This of course requires extra handling and
setup time, which hinders throughput, so accomplishing it all in a single setup on a combo
machine makes sense. And increasing throughput is the ultimate goal, particularly at a heavy
fab operation in which cutting is the first operation in a routing. Problematic plate cutting
can starve the rest of the shop. This ultimately
is what those who have combination plate cutting machines aim to avoid.
Variation: Plate Thickness
and Thermal Expansion
Of course, it’s not as straightforward as just adding a gantry carrying drills and mills onto a plasma cutting table. Milling and drilling large plate
does abide by many of the speeds and feed conventions of machining, but in certain ways the
process can be a different animal.
For instance, during drilling and milling of a
workpiece fixtured in a vertical machining center (VMC), the part is fixtured in place, and the
operator sets the tool length offset and work
offset values. The machine knows where the
workpiece is and where to start.
A small workpiece in a VMC is one thing; a
1-in.-thick plate spanning more than 15 feet
wide or long is quite another.
When a heavy fabricator accepts plate material, the inventory manager or helper measures
the plate thickness, and the operator loading
material verifies it. Say it’s Grade 50 hot-rolled
carbon steel. The combination plate cutting
machine’s controller should have that material
in its library, so it knows its average hardness,
tensile values, and nominal thickness. What it
doesn’t know is the actual thickness. A 1-in. plate
that’s measured as 1.0625 in. is still within mill-spec tolerances. If the control doesn’t know the
actual thickness, a drilling operation might not
break through the bottom of the plate, and the
operator just created rework.
Operators can change the plate thickness
measurement manually in the control, of
course, but this might not be adequate for
every situation, particularly on a large plate.
Just because a plate is 1.0625 in. in one place
doesn’t mean it’s exactly that thick everywhere.
Moreover, many plates bow. A drill or mill
descending to 1-in. plate could find the plate
surface to be a quarter inch higher or more,
which can throw the entire operation off-kilter.
Effective workholding can help remedy these
complications. Typical oxyfuel and plasma cutting operations don’t require workholding—it’s
one of the benefits of using a “soft tool” thermal cutting process. But drilling and milling
do. The mass of large plates might be heavy
enough to withstand the torque from certain
drilling or milling operations. But again, many
plates received from the mill aren’t entirely flat.
Historically, combination machines have used
pneumatic clamps. Some systems now use a
servo-driven clamp foot—a small frame that
descends directly below the spindle to clamp
the work area and push it down flat. Once the
workpiece is clamped, the system senses where
exactly those clamps are in the Z direction, then
adjusts the cutting program accordingly.
For precision work, combo machines need
to cut in a strategic order that accounts for,
among other things, thermal expansion. Say
you have a nest layout that incorporates a series of blank profiles, each with a single hole in
it. The position of the hole as it relates to the
part perimeter is critical.
If the machine were to drill all those holes at
once and then come back with the plasma torch
to cut the profiles, the position of those holes
likely would be off slightly, as measured from
the part edge. Why? It has to do with thermal
expansion. As the plasma cuts from one end of
the plate to the other, the sheet “grows” ever
so slightly, which in turn throws off the plasma
torch’s position in relation to those holes.
To cut this sequence in a precise way, the ma-
chine uses a drilled hole as a datum point for ref-
erencing the profile; the machine switches to the
plasma and makes the cut based on that refer-
ence. This process is repeated from part to part.
In this way, the system accounts for that thermal expansion. Sure, this is slower than doing
plasma cutting all at once followed by drilling,
but in precision work, such a cutting and drilling sequence wouldn’t reliably produce quality parts within tolerance. Regardless, it’s likely
much faster, not to mention much less labor-intensive, than having to carry the work to a
secondary drilling or machining operation.
Visit, say, a mine with a large rock chute and
you’ll see a plow bolt sitting flush to a plate surface. Release the screw on the other side, lift out
the large square bolt, and you’ll see a plow bolt
hole that shows what a combination plate cutting machine can do in one setup.
Starting at the plate surface, the machine
mills a shallow pocket to create a step offset,
so the bolt sits slightly below the chute surface, protecting the bolt from excessive wear.
» These parts involved a variety of milling and cutting
processes, all performed on a combination plate cutting
» Along with their plasma and oxyfuel torches, today’s
combo machines have a tool magazine that feeds a high-speed spindle with a variety of tools.
» A servo-driven foot clamps the workpiece during a drilling operation, hidden by an enclosure that protects the work
area and aids chip evacuation.