Thursday, 27 February 2014

SXT Construction Tips

SXT with added details, ready for painting.

Removing Support Structures

Carefully remove the support structure from both parts of the wagon.  It is suggested to use a sharp knife  carefully cut the supports away from the roof, ends, and the underframe.  Take particular care around the brake handles, brake hoses and brake rigging on the underframe.  The ends of the brake handle have a "sacrificial" guard to reduce the risk of breakage during production and shipping.  You may wish to leave the guards in place until the majority of the assembly work has been complete to minimise the risk of breakage.

Because of the fine ribbing on the brake hoses, the 3D printer's automatic software has generated many support points, which have merged into one.  Carefully cut between the brake hose and the support structure, using repeated cuts with a sharp knife (e.g. Olfa snap-off blade type or X-Acto knife with #11 blade), to separate the support structure, then carefully trim the remnants from the hose.

Interlocking Parts (CXB shown, but SXT is the same)
Note that on the underside of the roof, alternating ribs extend a short distance beyond the edge of the roof to interlock the two parts when it is time for final assembly.  

The support structure extends inside the upper and lower decks of the wagon to support the inside of the wagon sides.  From the outside, cut through supports attached to the small circular brackets for the horizontal wires above the side planking on each deck, on the underside of the roof, and the underside of the middle floor.  When you have cut as many of these supports as possible from the outside of the wagon, gently wiggling the supports structure (e.g. with needle nose pliers) from the open side, will break the remaining supports and allow pieces of the structure to be torn away. Be careful not to apply excessive force to avoid damaging the side planks and door areas.

Once the majority of the support structure has been removed, carefully go over both parts and cut away the small supports which typically extend from one part to another, for example, around the brake rigging and between the frame members of the underframe.  An X-acto type hobby knife with a sharp pointed blade (Exacto #11 or similar) is quite useful for getting into the nooks and crannies.

Go around the surfaces of each part where they will join together, and smooth off any remnants of the fine supports, on the roof, ends, middle and lower floors, etc.  A sanding stick or small file can be useful for this, but be careful not to damage the projecting parts which are designed to interlock.  Also remove any remnants from the very ends of the projecting parts.  When you think you have finished, test fit the two halves together.  Interlock the roof ribs first, then gently bring the underside of the wagon together to see how it fits.  There are several frame members in the underframe which have retangular pins which fit into oval holes in their opposite number (a variation on the square peg in the round hole concept). 

If the two halves don't close completely, dismantle again and check for and remove any tiny bumps from the support structure attachments.

Near each end of the wagon and in the centre, there are four small tabs designed for 1 mm x 3 mm long pan-head self tapping screws, to secure the two halves of the wagon together.

In the long term, you have the option of gluing the body together, e.g. with superglue, or continuing to use the screw fixings..

Non-Tapping Holes


The holes for the couplers and bogies are designed to accommodate 2-56 or 8BA screws without tapping.  This also has the advantage that the screws remain tight, so they won't accidentally work loose (especially for the bogies).

Note:  Tapping the holes is not recommended as it will remove more material and potentially lead to stripped threads.

Adding Wire Detailing


(CXB wagon shown, SXT is similar.)
Several lengths of 0.4 mm brass wire are required as follows:
  • A - 59.5 mm, above timber side rails, 8 pieces per side
  • B - 13.5 mm, above timber side rails, upper deck only, 4 pieces per side
  • C - 12 mm, upper doors, 4 pieces per side
  • D - 24 mm, lower doors, 4 pieces per side
  • E - 28.5 mm, upper door top guide, 2 piece per side
  • F - Brake pull rod, not required for SXT
  • G - 27.5 mm for centre linking section, 6 pieces per wagon (not shown in diagram)
The holes for the above have all been incorporated in the 3D-printed model.  Once in position, the various wire parts can be secured with super glue.

Wire "drill bit" in use for cleaning out holes in centre section.
The horizontal rods in the centre linking section are each secured by a hole in the centre vertical angle iron, and the ends sit in notches in the outer vertical angle irons.  Cleaning out the holes in the centre vertical angles requires a special technique, as the body of the wagon prevents use of a normal drill bit.  The suggested technique is to use a piece of 0.4 mm brass wire approx 75 mm long as an improvised drill but, by cutting one end at a 45 degree angle.  The rough edges of the cut end form a crude but effective drill bit when the wire is held in a pin vice.  To reach the centre holes, the end of the long "drill but" can be passed through the gaps between the side board and then through the gaps between the end boards.  If gently guided by tweezers, the wire "drill bit" will bend into a gentle curve to drill a fairly straight hole in the centre angle iron (See photo above.).  Once these holes in the angle irons have been cleaned out, the cut pieces of wire also have to be fed back through the slots in the end boards to allow inserting on the hole, then slid into their correct position.

Wire "drill bit" in use for cleaning out holes in doors. Note wires already inserted in centre section.
The holes for the horizontal rods above the side planks have a nominal diameter of 0.6 mm.  If a wire does not go in easily, do not force it.  Clean out the hole(s) with a 0.45 mm or 0.5 mm drill bit in a pin vice.  The holes for the wires in the doors and the upper door top guide are smaller, and will definitely need cleaning out with a 0.45 mm or 0.5 mm drill bit.  When cleaning out the holes in the doors on the left end of each side, the 75 mm long wire "drill bit" may also be useful as access to the holes in the doors is limited for a normal drill bit.

Also using 0.4 mm wire, form the 4 handrails which are located vertically on the ends of the wagon, above the brake lever, and also on the inner ends of each of the former CXB bodies.  The spacing between the holes is 6.5 mm.  

Bogies


Railwest Models sell suitable bogies.

An alternative is American Models S Scale standard gauge Bettendorf bogies which can be regauged to Sn3½ as described on the Sn3½ Blog.  (Marbelup models has 3D-printed replacement bolsters available which simplify regauging of the American Models bogies.)

If necessary, adjust the height of the van on the bogies so that the coupler mounting surface is 11.5 mm above rail level.

Couplers


The VG is designed for Kadee "whisker" couplers.  Either the #158 (scale size) or #148 (normal size) couplers can be used, with #262 draft gear boxes.

The draft gear boxes supplied with the #148 and #158 couplers do not fit as they have a different mounting hole position.  The #262 draft gear boxes are narrower and have been used because they allow details such as the brake hoses to be positioned the scale distance from the van centre line.  Also, the #262 draft gear boxes are easier to use as the lid snaps into position.

Note that the height from rail level to the coupler mounting surface should be 11.5 mm, the standard for Kadee couplers.

Brake Pull Rods and Hangers


(VG Van Shown - SXT brake arrangement is the same.)

On each side, there is a brake pull rod which can be formed from 0.4 mm brass wire.  Starter holes are provided which should be opened out with a 0.45 mm or 0.5 mm drill bit.  On the brake gear hanger nearest the vacuum cylinder, support the underside of the small lever from the inside when drilling to avoid braking the small lever.  The distance (centre to centre) between the holes is 44.5 mm.

The hangers for the brake pull rods can be formed from brass ministrip 0.8 mm x 0.25 mm (made by North Yard NZ and available from Railwest Models).  Small slots have been provided in the underside of the underframe for attaching the top end of the brake hangers.  The brass ministrip should be cut to lengths of 11 mm and 13.5 mm, with 2 mm at the bottom bent over to form a loop to support the brake pull rod.  (Refer to prototype photos for arrangement.)

Note: Due to the print orientation for the SXT, the slots in the underframe may be blocked by a tiny (0.5 mm diameter) support piece.  Test fit some brass ministrip in each slot - it should go in 2 mm.  If the slot is blocked, clear it with a fine knife blade, e.g. Xacto #11.

Weighting


With no additional weight, the completed wagon including wheels and couplers weighs around 98 grams.  The "desirable" weight for a wagon of this length (200 mm) is around 125 grams to ensure optimum operation including operation of Kadee couplers.

Addition of a Marbelup Models sheep load in Polyamide material adds approximately 32 grams, bringing the total weight up to around 130 grams. 

If modelling an empty wagon, the weight can be increased by gluing lead shot or sheet lead between the various frame members of the underframe, where it would not be seen in normal operation. 

Painting


Either enamel or acrylic hobby paints can be used to paint the finished model.  

If desired, the interior of the wagon can be painted (airbrushed) prior to assembly.  Experience to date suggests that this is probably not necessary as overspray from painting the outside tends to take away the "bare plastic" look on the inside surfaces.  

Obviously, if a sheep load is to be included, it should be added after spray painting the wagon itself. Tamiya XF-78 Wooden Deck Tan is a reasonable colour for the sheep load.

Once the wagon is completely assembled, the join line on the roof can be filled and smoothed off. The join lines on the ends are virtually invisible as they are aligned with the angle iron end braces.