Machining Small Castings

Recently I was asked some questions about machining the flywheel casting from a
small model engine casting set.  Now, I've never actually built an *engine*
from a casting kit but I have built all the PMR machine tool models from
casting kits and the range of part types there is wide enough to qualify me to
talk about machining small castings, at least from a hobbyist viewpoint.

I've long preached that the newbie should select for his first engine project
an engine built from bar stock rather than a (seemingly simpler) engine built
from a purchased kit of castings.  The reasons for this are legion but I don't
want to discuss them here. In fact, the newbie's first exercise in thinking
like a model engineer should be to write a list of reasons why my advice is
good advice.

Nevertheless, folks buy casting kits.  Then they discover that the castings,
out of the box, expose a whole new set of problems for which they aren't
prepared.  Casting halfs don't align precisely, cylindrical pieces aren't
cylindrical and right angles aren't ninety degrees because draft angles are
required to get castings out of the mold.  Surface finish is uniformly rough
and there isn't a reference surface in sight on any of the parts to use as a
starting point for machine operations.

Moreover, small models are much less forgiving about minor irregularities. A
0.005" TIR that might go totally unnoticed on a 12" flywheel will be all too
depressingly evident on a 3" wheel.  Getting small engines to operate visually
'smoothly' requires a lot of attention to detail.

A lot of planning is required when machining castings. First decide the order
in which the individual castings will be machined and record this on paper.  If
you think about what needs to fit what, your list will quickly show that
almost none of the castings can be finish machined before moving on to the
next casting.  You can't machine the locators on the cylinder heads until the
cylinder is bored and you can't bore the cylinder until the cylinder is squared
on the base...etc. ad nauseum. In fact, it's very easy to convince yourself
that work on this engine can never be started, much less finished.

Once you've got the machining sequence for the castings you need to establish
a (preferably written) machining sequence for each operation called out on
your master list.  Here you'll frequently deal with the problem of
establishing an initial reference surface or location. More on that later.
All this planning will test your patience but it's necessary.  Resist the urge
to "get in there and cut some metal" or all this will come back to bite you on
the a...

An example will be helpful. One of the few parts of a model engine that can be
completely machined without much consideration of the other castings is the
flywheel so we'll start with that. Let's assume the classic flywheel with a
nominally cylindrical HUB, through-drilled to fit on the crankshaft, to which
are attached SPOKES or a web that supports the RIM.  Our goal is to machine
the hub, punch an accurately sized hole through it and machine the rim so that
its outer contour is concentric with the hub itself and the hub hole.  (We'll
ignore any spoke machining for now.  Since the spokes are discontinuous, it's
difficult to detect minor flaws anyway.)

Now the hub is probably a frustum of a cone because of the mold draft angles.
It can't be grasped firmly in a chuck or collet so we're going to have to
grasp the part on the rim. But the rim casting isn't a nice cylinder either.

Our procedure will be to mark the 'center' (see aside below) of the not quite
circular hub somehow; grasp the flywheel by the rim in the 4jaw, get the
marked 'center' running true; drill, bore and ream the central hole; machine
the exposed side of the hub to a cylinder and machine the exposed side of the
rim. The rim periphery is not touched at this stage.

-----------------
Aside - finding the center of the 'wonky' hub...

We're really trying to find the center-of-gravity of all those shapes that
make up the as-cast hub.

Use your drill gage (or drill bushings if you have them), to find the smallest
hole into which the (non-circular) hub will fit. Drill a hole of that
diameter axially partway into a short length of scrap. Now drill
through with a close-fitting hole that matches one of your transfer punches.

Push this jig over the hub, drop in a transfer punch and tap with a hammer.
The resulting mark is about as good as you'll be able to define 'center' for
something that isn't cylindrical.  Use a pump center to get this mark running
true in the 4jaw.
-----------------

Once this much is done, there are several ways to go.

Mount the flywheel on an expanding arbor in a collet chuck and finish the
unfinished side of the wheel and the outer surface of the rim. I don't like
this approach since expanding arbors (or tapered mandrels) in small sizes
don't have a lot of 'grab' to resist the huge torque when machining the rim.
Plus they tend to mark up the bore of the hub which can cause problems down
the line.  A flywheel that fits loosely on the crankshaft is just as bad as
one that's not machined concentrically.  A variation of this is to grip the
finished half to the hub in a collet.  It's just as bad. I'll guarantee that
it will slip in the collet when you begin to turn the rim periphery.

Another approach is to finish half the rim periphery after drilling the hub
hole.  Then flip the flywheel in the 4jaw (clamping on the machined surface),
recenter on the hub hole, and finish the other half of the rim.  I detest this
procedure because it's almost impossible to perfectly blend the two cuts on
the rim, at least for me.

My preferred method goes like this:
Clamp the flywheel (already machined side down) to a sacrificial plate
(aluminum is good) using strap clamps acting on the spokes.  Mount the
sacrificial plate in the 4jaw (or on the faceplate) with the hub hole centered.
Machine the hub. The rim is machined across its entire periphery by cutting
into the sacrificial plate.

[Note that at no point in any of these procedures was use of a 3jaw chuck
suggested. 3jaws are not precision tools and will only produce concentric part
features if all machining can be done without reclamping the part.]

Machining castings is yet another learned metalworking art.  Each casting
presents unique problems.  The best I can do is provide some hints learned
from (oft bitter) experience:

Plan your cuts to 'average out' the casting irregularities.  If there's a
cast-in port that ought to 'look centered' in the final product, plan your
cuts to remove material equally on both sides of it so it doesn't end up
visually off center.

Make shallow cuts on all machined surfaces (iow, rough in the part) to
establish whatever reference surfaces you need prior to cutting to size.

Holding the part to machine the first one or two reference surfaces can be a
real trial.  I made a small block, equipped with fences, that will fit in my
milling machine vise in all possible orientations. In essence, it's a set of
clamp-on reference surfaces. The part is clamped to it and then, by flipping
the block cum casting in the vise, I can machine preliminary orthogonal
surfaces on the casting.

Clamping parts that have no flat surfaces is a bitch.  On occasion, I've
resorted to milling a pocket in a (squared) block of aluminum and securing the
casting in the pocket with Cerrosafe (Wood's metal). Make the pocket with
flared walls (kind of the opposite of what you want in a mold) so the
Cerrosafe has something to 'grab onto'.

When possible, drill critical holes (e.g., cylinder bore) undersize and then
trial assemble parts to establish that everything lines up as required.  While
this may often necessitate making an undersized jig 'piston', any errors
detected can still be corrected if the cylinder casting still has a little
'meat' left to cut on.

In fact, accurately machining castings generally ends up being a huge exercise
in creative one-off jig and fixture design.  It's not unusual to spend more
time making jigs than making parts.  (Maybe now you're beginning to see why
making engines from bar stock is a lot easier.)

Marv Klotz