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PROFESSOR: Now we consider the process of thermo-forming.
In this process, we will make a simple circuit
apart by heating a flat sheet of polystyrene
and then pulling it over a tool under vacuum.
After forming, the part is usually
trimmed to a final shape.
To begin the process, the flat sheet
is loaded into the machine and then clamped into place.
The sheet is moved into an oven and heated
for a specified amount of time.
The oven temperature is under feedback control
and the timing of entering and exiting the oven
is also under program control.
After residing in the oven the desired time,
the sheet is moved over the tool,
it comes up to contact the sheet,
and the vacuum is applied through the tool.
This draws the sheet onto the tool, taking its form
and, at the same time, cools the sheet.
The part is then removed from the clamps
and measured using a vernier caliper.
We will look at two different thickness sheets
and have two different resonance times
in the oven for a total of four different conditions.
Run charts for the part diameter can then
be plotted noting the effective thickness and heating time.
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Now we consider the process of sheet metal shearing.
In this process, we take 5-inch-wide strips
of sheet metal and cut off 1-inch-wide coupons
to be used in the brake-forming process.
The key dimensions for the part are the average width
and the taper along the length determined by measuring
the width at each end.
The cutting is done on a simple treadle
shear operated manually.
The cut width is determined by the location
of the back gauge, which can be set using
the graduations on the gauge.
Note that these can resolve to only about 1/16 of an inch.
And citing errors are likely to happen.
A sheet is inserted into the shear
as shown and pushed into contact with the back gauge.
To ensure a square part, the long edge of the sheet
should be held flush to the edge fence as shown.
If instead we keep the cut edge of the sheet
flush to the back gauge, we could not
eliminate initial tapered edges on the sheet.
As the treadle is pushed down, notice that a clamp comes down
to hold the sheet just before the blade moves
to cut the sheet.
The part falls off at the rear and should
be collected and numbered.
The width at each end is then measured
using a vernier caliper.
Both measurements are recorded so we can get an average width
and taper.
For this process, we will share both aluminum and steel sheets
and create a run chart for each.
[MUSIC PLAYING]
Here, we consider the process of brake forming.
The purpose of this process is to bend flat coupons of sheet
metal to a finite angle.
This is accomplished by applying a three-point load
using a punch and die, in this case, mounted in a lathe.
The lathe provides a stiff foundation
and, with the tailstock, a means of controlling
the displacement of the punch.
Here, we see the tooling mounted in the lathe
and show the punch moving as the tailstock lead
screw is rotated.
Notice that the displacement of the punch
is determined by the rotations of the hand wheel
on the tailstock.
The bend angle is determined by the displacement
of the punch relative to the die but will also
depend on the thickness and constitutive
properties of the sheet.
The rotational displacement is measured using the vernier
on the lead screw.
In this machine, we can resolve 1 part in 100 per rotation.
For a lead screw ratio of 1 inch per 10 revolutions,
this gives a displacement of 1/1,000 of an inch per line
on the vernier.
Here, we see the machine being cycled while forming parts.
Notice that, when the load is released,
the part shows significant spring back.
We can show that this spring back
is a function of the thickness, yield stress, elastic modulus,
and strain-hardening properties of the sheet
as well as the degree of bending.
The specifications call for a depth of either 3/10 of an inch
or 6/10 of an inch or three or six full revolutions
of the lead screw.
The key is determining the zero angle point where
the punch just touches the sheet and then moving three
or six revolutions further in.
As each part is formed, we can measure the resulting angle
using the machinist's protractor.
This is a difficult measurement and must be made with care.
In this case, we can resolve to better than 1/10 of a degree.
But it is important to stay tangent
to the flanks of the part.
Finally, we record the angle, the material type,
and the depth of the punch on a datasheet.
A run chart for the process is then plotted.
Data is most logically grouped by material and depth
since each has a strong effect on the final angle.
[MUSIC PLAYING]
Now we consider the process of injection molding.
In this process, we are manufacturing simple snap rings
to a specified diameter.
We start with clear polypropylene pellets,
which are melted and injected into the tool under pressure.
The injection-molding machine is rather complex.
It includes a compounding and melding screw,
an injection barrel, as well as the tooling and tooling clamp.
The pellets move from a hopper into a heated barrel
with a single screw.
During the melding phase, the screw rotates and moves
rearward, storing the melded plastic
in the nose of the barrel.
The melding is affected mainly by the mechanical shear
on the pellets and not by heat transfer from the barrel.
The barrel is brought into contact with one tool half
to create a flow path into the tool cavity.
During the injection cycle, the screw
first rotates and then moves backward
as the plastic is melded.
It then moves forward at a specified velocity
to fill the mold.
Once the mold is filled, the screw piston
switches to pressure control.
And the additional forward motion, called packing,
is caused by the plastic compressing in the mold
before it hardens fully.
The two halves of the tool are brought together
under the action of a large hydraulic clamping
cylinder, which keeps them closed during injection.
The tool itself serves to cool the part.
And the temperature of the part when the tool opens
depends strongly on the hold time in the tool.
This time can also be programmed into the machine.
After the hold time, the tool separates,
and the part is removed.
In the production process, the part
would be ejected by a set of actuated pins.
But here, it is done manually.
After the part is removed and cooled,
it can be measured using a vernier caliper
and the diameter recorded.
In this experiment, we will vary the injection speed
and the hold time to yield four different production
conditions.
A run chart can then be plotted for all the data, keeping track
of the machine settings.
Now we consider the process of CNC turning.
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The part to be manufactured here is a simple cylinder
of aluminum.
The workpiece is a bar of 0.75-inch-diameter aluminum,
which is to be turned to 0.675-inch-diameter.
After turning three parts of 0.75-inch,
length will be cut off from the bar.
The workpiece is clamped in the chuck
with a stick out of 3 inches.
The turning and cutoff cycles are controlled by the NC
controller so the only operator intervention
is loading of material and changing of tools.
Note that the machine frame for this process
is massive so as to minimize any deflection under machining
loads.
The cycle begins with a single turning
pass with fixed spindle speed and feed.
Notice that the longitudinal motion
is controlled by a servomotor driving the long-axis lead
screw under program control.
During the experiment, the operator
will change the spindle speed once and the feed rate once
for a total of four different operating conditions.
After turning, we change to a simple cutoff tool.
And a fast plunge cut motion separates the parts.
It is important that each part is captured
as it falls so we can tell which part of the bar
it was made from--
inner, outer, or middle.
Each part diameter is measured using a vernier caliper
and recorded in order of production.
The spindle speed and feed rate should also
be recorded for each part.
Run charts are then created for the part in sequence
or by location on the bar.