Lathe
A Lathe in Building 35
Labeled Photograph
Description
Choosing a Cutting Tool
Installing a Cutting Tool
Positioning the Tool
Feed, Speed, and Depth of Cut
Turning
Facing
Parting
Drilling
Boring
Single Point Thread Turning
Advanced Work Holding
The purpose of a lathe is to rotate a part against a tool whose position
it controls. It is useful for fabricating parts and/or features that
have a circular cross section. The spindle is the part of the lathe
that rotates. Various workholding attachments such as three jaw chucks,
collets, and centers can be held in the spindle. The spindle is driven
by an electric motor through a system of belt drives and/or gear trains.
Spindle speed is contolled by varying the geometry of the drive
train.
The tailstock can be used to support the end of the workpiece
with a center, or to hold tools for drilling, reaming, threading, or
cutting tapers. It can be adjusted in position along the ways to
accomodate different length workpices. The ram can be fed along the
axis of rotation with the tailstock handwheel.
The carriage controls and supports the cutting tool. It consists of:
- A saddle that mates with and slides along the ways.
- An apron that controls the feed mechanisms.
- A cross slide that controls transverse motion of the tool
(toward or away from the operator).
- A tool compound that adjusts to permit angular tool movement.
- A toolpost T-slot that holds the toolpost.
Cutting Tool Terminology
The figure above shows a typical cutting tool and the terminology used
to describe it. The actual geometry varies with the type of work to be
done. The standard cutting tool shapes are shown below.
- Facing tools are ground to provide clearance with a center.
- Roughing tools have a small side relief angle to leave more
material to support the cutting edge during deep cuts.
- Finishing tools have a more rounded nose to provide a finer finish.
Round nose tools are for lighter turning. They have no back or side
rake to permit cutting in either didection.
- Left hand cutting tools are designed to cut best when traveling from
left to right.
- Aluminum is cut best by specially shaped cutting tools (not shown) that
are used with the cutting edge slightly above center to reduce chatter.
Standard Cutting Tools
Lathe cutting tools are held by tool holders. To install a tool, first
clean the holder, then tighten the bolts.
(click here for video) 2.11MB
The tool post is secured to the compound with a T-bolt. The tool holder
is secured to the tool post using a quick release lever.
(click here for video) 1.99MB
In order to move the cutting tool, the lathe saddle and cross slide can
be moved by hand.
(click here for video) 860kB
There are also power feeds for these axes. Procedures vary from machine
to machine.
A third axis of motion is provided by the compound. The angle of the
compound can be adjusted to allow tapers to be cut at any desired angle.
First, loosen the bolts securing the compound to the saddle. Then
rotate the compound to the desired angle referencing the dial indicator
at the base of the compound. Retighten the bolts. Now the tool can be
hand fed along the desired angle. No power feed is available for the
compound. If a fine finish is required, use both hands to achieve a
smoother feed rate.
(click here for video) 2.05MB
The cross slide and compound have a micrometer dial to allow accurate
positioning, but the saddle doesn't. To position the saddle accurately,
you may use a dial indicator mounted to the saddle. The dial indicator
presses against a stop (often a micrometer as shown in the clip below).
(click here for video) 1.29MB
Cutting speed is defined as the speed at which the work moves with
respect to the tool (usually measured in feet per minute). Feed rate is
defined as the distance the tool travels during one revolution of the
part. Cutting speed and feed determines the surface finish, power
requirements, and material removal rate. The primary factor in choosing
feed and speed is the material to be cut. However, one should also
consider material of the tool, rigidity of the workpiece, size and
condition of the lathe, and depth of cut. For most Aluminum alloys, on
a roughing cut (.010 to .020 inches depth of cut) run at 600 fpm. On a
finishing cut (.002 to .010 depth of cut) run at 1000 fpm. To calculate
the proper spindle speed, divide the desired cutting speed by the
circumference of the work. Experiment with feed rates to achieve the
desired finish. In considering depth of cut, it's important to remember
that for each thousandth depth of cut, the work diameter is reduced by
two thousandths.
The lathe can be used to reduce the diameter of a part to a desired
dimension. First,
clamp the part securely in a lathe chuck (636kB).
The part should not extend more that three times its diameter. Then
install a roughing or finishing tool (whichever is appropriate). If
you're feeding the saddle toward the headstock (as in the clip below)
use a right-hand turning tool. Move the tool off the part by backing
the carriage up with the carriage handwheel, then use the cross feed to
set the desired depth of cut. In the clip below, a finish cut is made
using the power feed for a smoother finish. Remember that for each
thousandth depth of cut, the work diameter is reduced by two
thousandths.
(click here for video) 821kB
A lathe can be used to create a smooth, flat, face very accurately
perpendicular to the axis of a cylindrical part. First,
clamp the part securely in a lathe chuck (636kB).
Then, install a facing tool. Bring the tool approximately into
position, but slightly off of the part. Always
turn the spindle by hand (248kB)
before turning it on. This ensures that no parts interfere with the
rotation of the spindle. Move the tool outside the part and adjust the
saddle to take the desired depth of cut. Then, feed the tool across the
face with the cross slide. The following clip shows a roughing cut
being made; about 50 thousandths are being removed in one pass.
(click here for video) 2.35MB
If a finer finish is required, take just a few thousandths on the final
cut and use the power feed. Be careful clearing the ribbon-like chips;
They are very sharp. Do not clear the chips while the spindle is
turning. After facing, there is a very sharp edge on the part.
Break the edge (205kB) with a file.
A parting tool is deeper and narrower than a turning tool. It is
designed for making narrow grooves and for cutting off parts. When a
parting tool is installed, ensure that it hangs over the tool holder
enough that the the holder will clear the workpiece (but no more than
that). Ensure that the parting tool is perpendicular to the axis of
rotation and that the tip is the same height as the center of the part.
A good way to do this is to hold the tool against the face of the part.
Set the height of the tool, lay it flat against the face of the part,
then lock the tool in place.
(click here for video) 2.45MB
When the cut is deep, the side of the part can rub against sides of the
groove, so it's especially important to apply cutting fluid. In this
clip, a part is cut off from a piece of stock.
(click here for video) 246kB
A lathe can also be used to drill holes accurately concentric with the
centerline of a cylindrical part. First, install a drill chuck into the
tail stock. Make certain that the tang on the back of the drill chuck
seats properly in the tail stock. Withdraw the jaws of the chuck and
tap the chuck in place with a soft hammer.
(click here for video) 1.93MB
Move the saddle forward to make room for the tailstock. Move the
tailstock into position, and lock the it in place (otherwise it will
slide backward as you try to drill). Before starting the machine, turn
the spindle by hand. You've just moved the saddle forward, so it could
interfere with the rotation of the lathe chuck. Always
use a centerdrill to start the hole.
(click here for video) (2.30MB).
You should use cutting fluid with the centerdrill. It has shallow
flutes (for added stiffness) and doesn't cut as easily as a drill bit.
Always drill past the beginning of the taper to create a funnel to guide
the bit in.
(click for computer generated animation of funnel effect,
not yet available).
In this clip, a hole is drilled with a drill bit.
(click here for video) 1.07MB
Take at most one or two drill diameters of material before backing off,
clearing the chips, and applying cutting fluid. If the drill bit
squeeks, aplly solvent more often. The drill chuck can be removed from
the tail stock by drawing back the drill chuck as far as it will easily
go, then about a quarter turn more. A pin will press the chuck out of
the collet.
(click here for video) 554kB
Boring is an operation in which a hole is enlarged with a single point
cutting tool. A boring bar is used to support the cutting tool as it
extends into the hole. Because of the extension of the boring bar, the
tool is supported less rigidly and is more likely to chatter. This can
be corrected by using slower spindle speeds or by grinding a smaller
radius on the nose of the tool.
Boring On a Lathe
External threads can be cut with a die and internal threads can be cut
with a tap. But for some diameters, no die or tap is available. In
these cases, threads can be cut on a lathe. A special cutting tool
should be used, typically witha 60 degree nose angle. To form threads
with a specified number of threads per inch, the spindle is mechanically
coupled to the carriage lead screw. Procedures vary for different
machines.
Some parts require special techniques to hold them properly for lathe
work. For instance, if you wish to cut on the entire outside diameter
of a part, then the part cannot be held in a chuck or collet. If the
part has a hole through it, you can press it on to a lathe arbor (a
slightly tapered shaft), and clamp onto the arbor rather than the part
itself. The hole must have an adequate aspect ratio or the part will
not be firmly supported.
(click here for video) 554kB
If the part has a very large hole through it, a lathe arbor may not be a
practicable solution. You may instead use the outside of the jaws to
hold the inside diameter of the part.
(click here for video) 983kB
If the part has a very complex geometry, it may be neccesary to install
the part onto a face plate. The face plate is then attached to the
spindle.
(click here for video) 452kB
On to the belt sander.
Return to the machine shop.
Return to the Mechanical Engineering Department.
Maintained by
2.670adm@mit.edu