Peckett "Y" Class - Framed! (part 2)
Entry posted by 2mmMark
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We last left our intrepid adventurer holding a set of parts for a 2mm chassis but with no way of assembling them. So the first task is to find a way to hold the frames & spacers together. In this case, the frames will be screwed to the plastic spacers using 12BA screws. The first step is to mark out and centre-punch the screw locations. The actual position isn't critical so I decided to place the screws behind where the footsteps will be located.
In this photo you can just about see the punch marks. They don't need to be huge, just enough to locate a 1mm drill.
Then the frames are placed onto the assembly jig, held down by some tube which is tight fit on the 1.5mm rods and the 1mm diameter holes are drilled. While about it, I also put in some extra 1mm holes, the reason for which will become clear soon.
Inserting some 1mm rods to check things are square, the frames are used as a pattern to drill the screw holes only in the spacers. The extra 1mm holes are left alone for now. The screw holes are tapped to 12BA. The correct tapping size is 1.02mm but for this plastic, 1mm will suffice.
While on the jig, the extra 1mm holes are drilled to a short depth, 2mm or so and left as blind holes. Into those holes are inserted short lengths of 1mm rod, to act as locating pins for the frames in addition to the screws. The 1mm pins are fixed in place by Loctite 639 retaining compound.
This photo shows the frame dropped onto the pins. No need to go mad with the pins, I use just enough to keep things in place. The pins are filed off flush with the outer faces of the frames. The screw hole is enlarged to clearance size for 12BA (1.2mm) and also countersunk. For this I simply used a larger drill (4mm) twirled in my fingers.
With the pins in place and the screw holes ready, the frames can be assembled.
This results in a fairly solid frame block. There's no need to overtighten the fixing screws, in fact doing so risks stripping the thread in the plastic spacers. Finger tight on a jewellers screwdriver is sufficient.
Now its time to fit the wheels and gears. Here's the parts for the unpowered front axle:
Going from left to right, we have a wheel, M1.6 washer, axle muff, M1.6 washer and a wheel. The washers are used to take up some sideplay, which was a bit excessive.
Here's the rear driving axle:
From left to right we have a wheel, a integral axle muff and 14 tooth gear, a M1.6 spacing washer and the other wheel.
Mounted in the chassis, they look like this:
Now let's look at the intermediate gear shaft:
Left to right is a 1.5mm stub axle, 28 tooth 100DP worm gear, another integral 14 tooth gear/muff, a M1.6 spacing washer and the other stub axle.
Mounted, they look like this:
Because everything has been checked and aligned previously in the jig (see part 1 of "Framed"), the gears mesh sweetly and the chassis rolls very nicely.
The chassis block is still overlength at present but it will be trimmed back soon. Also, nothing is locked or glued in place yet. This will be done at final assembly stage when the motor mount is ready.
At this point, it's good to have a little gloat at having made a free-running chassis with all the wheels square and gears meshed correctly.
Coming in part 3, mounting the motor and meshing the worm & wormwheel.
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