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Axle mounts for PCB chassis

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wixwacing
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Axle mounts for PCB chassis

Post by wixwacing »

Making Axle Mounts for PCB chassis.

By Phil Wicks

In recent times I have had two or three enquiries on a narrative on how to make the axle mounts on my pcb chassis as found in the scratchbuild section, so. After a bit of a delay I have managed to sit down and do some typing and photography. I apologise now for some of the pictures as I had the camera set on the wrong light setting and rather than retake the whole lot ( there are several not shown here) I touched them up in Paintshop Pro but I think the gist of the article is still conveyed.


Making the axle mounts for my pcb chassis is a fairly simple operation but the emphasis is on accuracy at all times. If you approach the task half prepared then expect a half decent job. As the tubing is sold here in imperial dimensions then so will be the dimensions in the narrative.

Firstly, the correct choice of tube is required. I will be using standard slot car 3/32” axle rod (music wire) so the tube choice is going to be 1/8” o.d. (3/32” i.d.) for the cross tube and 5/32” o.d. (1/8” i.d.) for the upright tubes. One end of the large tube is checked for square. Remove any burrs and clean the end by rubbing the tube end on some fine wet and dry in a circular motion while rotating it slowly. The side walls of the tube end are then cleaned up with the fine paper in a radial motion for reasons I will show later.



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Place a 1/8” drill bit horizontally in a vice and slide the clean tube end over this, then, holding a pencil or similar marker about ¼” in (about six or seven m.m.) from the end, rotate the tube. This will give you a circle on the clean part of the tube.



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Next, grip the clean, marked tube end moderately firmly in a pair of parallel jaws (a vice is the best thing, plier’s jaws aren’t parallel when opened) just above the pencil marks. Pull the tube from the jaws and a score mark will be seen to cut across the pencil mark on both sides. If the tube is round then these marks will be directly opposite each other!



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The marks are the starting point for drilling. Using a round needle file, file a small radius where the score and the line intersect.


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Once done, using a small (1/16” or similar) drill bit, drill in the centre of the radius on both sides. Don’t drill right through in one go or you may loose accuracy and have an offset hole.


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Once drilled from both sides, pass the drill through both holes to check they are at right angles to the large tube, this is important. If there is any discrepancy it can be rectified at this stage by carefully working the drill and applying side pressure to the hole and reaming it square.


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Next enlarge the holes with a 1/8” drill bit, again from either side. At this stage take care as the 1/8” bit may pick up in the irregular sides of the tube. Once complete, check the new hole for squareness.


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You will find the smaller tube will pass through as a snug fit and hopefully at 90 degrees to the large tube. If it is slightly out it can be rectified later on.


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The drilled end is then cut off with enough spare to mount on the chassis lets say ¾” for now. Repeat this another three times if you are doing four mounts.

Next is to drill the pcb. I’m assuming at this stage the chassis has been marked and cut out! The chassis should have the front and rear axle centres marked on it and the hole positions will need to be marked on these lines a little way in to allow enough land on the chassis to support the uprights once in position.


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Pilot drill the hole centres with a 1/16” bit drilling in from the pcb copper side. Then open up the holes to large tube diameter (5/32”) from the board side, this will reduce the risk of tearing the copper from the board at the edges of the holes.


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Once this is done, cut some 1/8” o.d. cross tubes which will become the axle tube mounts. These only need to be as long as necessary but at least the width of the uprights. these are placed through the uprights. Next is to solder all in place as accurately as possible. You will need some axle material now.



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Mount all the uprights with their cross tubes into the pcb copper side. Then place the front and rear axles in place through the tubes. The whole lot is then pushed down to the pcb copper side surface, this automatically squares every thing up. There should only be an 1/8” or so of upright above the cross tubes. With everything cleaned and held down, solder into the open tops of the uprights. The solder should flow readily around the axle tube, fixing it in place. Once all four are done, allow it to cool.



I don’t have a model in progress so there aren’t any illustrations for the next section but hopefully it should be self explanatory as it only requires a bit of common sense:

Next bit is tricky, three hands are required here!! It is necessary at this stage that the body of the model is prepared and ready to accept the chassis. It doesn’t have to be painted or decaled, just physically finished and ready for details and paint. This includes chassis fit. The chassis must have already been cut to shape and made a good fit into the body. When this is so, we can set the axle heights.

With the rear axle and mounts in place, offer the body up to it and set the axle height so the wheels sit in the correct position in the arches. If you have used the correct drill, the uprights should be a snug fit in the board. Without disturbing anything, drop the chassis out and apply the soldering tools to spots front and rear of the uprights. It isn’t necessary to solder all the way round now. Just tack them in place. This is repeated on the front and you should now have a model that sits comfortably on all four wheels at the correct ride height with wheels central in the arches. That just leaves the rest of the chassis detail to be finished before fixing to the model.


One other extra I have incorporated into the design is a simple fix for oiling. Once all the mounts are in place you will get a pcb chassis with four open tubes protruding from the underside. I trim these to length with a Dremel type cut off disc and using a couple of inter fitting brass tubes I make a sleeve which will fit snugly inside the uprights temporarily. I then use this sleeve as a guide to drill a 1/16th hole centrally in the axle cross tube from underneath. This will allow me to put a couple of drops of oil on the axle from time to time without dismantling the model!


I have short circuited the lengthy process of hole alignment by using the same technique to make a hole boring jig. This greatly speeds up the process and reduces drilling error. It also consists of two pieces of brass tube but this time the tube dimensions are increased to take the drill bit and the upright tubing.


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For other tips read the articles in the scratchbuild forum like the Aston Martin DBR1 and especially the Lotus Cortina

Happy modelling!
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When I'm not racing slotcars,
I'm out in the back yard, burning food!!

When I win, it's because of my talent, not my car or my controller!
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