A Brutalised Barclay

[Brian McKenzie]

Thanks guys. Wheel centres for the tiny 30inch diameter drivers were SLS printed in China from tough Somos® Taurus material.



Don't know the orientation of the printing. Has the shiny side been vapour smoothed? The matt side is a bit rugged in places, but will be fine facing inwards.

The 3D design follows the practice used for cast brass wheel centres, with a preferred machining allowance added to all flat faces and outer diameter.

First up, a 1/8inch hole is drilled at the axle centre (for a later operation). An aluminium collar was prepared to accept the mouldings which are a light press fit inside, done for better concentricity than the three jaw chuck would deliver. At a later stage this hole will be bored out (not drilled) to the axle's scale diameter of 0.162".



Further use of the aluminium collar was made by screwing it to a piece of brass bar for ease of clamping to the mill table. Front and back faces of the mouldings were then milled to leave a thickness equalling the width of the tyres to be fitted, matching the prototype's appearance. As the crank boss needed to be 10thou (0.25mm) proud of the counterweight and rim, it was easier to mill around the rim and thus dodge the crank portion when facing the counterweight (although the wheel could be mounted eccentrically for turning this).



Next step was to turn the OD to be a press fit inside the tyres. The 1/8" hole drilled at the start was used to push the mouldings onto a short spigot for turning. The robustness of the 'Somos Taurus' material allows for this, but that may not be a practical method for less rigid 3D print materials. Although easily turned, the residue comes off as a very fine dust, so light that it drifts in the air almost like smoke. I wore a mask when turning this material once before, but this time a vacuum cleaner hose was tied to the toolpost.



Despite aiming for an interference fit of about 3/4 of 1 thou of an inch ( yes I know, but it is how I work with my mostly imperial equipment ), Loctite retainer is also applied. For this to work as intended by the manufacturer, a needle file is used to score a shallow clearance in the centre of the turned perimeter.

The moulded wheel centre is then pressed a little way into the tyre, and Loctite 680 (Loctite numbers jump around globally) is spread around the plastic perimeter, before the centre is fully squeezed home. Parts were cleaned with Loctite 7471 primer beforehand.



As the crank boss protrudes 0.25mm eitherside of the tyre width, the wheel is placed inside a folded wrapper of 0.25mm brass, with apertures cut to clear these bosses, before pressing home fully. This ensures correct positioning of the wheel centre inside the tyre.



Several sets were made, giving opportunity to experiment with (as pictured), stainless steel, an alloy steel, and cast iron for tyres.



Next job is boring for the axles and fitting of crankpins.
-Brian McK.

 

[Brian McKenzie]

Loco wheels were clamped into the end of an aluminium bar to finish bore the axle hole to a scale 5-1/2" dia.

As all of the crank boss protrudes slightly from the rear of the wheel, the clamp plate has a cut-out to avoid it. Original intention was to use a 1 in 48 tapered reamer after boring (hence the mounting of the wheel backwards), for similarly tapered axle ends, but the reamers at hand were not ideal for that size.



So the wheels remained bored parallel, and the axle ends were given a lesser taper, equal to an increase of 1-1/2 thou (0.0015" or 0.04mm) in diameter over the length of the seating, for a tight secure fit in the plastic of the wheel centre.


Previously, a steeper 1 in 48 taper was given to axle ends for the many brass centered driving wheels made (circa Gauge 1), to fit matching tapered wheel bores. To get axle material sized to display the correct outer end diameter, appropriate HSS drill blanks were purchased. As these blanks come in a hardened state (i.e. a 'drill' before the flutes are ground or twisted in), the cylindrical grinding device shown below was cobbled up to grind tapers concentrically - while the axle material rotates about its own axis in brass bushes. More recently I have gone away from doing that as the supplier began selling drill blanks from Asia - that are anything but round, and/or are heavily surface indented with size or branding.


For this Barclay loco model, 3/16" dia. silver steel was used, with the ends turned down a little. The $8 motor (ex a scrapped record player) is powered by a Hammant & Morgan Duette, and the drive belt is a slice of mountain bike inner tube. With the axles ends coated by black marker pen, the spinning grinding wheel is lowered, and the taper appears as a bright metal ring steadily growing wider as it produces the shallow angle and removes the black ink. The taper angle applied is determined by the amount the device is 'tipped' over - in this instance by placing a strip of regular 0.004" thick notepaper under one edge. (When grinding the steeper 1 in 48 tapers, a 6inch steel rule was used as packing.)



Why do these tapers? The main reason - and my fetish, is to avoid any wheel wobble or eccentricity, sadly something not uncommon. But with this loco model, there was also a chance to compare the traction adhesion between stainless steel, plain steel and cast iron wheel tyres. Stainless steel has been my go-to in the past, with Avesta pickling paste sometimes used to tone down the shiniest. The intention was to swap wheels around with the wheels only partially pressed on to the axles for tests.



A 'Wheel Quartering' jig was made from two squares of 12mm steel plate. Both were bored to accept the wheels neatly over the flanges. Four holes were drilled equally spaced in a square pattern off those bores for 5mm dia pins - while the plates were stuck face-to-face to each other, using double-sided tape.


Working off the 3/16" hole in the centre, a 1.15mm dia hole was drilled right through all, to capture the undernourished M1.2 crankpin screws.

90degree quartering is thus assured by rotating one of the plates 90 degrees on the 5mm pins. With the wheels and axle placed in jig position, the whole lot is squeezed in the vice.


The next pic. shows at right a short steel pin sitting inside the 3/16" centre hole. The length of the pair of pins used is carefully calculated, to centralize the wheels on the axle, and stops the vice being squeezed to less than the required back-to-back measurement. When the wheel at right is fully pressed on, the axle end will protrude 0.005".



For now, the cast iron tyred wheels have been installed (I rather like the iron colour for the intended rusty finish loco) - and I'm not so keen to pull them off again . Have already been there and done that. Forgot to add the axleboxes .


Photo below shows a stainless tyred wheel in the wheel puller, that has a 6mm cap head screw in the back, fitted with a brass peg at the business end.


The Barclay model is not liking what was done to its prototype - so it tried to do a runner.