This section is from the book "Lathe Design, Construction And Operation, With Practical Examples Of The Lathe Work", by Oscar E. Perrigo. Also available from Amazon: Lathe Design: Construction And Operation.
With these preliminary statements relative to the conditions and requirements of good and successful designing, we may take up the designing of lathes somewhat in detail and inquire into the design of the individual parts and groups of parts, giving some of the ideas of men prominent in this field, and adding such comments and suggestions as seem proper and pertinent to the case as the matter is proceeded with.
In carrying out this plan it will be natural to commence with the bed, considering its use and purpose, and the proper form to fulfil the requirements of this particular part.
The lathe bed, considered in an elementary way, and in the case of a lathe of moderate length, may be taken as a beam, supported at each end and in its turn supporting at one end the head-stock, at the other end the tail-stock, and in the center the carriage, as represented in Fig. 25.
Fig. 25. - Elementary Form of Lathe Bed.
This being the problem, and as the head-stock and the tail-stock stand directly over the legs or supports, we might consider the problem as that of a beam loaded at the center, which would naturally suggest that the under side of the bed instead of being straight should be a parabolic curve. This would result in the form shown in Fig. 26, which would, if the carriage was stationary, conform to the conditions of the problem. But, while the carriage is not stationary, it is located at what would normally be the weakest point along the length of the bed, namely, a point farthest from either support. So far the parabolic curve, then, is correct. But while we have been placing our supports at the extreme end of the bed we have no condition of the case which makes it incumbent upon us to do so. In other words, we may add a portion to each end of the bed, outside of, or beyond the line of these supports, in the form shown in Fig. 27, showing a modified form of the lathe bed, the extensions at each end being in the form of a beam supported at one end. Theoretically, then, this would seem to be the proper form of a lathe bed in order that it might conform to the necessary requirements as to form and its ability to sustain the usual weights and strains to which it will be subjected, and at the same time not be of excessive weight, which would entail unnecessary expense.
Fig. 26. - Parabolic Form of Lathe Bed.
This is substantially the view taken by Prof. John E. Sweet in reference to machine beds. He says:
"No reasoning can make it out that the place for the support of an ordinary sized lathe bed at the tail-stock end of the lathe is at the end. If placed a considerable distance from the end, and the tail-stock is at the end, it is better supported than when in the middle of the present style of lathes and also better supported at all other points. At the head-stock end it is quite a different matter as the head-stock is always fixed and is usually heavier loaded, exclusive of its own greater weight. Where the head-stock end support is a closet, there is no way to make it look right except to have the closet the same width as the head-stock is long.
Fig. 27. - Modified Parabolic Form of Lathe Bed.
"In the case of a planing machine bed up to 12 or 15 feet in length there is no reason for having three pairs of supports. Unless the foundation is absolutely unyielding - a thing that is more rare than the other kind - the three or more pairs of supports are especially bad, and to attempt to hold the foundation true with a frail planer bed is foolish. The distance between the supports in Fig. 28 is no greater than in 29, and as in no case would the center of the load in planing overhang the supports more than a slight distance the style shown in Fig. 28 is quite as well supported as the other; and when the iron in the legs and the work to fit them are taken into account, if they were all put into the casting the bed could be brought down to the floor as in Fig. 30, greatly improving the structure.
"Another improvement is to use the iron usually put in the crossgirts - which do not stiffen the bed in any way to any great extent - and use it in bottom and top webs, making the thing a foursided box, which is from four to a dozen times stiffer in all directions, and then rest the whole thing on three points, one under the back of each housing and one under the middle toward the other end. The whole thing, including patterns and setting, will cost no (or very little) more and be four times better than present practice.
Fig. 28.-Prof. Sweet's Form of Bed, supported at Two Points only.
Fig. 29.-A Common Form of Lathe Bed, Supported at Three Points.
Fig. 30. - Prof. Sweet's Design Applied to a Planer Bed.
"If the bed is supported at the same points when it is planed and fitted up, no attention or skill is required in the erection -just set it anywhere and on anything solid, and that is all that need be or can be done."
There is "meat for reflection" in what Professor Sweet says (as there usually is), and the principle upon which he makes his deductions is undoubtedly correct.
To render the comparison more apparent and in a practical manner the two views shown in Figs. 31 and 32 are given. In Fig. 31 the parabolic design is shown in proper proportion for supporting the head-stock, tail-stock, and carriage, and the proportions laid out are ample for all purposes, as is also the supports and their distance from each other. In Fig. 32 is shown a rectangular design of bed of like length and of sufficient depth to give the requisite strength, provided there is a central support added to prevent a sinking in the center of the bed, as the distance between supports would otherwise be too great. While nothing has been added to the strength or the stiffness of the bed, we have been obliged to add the central support and in addition to this the weight of the parabolic form of bed is 1,390 pounds, while the rectangular form is 1,550, a very material addition without compensating advantages; and at the same time we have the disadvantage referred to by Professor Sweet, that the nearer together we can get the supports and still retain the condition of rigidity the less we shall have to depend upon the correctness of the foundations, and this of itself is a matter of very important consideration, since in some of the popular forms of machines their truth and correctness depends to a very considerable extent upon the accuracy and continued stability of the foundations upon which they rest.
Fig. 31. - The Parabolic Design of a Lathe Bed.
Fig. 32. - The Rectangular Form of Equal Strength.