Design of head-stock for wooden bed lathes. Early design for use on a cast iron bed. An old New Haven head-stock. The arch form of the bottom plate. Providing for reversing gears. The Hendey-Norton head-stock. The Schumacher & Boye head-stock. The Le Blond head-stock. The New Haven head-stock. The arch tie brace of the new Hendey-Norton design. Generalities in describing a lathe spindle. Designing a spindle. Governing conditions. The nose of the spindle. Spindle collars. Proper proportions for lathe spindles. Large versus long bearings. Design of the spindle cone.
THE subject of lathe design is continued by the consideration of the design and construction of the head-stock, which in some respects is the most important part, and with it and the parts which go to make up the complete head-stock, the most important group of parts in the lathe.
In the earlier form of lathes this piece was, like most of the other parts, simple and crude in design as well as in the workmanship bestowed upon it. It generally consisted of a base and the two upright ends in which provision was made to receive the boxes, and when wooden beds were thought sufficient for a lathe a strip was added beneath that filled the space between the two timbers forming the bed. Such a design for a head-stock is shown in Fig. The Hendey-Norton Form of Cabinet for Large Lathes, which is taken from an old lathe that did many years’ service in a general repair shop. It will be noticed that the housing for the spindle boxes do not have square edges’, but are of V-shaped form. They were finished with a file only and the boxes made of cast iron, filed to a fit and lined with babbitt metal which was said to have been poured around the lathe spindle after it was finished, set in place, and lined up as well as might be with the crude appliances at hand. The top portion of the boxes were held down by a straight bar cap with two holes which fitted over fixed threaded studs that had been cast into the head-stock for this purpose.
The lathe was devoid of a back gear and the spindle carried a three-step cone, the largest part of which was as large as was possible to get into the head, and a belt quite wide, considering the power then thought necessary to drive a lathe carrying the diminutive chip which was considered proper for a lathe to take at the time this lathe was in use.
Later on, when the cast iron bed was adopted and when back gears were added to the lathe, the requirements of additional strength were recognized and not only the base plate, but the up-rights or housings at the front and rear end, were made thicker and heavier. One of these head-stocks is shown in Fig. Early Form of Head-Stock for Wooden Lathe Bed, which gives a good general idea of the form of the casting and shows also a strengthening brace A. While it would seem at first thought more necessary to brace the housing of the front box than that carrying the rear journal, it should be remembered that the latter must withstand the strain of the “thrust” or end wise pressure of the spindle due to holding work upon centers, and the pressure of drilling work, one end of which is held in a chuck and the other in a center rest, and similar kinds of work. While in the modern lathes this thrust device is usually a part of the rear box, the earlier method was to fix two studs in the rear of the head-stock, one in each side of the rear box and on a horizontal line with it, and across these to fix a strong bar carrying an adjustable thrust screw for taking the end thrust of the spindle. The details and design of this important device will be taken up further on.
In Fig. A Later Form of Head-Stock with Back Gears and a Strengthening Brace is shown a peculiar form of head-stock upon an old lathe in one of the older shops in New Haven, Conn. The lathe was broken up for old iron after an indefinite period of idleness. It was of about 16-inch swing and the various members of the headstock were about one and one-half inches square. The bed of the lathe, and the legs which supported it, were of cast iron and very much like those shown in Fig. Early Form of Cast Iron Legs with Braces. The head was provided with back gears of very light design and the lathe had a lead screw and feed-rod adapting it for thread cutting. It was undoubtedly considered a proper engine lathe “in its day.”
The next form of head-stock which followed that shown in Fig. Early Form of Head-Stock for Wooden Lathe Bed seems to have been of the form shown in Fig. Form of Head-Stock on Old Lathe found in New Haven, Conn. In this case the base of the casting was raised in arch-like form and the under side recessed to the same form so as to maintain an equal thickness of metal throughout. This form seems to have been a favorite one and many lathes were built by various makers with substantially this form, the variations from it not being of sufficient importance to justify a further classification.
As yet the housings had not been made thick enough to suggest coring them out in order to save iron or for the purpose of avoiding unequal contraction of the metal upon cooling after casting, by making all members of the casting of as nearly an equal thickness as possible. Of late years these points have received much attention and study by the designers of machine tools, and rightly so, as their importance was to a large extent overlooked in the earlier designs, the reason probably being that all castings were made so much lighter and had much less strain to withstand in the regular service to which the machine was put.
In Fig. One of the Older Favorite Forms of Head-Stock is shown a modification of the arch form shown in Fig. Form of Head-Stock on Old Lathe found in New Haven, Conn, which has for its purpose the strengthening obtained by the rib A in Fig. Early Form of Head-Stock for Wooden Lathe Bed, only in a better form, as the method is “cored out,” or formed with a “green sand core” under the head-stock, so as to provide for an equal thickness of metal over the entire base. This raised portion could be introduced quite conveniently as the small end of the spindle cone was located over it, thus insuring ample space for building it up.
In the examples thus far shown of lathe heads the feed gears were located outside the housings, except in the case of that shown in Fig. A Later Form of Head-Stock with Back Gears and a Strengthening Brace. As the change came to be made of locating ” tumbler gears,” or reversing gears, inside of the housing, it naturally followed that the metal of the head-stock base must be cut away under that part of the main spindle upon which was fixed the spindle gear or feed gear from which the feed mechanism was driven. This was the case for perhaps fifty years, and at the present time, now that reversing devices are constructed as a part of the apron mechanism, the feed gears may be placed outside of the housing, although some good builders still keep it inside and connected in practically “the same old way” even if the “yoke gears” or reversing gears are omitted.
When reversing gears were thought necessary to be upon the inside of the housing, a hole was cut out for them in the raised arch A, Fig. One of the Older Favorite Forms of Head-Stock, and this practice was followed in any head-stock having this or a similar obstruction to these gears, and provided, of course, that they were to be located inside of the rear housing.
One of the recent modifications of the above form is that shown in Fig. Another Form of Strengthening Brace, which is a type of the Hendey-Norton manufacture. The central figure is a front elevation with the sectional form indicated by dotted lines. The figure at the left is a rear end elevation with the internal form on the line A, A, of the central figure, while the figure on the right is a similar elevation of the front end with dotted lines showing the section on the line B, B.
It will be seen that the portion of the base on the line A, A, is of arched form, somewhat as shown at A, Fig. One of the Older Favorite Forms of Head-Stock, while the form at the line B, B, is of an inverted arch, or as frequently called by the shop men a “pig trough” shape. This latter form enables the metal to be carried higher up at the front and back while the center is depressed to give proper clearance for the larger steps of the cone and the face gear. At the lowest part of this depression there is usually an opening through which oil may drip so as not to collect inconveniently at this point. The arch-like form near the rear housing adds very much to the strength and rigidity of the casting. It will be noticed that in this design the main spindle boxes are not “capped in,” that is, held down by removable caps. More will be said of this peculiarity in describing boxes and spindles.
The cores beneath the base are carried up into the housings in many of the modern head-stocks as far as possible, and still leave ample support for the boxes and spindles. The advisability of this method of lightening the weight of the casting is still an open question among machine tool designers who have endeavored to avoid unequal strains in the shrinkage of castings by making all members of as nearly equal thickness as possible. Sometimes this idea is carried too far and the result is liable to be that of sacrificing the necessary rigidity to prevent vibration, in the effort to follow out the ideal as to strains.
Fig. The Hendey-Norton Form of Head-Stock shows a head-stock in which the inverted arch form is continued the entire length between the housings, but is carried upon a curved line as shown and forms a very graceful curve. The three figures are arranged the same as those comprising Fig. 58. The height of the curve might be greater at the line A, A, as will be shown in some others further on in this chapter, and the strength of the casting considerably increased.
This form is used with few modifications to adapt it to the diameters of driving-cones, the nature of the back gears and the feed gears and similar conditions that tend to somewhat alter the construction outlines of its design. This form seems to be a favorite one with designers, since among all the different builders and the variety of designs there are more builders using this form than all the others put together.
While the above form of design carries a reversed curve for the top of the base, the form used by Shumacher and Boye, shown in Fig. Form of Head-Stock built by a Majority of Lathe Builders, is of a single curve from rear to front housing and the inverted arch in its transverse sectional form. In this design the front and back is carried high up near the rear housing and comparatively low down near the front housing.
This is ‘a design of much strength and rigidity in proportion to the weight of the casting, the metal being well distributed to resist heavy strains in the operation of the lathe.
The Le Blond type is shown in Fig. The Schumacher & Boyce Form of Head-Stock. In this we have a straight line at the back and front, with a modification of the reversed curve and the combination of the arch proper and the inverted arch as shown in Fig. Another Form of Strengthening Brace. The form is pleasing to the eye, and the strength of the casting is quite sufficient for the requirements. In this case the housings are made of ample width, especially the front one. They are cored out inside so as to have substantially an equal thickness of metal at nearly all parts. The New Haven type of head-stock is shown in Fig. The Le Blond Form of Head-Stock. In this case the inverted arch is used all the way through, but it is upon straight lines, that form a cross section at A, A, continuing straight and on a proper incline to a point near the line B, B, from whence it is horizontal.
This design gives great strength, and with the proper proportions and thickness of metal throughout it is as rigid as it is possible to design a head-stock. The housings are unusually thick and cored out underneath as shown by dotted lines.
The design shown in Fig. The New Haven Form of Head-Stock is by Hendey-Norton, and is practically the same as that shown in Fig. Another Form of Strengthening Brace, except for the arched brace C, from the front to the rear housing, effectually tying them together and thus adding considerably to the rigidity of the spindle-bearing boxes, which is always an excellent point to be considered.
The fact that these housings are solid, that is, not held by separate caps, permits the addition of this very strong brace, which could not be efficiently added to a head-stock whose boxes are held in by a separate cap.
While this idea is now quite common in the design of milling machines, it has not been applied to the head-stocks of lathes by any builders but these so far as is known.
There are many classes of work in which a head-stock so braced would be very valuable, as its strength and rigidity is much increased by it and the strain and vibration is considerably reduced, which has the effect of increasing the efficiency and also the life of the cutting-tools. This question of increased rigidity and the importance of obtaining it has received much attention in the past few years, and the result has been the constant increase in the proportions and the weights of all parts of metal-working machinery which form the supports of cutting-tools or their intermediary parts. It is altogether probable that in this increase in weight the limit has not been reached, but that it will continue in years to come, although not perhaps in the same proportion that it has during the last decade. The use of high-speed steel will, doubtless, be extended to other uses than at present, and its price will be materially reduced, thus increasing the amount used and consequently demanding stronger machines and more power to drive them, so as to continually reduce the cost of the product by reducing the time of machine operations.
Having designed a good head-stock with ample proportions in general, the metal so distributed as to withstand not only the strains to which it will be subjected in performing its appointed functions, but with proper considerations for the changes which will take place in the process of casting and cooling, and not forgetting that castings will change their form more or less for weeks after being cast, our next concern will be the spindle.
It is not enough to say, as catalogues sometimes do, that “the spindle is of hammered crucible steel of large diameter and runs in hard bronze boxes.” This may all be relatively true and yet it may be neither properly designed or properly constructed for the uses to which it is to be put.
To design a lathe spindle we must consider the work it has to do, the points at which it will be supported, the points where it must support the material that is to be machined, and the parts with which it is loaded and which become a part of its attendant mechanism ; not only these points, but others that are equally important, the torsional strains to which it will be subjected in performing its regular functions, and which include that of driving the piece to be turned, of the strains of the cone when driving direct, or the back gears or triple gears when they are in action, and of the feeding mechanism which derives its motion from the rear end of the spindle.
If we are to consider principally the weight of the face-plate and the material to be turned, which falls almost entirely upon the front journal, we should have the form of the lathe spindle as represented in Fig. Lathe Spindle showing Principal Weight on Front Bearing. In this case the front bearing would necessarily be very large and strong and with ample support. The rear bearing need not be a matter of serious consideration, as it is quite a distance from the front bearing, while the weight of the face-plate or chuck carrying the work, or the center which supports one end of the work, if supported by this means, carries nearly all the strain. Therefore the rear bearing may be small and short as shown.
Again, if the weight of the cone and its parts are to be principally considered, we should have a spindle more nearly conforming to the outline shown in Fig. Form of Lathe Spindle when undue prominence is given to Cone Pulley, the rear bearing being larger and the front bearing smaller than is shown in Fig. Form of Lathe Spindle when undue prominence is given to Cone Pulley. This would also be the case if the upward pull of the belt were a governing factor in determining the form and proportion of the spindle. But the fact is that the cone and its action upon the spindle, so far as its weight or the belt pull upon the spindle, while in reality a factor to be considered, as will be referred to later on, is not the prime factor by any means. Therefore we must recur to the form shown in Fig. Special Form of Hendey-Norton Head-Stock for the points necessary for the proper consideration of forms, the determination of the contour, and the proper proportions of the lathe spindle.
This view of the case leads us to the choice of a medium between the two extremes presented and an ideal form as shown in Fig. Ideal Form of Lathe Spindle, wherein the conditions governing both the former examples are properly considered and met.
There is one more condition to be considered, however. This is the upward or lifting tendency supposed to exist by reason of the cutting-tool forming a fulcrum, which, in connection with the circular motion of the piece being turned, tends to lift the spindle in the front box and so throws an upward strain on the cap over the front journal. This tendency is represented in Fig. The Ideal Spindle shown in Practical Form, wherein the arrow shows the direction of the belt and revolution of the material being turned. It is doubtful, however, if this point is of much importance, particularly in a lathe properly designed as to the dimensions and weights of its parts, especially of the spindle and its appendages.
Taking all these matters into consideration we shall find that the proper proportion and design of the spindle with the face gear, cone pinion, and the feed gear, will be substantially as shown in Fig. The Ideal Spindle shown in Practical Form, leaving out of the design for the time being the special form of journal oiling devices, the thrust bearing for the rear end and the special form of the nose of the spindle, which will next receive attention.
As to the nose of the spindle. It is customary by many builders to cut the thread on the nose of the spindle nearly up to the collar, against which the chuck-plate or the face-plate takes its bearing. It is a well-known fact that it is extremely difficult to accurately center such a plate upon a threaded portion of a spindle. As the purpose of the thread is simply to prevent the plate from coming off the spindle, it naturally follows that the length of this thread may be very much reduced without in any way reducing its capacity to securely hold the plate in place. It is also quite as evident that we can hold the plate perfectly true in its place and exactly concentric with the front bearing if we grind a portion of the nose of the spindle to a truly cylindrical form when we grind the front bearing and then fit a sufficient portion of the bore in the plate to this round surface. This may be accomplished by threading the nose of the spindle through only one third of its length, and grinding the remaining two thirds to which the chuck-plate or face-plate is fitted. This centers the plate accurately with the axis of the spindle. If the face of the collar is accurately ground, and the hub of the chuck-plate or face-plate fits fairly against it, there will be no difficulty when removing the plate of always being able to replace it in exactly its former position, perfectly true in the running of its face and perfectly concentric with the ground bearings of the spindle. Even the wearing of the thread will not effect its true running, since the only office of the thread is to hold it on, while the ground surfaces insure its trueness. This is shown in Fig. Nose of sPindle.
In this connection it is noticeable that some manufactures omit the large collar on the front end of the spindle and furnish only a small shoulder on the spindle, clue to the nose being some-what smaller than the front bearing, against which the face-plate or chuck-plate rests, and assuming that its close fit upon the ground surface between this shoulder and the threaded portion will be quite sufficient for all purposes. This would seem to be an erroneous view of the question as this comparatively small shoulder cannot possibly afford the support and rigidity that may be obtained by a collar or thrust surface of two or three times the area. It is true that as a matter of economy in furnishing the stock for these spindles the question favors the omission of the shoulder. But as a matter of good design and proper shop practice it will hardly be disputed that the larger collar, forged on, is the proper design and construction.
Referring again to Fig. The Ideal Spindle shown in Practical Form, there are several points to which it is proper to call attention. The spindle boxes represented are of bronze and such as are now commonly used in good lathes. The formation of the front end of the spindle with its fixed collar formed in the forging is also the usual practice, except in some of the lathes of newest design and development, in which it is probably omitted as being considered an unnecessary expense. The thrust bearing is similar to that represented in Fig. Lodge & Shipley Thrust Bearing, but an improvement upon it, since a hardened steel ring is interposed between two bronze rings, which render cutting well-nigh impossible.
The cone pinion is made of machine steel and has a long sleeve forced into the small end of the spindle cone. While it is not good practice to run two steel surfaces together unless one is hardened, it is still perfectly practicable in this case as the pinion is of ordinary soft machine steel while the spindle is 50 to 60-point carbon crucible steel, which answers the conditions in practice and many lathes are now built in this manner.
The spindle is shown bored out, as a large majority of lathes are now so constructed and the demands of the customers require hollow spindles in nearly every instance when the lathe is over 12-inch swing.
The proportions upon which this design is made may be interesting. Using the full swing of the lathe in inches as a unit, represented by A, the proportions of the spindle will be as follows :
Diameter of the front bearing, A ÷ 5.7″
Length of the front bearing, A H ÷ 3.6″
Diameter of the rear bearing, A ÷ 6″
Length of the rear bearing, A ÷ 4.5″
Length of the nose of the spindle, A ÷ 6″
Distance between bearings, A X 1.2″
Diameter of bore through spindle, A ÷ 10″
In Fig. Lathe Spindle with Extra Large Bearings we have a spindle of somewhat overgrown proportions, yet one of proportions advocated by an eminently practical mechanic who is said to have remarked that he “didn’t want a lathe spindle with a front bearing so many inches diameter and so many inches long, but he wanted it with a bearing so many inches large and so many inches short,” by which we may readily understand his idea that a large and short front bearing was much better adapted to the work than one of medium diameter and extra length.
Thus if we have a front bearing of 3¼ inches diameter and 5 inches long, and we increase the diameter 50 per cent and reduce the length in the same proportion, viz., one third, we shall have about the same area of bearing surface, but we shall gain the advantage of bringing the driving-cone closer to the work, of shortening the whole length of the spindle, and of making the front end of the spindle much more rigid and better adapted to withstand the strain of a heavy cut on work of the usual diameters, and still better when large facing work is to be done and the cut is carried out near the periphery of the largest diameter that can be handled.
It does not follow, however, that the proportions of the enlarged diameter of the front bearing need be carried all the way through, by which a spindle of unnecessary weight would be produced, as practically all important advantages may be gained if its dimensions are as shown in dotted lines in the engraving.
In Fig. Lathe Spindle with Extra Long Bearings is shown the opposite method of designing a lathe spindle, that is, by making the bearings of the usual diameter, but increasing the length to a considerable extent. It is evident that while there are always certain advantages in increasing the distance between the supporting boxes, there is an apparent tendency to weakness, or lack of rigidity of the spindle at the vital point, namely, the overhanging portion of the front end of the spindle which supports the face-plate, the chuck, or the work as it bears upon the lathe center.
As between the two designs of extra large bearings and extra long bearings, the practical advantages seem to be in favor of the former.
The spindle cone should receive due attention. The method of introducing the cone gear sleeve into the small end of the cone has been referred to in connection with Fig. The Ideal Spindle shown in Practical Form. The large end of the cone may have an inwardly projecting ﬂange cast integral with it or made separate and attached by screws. In either case the locking bolt must be accommodated in it. Between this head and its bearing it should be well supported from the central quill. This may be done by providing for four or more radial plates extending from the connection with the central quill under the smallest step to one half the remaining distance toward the large end of the cone, as shown in Fig. The Ideal Spindle shown in Practical Form.
In finishing the outside of the cone the rising steps should be faced up as shown in Fig. Form of Cone Steps, that is, with the face cut back from 1/32 to 1/16 of an inch, according to the size of the cone, for the purpose of lessening the friction on the edge of the belt. In cases where this relief is not given to the belt it is not an unusual condition to find the edges of belts running over cones, particularly at high speeds, to be turned up, the corners where the belt is joined to be distorted or worn away, and in a short time the belt well-nigh ruined.
In purchasing lathes or other machines provided with speed cones, the purchaser should insist that the faces of the cones should be made as shown, as it is a matter of much importance in belt economy and belt efficiency.