The design of a mechanical interlocking frame is much like a mechanical puzzle, but once understood, the principles can be applied to any track and signal arrangement.
In the frame are two sets of bars which interact with each other to prevent the operator from making dangerous moves.
The main set of bars are the "tappets" and one tappet is connected to each lever.
If the lever is pulled to clear a signal or move a switch, the tappet moves a short distance lengthwise at the same time.

Close behind the plane of the tappets are the locking bars.
These can also move a short distance but at right angles to the tappets.
The number of locking bars required depends on how many false moves must be prevented.

In the sides of the tappets are notches with sloping sides, and connection between the tappets and locking bars consist of cams called "dogs".
Two or more dogs are mounted on each locking bar.
These slide into and out of the notches in the tappets as the tappets are moved, locking and unlocking them.

Here's how the scheme works: Suppose the operator pulls the lever to clear a particular signal.
This also pulls the tappet connected to the particular lever and forces any dogs seated in the notches to the side, thus moving one or more locking bars.
The dogs on the other ends of these locking bars are thus forced into notches in other tappets.
By this scheme, pulling one signal to clear locks all the other switch and signal levers in safe positions until the first signal is again restored to normal.

Interlocking signals are normally at stop or "red" position, and a lever must be pulled to "clear" the signal.
This is not necessarily to green, however, for in some situations only a yellow indication is given to a train to let it into the "plant".

There are other basic rules.
A turnout may have two levers, one to actually move the switch points, the other to lock the points.
A signal cannot be cleared until all the related turnouts are properly thrown and locked.
Such locks are nearly always used where the switch points "face" oncoming traffic.
The lock insures that the points are thrown all the way with no chance that a wheel flange will snag on a partly thrown point.
If the points aren't thrown all the way, the Turnout cannot be locked, and in turn, the signal cannot be cleared.
Generally, these locks on turnouts are called "facing point locks".

Figs. 1-6 show typical arrangements of track and signals.
Each diagram is accompanied by a "dog chart", a list of the levers that show which other levers any particular lever will lock if pulled.
The lines connecting the wedge-shaped dogs represent the locking bars at right angles to the tappet bars.

By studying the track-signal diagrams you'll note several other details.
Derails -- mechanical track devices that actually guide the wheels off the rails if a train passes a "stop" signal -- are used in many instances.
"Home" signals have two blades.
The blacked-in blades indicate a fixed aspect -- the blade does not move.
As an engineer approaches the plant the position of the home signal is seen in advance when he passes the "distant" signal located beyond the limits of the interlocking plant.
In some low-speed situations, the distant signal is fixed at caution.
In other instances where there is no automatic block signaling, the distant has only green and yellow aspects.

So much for the prototype.

The interlocking frame we built at the model railroader workshop and then installed on Paul Larson's railroad follows the Fig. 1 scheme and is shown beginning in Fig. 7, page 65, and in the photos.
Here's how it can be built.

Frame

The sizes of pieces needed for the interlocking frame are shown in the notes within Fig. 7, most of the bars being 1/8'' brass in 1/4'' and 1/2'' widths.
You may change the dimensions to suit a frame for more or fewer levers and locks as you wish.
Our instructions assume you are building this particular frame, which is for a junction.

When cutting the pieces, dress the ends smooth, and square with a smooth file or sanding disk.
Start with the right-hand piece "B", Af, soldering it to the lower piece "A" of the same material but 12'' long.
Let exactly 1'' of "A" extend beyond "B" and use a square to check your angle to exactly 90 degrees.

Now lay 12 pieces of Af cut 5-3/4'' long side by side but separated by 12 pieces of the same material 1/2'' sq.
This gives you the spacing for locating the left-hand piece "B".
Compress the assembly when you make the mark to show the location for "B".
Solder this second "B" to "A" at right angles.
There should be 10'' between the two parallel members and each should be 1'' from an end of the long piece.

Cap this assembly (with spacing bars in place) with a Af bar.
Tack-solder all the 1/2'' sq. pieces to the 10'' and 12'' members.
These will be drilled and tapped later on.

Now cut five Af locking bar spacers (which run horizontally).
Position these using six intermediate temporary Af spacers and locate the upper 12'' bar "A".
Solder it and the five locking bar spacers to the frame.
Now place 12 pieces 1/2'' sq. on this edge as we did before and space them with the 5-3/4'' long "tappets", as they are called.
Cap with a Af bar and tack-solder in place.
Cap the locking bar spacers with two Af directly under the first two "B" pieces.
Remove all the loose spacing bars.

Mark and center-punch all the holes required for screws to hold this assembly together.
See Fig. 7.
Placement of these holes is not critical, but they should be located so that the centers are about 1/8'' from any edge.
Drill all No. 50 and counter-drill all except the "A" pieces size 43.
Tap the "A" pieces 2-56.

Now unsolder and disassemble the frame except for the two 12'' and the first two 3-3/4'' bars ("A" and "B" pieces), which are soldered together.
Either lay the components aside in proper order or code them with numbers and letters so they may be replaced in their proper positions.
Dress all surfaces with a file, cleaning off all solder and drilling burrs.

Drill 20 No. 47 holes in the upper piece "A" as shown in Fig. 7.
Tap these 3-48 for mounting the electrical contact later on.
Note, 6 and 8 lock levers don't require holes for contacts.

Now reassemble the frame, using Af roundhead steel screws and nuts.
Put the 12 tappets and some Af locking bar spacers in the frame to help align all the components before you tighten the screws.
Be sure the tappets are not pinched by a twisted 1/2'' sq. spacer.
As an anchor for the spring lock, insert a Af bar in the lower left corner of the frame as shown in Fig. 7.
Drill a No. 43 hole through the pieces and secure with a 2-56 nut and screw.
Drill two No. 50 holes, one in the insert and one in the locking bar spacer directly above it, and tap 2-56.
Number all the tappet bars before removing them so they can be replaced in the same slots.
Remove all other loose pieces and file the edges of the basic frame smooth.
Cut five pieces of Af brass bar stock 3-3/4'' long.
These are supporting members for the short locking bars.
Locate their positions in Fig. 7 and drill No. 43 to match the corresponding holes in the frame.
Cut off excess screw lengths and file flush with either frame or nut.
Drill four No. 19 and four No. 28 holes in the 12'' long "A" pieces.
Locate the position from Fig. 7.

Tappets and locking bars

Draw-file No. 1 tappet to a smooth fit in its respective slot and square the ends.
Break the end corners with a slight 45 degree chamfer.
Drill a No. 50 hole 1-1/4'' from one end and tap 2-56.
(See Fig. 7.
) Put a 2-56 roundhead screw into the hole, cut off the excess threads and file flush with the underside of the bar.
To find the other stop screw position, insert the tappet into the frame and hold the screw head tight against the frame edge.
Scribe a line across the bar on the other end of the tappet, 1/4'' plus half the diameter of the 2-56 screw head (about 5/64'') away from the frame edge.
Total distance is about 21/64''.
Tend to make this dimension slightly undersize so you can file the screw head to get exactly 1/4'' tappet movement.
Drill a No. 50 hole, tap 2-56 and insert a roundhead 2-56 screw as you did on the first end.
Drill a No. 47 hole crosswise through the tappet at the position shown in Figs. 7 and 8.
Repeat these drill and tap operations for each of the tappet bars.

To each tappet except 6 and 8, solder a Af piece of brass and file to the tapered shape shown in Figs. 6 and 8.
These will serve as lifting pads for the electrical contacts.

Fitting the locking bars and making the locking pieces is a rather tedious job since stop screws, tappets and locking bars must be removed and replaced many times.
As the work progresses the frame and moving parts become a sort of Chinese puzzle where several pieces must be removed before the part you are working on is accessible.
A little extra work here will pay off with a smooth, snug-fitting machine when you are finished.
Each completed locking bar should remain in place as the work progresses to insure snug fitting.

The order of fitting is not too important.
However, we started with the first row of bars and worked our way back.
Since the same method of shaping and fitting the dogs and notches is used throughout, we will only describe the construction of one locking bar.
Figs. 7 and 8 give all pertinent dimensions.
All the bars are cut from Af brass.
The lengths of each piece are listed at the bottom of Fig. 7.
Bar "C" is 2-3/4'' long.
Draw-file the edges, square up the ends and put a slight chamfer on the edges so they will not snag in the frame.

Fig. 8 gives the dimensions for locating the dog-pin holes.
Center-punch and drill the No. 31 hole 7/16'' from one end of the bar.
Chuck a length of 1/8'' dia. drill rod into a drill press or some similar turning device and while it is rotating file the end square and then file a slight taper 1/8'' long.
Cut the piece about 9/32'' or 5/16'' long and drive it into the No. 31 hole drilled in the locking bar.
File the bottom edge flush with the bar and the top 1/8'' above the bar.
This dog will engage a notch to be cut in tappet 3.

Place the locking bar in proper position and insert tappet 3.
Scribe a line through the center of the pin and across the face of tappet 3, parallel to piece "A".
See the drawings for the shape of the notch.
Scribe V-shaped lines on the bar and rough out with either a hack saw or a cutting disk in a hand power tool.
We used the latter equipped with a carborundum disk about

.020'' thick and 1'' dia. fitted on a 1/8'' dia. mandrel.
Such disks are very handy for cutting and shaping small parts.
File to a smooth finish.
A Barrette Swiss pattern file is handy since its triangular shape with only one cutting face will allow you to work a surface without marring an adjoining one.
Endeavor to get the notches as much alike as possible.
The notch should have a smooth finish so that the steel dog will slide easily over it.
Assemble the parts in the frame and test the sliding action of the mating pieces.
All matching surfaces should be checked frequently and mated on a cut and fit basis.

Chuck a 2'' or 3'' piece of 1/8'' dia. drill rod in a drill press or electric hand tool.
Fashion a sharp scribing point about 3/64'' long on one end, using Swiss pattern files.
This tool can also be made with a lathe.