LTM 51 in HO

Checking and measuring

Introduction

Now that the first test run has been successfully completed, it is time to check how accurately I have built the locomotive. If anything needs to be corrected, now is the time to do it.

The boiler cradle must lie horizontal in both the longitudinal and transverse directions. However, the position of the boiler cradle is not determined by one single measurement, but by a number of measurements that add up. These include

the wheel diameter, particularly at the pivot point;
the position of the axle bearings in the frame, again at the pivot point;
the height of the frame, again at the pivot point;
the height of the pivot point itself;
the internal dimensions of the boiler cradle.

The position of the boiler cradle is therefore a cumulation of dimensions that were determined at an earlier stage of construction and the consistency therein. If, for example, the wheels of the locomotive are not all the same, but differ slightly, a difference in height may arise, perhaps too small to be noticeable. But if the two frames of the bogies also differ slightly, the combined effect can lead to undesirable results.

Suppose that in the above list a tolerance of +/- 0.2 mm applies. In general, this will not be noticeable. But imagine that, by a bizarre coincidence, all the dimensions at the front of the locomotive are at the lower limit of the tolerance, i.e. all -0.2 mm, and at the rear all the dimensions are at the upper limit of the tolerance, i.e. +0.2 mm. In that case, none of the 2 x 5 measurements would be rejected. But the cumulative effect is that the boiler cradle at the front is 5 x 0.2 mm = 1.0 mm too low and at the rear 1.0 mm too high. And that is most certainly noticeable!

Boiler cradle

First, I wanted to know to what extent the boiler cradle was already horizontal, separate from the bogies. To this end, I turned two plugs on the lathe that were almost identical.

11,35 mm exactly.

11,35 plus ca. 0,006 mm.

Those plugs were then bolted under the locomotive and the height gauge was set to zero.

First, the position of the boiler was checked in three steps. This measurement is fairly simple. Because the boiler is well supported, you can lower the arm of the height gauge until it rests on the boiler. You repeat the measurement a few times to see how consistent they are.

These are the results:

Where	Height
Rear, close to the cab	35.50 mm
Middle, ahead of the dome	35.47 mm
Front, on the rim of the smokebox*)	35.42 mm

In other words: over a distance of 50.0 mm, I have a forward slope of 0.08 mm. I am very satisfied with that.

*) The measurement on the rim of the smokebox was corrected for the extra width of the smokebox relative to the boiler proper.

The next relevant measurement concerns the corners of the footplate of the boiler cradle. I know that I measured them at the time and was very satisfied with the results, but apparently I did not record them. I no longer have the plugs either, so I cannot reconstruct them.

Boiler cradle on the bogies

I did not record those measurements either, but I can reconstruct them. So the photos below are from a later, reconstructed measurement. The locomotive was already well into the detailing phase at that point, so you can see much more detail in the photos than in the previous measurements. In principle, it does not matter, because the detailing does not affect the basic construction and its dimensions.

Measuring left front

The measurement method was slightly different here. The parts of the locomotive are now freely moving, and simply placing the height gauge on the corners would cause the locomotive to tilt and thus interfere with the measurement. I did just that though as a first rough setting: lowering the height gauge until I saw the locomotive move. Then I proceeded as follows:

I moved the gauge backwards, away from the locomotive;
and moved it forward again.
Then the gauge arm touched the edge of the base plate. This was visible because the locomotive moved away.
Then I set the gauge slightly higher
and repeated the process until the height gauge audibly scraped across the base plate without moving the locomotive.

That was the decisive measurement.

Measurement left rear

And here too, I am completely satisfied with the results.

Where	Measurement
Left front	18.72 mm
Left rear	18.76 mm
Right front	18.77 mm
Right rear	18.66 mm

The maximum difference of 0.11 mm over a length of approximately 115 mm is a more than satisfactory result.

Finally, I took the same measurements for both bogies separately.

Where	Front drive unit	Rear drive unit
Left front	13.96 mm	13.82 mm
Left rear	13.98 mm	13.94 mm
Right front	13.87 mm	13.98 mm
Right rear	13.80 mm	13.98 mm

Here too, I have no further comments.

Conclusion

The conclusion is therefore that the locomotive is perfectly level, both in terms of its individual components and its overall composition. This meant that I did not need to take any corrective action.