Have fun while learning to fabricate with plywood using CNCRouters
Although linker logs are made with the
aid of a computer—once complete, they’re a great way to get kids (and parents)
off the computer for an afternoon of old-fashioned fort building and pretend.
The techniques shown here—including pre-finishing the plywood, programming
correct joint tolerances, arranging parts, locating hold-downs and placing
tabs—can also be used for other plywood projects.
Th e inspiration for making these building planks comes from a story titled
“Plywood Play Planks” that appeared in the December 1953 issue of Mechanix
Illustrated magazine.
Th e original designers used 3/4″ plywood and a dado blade to cut the joints. That technique can
still be used, but using a CNC ROUTER provides more freedom when designing the
shape of the planks. Bill Young (a ShopBot guru from Virginia) adapted the idea
for the CNC ROUTER by creating a wide variety of planks, all with a standard
notch spacing. The playhouse shown below is similar to the original Mechanix Illustrated
design—but I added the puppet theater window
and marquee board. Also check out the Blanket Hut below, with with its custom
barrel vault roof.
Start by finishing
A coat of shellac followed
by a water-based topcoat provides a durable finish for Linker Logs —and most
other plywood projects. Finishing a sheet of plywood is much easier than finishing
dozens of individual pieces, so applying the finish first makes lots of sense.
Shellac dries quickly and seals the wood so the water-based topcoat won’t raise
the grain, and gives the plywood a nice amber tone. See “Pre-finishing Plywood”
(page 68) to learn more about finishing plywood.
Measure the plywood
The thickness of the plywood must
be consistent so the cross-lap joints that fit properly. I recommend a
tolerance between sheets of plus or minus .005″. Most good-quality AC or AB sanded plywood will be consistently
sized. The sheets of 1/2″ plywood
that I used to make this playhouse measured .47″ thick. Measuring after applying the finish ensures the most
accurate measurement.
Cut a single part to test the joint
Rout a test part. When
you test the fit in the next step, the width of the notch is the only thing
that matters, so you can make the test part out of almost anything. I used 1/2″ MDF.
The test notch measured .48″ wide, which should provide the desired .01″ clearance when the cross-lap joint is
assembled.
Check the joint’s fit
A mechanic’s feeler gauge works
well to measure the gap between the parts of the cross-lap joint. Ideally, this
gap will be .01″,
but anything from .005″ to
.015″ is
acceptable and will hold the Linker Logs together while still allowing them to
be easily assembled and disassembled.
Nest parts and locate screws
Most CNC ROUTER design programs include
a “parts nesting” feature that automatically fits multiple parts to the
plywood. The Linker Log parts shown here are nested .27″ inches apart—slightly larger than the
1/4″ bit
that will cut them out. Once the parts are nested you can still move them
around to attain the exact layout you want. This allows you to safely locate
the hold-down screws that secure the plywood sheet.
PROJECT PLANS ARE AVAILABLE AT THE
BOTTOM OF THIS PAGE.
Add connecting tabs
The next step is
to add tabs between the parts to keep them attached during routing. Since there
is no waste material between most of the parts, tabs need to be added opposite of
each other. The tabs added here measure .1″ thick x 1″
long. An alternative to adding tabs is to adjust the routing depth to leave a
thin layer of material (a skin) at the bottom of the kerf. A skin of .05″ would work fine for this project.
Secure the plywood with screws
Program the CNC ROUTER to
mark the location of each anchoring screw with a shallow plunge cut. Locating
the screws in your drawing and transferring them to the plywood guarantees that
the router bit won’t run into the screws while cutting the parts.
Use a down spiral bit
A down-cut spiral bit pushes
the wood down while cutting, so it leaves a nice, clean edge at the top of the
plywood. The bottom of the cut is also clean, because the spoil board under the
plywood prevents blow-out.
Separate and roundover in one step
Remove the connecting tabs. A
straight flush-trim bit works well for this, but I used an Amana 1/8″ roundover bit (#MR0112) with a
miniature bearing. This bit removed the tabs and rounded the sharp edges. It
also allowed the joints to fit fully
together, because the logs’ rounded-over edges match the rounded inside corners
of their CNC ROUTER-routed notches. I left the edges and notches unfinished.
Combo Puppet Theater and Play House
A combo puppet heater and play
housethat all kids seem to enjoy, It features a marquee with room to
engrave or paint your kid’s theater logo.
Project plans, Vectric Aspire, .dxf,
and SketchUp files for the two playhouses can be downloaded at the bottom of
this page.
Additional plank designs that included
angles and curves can be found at LinkerLogs.com. You supply the kids.
Alternate T-bone notch
If you want to keep the plywood edges
square, use a “t-bone” notch design. Most CNC ROUTER drawing programs have a
built-in tool that lets you quickly add the right size “t-bone” to your joints.
As you can see, this modification lets the plywood fully seat in the bottom of
the notch.
