John Cutler
Oh dear!
I recently gave up on a 64xx chassis kit from High Level, so you are not alone!
In my case it is a compensated affair. Yours should be more straightforward, being rigid.
I too thought I had produced a square chassis and at one point this was true with the front and rear spacers fixed. I think things went downhill when I then inserted 2 intermediate spacers. For whatever reason, one spacer ended up wider than the rest after assembly. I suspect that soldering bits onto or near the spacers using a Resistance Soldering Unit may have distorted the chassis without me realising. The result was a noticeable banana! To compound the axle alignment error, I also had trouble getting the HL hornblock assemblies to lie flat against the frames; not relevant to your problem. I will start again!
You state your chassis is square. Did you use a jig or tapered axles to check that the axles through the frames match the coupling rods? This is fairly fundamental for getting a chassis to run smoothly. A chassis need not be precisely square so long as the axle centres match the coupling rod hole centres.
The HL kits are precisely etched so the frames and coupling rod centres match. Errors that can cause misalignment on assembly are:
1. The spacers are not square to the frames. This may not be fatal but if visible is likely to be so.
2. The axle bushes are not centred in the frame holes. The holes should be reamed out, not drilled for the bushes to fit; they must be a reasonably tight fit in the frames, not loose enough to fall out.
3. The axle bushes are not at right angles to the centre line of the frames.
4. The coupling rod hole centres have wandered because the holes have been drilled out instead of being reamed; drills have a tendency to wander off-centre. If bearings are fixed inside widened coupling rod holes, these need to be carefully centred.
5. Misalignment of coupling rod layers on assembly. This can be avoided by using 3 drill bits inserted vertically into a block of wood to act as a jig for soldering the layers together.
6. If coupling rods are articulated, as in the HL kits, a loose fit of the rivet in the knuckle joint means the rod can be over-long.
If, after following the above, one ends up with a chassis where the rods do not match the axles in the frames, there may be a solution. This involves dismantling some or all of the coupling rods or replacing them. New rods need to be made up using tapered axles (from London Road Models or those from the Poppy chassis jig) in the frames as a jig. The rod layers are made up in this jig using aluminium hair clips or similar to hold them in place whilst soldering. A bit fiddly, especially with compensation or springing in place, but it works (credit to John Brighton). The rod holes now match the axle centres. I suspect this solution may not work if the error is more than a millimetre on a rod.
OK so the chassis and rods are now perfect but the dratted thing still does not run smoothly. If you have fitted the motor, disengage the worm and run the chassis using gentle hand pressure. This might disclose any obstructions (see below) or obviously wobbly wheels.
If the axles and coupling rods match, the remaining possible mismatch is between the axle and the crankpin of each wheel. Not only do they look awful, but wobbly wheels introduce eccentricity in running. How to avoid them?
1. Before wheel assembly, ensure that the axle ends are lightly chamfered or rounded so that there are no sharp edges to dig into the wheel plastic and allow the axle to drift off-centre on fitting.
2. Lightly countersink the inner sides of the wheel axle holes to help locate the entry of the axle into the wheel.
3. Apply a drop of spit or water to the axle ends just before entering them into a wheel. This acts as a temporary lubricant.
4. If you use a GW press for quartering, use spacers to match the thickness of any protruding wheel boss. These should support the wheel tyres when the press is applied; otherwise the axle can easily end up out of true. (Incidentally, make sure the axles are the correct length before using the GW press or it may not work properly; Back-to-Back + 2x wheel width including any outside boss).
5. If you use Alan Gibson wheels, make sure the tyres are not loose and each plastic centre is fully pushed down into the tyre. This often seems to catch me out. (Taking wheels off their axles and reassembling does not help).
Assuming the chassis frames and coupling rods match and the wheels fitted truly, we need to ensure the crankpins engage the coupling rods correctly.
1. The crankpins need to be set at 90 degrees to the wheel. If the crankpin holes must be drilled into the wheel, it is best to use a pillar drill to ensure verticality. I do not possess one. One alternative is to use a crankpin bush over the proposed hole location and drill through the bush. Much to my surprise, this tends to work quite well.
2. I find this to be very difficult to achieve with many AG wheels. If there is an inside boss to the back of the wheel, it is difficult to countersink to take the crankpins. The boss or its remains seem to force the crankpin away from 90 degrees. I confess I usually end up using pliers to bend the crankpin so it sits vertical to the wheel profile by eye.
3. Quartering of wheels need to be consistent. Not necessarily at 90 degrees but all wheels must be set with the same angle. The easiest way to ensure a consistent setting is to use the GW wheel press or jig. This cannot be used in all cases. I have tried using a Bill Bedford jig but not very successfully (note this jig seems not to set the quartering at exactly 90 degrees so all wheels must use the same jig to ensure consistency). So sometimes the quartering can only be set by eye. I use bits of sellotape, longer than the wheel diameter, across the wheels at right angles to the cranks to act as a guide; the spokes help to align the tape.
4. Coupling rod holes should only be minimally opened up by reaming to allow the crankpin bushes to turn in them. Iain Rice recommended that the bushes should just about fall out of the rods with gravity. Oval holes are not good engineering!
One thing most modellers cannot do is to check that the crankpin throw on all wheels is correct. This is extremely difficult to measure, let alone accurately, so we can only really visually identify an obvious error; the only remedy is a new wheel.
Many running issues are often nothing to do with the chassis set-up. These are often of the one click per revolution variety (although poor quartering can yield the same). Usually something is obstructing or catching moving parts.
1. Pick-ups are the bane of my life! The darned things catch in spokes. They are fine in forward but catch something in reverse. Just as you think all is OK, they bend and catch somewhere else. Then there are those that misbehave only on a curve, usually midway across a turnout so you are fooled into thinking it is an electrical fault; the lateral movement of the wheels opens up a fresh dimension for the pick-ups to catch on something. Watch out for plastic moulding bumps on the backs of wheels; even if the pick-ups do not catch on them, they will be bent by them or a wheel wobble may be induced. So you clear them from any obstruction and discover no current is being collected at the motor! Have you wondered why chassis kit suppliers hardly ever include pick-ups and are usually vague about them in their instructions? The exception was Alan Gibson who used and supplied plunger pick-ups. Fine for rigid axles but problematic if they are compensated or sprung. Me? I give up! In the past I built split chassis if possible; no pick-ups. Now all my new builds and conversions are destined for radio control; no pick-ups!
2. Brakes are another high risk cause of poor running. Make sure the brakes are removable (as in the HL kits) so you can pinpoint or eliminate this as a cause. Again lateral movement of wheels on curves may cause brake contact when running is perfect on the straight. Do not assume it is just the brake shoes that are responsible. Wheel balance weights are prone to catch on brake rods if they are too proud of the wheel, for example.
3. If, as above, you tested the chassis for running without the motor and worm attached, you will have already found any obstruction or dirt in the gear train. Beware of swarf from filing etc. Check the worm for the same and also that the worm and motor are correctly seated.
4. Sometimes the obvious gets missed. If the chassis runs perfectly without the body but not with, lack of running clearances must be the issue. The usual problem is that the coupling rods or their oil boxes clout the footplate or the steps or the insides of splashers. Sometimes the (wobbly?) wheels follow suit.
Sorry if you know this already. I just felt a fair.y comprehensive reply is in order to help anyone suffering the same issue(s). No doubt others will comment on issues I have omitted.
Good Luck!