The first, or roughing cut, is taken with a high-speed steel tool, or
bit, fastened in a tool holder. The tool holder is clamped in the tool
post of the lathe so that the point of the tool is at, or a little
above, the center, or axis, of the lathe, as in Fig. 8.
If the point of the bit is too high, it is easy to see that, as the
shaft rotates, the tool will not cut at all, Fig. 9. In case the tool
is set below the center, the cutting action is very poor so that
turning tools are never set as in Fig. 10.
[Illustration: Fig. 8]
[Illustration: Fig. 9]
[Illustration: Fig. 10]
[Illustration: Fig. 11]
=Speed of the Lathe.=—In taking the heavy roughing cuts, the belt
may be placed on the second largest step of the cone, while for the
finishing cuts the lathe should run a little faster, say with the belt
in the next smaller step.
=Grinding Turning Tool.=—The front, or point, and the sides of the tool
are ground at an angle, which is called the clearance. If the tool has
too little clearance, it will not cut freely, while if it has too much
clearance, the point will be so thin that it will break off or become
dull quickly.
The top of the tool is also ground at an angle. This is called the
rake. If the tool has too little rake, it will not cut freely and if it
has too much, the edge will soon break down.
It requires some practice for a beginner to learn the proper rake and
clearance that should be given to a tool. Fig. 11 shows a tool ground
with clearance and rake that will give very good results.
=Direction Tool Should Travel.=—The depth of the first cut should be
about ¹⁄₁₆ of an inch and the travel of the tool should always be from
the tail-stock end towards the head-stock. If the travel is in the
opposite direction, the pressure on the tail-stock center is increased,
causing it to heat quickly.
The length of the cut should be as great as possible without the lathe
dog striking the tool, or cross-rest.
=Adjusting the Lathe to Turn Straight.=—After the first cut, the work
should be calipered and if it is not the same diameter at both ends
the tail-stock should be adjusted so that the lathe will turn straight.
The tail-stock adjustment is made by loosening the main clamping nut B
and one of the screws C and then tightening the other screw C on the
opposite side of the tail-stock, Fig. 5.
If the shaft is larger at the tail-stock end, the tail-stock should be
moved towards the front of the lathe one half the difference between
the diameters of the shaft at the two ends.
In doing close work, the tail-stock should be adjusted as closely as
possible, but in this case if it is off center only a little, say 0.002
or 0.003 of an inch, it will be close enough providing it is set so
that the shaft will be turned larger at the head-stock end. If the
tail-stock is set so that the shaft is turned larger at the tail-stock
end, the shaft will be too small at the other end after the finishing
cut is taken.
=Fitting Shaft to the Collar.=—After the roughing cut is taken and the
lathe has been adjusted so that it turns approximately straight, the
end of the shaft is turned for about ¼ of an inch so that it will just
fit the hole in the collar, shown in the drawing of Exercise 1. To
measure this: first set the inside calipers to the diameter of the hole
in the collar, then set the outside calipers to the inside calipers and
caliper the shaft as accurately as possible. For a final test of this
diameter, remove the work from the lathe and try it with the collar
itself.
The advantage of turning but ¼ of an inch at the end of the shaft is
this; if the finishing cut were set too deep, only ¼ of an inch of
the shaft would be too small, while if this cut were taken the whole
length, the entire shaft would be too small.
After the shaft has been turned at the end so that it fits the collar,
the rest of the shaft should be turned a little larger, say 0.002 or
0.003 of an inch, in diameter. This will leave enough to finish with a
file.
=Filing.=—The object of filing is to take out the tool marks, but it
is also found to be much easier to make a close fit by filing off the
last 0.002 or 0.003 of an inch than to take so small a cut with a tool.
The amount of allowance for filing depends upon the character of the
finishing cut. Since the less filing required the better, the finishing
cut should be made as smooth as possible.
The tool used for the roughing cut may also be used for finishing, but
it is usually necessary to re-sharpen it. After it is reset in the tool
post, the point should be flattened a little wider than the pitch of
the feed, say about ¹⁄₃₂ of an inch, and parallel with the work. This
is done with an oilstone.
For filing work on a lathe, a single cut file is used. This is called a
lathe, or mill file.
The stroke of the file should be slow, steady, and straight across
the shaft. The lathe should run a little faster for filing than for
turning, the object being to have the work make several revolutions for
a single stroke of the file. If the lathe runs too slow and the stroke
of the file is too fast, the shaft, instead of being filed round, will
have a series of flat places on the surface.
After the work is finished as close to the dog as possible, reverse it
in the lathe and finish that part where the dog was fastened.
EXERCISE No. 2.
[Illustration]
The finished shaft in Exercise No. 1 may be used for Exercise No. 2.
Place the shaft in the bench vise and with a rule and scriber lay off
the dimensions: 1¾ in., 3 in., and 1¼ in. Then center punch the lines
just deep enough so that they can be easily seen when the work is in
the lathe.
Turn the large end first.
When it is necessary to turn a fixed distance, or to a line as in this
case, it is well to disconnect the feed when the tool is within about ⅛
of an inch from the end of the cut and to feed the tool the rest of the
distance by hand. If this is not done, the tool may travel farther than
it is intended to.
It is better to turn the portions to be threaded a little under rather
than over size. For if they are over size, the threads will not fit the
standard size nut, but if under size the threads do not need to be cut
so deep in order to fit the nut.
For measuring the diameters of this piece set the calipers as
accurately as possible by measuring from the end of the rule, as shown
in Fig. 12.
[Illustration: Fig. 12]
[Illustration: Fig. 13]
=Cutting Recess.=—The surface at the end as well as the recesses
between the threads and the taper are cut with a square-nose, or
cutting-off tool, Fig. 13.
This tool should have a sharp smooth edge, the point being set level
with the center of the lathe.
To produce a smooth finished surface lard oil should be used with a
slow feed and lathe speed.
After the end is turned to size, reverse the work in the lathe and turn
the other end and the taper before cutting the threads.
=Turning Taper.=—The drawing calls for a taper of 1 inch per foot. This
is cut by using a taper attachment, or by setting the tail-stock off
center. As most lathes are not provided with a taper attachment, the
latter method will be used.
If the work was 12 inches long, the tail-stock would be moved off
center ½ inch to turn a taper of 1 inch per foot. It being only 6
inches long, the tail-stock is set off center but half that amount, or
¼ inch.
Before taking the finishing cut, caliper both ends to prove that the
lathe is cutting the correct taper.