Tips and tricks
Deep hole drilling
DEEP HOLE DRILLING
MACHINERY'S REFERENCE SERIES
The Industrial Press, New York City, 1910
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CONTENTS
- Introduction
- Principles of Deep Hole Drilling,
- Deep Hole Drilling in Gun Construction,
- Construction of Deep Hole Drills,
INTRODUCTION
The difficulties to be overcome in producing deep drilled holes can be classified in three groups. In the first place the drill has a great tendency to "run out," thus producing a hole that is neither straight, nor uniform in diameter; in the second place great difficulties are encountered in trying to remove the chips in a satisfactory manner, and in the third place the heating of the cutting tool is difficult to prevent.
The principle involved in common drill presses where the drill is given a rotary motion simultaneously with the forward motion for feeding is the one least adapted to produce a straight and true hole. Better results are obtained by giving only a rotary motion to the drill, and feeding the work toward it. It has been found, however, that for drilling deep holes the reversal of this, that is, imparting a rotary motion to the work, and the feed motion to the drill will answer the purpose still better. It seems as if there could be no material difference between the two latter methods. An analysis of the conditions involved will show, however, that there is a decided difference in the action of the drill. If the drill rotates, and the work is fed forward as shown to the left in Fig. 1, the drill, when deviating from its true course, will be caused to increase its deviation still more, by the wedge action of the part B, which tends to move in the direction BA when the work is fed forward. In the case of the work rotating and the drill being fed forward, the point of the drill when not running true will be carried around by the work in a circle with the radius a, thus tending to bend the drill in various directions. The drill is by this action forced back into the course of "least resistance," as it is evident that the bending action, being exerted on the drill in all directions, will tend to carry the point back to the axis of the work where no bending action will appear. The chips, as is well known, are carried off by forcing a fluid into the hole, which upon its return carries with it the chips. This fluid being oil will serve the double purpose of carrying away the chips and lubricating the cutting tool, keeping it at a normal temperature.
In the following chapters we shall deal with the practice of deep hole drilling as met with in a number of prominent American shops, presenting at the same time a collection of useful data covering different classes of work. The relation between ordinary drilling and deep hole drilling, dealing with first and fundamental principles, is treated in Chapter I, followed by a detailed account of the practice of deep hole drilling at the Pratt & Whitney Works, Hartford, Conn. In Chapter II the boring of large guns, according to the practice employed at the Watervliet Arsenal, is described. Chapter III is devoted to illustrating and describing various constructions of deep hole drills of merit, together with hints regarding their making, thereby completing the treatise.
CHAPTER I
PRINCIPLES OF DEEP HOLE DRILLING
The process of drilling deep holes in metal is a familiar one in many shops, particularly where firearms are manufactured, or heavy ordnance is constructed. Since the adoption of hollow spindles for lathes and other machine tools, the methods for machining the bores of guns have been employed in machine tool shops for drilling these spindles; and through this and other means the principles of the operation have become better understood. It is not an easy matter, however, even with the best appliances, to drill or bore a deep hole smooth and round, of exactly the required diameter from end to end, and perfectly straight. While many mechanics are familiar in a general way with the methods and tools for doing this work, specific information upon the subject will be appreciated by those who have not had actual experience in deep hole drilling.
It is well known that a long, or deep, hole that is, one long in proportion to its diameter is best roughed out and finished by using a tool on the end of a long bar which enters the work from one end. This is true, whether drilling into solid metal, or boring and reaming a hole that has already been drilled or bored out. A boring bar which extends through the piece, and on which is either a stationary or a traveling head, is not satisfactory for very long work, owing to the spring and deflection of the bar, which is made worse by the fact that the bar must be enough smaller than the bore to allow room for the cutter head. While a long hole may sometimes be finished satisfactorily by means of such a boring bar, by packing the cutter head with wooden blocks which just fill the part of the bore that has been machined, and so support the bar, the method is fundamentally incorrect for long work.
The best methods for machining deep holes are nothing more nor less than an adaptation of what has been found successful in ordinary drilling and boring in the engine lathe or chucking machine. We will therefore first discuss certain types of chucking tools and drills, and show their relationship to tools that may be used for deep hole drilling.
The Flat Drill
To start with first principles, consider the ordinary flat drill. It is useful for rough work or in drilling hard metals, because it can be easily made and tempered; but it has too much of an inclination for drilling holes that are neither round nor straight, and whose diameter seems to bear no relation to the diameter of the drill. When a flat drill runs into a blow hole or strikes a hard spot, it is deflected, as in Fig. 2, the only resistance to this deflection being the narrow edges of the drill. Under such conditions the hole will be out of round, and crooked. Add to this natural tendency of a flat drill to run out the fact that such drills are often carelessly made, and one understands why they have a reputation for poor work. Thus, if the point is not in line with the axis of the drill, and if the lips are of unequal length, or do not make equal angles with the axis, the hole will be larger than the drill diameter. This is illustrated in Pig. 3, where one lip is longer than the other, and the point does not lie In the central axis of the drill. The tendency of the drill is to rotate about its point, and thus the axis will move in a small circle about this point, causing the hole to be of larger diameter than the drill.
It is obvious that to improve the action of a flat drill, it should be so guided as to prevent its wabbling and to compel it to move forward in a straight line. This is partially accomplished with the flat chucking drill, which is a near relative of the ordinary flat drill, differing from it in that it is generally more carefully made and is adapted for use in the engine lathe. In Fig. 4 is an illustration of a chucking drill at work on a piece in the lathe, and to make the comparison fair it is shown with one lip longer than the other, as was the flat drill in Fig. 3. The work is held in the lathe chuck and turns with the spindle. A rest steadies the drill at a point near the work, and in starting the hole, the drill is held firmly against the rest by means of a monkey wrench. It will be noted that while a poorly ground chucking drill will make a large hole, just as does the drill in Fig. 3, if properly started it will not wabble, and it will drill the hole where it is wanted and approximately in line with the lathe centers. To attain these results, however, the drill must be started right. If it is found to wabble when left free, it must be started over again, before the full size of the hole has been attained, by crowding it into the work and toward the operator at the same time, causing only one edge of the drill to do the cutting. This edge will then true up the hole, and in proceeding with the drilling the trued hole will guide the drill. The latter will thus be continuously sup- ported near the cutting edges by the cylindrical surface of the hole, and the drill will tend to advance in the direction in which it was started. After the hole is drilled, it is usually brought to size by a flat reamer, like Fig. 5. For reasons that will be explained, the flat drill is not an accurate tool, even when well made and used in the lathe, and the flat reamer is not as reliable as one with more blades.
The general principle, however, of first starting with a true hole, and then having the drill body designed to follow in its path and so guide the cutting edges, is the fundamental principle of deep hole drilling.
Fig. 6 shows how a flat drill may be adapted for deep hole drilling. The drill from which the illustration was made was employed for drilling a four-inch hole through steel rolls seven feet long. Instead of depending upon the narrow edges of the drill proper to guide and support the cutting edges, the cutting edges are formed on a blade inserted in a cylindrical cast-iron head, the outside diameter of which is turned to a sliding fit in the hole that is being bored. The cutting edges are grooved to break up the chips, enabling the latter to pass out through the passage E, on each side of the head. The grooves are laid out so that those in one blade come opposite to the lands in the other blade. In the illustration, A is the cutter, B one of the screws holding the cutter to the head C, and the head is attached to the bar by the shank D.
The Twist Drill
The modern twist drill accomplishes all that is attained by the arrangement in Fig. 6, and in addition can be ground without seriously affecting the rake, and will free itself from chips more readily, owing to its spiral flutes. The lands of a twist drill present a large cylindrical surface to bear against the sides of the hole and take the side thrust. If the drill is also guided by a hardened bushing, at the point where it enters the metal, as in the case of jig work, the drill will have very little chance to deflect, and the hole will be accurately located and will be quite true and straight.
The twist drill in a modified form is also employed for deep hole drilling. The hollow drill introduced by the Morse Twist Drill Co., New Bedford. Mass., is adapted for this purpose, and in Fig. 7 is shown the arrangement recommended by this company for feeding this drill into the work. The drill has a hole lengthwise through the shank, connecting with the grooves in the drill, as indicated. The shank can be threaded and fitted to a metal tube which acts as a boring bar and through which the chips and oil may pass from the point of the drill. Oil is conveyed to the point on the outside of the tube, as shown in Fig. 7.
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The difficulties to be overcome in producing deep drilled holes can be classified in three groups. In the first place the drill has a great tendency to "run out," thus producing a hole that is neither straight, nor uniform in diameter; in the second place great difficulties are encountered in trying to remove the chips in a satisfactory manner, and in the third place the heating of the cutting tool is difficult to prevent.
The principle involved in common drill presses where the drill is given a rotary motion simultaneously with the forward motion for feeding is the one least adapted to produce a straight and true hole. Better results are obtained by giving only a rotary motion to the drill, and feeding the work toward it. It has been found, however, that for drilling deep holes the reversal of this, that is, imparting a rotary motion to the work, and the feed motion to the drill will answer the purpose still better. It seems as if there could be no material difference between the two latter methods. An analysis of the conditions involved will show, however, that there is a decided difference in the action of the drill. If the drill rotates, and the work is fed forward as shown to the left in Fig. 1, the drill, when deviating from its true course, will be caused to increase its deviation still more, by the wedge action of the part B, which tends to move in the direction BA when the work is fed forward. In the case of the work rotating and the drill being fed forward, the point of the drill when not running true will be carried around by the work in a circle with the radius a, thus tending to bend the drill in various directions. The drill is by this action forced back into the course of "least resistance," as it is evident that the bending action, being exerted on the drill in all directions, will tend to carry the point back to the axis of the work where no bending action will appear. The chips, as is well known, are carried off by forcing a fluid into the hole, which upon its return carries with it the chips. This fluid being oil will serve the double purpose of carrying away the chips and lubricating the cutting tool, keeping it at a normal temperature.
In the following chapters we shall deal with the practice of deep hole drilling as met with in a number of prominent American shops, presenting at the same time a collection of useful data covering different classes of work. The relation between ordinary drilling and deep hole drilling, dealing with first and fundamental principles, is treated in Chapter I, followed by a detailed account of the practice of deep hole drilling at the Pratt & Whitney Works, Hartford, Conn. In Chapter II the boring of large guns, according to the practice employed at the Watervliet Arsenal, is described. Chapter III is devoted to illustrating and describing various constructions of deep hole drills of merit, together with hints regarding their making, thereby completing the treatise.
CHAPTER I
PRINCIPLES OF DEEP HOLE DRILLING
The process of drilling deep holes in metal is a familiar one in many shops, particularly where firearms are manufactured, or heavy ordnance is constructed. Since the adoption of hollow spindles for lathes and other machine tools, the methods for machining the bores of guns have been employed in machine tool shops for drilling these spindles; and through this and other means the principles of the operation have become better understood. It is not an easy matter, however, even with the best appliances, to drill or bore a deep hole smooth and round, of exactly the required diameter from end to end, and perfectly straight. While many mechanics are familiar in a general way with the methods and tools for doing this work, specific information upon the subject will be appreciated by those who have not had actual experience in deep hole drilling.
It is well known that a long, or deep, hole that is, one long in proportion to its diameter is best roughed out and finished by using a tool on the end of a long bar which enters the work from one end. This is true, whether drilling into solid metal, or boring and reaming a hole that has already been drilled or bored out. A boring bar which extends through the piece, and on which is either a stationary or a traveling head, is not satisfactory for very long work, owing to the spring and deflection of the bar, which is made worse by the fact that the bar must be enough smaller than the bore to allow room for the cutter head. While a long hole may sometimes be finished satisfactorily by means of such a boring bar, by packing the cutter head with wooden blocks which just fill the part of the bore that has been machined, and so support the bar, the method is fundamentally incorrect for long work.
The best methods for machining deep holes are nothing more nor less than an adaptation of what has been found successful in ordinary drilling and boring in the engine lathe or chucking machine. We will therefore first discuss certain types of chucking tools and drills, and show their relationship to tools that may be used for deep hole drilling.
The Flat Drill
To start with first principles, consider the ordinary flat drill. It is useful for rough work or in drilling hard metals, because it can be easily made and tempered; but it has too much of an inclination for drilling holes that are neither round nor straight, and whose diameter seems to bear no relation to the diameter of the drill. When a flat drill runs into a blow hole or strikes a hard spot, it is deflected, as in Fig. 2, the only resistance to this deflection being the narrow edges of the drill. Under such conditions the hole will be out of round, and crooked. Add to this natural tendency of a flat drill to run out the fact that such drills are often carelessly made, and one understands why they have a reputation for poor work. Thus, if the point is not in line with the axis of the drill, and if the lips are of unequal length, or do not make equal angles with the axis, the hole will be larger than the drill diameter. This is illustrated in Pig. 3, where one lip is longer than the other, and the point does not lie In the central axis of the drill. The tendency of the drill is to rotate about its point, and thus the axis will move in a small circle about this point, causing the hole to be of larger diameter than the drill.
It is obvious that to improve the action of a flat drill, it should be so guided as to prevent its wabbling and to compel it to move forward in a straight line. This is partially accomplished with the flat chucking drill, which is a near relative of the ordinary flat drill, differing from it in that it is generally more carefully made and is adapted for use in the engine lathe. In Fig. 4 is an illustration of a chucking drill at work on a piece in the lathe, and to make the comparison fair it is shown with one lip longer than the other, as was the flat drill in Fig. 3. The work is held in the lathe chuck and turns with the spindle. A rest steadies the drill at a point near the work, and in starting the hole, the drill is held firmly against the rest by means of a monkey wrench. It will be noted that while a poorly ground chucking drill will make a large hole, just as does the drill in Fig. 3, if properly started it will not wabble, and it will drill the hole where it is wanted and approximately in line with the lathe centers. To attain these results, however, the drill must be started right. If it is found to wabble when left free, it must be started over again, before the full size of the hole has been attained, by crowding it into the work and toward the operator at the same time, causing only one edge of the drill to do the cutting. This edge will then true up the hole, and in proceeding with the drilling the trued hole will guide the drill. The latter will thus be continuously sup- ported near the cutting edges by the cylindrical surface of the hole, and the drill will tend to advance in the direction in which it was started. After the hole is drilled, it is usually brought to size by a flat reamer, like Fig. 5. For reasons that will be explained, the flat drill is not an accurate tool, even when well made and used in the lathe, and the flat reamer is not as reliable as one with more blades.
The general principle, however, of first starting with a true hole, and then having the drill body designed to follow in its path and so guide the cutting edges, is the fundamental principle of deep hole drilling.
Fig. 6 shows how a flat drill may be adapted for deep hole drilling. The drill from which the illustration was made was employed for drilling a four-inch hole through steel rolls seven feet long. Instead of depending upon the narrow edges of the drill proper to guide and support the cutting edges, the cutting edges are formed on a blade inserted in a cylindrical cast-iron head, the outside diameter of which is turned to a sliding fit in the hole that is being bored. The cutting edges are grooved to break up the chips, enabling the latter to pass out through the passage E, on each side of the head. The grooves are laid out so that those in one blade come opposite to the lands in the other blade. In the illustration, A is the cutter, B one of the screws holding the cutter to the head C, and the head is attached to the bar by the shank D.
The Twist Drill
The modern twist drill accomplishes all that is attained by the arrangement in Fig. 6, and in addition can be ground without seriously affecting the rake, and will free itself from chips more readily, owing to its spiral flutes. The lands of a twist drill present a large cylindrical surface to bear against the sides of the hole and take the side thrust. If the drill is also guided by a hardened bushing, at the point where it enters the metal, as in the case of jig work, the drill will have very little chance to deflect, and the hole will be accurately located and will be quite true and straight.
The twist drill in a modified form is also employed for deep hole drilling. The hollow drill introduced by the Morse Twist Drill Co., New Bedford. Mass., is adapted for this purpose, and in Fig. 7 is shown the arrangement recommended by this company for feeding this drill into the work. The drill has a hole lengthwise through the shank, connecting with the grooves in the drill, as indicated. The shank can be threaded and fitted to a metal tube which acts as a boring bar and through which the chips and oil may pass from the point of the drill. Oil is conveyed to the point on the outside of the tube, as shown in Fig. 7.
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