Modified 3D Printed Servo Mount

This unit is a variation of the MERG horizontal point motor mount designed by Bob Gledhill to whom due credit is acknowledged..
For MERG members, the original unit details can be accessed at:


The basic mechanism as supplied is shown below as the supplied parts and also fully pictured assembled. The 3D printed parts are; the original Horizontal Sliding servo mount, the original microswitch mount and the new modified slider arm.

Additional items required:

  • 1 off SG90 sized servo with mounting screws, etc.
  • 1 or 2 micro switches for frog switching and feedback if required.
  • 1 set of M1.6 x 8 machine screw with flat washer and nut. Required for all versions.
  • 2 off M2 x 10 machine screws and nuts for each micro switch.
  • 1 off No2 x 9.5 mm (3/8”) self tap screw to retain the micro switch mount.
  • 1 off M3 x 8mm screw with flat washer and self locking nut. Output drive link.

Initial construction.

Enlarge one of the holes in a servo arm with a 1.5mm drill. The second hole from the boss is good for about 7mm Slider movement with 90 degree movement of the servo arm. Use the hole nearer the boss for less movement.

Add the M1.6 machine screw ensuring it is screwed from the boss side of the servo arm. Lock it off with the nut with a washer underneath.

Centre the servo with your setting box and screw on the servo arm so it will sit in the Slider in its central position or as near as the output splines will allow.

Place the slider in the main mount and screw the servo to the main mount ensuring the machine screw on the arm is in the slot in the Slider.

Mount either 1 or 2 micro switches in the micro switch mount as required. The slots in the mount keep the nuts from turning. Do not over tighten! Mount them at the outer extremities for now.

The micro switch mount is a friction fit and is secured with the No2 x 9.5 mm self tap screw

Output options:

There are several methods of connecting the output to the point tie bar.

As used on SJ the link to the servo tie bar uses 0.35mm or 0.51mm piano wire attached to the slider arm with a M3 x 8 screw and washer plus an M3 nylon type nut. The wire end is formed as a half loop and trapped behind the washer.

This method was chosen so the servo slider actually bends the wire at each limit and springs the point blades against the out rails in a similar way to stall motors.

The alternative is to use one of the connection ends to hold the piano wire especially if thicker wire is used. There 2 types of connection end. One screws to the end of the slider using the M3 x 8 nut and bolt while the other has a stud and is glued in.

As well as the top/bottom entry hole there is a hole in the end for Bowden type cable terminations.

Both use a No 2 x 6 mm self tap screw to hold the output wire in place. 

Fitting the servo mount.

The servo mount is straightforward to mount using two pan-head (FLAT HEADED—NOT COUNTERSUNK!) screws. If flat headed screws are not available then use a flat washer to spread the loading.

Adjust the servo throw to suit the point mechanism and then set the micro switch position(s) so they operate at each limit.


If the servo mount has to be mounted away from the tie bar then there is an optional extender bar available. This uses the same end connections as the slider and the length can be adjusted by cutting off sections as required. The reason for this part being designed was that the SJ the servo mounts could not be fitted where the legs were positioned for storage and would have crushed the mounts.

 Neale Brodie 5-Mar-18

Alternative Extender

This alternative extender is useful for situations where there isn’t enough room to position the servo mount to suit the holes in the standard extender:

The servo wire mounting hole can be drilled anywhere along the grooved line.

Neale Brodie 26-Oct-18