Wednesday, 22 April 2020

DB Loco Underframe Reinforcement with Brass Strips

This post describes a fairly simple modification which can be retrofitted to Marbelup Models DB locomotives to correct or prevent any sag in the underframe.

A characteristic of the 3D print material that it will "relax" slightly under continuous, moderate stress, which can cause a "sag", especially for a long model such as the DB locomotive.  The good news is that most of the "sag" can be corrected.

It is fairly easy to retrofit 0.5 x 6 mm strips of brass on edge inside the long hood to provide more support to the underframe when the underframe and body are assembled.  K&S item #9840, available from good hobby shops, includes 3 strips.  Two are required per loco.

The modification is applicable to both the Hollywood Foundry and ViTrains versions of the DB underframe.  
General View of Underframe with Brass Reinforcements
Cut and bend the brass strips as per the diagram below.  ( A full-size PDF template is available on request.)  Join the strips, e.g. by soldering, on a flat surface to ensure the completed frame is flat and even.

Detail of Fabrication of Brass Reinforcements
For the Hollywood Foundry version only, use a cutting disk in a Dremel or similar rotary tool to remove approx. 0.5 mm from each side of the motor mounting brackets, as per the diagram below, to provide clearance for the brass strip.  Use a low speed to avoid overheating the resin material.

Motor Mount modification for Hollywood Foundry version
If the front bogie of the Hollywood Foundry mechanism is powered, it will be necessary to cut a shallow notch in the underside of the front end of the brass to provide clearance for the front drive shaft.  The notch should be approx. 10 mm wide x 2.5 mm high.

A modification is required to the 4 central fixing posts inside the body of the loco, to provide clearance for the brass reinforcement strips.  This can be done with a cutting disk in a Dremel or similar rotary tool, but use a low speed to avoid overheating the resin material.

Step 1 is to cut away the lower portion of each fixing post, as per the first diagram below.

Fixing Post Modifications - Step 1
 Step 2 is to cut a groove in each fixing post to a total depth of 6 mm, so that the body will rest on the brass strips when the loco is reassembled.  Cut the groove in stages, so as not to cut too deeply.  Use an offcut of the brass strip to check progress on each fixing post to ensure the bottom of the brass strip is flush with the bottom of the body side.
Fixing Post Modifications - Step 2
Any potential sagging can be further minimised by avoiding adding too much weight to the fuel tank and, instead, locating additional weight above each bogie.  Suggested weighting is no more than 60g in the fuel tank and an additional 50-60g above each bogie.

Also, make sure you insert all 10 screws to attach the underframe to the body, as this will help keep them in alignment.  Be careful to not overtighten the screws though.  Likewise, don't overtighten the screws securing the fuel tank to the underframe.  All screws should be snug but not too tight.