edge set parallel to the tool's travel, and a coarse feed for finishing, a smooth cut can be taken in a comparatively short time. Castings which are close to the finished size in the rough can often be finished to advantage by taking a single cut with a broad tool, provided the work is sufficiently rigid. It is not always practicable to use these broad tools and coarse feeds, as they sometimes cause chattering, and when used on steel, a broad tool tends to gouge or “dig in” unless the part being turned is rigid. Heavy steel parts, however, are sometimes finished in this way. The modern method of finishing many steel parts is to simply rough them out in a lathe to within, say,1/32inch of the required diameter and take the finishing cut in a cylindrical grinding machine.
Effect of Lubricant on Cutting Speed.—When turning iron or steel a higher cutting speed can be used, if a stream of soda water or other cooling lubricant falls upon the chip at the point where it is being removed by the tool. In fact, experiments have shown that the cutting speed, when using a large stream of cooling water and a high-speed steel tool, can be about 40 percent higher than when turning dry or without a cooling lubricant.For ordinary carbon steel tools, the gain was about 25 per cent. The most satisfactory results were obtained from a stream falling at a rather slow velocity but in large volume. The gain in cutting speed, by the use of soda water or other suitable fluids, was found to be practically the same for all qualities of steel from the softest to the hardest.
Cast iron is usually turned dry or without a cutting lubricant. Experiments, however, made to determine the effect of applying a heavy stream of cooling water to a tool turning cast iron, showed the following results: Cutting speed without water, 47 feet per minute; cutting speed with a heavy stream of water, nearly 54 feet per minute; increase in speed, 15 per cent. The dirt caused by mixing the fine cast-iron turnings with a cutting lubricant is an objectionable feature which, in the opinion of many, more than offsets the increase in cutting speed that might be obtained.
Turret lathes and automatic turning machines are equipped with a pump and piping for supplying cooling lubricant to the tools in a continuous stream. Engine lathes used for general work, however, are rarely provided with such equipment and a lubricant, when used, is often supplied by a can mounted at the rear of the carriage, having a spout which extends above the tool. Owing to the inconvenience in using a lubricant on an engine lathe, steel, as well as cast iron, is often turned dry especially when the work is small and the cuts light and comparatively short.
Lubricants Used for Turning.—A good grade of lard oil is an excellent lubricant for use when turning steel or wrought iron and it is extensively used on automatic screw machines, especially those which operate on comparatively small work. For some classes of work, especially when high-cutting speeds are used, lard oil is not as satisfactory as soda water or some of the commercial lubricants, because the oil is more sluggish and does not penetrate to the cutting point with sufficient rapidity. Many lubricants which are cheaper than oil are extensively used on “automatics” for general machining operations. These usually consist of a mixture of sal-soda (carbonate of soda) andwater, to which is added some ingredient such as lard oil or soft soap to thicken or give body to the lubricant.
A cheap lubricant for turning, milling, etc., and one that has been extensively used, is made in the following proportions: 1 pound of sal-soda, 1 quart of lard oil, 1 quart of soft soap, and enough water to make 10 or 12 gallons. This mixture is boiled for one-half hour, preferably by passing a steam coil through it. If the solution should have an objectionable odor, this can be eliminated by adding 2 pounds of unslaked lime. The soap and soda in this solution improve the lubricating quality and also prevent the surfaces from rusting. For turning and threading operations, plain milling, deep-hole drilling, etc., a mixture of equal parts of lard oil and paraffin oil will be found very satisfactory, the paraffin being added to lessen the expense.
Brass or bronze is usually machined dry, although lard oil is sometimes used for automatic screw machine work. Babbitt metal is also worked dry, ordinarily, although kerosene or turpentine is sometimes used when boring or reaming. If babbitt is bored dry, balls of metal tend to form on the tool point and score the work. Milk is generally considered the best lubricant for machining copper. A mixture of lard oil and turpentine is also used for copper. For aluminum, the following lubricants can be used: Kerosene, a mixture of kerosene and gasoline, soap-water, or “aqualine” one part, water 20 parts.
Lard Oil as a Cutting Lubricant.—After being used for a considerable time, lard oil seems to lose some of its good qualities as a cooling compound. There are several reasons for this: Some manufacturers use the same oil over and over again on different materials, such as brass, steel, etc. This is objectionable, for when lard oil has been used on brass it is practically impossible to get the fine dust separated from it in a centrifugal separator. When this impure oil is used on steel, especially where high-speed steels are employed, it does not give satisfactory results, owing to the fact that when the cutting tool becomes dull, the small brass particles “freeze” to the cutting tool and thus produce rough work. The best results are obtainedfrom lard oil by keeping it thin, and by using it on the same materials—that is, not transferring the oil from a machine in which brass is being cut to one where it would be employed on steel. If the oil is always used on the same class of material, it will not lose any of its good qualities.
Prime lard oil is nearly colorless, having a pale yellow or greenish tinge. The solidifying point and other characteristics of the oil depend upon the temperature at which it was expressed, winter-pressed lard oil containing less solid constituents of the lard than that expressed in warm weather. The specific gravity should not exceed 0.916; it is sometimes increased by adulterants, such as cotton-seed and maize oils.
It is often necessary, in connection with lathe work, to turn parts tapering instead of straight or cylindrical. If the work is mounted between the centers, one method of turning a taper is to set the tailstock center out of alignment with the headstock center. When both of these centers are in line, the movement of the tool is parallel to the axis of the work and, consequently, a cylindrical surface is produced; but if the tailstockh1is set out of alignment, as shown inFig. 1, the work will then be turned tapering as the tool is traversed fromatob, because the axisx—xis at an angle with the movement of the tool. Furthermorethe amount of taper or the difference between the diameters at the ends for a given length, will depend on how much centerh1is set over from the central position.
Taper Turning by the Offset-center MethodFig. 1. Taper Turning by the Offset-center Method
Taper Turning by the Offset-center Method
Fig. 1. Taper Turning by the Offset-center Method
The amount of taper is usually given on drawings in inches per foot, or the difference in the diameter at points twelve inches apart. For example, the taper of the piece shown atA,Fig. 2, is 1 inch per foot, as the length of the tapering surface is just twelve inches and the difference between the diameters at the ends is 1 inch. The conical roller shown atBhas a total length of 9 inches and a tapering surface 6 inches long, and in this case the taper per foot is also 1 inch, there being a difference of1/2inch in a length of 6 inches or 1 inch in twice that length. When the taper per foot is known, the amount that the tailstock center should be set over for turning that taper can easily be estimated, but it should be remembered that the setting obtained in this way is not absolutely correct, and is only intended to locate the center approximately. When a taper needs to be at all accurate, it is tested with a gage, or by other means, after taking a trial cut, as will be explained later, and the tailstock center is readjusted accordingly. There are also more accurate methods of setting the center, than by figuringthe amount of offset, but as the latter is often convenient this will be referred to first.
Examples of Taper WorkFig. 2. Examples of Taper Work
Examples of Taper Work
Fig. 2. Examples of Taper Work
Setting Tailstock Center for Taper Turning.—Suppose the tailstock center is to be set for turning partC,Fig. 2, to a taper of approximately 1 inch per foot. In this case the center would simply be moved toward the front of the machine1/2inch, or one-half the required taper per foot, because the total length of the work happens to be just 12 inches. This setting, however, would not be correct for all work requiring a taper of 1 inch per foot, as the adjustment depends not only on theamountof the taper but on thetotal lengthof the piece.
Detail View of Lathe TailstockFig. 3. Detail View of Lathe Tailstock
Fig. 3. Detail View of Lathe Tailstock
For example, the taper rollerBhas a taper of 1 inch per foot, but the center, in this case, would be offset less than one-half the taper per foot, because the total length is only 9 inches. For lengths longer or shorter than twelve inches, the taper per inch should be found first; this is then multiplied by thetotallength of the work (not the length of the taper) which gives the taper for that length, and one-half this taper is the amount to set over the center. For example, the taper per inch of partBequals 1 inch divided by 12 =1/12inch. The total length of 9 inches multiplied by1/12inch =3/4inch, and1/2of3/4=3/8, which is the distance that the tailstock center should be offset. In this example if the taper per foot were not known, and only the diameters of the large and small ends of the tapered part were given, the difference between these diameters should first be found (21/2- 2 =1/2); this difference should then be divided by the length of the taper (1/2÷ 6 =1/12inch) to obtain the taper per inch. The taper per inch times thetotallength represents what the taper would be if it extended throughout the entire length, and one-half of this equals the offset, which is3/8inch.
Taper Plug and GageFig. 4. Taper Plug and Gage
Taper Plug and Gage
Fig. 4. Taper Plug and Gage
Example of Taper Turning.—As a practical example of taper turning let us assume that the piece A,Fig. 4, which has been centered and rough-turned as shown, is to be made into a taper plug, as indicated atB, to fit a ring gage as atC. If the required taper is 11/2inch per foot and the total length is 8 inches, the tailstock center would be offset1/2inch.
To adjust the tailstock, the nutsN(Fig. 3) are first loosened and then the upper partAis shifted sidewise by turning screwS. Scales are provided on some tailstocks for measuring the amount of this adjustment; if there is no scale, draw a line across the movable and stationary partsAandB, when the tailstock is set for straight turning. The movement of the upper line in relation to the lower will then show the offset, which can be measured with a scale.
When the adjustment has been made, nutsNare tightened and the part to be turned, with a dog attached, is placed between the centers the same as for straight turning. The taper end is then reduced by turning, but before it is near the finished size, the work is removed and the taper tested by inserting it in the gage. If it is much out, this can be felt, as the end that is too small can be shaken in the hole. Suppose the plug did not taper enough and only the small end came into contact with the gage, as shown somewhat exaggerated atD; in that casethe center would be shifted a little more towards the front, whereas if the taper were too steep, the adjustment would, of course, be in the opposite direction. A light cut would then be taken, to be followed by another test. If the plug should fit the gage so well that there was no perceptible shake, it could be tested more closely as follows: Draw three or four chalk lines along the tapering surface, place the work in the gage and turn it a few times. The chalk marks will then show whether the taper of the plug corresponds to that of the gage; for example, if the taper is too great, the marks will be rubbed out on the large end, but if the taper is correct, the lines throughout their length will be partially erased.
Another and more accurate method of testing tapers is to apply a thin coat of Prussian-blue to one-half of the tapering surface, in a lengthwise direction. The work is then inserted in the hole or gage and turned to mark the bearing. If the taper is correct, the bearing marks will be evenly distributed, whereas if the taper is incorrect, they will appear at one end. Tapering pieces that have to be driven tightly into a hole, such as a piston-rod, can be tested by the location of the bearing marks produced by actual contact.
After the taper is found to be correct, the plug is reduced in size until it just enters the gage as atC. The final cut should leave it slightly above the required size, so that a smooth surfacecan be obtained by filing. It should be mentioned that on work of this kind, especially if great accuracy is required, the final finish is often obtained by grinding in a regular grinding machine, instead of by filing. When this method is employed, a lathe is used merely to rough-turn the part close to size.
Setting Work for Taper Turning by use of Caliper GageFig. 5. Setting Work for Taper Turning by use of Caliper Gage
Setting Work for Taper Turning by use of Caliper Gage
Fig. 5. Setting Work for Taper Turning by use of Caliper Gage
When the amount that the tailstock center should be offset is determined by calculating, as in the foregoing example, it is usually necessary to make slight changes afterward, and the work should be tested before it is too near the finished size so that in case one or more trial cuts are necessary, there will be material enough to permit this. When there are a number of tapered pieces to be turned to the same taper, the adjustment of the tailstock center will have to be changed unless the total length of each piece and the depth of the center holes are the same in each case.
Side View showing Relative Positions of Gage and WorkFig. 6. Side View showing Relative Positions of Gage and Work
Side View showing Relative Positions of Gage and Work
Fig. 6. Side View showing Relative Positions of Gage and Work
Setting the Tailstock Center with a Caliper Tool.—Another method of setting the tailstock center for taper turning is illustrated inFig. 5. The end of an engine piston-rod is to be made tapering as at A and to dimensionsa,b,candd. It is first turned with the centers in line as atB. The enddis reduced to diameterbup to the beginning of the taper and it is then turned to diameteraas far as the taper partcextends. The tailstock center is next set over by guess and a caliper tool is clamped in the toolpost. This tool, a side view of which is shown inFig. 6,has a pointerpthat is free to swing about pivotr, which should be set to about the same height as the center of the work. The tailstock center is adjusted until this pointer just touches the work when in the positions shown by the full and dotted lines atC,Fig. 5; that is, until the pointer makes contact at the beginning and end of the taper part. The travel of the carriage will then be parallel to a linex—x, representing the taper; consequently, if a tool is started at the small end, as shown by the dotted lines atD, with the nose just grazing the work, it will also just graze it when fed to the extreme left as shown. Of course, if the taper were at all steep, more than one cut would be taken.
Obtaining Tailstock Center Adjustment by use of SquareFig. 7. Obtaining Tailstock Center Adjustment by use of Square
Obtaining Tailstock Center Adjustment by use of Square
Fig. 7. Obtaining Tailstock Center Adjustment by use of Square
If these various operations are carefully performed, a fairly accurate taper can be produced. The straight enddis reduced to size after the tail-center is set back to the central position. Some mechanics turn notches or grooves at the beginning and end of the tapering part, having diameters equal to the largest and smallest part of the taper; the work is then set by these grooves with a caliper tool. The advantage of thefirst method is that most of the metal is removed while the centers are in alignment.
Setting the Tailstock Center with a Square.—Still another method of adjusting the tailstock for taper turning, which is very simple and eliminates all figuring, is as follows: The part to be made tapering is first turned cylindrical or straight for 3 or 4 inches of its length, after the ends have been properly centered and faced square. The work is then removed and the tailstock is shifted along the bed until the distancea—bbetween the extreme points of the centers is exactly 1 foot. The center is next offset a distanceb—cequal to one-half the required taper per foot, after which a parallel stripD, having true sides, is clamped in the toolpost. PartDis then set at right angles to a line passing from one center point to the other. This can be done conveniently by holding a 1-foot square (preferably with a sliding head) against one side ofDand adjusting the latter in the toolpost until edgeEof the square blade is exactly in line with both center points. After partDis set, it should be clamped carefully to prevent changing the position. The angle between the side ofDand an imaginary line which is perpendicular to axisa—bis now equal to one-half the angle of the required taper.
Second Step in Adjusting Tailstock Center by use of SquareFig. 8. Second Step in Adjusting Tailstock Center by use of Square
Second Step in Adjusting Tailstock Center by use of Square
Fig. 8. Second Step in Adjusting Tailstock Center by use of Square
The axis of the part to be turned should be set parallel with lineE, which can be done by setting the cylindrical surface which was previously finished, at right angles to the side ofD. In order to do this the work is first placed between centers, the tailstock being shifted along the bed if necessary; the tail-center is then adjusted laterally until the finished cylindrical surface is square with the side ofD. A small try-square can be used for testing the position of the work, as indicated inFig. 8. If the length of the work is less than 1 foot, it will be necessary to move the center toward the rear of the machine, and if the length is greater than 1 foot, the adjustment is, of course, in the opposite direction.
The Taper Attachment.—Turning tapers by setting over the tailstock center has some objectionable features. When the lathe centers are not in alignment, as when set for taper turning, they bear unevenly in the work centers because the axis of the work is at an angle with them; this causes the work centers to wear unevenly and results in inaccuracy. Furthermore, the adjustment of the tailstock center must be changed when turning duplicate tapers, unless the length of each piece and the depth of the center holes are the same. To overcome these objections, many modern lathes are equipped with a special device for turning tapers, known as a taper attachment,which permits the lathe centers to be kept in alignment, as for cylindrical turning, and enables more accurate work to be done.
A Lathe Taper AttachmentFig. 9. A Lathe Taper Attachment
Fig. 9. A Lathe Taper Attachment
Taper attachments, like lathes, vary some in their construction, but all operate on the same principle. An improved form of taper attachment is illustrated inFigs. 9and10.Fig. 9shows a plan view of a lathe carriage with an attachment fitted to it, andFig. 10a sectional view. This attachment has an armAon which is mounted a slideSthat can be turned about a central pivot by adjusting screwD. The armAis supported by, and is free to slide on, a bracketB(see also sectional view) that is fastened to the carriage, and on one end of the arm there is a clampCthat is attached to the lathe bed when turning tapers. On the slideSthere is a shoeFthat is connected to barEwhich passes beneath the toolslide. The rear end of the cross-feed screw is connected to this bar, and the latter is clamped to the toolslide when the attachment is in use.
Sectional View of Taper AttachmentFig. 10. Sectional View of Taper Attachment
Fig. 10. Sectional View of Taper Attachment
When a taper is to be turned, the carriage is moved opposite the taper part and clampCis fastened to the bed; this holds armAand slideSstationary so that the carriage, with bracketBand shoeF, can be moved with relation to the slide. If this slideSis set at an angle, as shown, the shoe as it moves along causes the toolslide and tool to move in or out, but if the slide is set parallel to the carriage travel, the toolslide remains stationary. Now if the tool, as it feeds lengthwise of the work, is also gradually moved crosswise, it will turn a taper, and asthis crosswise movement is caused by the angularity of slideS, different tapers are obtained by setting the slide to different positions.
By means of a graduated scaleGat the end of slideS, the taper that will be obtained for any angular position of the slide is shown. On some attachments there are two sets of graduations, one giving the taper in inches per foot and the other in degrees. While tapers are ordinarily given in inches per foot on drawings, sometimes the taper is given in degrees instead. The attachment is set for turning tapers by adjusting slideSuntil pointerpis opposite the division or fractional part of a division representing the taper. The whole divisions on the scale represent taper in inches per foot, and by means of the sub-divisions, the slide can be set for turning fractional parts of an inch per foot. When slideSis properly set, it is clamped to armAby the nutsN. BarEis also clamped to the toolslide by boltH, as previously stated. The attachment is disconnected for straight turning by simply loosening clampCand the boltH.
Lathe with Taper Attachment arranged for Boring Taper Hole in Engine PistonFig. 11. Lathe with Taper Attachment arranged for Boring Taper Hole in Engine Piston
Lathe with Taper Attachment arranged for Boring Taper Hole in Engine Piston
Fig. 11. Lathe with Taper Attachment arranged for Boring Taper Hole in Engine Piston
Application of Taper Attachment.—Practical examples of lathe work, which illustrate the use of the taper attachment, are shown inFigs. 11and12.Fig. 11shows how a taper hole is bored in an engine piston-head, preparatory to reaming. The casting must be held either in a chuckCor on a faceplate if too large for the chuck. The side of the casting (after it has been “chucked”) should run true, and also the circumference, unless the cored hole for the rod is considerably out of center, in which case the work should be shifted to divide the error. The side of the casting for a short space around the hole is faced true with a round nose turning tool, after which the rough-cored hole is bored with an ordinary boring toolt, and then it is finished with a reamer to exactly the right size and taper.
Taper Attachment Set for Turning Taper End of Piston-rodFig. 12. Taper Attachment Set for Turning Taper End of Piston-rod
Taper Attachment Set for Turning Taper End of Piston-rod
Fig. 12. Taper Attachment Set for Turning Taper End of Piston-rod
This particular taper attachment is set to whatever taper is given on the drawing, by loosening nutsNand turning slideSuntil pointerPis opposite that division on the scale which represents the taper. The attachment is then ready, after boltHand nutsNare tightened, and clampCis fastened to the lathe bed. The hole is bored just as though it were straight, andas the carriage advances, the tool is gradually moved inward by the attachment. If the lathe did not have a taper attachment, the taper hole could be bored by using the compound rest.
The hole should be bored slightly less than the finish size to allow for reaming. When a reamer is used in the lathe, the outer end is supported by the tailstock center and should have a deep center-hole. The lathe is run very slowly for reaming and the reamer is fed into the work by feeding out the tailstock spindle. The reamer can be kept from revolving, either by attaching a heavy dog to the end or, if the end is squared, by the use of a wrench long enough to rest against the lathe carriage. A common method is to clamp a dog to the reamer shank, and then place the tool-rest beneath it to prevent rotation. If theshank of a tool is clamped to the toolpost so that the dog rests against it, the reamer will be prevented from slipping off the center as it tends to do; with this arrangement, the carriage is gradually moved along as the tailstock spindle is fed outward. Some reamers are provided with stop-collars which come against the finished side of the casting when the hole has been reamed to size.
After the reaming operation, the casting is removed from the chuck and a taper mandrel is driven into the hole for turning the outside of the piston. This mandrel should run true on its centers, as otherwise the outside surface of the piston will not be true with the bored hole. The driving dog, especially for large work of this kind, should be heavy and stiff, because light flexible clamps or dogs vibrate and frequently cause chattering. For such heavy work it is also preferable to drive at two points on opposite sides of the faceplate, but the driving pins should be carefully adjusted to secure a uniform bearing on both sides.
The foregoing method of machining a piston is one that would ordinarily be followed when using a standard engine lathe, and it would, perhaps, be as economical as any if only one piston were being made; but where such work is done in large quantities, time could be saved by proceeding in a different way. For example, the boring and reaming operation could be performed much faster in a turret lathe, which is a type designed for just such work, but a turret lathe cannot be used for as great a variety of turning operations as a lathe of the regular type. There are also many other classes of work that can be turned more quickly in special types of machines, but as more or less time is required for arranging these special machines and often special tools have to be made, the ordinary lathe is frequently indispensable when only a few parts are needed; in addition, it is better adapted to some turning operations than any other machine.
Fig. 12illustrates how a taper attachment would be used for turning the taper fitting for the crosshead end of an engine piston-rod. Even though this taper corresponds to the taper of the hole in the piston, slideSwould have to be reset to the correspondingdivision on the opposite side of the central zero mark, because the taper of the hole decreased in size during the boring operation, whereas the rod is smallest at the beginning of the cut, so that the tool must move outward rather than inward as it advances. The taper part is turned practically the same as a cylindrical part; that is, the power feed is used and, as the carriage moves along the bed, the tool is gradually moved outward by the taper attachment.
If the rod is being fitted directly to the crosshead (as is usually the case), the approximate size of the small end of the taper could be determined by calipering, the calipers being set to the size of the hole at a distance from the shoulder or face side of the crosshead, equal to the length of the taper fitting on the rod. If the crosshead were bored originally to fit a standard plug gage, the taper on the rod could be turned with reference to this gage, but, whatever the method, the taper should be tested before turning too close to the finished size. The test is made by removing the rod from the lathe and driving it tightlyinto the crosshead. This shows how near the taper is to size, and when the rod is driven out, the bearing marks show whether the taper is exactly right or not. If the rod could be driven in until the shoulder is, say,1/8inch from the crosshead face, it would then be near enough to finish to size by filing. When filing, the lathe is run much faster than for turning, and most of the filing should be done where the bearing marks are the heaviest, to distribute the bearing throughout the length of the taper. Care should be taken when driving the rod in or out, to protect the center-holes in the ends by using a “soft” hammer or holding a piece of soft metal against the driving end.
After the crosshead end is finished, the rod is reversed in the lathe for turning the piston end. The dog is clamped to the finished end, preferably over a piece of sheet copper to prevent the surface from being marred. When turning this end, either the piston reamer or the finished hole in the piston can be calipered. The size and angle of the taper are tested by driving the rod into the piston, and the end should be fitted so that by driving tightly, the shoulder will just come up against the finished face of the piston. When the taper is finished, the attachment is disengaged and a finishing cut is taken over the body of the rod, unless it is to be finished by grinding, which is the modern and most economical method.