The Cutting Tool On The Lathe Exerts A Force
These forces obey Newton's third law of motion: Every action has an equal and opposite reaction. The elastic-recoil and chip-formation forces are both reactions of the blanking machinery to a blade striking the surface. The shear force balances the chip-formation forces, and the elastic-recoil is in response to the pressure of the blanking force. Studying these forces, engineers can manufacture foil, metal, paper, textile, plastic film and wire through the cutting force of their machines. Cutting Force of Scissors.
Kaup.Planer tools will first come under our notice as being the simplest and requiring the least skill in setting. You have doubtless observed that if the chip be unwound from the spiral shape it assumes in leaving the tool, and projected in a straight line it is shorter than the surface from which it came. This is due mainly to the compression of the metal in the direction of the cut, and you will thus see the possibilities of saving power and strain upon the machine by giving proper cutting angles to the tools and reducing this compression to a minimum.In Fig.
1 the cutting tool is at right angles to the work and without rake. It exerts its force in a direction nearly parallel to the surface of the work and having no side rake, either, it simply does not cut, but shoves or crowds the metal forward, producing a chip made up of little splints. It cannot exert any force tending to lift or curl the chip.This tool is wholly wrong; nor would it materially improve it to grind like the tool shown in the little sketch at the right, which goes to the other extreme, and would spring into the work. A tool must first of all be heavy enough at the back or heel to resist the horizontal cutting force, and consequently should have very little clearance.
The 7 degrees clearance shown in the lathe tool in the upper view, Fig. 2, is too much for a planer tool, while the 3 degrees of the lower sketch is as small as can be used safely. Theoretically if the point leads by only a thousandth or two it will perform its function.
There should be very little top rake on account of its tendency to make the tool dig into the cut; but this can be compensated for by giving considerable side rake. Another reason why a planer tool tends to dig into the work is illustrated in Fig. Point A in each sketch is the fulcrum. In the first sketch the tendency is for the tool to dig into the work in the direction of the arrow. This is not so serious as appears on the face of it, as planer tools are usually so stiff that they will spring but little and any error that might occur in the roughing cut would be eliminated in the finishing cut. What many mechanics take as an indication of the spring of the tool is really due to the chatter of the planer; since a rack and pinion planer will frequently chatter after it has become worn, while in a worm-driven planer the lost motion is all taken up at one end before beginning the cut and the screw action does away with the chatter.To obviate any spring the tool may be designed as in the second sketch, Fig. 3, where the deflection due to the force of cut is away from the work in the direction of the arrow.The tool in Fig.
4, approaches the ideal for a finishing tool, and gives the best finished surface of any that I have used on planer or shaper. It is made from a piece of ordinary tool steel and forged on the end in the shape indicated. It will be noticed that it has side rake and instead of being straight on the bottom, the line that comes in contact with the work is a little rounding.We will now take up the subject of the cutting edges of some of the many varieties of lathe tools, Fig. Here are shown a diamond point, a round-nose tool, a side tool, centering tool, thread-cutting tool, and cutting-off tool. We will first consider the diamond point as it is more of a universal tool than any of the others.
Before speaking of rake, clearance or the setting of the tool, I want to call attention to the general form of the cutting edges and the importance of maintaining the same throughout the life of the tool'Fig. 7 will best illustrate my meaning. The tool is shown at the left with depth of cut, and ground so that angle x shall not be less than 55°.
To the right is a tool in which the angle has been changed by grinding on both sides of the point, only, because the machinist claims that he is in a hurry and must make time on his work. But it will be seen that the line of cut b is much greater than the line of resistance a, showing loss in efficiency in the tool and requiring more power to drive it after it had been ground. Nor is this the only reason why careless grinding will produce a loss, as I will proceed to prove.This is true with proper rake, angles and clearances, but when the mechanic ignores all principles, and is careless besides, how much more serious it becomes, because more finishing cuts will be required to make the piece straight. The nearer the cutting edge of the tool comes to being parallel with the axis of the work the more power will be required to operate the tool.It will be interesting to note what really takes place in turning, as shown in diagrammatic form in Fig. Here is represented a piece of rough stock that is to be turned as indicated at the right. First, starting at the corner line A, and developing the line of circumference in a straight path, we will get a line like (1).
After turning and repeating the process, the developed line will look like the line at (2). It will be noted that the second line is somewhat irregular, showing that even after roughing it off, the surface of the piece has nearly all the irregularities of the rough stock, 'though on a smaller scale.This brings us to another important point, and that is the necessity of centering work as accurately as possible, for no matter how even the work may be on its circumference, if centered out of true, it will not be round after turning.
The Cutting Tool On The Lathe Exerts A Force F On The Shaft As Shown
Because the thickness of the chip or shaving is not uniform, hence does not offer uniform resistance to the cutting edge and the work will bend more at one point than the other. If the cut were uniform and offered the same resistance, of course we could expect round work.The bottom figure on the sketch illustrates the tool and method of obtaining the lines. A long has a knife edge or point at one end, near the fulcrum, which bears against the periphery of the work.
On the other end is a lead pencil attachment, the point bearing against the piece of paper indicated, the paper traveling at the same rate of speed as the work, only in the direction of the axis of the work. An uneven-ness in the surface of the work raises or lowers the point of the pencil and as the ratio is great (20 to 1), the variation in the line is marked.Continue to:. prev:. next.