Chassis for a Tyro (by Colin Binnie)

I hope that the more skilled among us will forgive an excursion into the sanguinary obvious but there does seem to be a number of our group who regard the construction of a simple electric chassis as something outrageously difficult. I therefore propose to describe the construction of an inexpensive four wheeled chassis for those gifted with ten left thumbs. When I started to explain the technique to Chris MacKenzie at Kew he became all excited (or agitated) and asked for sketches and words on the egroup. All credit may therefore be addressed to me and all criticism be sent to Chris.

To make the chassis we need the ability to clamp some sheet metal together, the ability to drill a few holes (not necessarily in the right place) and a very modest skill at sawing and filing. There is a bit of soldering involved but it is simple and absolutely Rogersproof.

I attach sketches of the finished chasses (or chassises).

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Here we go then.
First things first. You will need a set of wheels, axles to suit and some brass or bronze frame bushes. A set of metal wheels as sold for wagons will do fine and the axles can be used as well. Appropriate frame bushes for your axles should be available from commercial sources. If you have a lathe then there is a nice bit of turning practice for you. You will also need a can motor and a worm and wheel of around ten or twenty to one reduction. The chunkier OO ones will do fine unless you are building for superpower. The latheless constructor will have to ask someone to open out the hole in the worm to fit the axle.

So, to the first diagram. This purports to give the basic dimensions for inside and outside framed chasses for both the elegant 32mm chassis and the mundane or 'broad' chassis for 45mm. Wheels are assumed to be 6mm thick. Frames, coupling rods and spacers are 1,5mm (1/16'') bright mild steel. Axle diameter to your choice.

Next episode starts cutting metal!

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This is a description of a chassis system based on very simple tools and techniques but which can be used to power almost any style of loco. The rather longwinded description is aimed at the absolute beginner.

Step 1
Cut a strip of metal about 10mm wide and poke a couple of 1/8'' or 3mm holes in it at the right centres for the wheelbase (well approximately, anyway).

If the chassis is to be six wheeled a bit more care must be taken. Scribe a deep line along the strip and engage the point of your centre punch in the line when marking the hole centres. Drill the holes and insert a short length of dowel (posh name for a tight fitting bit of rod) in each hole. Hold a steel rule or straightedge against the dowels to check that the holes are in line. If not follow Digression 1 on sketch.

It is of course very difficult to get two holes out of line.

Be very upmarket and call it a template.

Step 2
Cut two pieces of metal about 25 mm longer and a little wider than actually required for the frames. Mark out the frame ends so that there is about 12mm of surplus metal at each end of the frames.

Mark out one of the axle holes. Clamp the two strips together and drill the axle hole through both the same diameter as in the template. Unclamp. Clean off the burr and reassemble with the template on top. Use a dowel in the holes to locate all three parts. Make sure the dowel looks square to the surface. Clamp up tight. Use the other hole(s) in the template and drill through the axle hole(s). Keep the drill as square as possible to the plates. Push another dowel in. Now drill two holes in the waste bits at the ends of the frames. Open up these holes to clear M5 or 2BA or whatever your local thread system might be. See sketch. Unclamp and clean up. Put the template away carefully.

Step 3.
Bolt frame plates together nuts and bolts through the end holes and with the dowels in the axle holes, trim the top edge of the frame so that the distance from dowel to edge is is equal at each axle position. If we can't be sure of putting the axle holes accurately in the frames, we make the frames around the holes. In my early years I was Chief Draughtsman for a well known company and we had a favourite saying in the office. '' The function of most bits of metal is to contain holes'' Get the holes right and it will go together, the outer shape is purely decoration!

Till next time.
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We now have our chassis side frames and are about to turn them into a chassis frame. Look at the attached sketch. All we have to do is fold up a couple of frame angles. Then solder them between the side frames using a couple of long screws or studding and nuts to hold things together as we do so. The motor mounting is sorted out as we go. That is today's target and by the time we have dealt with a few alternatives that will be quite enough for this decrepit modeller.

Frame Spacers.
The folding of the angles is simple enough and should really need no comments from me but, bearing in mind that this is intended for Mr Tyro, I have included some annotated sketches. See sketch of 'Bending frame angles'.

I cannot tell you how long to make the legs of the angles since it depends entirely on your choice of prototype but one of the angle spacers must be long enough to mount the motor. Note that the motor mounting spacer on an outside frame loco must be cut away to clear the wheels. See 'Frame Assembly' sketch.

Frame Bushes.
I have sketched a suitable frame bush for the benefit of those with lathes. Ordinary screw brass is quite adequate for these. Years ago 'Design Engineering ' ran a series of tests of small plain lubricated bearings. With an unhardened shaft there was little to choose between common brass, screw brass, bronze or phosphor bronze!. The degree of polish on the shaft did dramatically affect performanmce however. With a hardened and lapped shaft the bronzes came into their own, but if you are going to fit harden and lapped axles into bronze bearings you should be writing these notes not reading them!
I hope that some of our semi-commercial members will offer to make suitable bushes for the latheless.

Frame Assembly.
Open up the axle holes to suit the bushes and give the frames and spacers a good clean. Inside and outside framed chasses differ at this point so Inside frames first, then Outside.

Inside frames.
Put axle bushes into frames do not secure them at this stage as we want them to line up with the axles after assembly adjustment. Put two pieces of rod of axle diameter through the bushes and gently clamp the frames using long screws and nuts through the holes in the overlong ends. Slip the worm wheel on the appropriate rod and fit the motor and worm assembly to its spacer. Now for the jiggly bit. Wind a couple of loose turns of cotton around the worm wheel and press the worm to the wheel, trapping the cotton. Check that the rods are square with the frame. Squint at the rods from the front to ensure they are in line and not twisted. When you are happy that all is well, tighten the clamp screws a trifle more, check again and remove rods, frame bushes, worm wheel, motor and worm. We are ready to solder!

Outside frames.
Put wheel sets and worm wheel in place. Insert wheel bushes. Assemble motor and worm and mesh with worm as above. Check that axles are square with frames. Place embryo frames on a length of level track and check for wobble. Remove motor and worm. We are ready to solder!

So far so good. Wheels and wheel sets can logically be left till after soldering for the inside framers but should have been tackled first for the outside framers. I can only echo dear Mr. A.H. ; ''We are coming. We are coming.'' Except that he didn't and we will !.


Colin B
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Soldering the frame.
When we finished last time we had made a trial assembly of our chassis and coaxed it into some semblance of truth. Remove the motor and put it away for we are going a-soldering. For this the metal should be bright and shiny particularly on the joints.

There are more ways and implements for soldering than I wish to discuss here, so if you disagree with the simple tyro method feel free to do it your way. We are going to use the SWMBO's gas stove or, if completely under the tyrant's thumb. one of those cheapo miniature gas torches to provide the heat. Soldering is really all about heat management, getting enough heat in the right place to melt the solder without over cooking the joint.

So let us get on with it.
Lubricate the joints with flux. (Of which more later) Turn the chassis into the position shown and drop a short length of solder (equal to about 12mm of uncooked spaghetti )into the corner. Heat the corner from the outside, trying to heat both the sideframe and the spacer equally. Do not heat the solder directly. The solder will melt and flash into the joint showing as a bright line on both sides. Life is suddenly much easier. The solder now forms a thermal bridge between the sideframe and the spacer so that we can heat both sides of the joint from the outside of the sideframe. A pause at this point to examine the joint away from the flame will do no harm. If the solder extends the full length of the joint then do not bother to read the next bit.

If the solder forms a neat line but does not extend the full length of the joint,apply a drop more flux and heat the frame about 3mm farther along the joint. As the solder melts and and extends along the joint it will 'chase the heat'. Add more solder as necessary. Repeat for all joints. Solder wheel bushes into place last, using axle to align them.

So what can go wrong?
a) The solder melts and forms a shiny puddle on one side of the joint but perversely refuses to go in. Stop for a moment. Remember the bit about solder chasing the heat. One side of your joint was much hotter than the other. Apply some more flux. Lay some more solder in the joint and turn job so that Sir Isaac can assist the puddle to flow into place . Heat joint evenly and watch as it all draws down into the joint.

b) There is a black charred mess with dull grey beads of solder isolated within it. Go stand in the corner, place dunce's cap on head and write out 'I must not put direct flame on solder and flux'. Clean up the mess, apply fresh flux and solder and try again. The problem is that in the fierce direct torch flame the flux and solder burn before the joint is hot enough to receive them.

General note on soldering.
Solder is a gentle solvent for the solderable metals and in dissolving the surface forms a new alloy. Just like liquid solvent on a plastic card joint it runs into the joint by capillary attraction. There is a snag however. The metal has a coat of oxide upon it which persistently and instantly re-forms as we clean the surface. This coating forms a barrier between the the molten solder and the underlying metal. Application of a chemical which dissolves the coating as the joint is heated solves the problem. This is called flux. Presumably since workers who picked up hot work were heard to cry ''Flux it !''.
For this job I use either 'Baker's Fluid' (also known as killed spirits) or the paste flux sold for domestic copper pipe work. They are nasty, evil substances which promote rusting on anything around so take the finished job into the kitchen and use SWMBO's toothbrush with bath and toilet cleaner to scrub away any detritus. Rinse the chassis thoroughly and return toothbrush before placing chassis in a warm place to dry off.

Give your chassis a critical look over. All OK ? Remove the nuts and bolts from the extended ends of the frames and cut the frames back to their final length. 'Tis done

Any lumps and bumps along the joints can be cleaned up with a scraper .(See sketch) It is worth making a couple of these for you will need them if you are to do metal bodywork. For the really house proud, steel wool is great for cleaning up but keep it well away from the motor and borrow the toothbrush again to get rid of any bits.

Check that axles revolve freely. Stick some masking tape around any protruding axle ends and around the worm wheel. Paint the chassis the colour of your choice. Grey or red primer followed by top coat. Both from spray cans. No fancy etch primer needed for steel.

Wheels and cranks.
Because the next logical step is the fitting of wheels and cranks and the quartering thereof, it might be a good idea to talk about aforesaid wheels and cranks since their absence makes fitting difficult. Take a piece of steel between 2 and 3mm thick. Have a look in your scrap bin. We only need a little bit ( don't we all) and it is a common size for domestic fittings.
The first step is to make 'drill jig'; this is a posh name for a scrap of steel with a couple of holes in it. One is axle sized, the other whatever your chosen crankpin size. The distance between the hole centres is the crank throw, more or less. Into the axle sized hole push a short off cut of axle size rod and secure it with 'Loctite, solder, 'Araldite or whatever takes your fancy. If we now drill an axle size hole in our chosen material, we can locate the drill jig in the hole and drill our crank pin hole using the jig to locate the drill. We can do it again and again and again. And the hole centres are always the same! Cut out and shape the cranks. With counterweights or without, it is up to you.

My apologies for taking so much time on soldering but it can be the one thing that scares the tyro off. It is easy, it is quick and a skill easily mastered.

Colin B
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Crankpins and coupling rods.
And now for the home straight. In instalment 5,5 (5 1/2 for imperialists) we made some outside cranks. With a short dissertation on crankpins and coupling rods we should be ready for assembly.

The sketch shows three variations on the crankpin theme. The first is a plain pin with a washer behind the coupling rod. A protective piece of thick paper/thin card is placed between the rod and the retaining washer before solderring the washer to the pin. Removal of the paper should leave a working clearance.

The second utilises a crankpin tapped for a retaining screw. I personally use 1/8'' dia crankpins tapped 8BA or the tapped crank scheme as detailed below.

Lastly, the crank is tapped 8BA and the crankpin is in the form of a sleeve with washers front and rear. This is particularly neat when the special steel screws from EKP with 9BA heads are used.
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The coupling rods are made from the same material as the cranks. Remember that template we used to drill the holes in the chassis ?. You were advised to wrap it up and take great care of it. I hope you did because now you need it to drill holes in the coupling rods !. Make the holes a close fit on the crankpins at this stage. As usual we drill the holes first and finish to shape afterwards.

Coupling rod ends come in several shapes and sizes. Modern production line locos tend to have round bosses that utilise circular bearing bushes or even roller bearings. They often have an oiling boss on top. Older locomotives could not depend upon the accurate matching of the rods to the axle centres so adopted a distinctive rectangular boss with adjustable brasses. A bit of packing between the end of the rod and the brasses allowed the length to be adjusted. Not that I suggest that we follow suit but it sometimes helps if we know what we are looking at !

To shape the rod ends it is helpful if we make another jig similar to the one we made for shaping the cranks (or poke another hole in that one ). I have a posh version of this jig that is in constant use for roundy bits

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But we can leave all that for the moment and concentrate on assembling the chassis with its cranks/wheels. I have read long and complicated accounts of jigs to overcome the problem of quartering the wheels but the problem isn't really there at all ! We are going to stick the cranks/wheels in place with Loctite. I'll describe that process in a moment. And by the way I shall use the term crank when it could equally mean wheel and hope that the Race Relations lot don't notice.

With the motor out of the way, put a crank on an axle. Give it a moment or so to grip and put a crank on the other end of the axle. With crank one pointing down wards, twist this crank into the horizontal position. The idea is to fit the cranks so that one leads the other by ninety degrees. i.e one points forward when the other points down. Do your best but it isn't too critical. Fit the third crank to the other axle and put the chassis aside overnight to set.

Fit one of your embryo coupling rods in place on the completed side and slip on an elastic band or use masking tape to hold it at bottom position. Assemble final crank and rest a steel rule on the crank pins. Twist the crank so that the edge of the rule can be lined up with the edge of the frames. That's it, all over ! Leave to set, go away and do something else but do leave the Loctite to set before playing.

Open up the holes in the rods slightly to give a running clearance and check that all is well. If OK, file up the rods to final profile and fit them.

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You should now have a running chassis. Roll it up and down the bench a few times. If it is merely stiff then oil it and roll it to and fro on a wooden surface until it is free. ( Being idle, I have a Meccano rubber tyred wheel on axle that goes in the lathe and drives against a wheel tread) Fit motor and watch the wheels go round. Wear soppy expression and bring in family, neighbours etc. to witness triumph.


If it locks solid at one particular point, check it out against the 'Rods and cranks' notes in next message

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Loctite is great stuff. It does just what it says on the bottle. It locks tight so keep it out of your bearings. When fitting wheels or cranks put a small drip on a plastic surface and apply to the the inside surface of the hole. Wipe the inside face of the crank clean and fit the crank so that the axle pushes any excess to the outside. There are many grades of Loctite but I use 601 or 603. They do much the same job but 603 is more tolerant of residual oil or grease. Loctite is easy to use but beware its limitations. Parts need to be degreased with cellulose thinners or methylated spirit. It 'grabs' quickly but takes aout 24 hours to develop full strength. This leads the unwary into trying to test a joint and weakening it before it has really set.

So that is that, thank you for your time and Merry Christmas.
Colin B

(Colin sent in this last part of the the article on Christmas Eve 2003.  - Dick S)

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