2- Crankshaft, Piston Pin, and
Piston Rod Bushing.
Basic Pipsqueak engine
Recognize machined surfaces that are purely for
appearance. Also recognize mechanical connections, fits
and alignments that are critical for function. Tolerances
on your drawings are intended to provide adequate
control of these features to assure proper function.
#10-24 X 3/8” oval head screw
#10-32 brass barbed fitting
#8-32 X 3/16” set screw
#6-32 X ½” Pan head screw
#10-32 X ½” Flat
Head Screws
¼” O.D. X 1/16” wall
PVC tubing
10-32 flat head
screws
3/8” O.D. X .028
wire dia. X ¾”
Spring
7/16” O.D. X 5/16” I.D. X ½”
long bronze bushings
Pipsqueak engine hardware available in Projects Lab
• Press fits, or interference fits are sometimes used in
manufacturing to reduce manufacturing costs or liabilities.
There are however disadvantages to using this assembly
method. Precise sizing procedures are necessary to get the
correct fit. Generally, precision fixtures are necessary to
consistently align the parts while they are assembled.
Excessive assembly force can distort parts or their alignment.
The option for disassembly for adjustment or additional
unforeseen finishing processes is eliminated.
• An alternate method of assembling pipsqueak parts using
screw threads is easier to accomplish and disassembly is a
simple option.
Threaded assembly option
Crank shaft to crank wheel, piston rod bushing to crank
wheel and pivot shaft to cylinder are good candidates for
screw thread assembly.
Flywheel Connection
The flywheel can also be connected with screw threads.
Flywheel secured to the crankshaft with a set screw
Or the Flywheel can be secured to the crankshaft with a
set screw. This is the more common method of securing a
low torque component to a shaft. It works well with the
geometry of this flywheel.
Crankshaft setup in vise
For the set screw option, mill a spot face on the crankshaft
for flywheel set screw to seat against. This prevents the
set screw from making a burr on crankshaft O.D., which
makes dis-assembly difficult. The spot face also serves as
a positioning feature.
Screw thread option
To use the screw thread option, first face the shaft to
finish length. This gives a good flat surface for the mating
part to seat against. It is not necessary to face the shaft
near the center of the shaft where a hole will be drilled.
Center Drilling
Center drill shaft to a diameter slightly larger than the
nominal thread size. This will leave a desirable chamfer
on the threaded hole. Use appropriate cutting speed and
cutting fluid.
Chamfer left by center drill after tapping
Drilling with a tap drill
Drill with correct tap drill (see tap drill size chart). Drill
deep enough to allow for starting threads on tap and
plenty of chip clearance in front of tap. Tap full threads
about two diameters deep.
Tap begins cutting full threads when
it gets in this deep.
A spiral point, or “gun” tap
This style of tap pushes the threading chips ahead of it.
Drill with your tap drill deep enough to accommodate
those chips. These taps work especially well in through
holes.
A standard hand tap
The chips created by this tap accumulate in the flutes of
the tap. Turn tap ½ turn then back it off ¼ turn to break
the chip. When tapping deeper than about 1 diameter,
the tap should be completely backed out to clear the
chips from the flutes at each diameter increment of
depth.
Tapping internal threads
Cut internal threads with tap. Use tap wrench and guide
in tailstock chuck to ensure straight threads. Be sure to
use cutting fluid.
Install threaded stock
Choose a screw that will bottom out in the threaded hole and
still leave enough threads showing to screw into the mating
part. Tighten screw in shaft end then saw or grind off the
head of the screw to where the correct length remains. Grind
small chamfer on fastener to clear away burrs left from
cutting to length as shown in the next picture.
Removing burr
Remove the burr created by cutting or grinding threaded
insert to length. Hold shaft angled down 45 degrees and
rotate as you sand or grind burr off.
Cylinder to pivot shaft connection
Threaded insert on cylinder pivot shaft must be shorter
than the wall thickness of the cylinder.
Machining the piston rod-end bushing
When specifying materials that will be in dynamic contact with
each other such as pistons and cylinders, shafts and bores etc., it
is best to use materials that are compatible with each other for
their application. Generally, dissimilar materials should be your
first choice. Brass and bronze are good bearing materials and
work well with steel. Aluminum on aluminum should be avoided.
Making the piston rod-end bushing
Make the piston rod-end bushing by first center drilling
then drilling the bushing stock. Drill the hole large
enough to clear the #6-32 connecting screw.
Turn bushing down to specified size
The diameter of bushing that fits inside piston rod-end
must be slightly longer than the width of the rod-end so
that the rod-end will have lateral clearance between the
bushing flange and crank wheel.
Part off finished bushing
Break sharp corners on bushing with small file or emery
cloth just prior to parting off if desired.
Parting blade adjustment
Parting blade must be adjusted to be perpendicular with
the lathe axis. This can be done by making it parallel
with the chuck jaw face.
Parting blade tip adjustment
Adjust the tip of the parting blade so that it will intersect
the axis of the work piece. Parting speeds should be less
than turning speeds, and because parting creates a
significant moment on the work piece it must be done
close to the chuck or collet.
Parting process using cutting oil
A constant drip of cutting fluid must be applied to the cutting edge
while parting. Parting tools will not generally create precise flat
surfaces. They will be either concave or convex. Facing operations are
needed when precise flat faces are required. Generally, parting more
than a half inch deep is not recommended. Band sawing and then
facing would be more advisable.
Piston to crank wheel assembly
The bushing is held tightly against the crank wheel with a
#6-32 pan head retaining screw. Piston rod-end turns
freely on bushing. By constraining the piston rod-end to
the crank wheel horizontally with a flanged bushing, the
axial movement of the crank shaft is controlled.
Vertical crankshafts
When designing an engine with a vertical crankshaft, a thrust bearing must be
added to support the weight of the crank shaft assembly so as to avoid putting a
unfavorable cantilever force on the piston head and rod-end bushing connection.
End