Peckett "Y" Class - Framed! (part 3)
Entry posted by 2mmMark
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A little white ago, we left the chassis soap opera with this cliff-hanger "Coming in part 3, mounting the motor and meshing the worm & wormwheel". Well, it's here at last!.
The chassis block and frames have been trimmed up thus
This chassis is a particularly unfancy set of ironmongery, referring back to the prototype photograph in part 1
http://www.rmweb.co.uk/community/index.php?/blog/1345/entry-14132-peckett-y-class-framed-part-1/
Note the lack of anything resembling guard irons. They've either been knocked off or deleted by a cost-conscious accountant. I suspect the latter. Anyway, it's jolly convenient for a lazy person like myself. The ends of the chassis have been given a recess where the buffer beam will sit, in order to avoid a short circuit across the split frames.
Next on the agenda is how to power the locomotive. I chose to use the flat sided can motor available from the 2mm Scale Association. This is a compact powerful and smooth-running unit. It has a pair of mounting holes in one end which accept an M1.0 thread (12BA can also be used). Here's the motor and worm. The worm & wormwheel are Association products from several years ago but are none the worse for that.
I needed to use the motor in an upright configuration which meant there was not a flat surface allowing the motor to be glued in position. To be perfectly honest, I'm not a fan of glueing motors in place. It's a perfectly sound practice and with the right adhesive, will give no problems. It's just that I prefer a slightly more engineered solution. What was needed was a strong right-angled section to mate up with the chassis block and hold the motor rigidly in place. I chose to use a section of 12mm aluminium angle of the type available from DIY stores. This has the hard work done as a slice of it will give an accurate 90 degree angle. It was drilled and shaped using my Proxxon MF70 mill and finished off with files. A mill isn't necessary, it's just a quick way of achieving the end result. Hopefully the photos will show the small amount of shaping and drilling needed. The base was tapped 12BA for the mounting screw.
The large countersink is to accomodate the bearing boss of the motor, so it sits flush against the mount.
With the motor attached, it should now be clear it's held in place. The front face of the mount has a small recess to allow the mount to be as close to the wormwheel as possible.
To fine-tune the mesh of the worm and wormwheel, the base is carefully sanded on some wet & dry paper glued to MDF. Working slowly using a circular motion, it's possible to reduce the dimension quite precisely.
The fixing screw is placed in the chassis
then the mounting is screwed in place. It is just a fraction narrower than the central chassis block thus electically isolated from the frames.
A single mounting screw has proved quite sufficient to hold the mount in place as there's quite a bit of friction between the mounting and the chassis block. If it becomes necessary, a small shear-pin can be inserted to lock everything in position.
The worm is slipped onto the motor shaft and as can be seen, the meshing is just about right. There are a number of ways to get the worm/wormwheel mesh correct. For every combination, there is a specific dimension of axle centres but as these are at right angles, I personally don't find it easy to use. Originally, I started off using either a "Rizla" cigarette paper or a sliver of aluminium kitchen foil to set the meshing clearance. Nowadays, I tend to eyeball it and work by feel, knowing from experience how much backlash there needs to be on the wormwheel. The photo below shows the result
Using a steel worm on the motor shaft, there is a way to check the mesh is a happy one. The worm is reamed out so it easily rotates on the motor shaft but has no perceptible clearance. This allows the worm to be spun on the shaft by fingertip to ensure there are no tight spots and it all rotates freely. A fingertip is a surprisingly sensitive measuring device! Once everything is confirmed OK, the worm will be secured with a drop of Loctite.
Now we can try the chassis against the body. The unused end of the motor shaft needs some trimming to fit. This is marked with a felt tip permanent marker and will be cut off using a carborundum disc in a mini-drill as the motor shaft is hardened steel. The shaft won't be completely cut back as I want to try mounting a small flywheel, simply to see what effect it might have.
This brings the progress on this loco up to date. The next instalment will hopefully cover the fitting of the coupling rods, flywheel and wires to the motor.
Oh yes, I fibbed about the "small recess allowing room for the wormwheel". I made a mistake with the position of the mounting hole and had to relieve some of the mounting to allow the wormwheel to rotate because I did not particularly want to make another mounting...!
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