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FLYING STEAM ENGINES - Lapping
Lapping
I have done a fair bit of this, reboring clapped out model
Diesel Engines, I have always managed to get them to run but I lacked the
'edge' in my technique to get it just right every time, however a final point
or two dropped into place some while ago and I think it may be of interest to
some of you.
The lap for the bore is turned and then at the business end
I drill and tap a hole of suitable size (about one third of the bore of the
engine). I use a taper (Number one) Tap to produce a tapered thread entry
where it will expand the lap as the adjusting screw is screwed into the
end,see sketch. This produces a smooth gradual adjustment as the lap does its
work and wears. The lap is then cross sawn to produce four sectors, I do this
in the mill out of choice but years ago before I bought the mill I had
managed it ok with a junior hacksaw. I use just three cuts if the lap is
smaller than say 0.250" (6mm) diameter and the smallest bore I ever lapped
was 0.125" (3.2 mm) and for that I split it in two with a single cut and a
12BA expanding screw.
This all works fine and the length seems to be of little
relevance to the cylinder but the lap for the piston is a bit more fussy, I
now believe the length of the piston lap to be of great
significance.
In my sketch you will find the piston lap is very
substantial at about 3 times the bore or more and made of solid brass. I also
use the lap as a jig for machining the piston internal form to reduce weight.
I am now as certain as I can be that it is always better to make the piston
lap shorter than the piston's finished length. I use a simple pipe clip to
squeeze the lap onto the piston. Also during lapping if I want to stop to
have a check on progress I always STOP THE MACHINE FIRST, it is of course a
bit safer AND it is obvious that as the lap leaves the component whilst it is
moving it can generate a taper where you don't want it.
The sketches and photograph shows the two laps I made to
fit a new piston in a 0.5cc Allbon Dart diesel of mine which I brought to
life again quite recently. I scrapped two pistons as they ended up 0.0001" or
so barrel shaped. I reduced the thickness of the lap and got it perfect. I
have a very nice Imperial/Metric digital micrometer, with a resolution of
0.00005"and so I can say with some certainty what taper there was at each end
of the piston. Whilst writing this I realise that the Allbon (later DC)
engines all had quite short pistons as did the later Frog engines and they
have usually presented more problems than earlier designs like the Mills and
ED 2 cc Comp Special series which used pistons nearly two diameters long.
This is due I now believe to the simple fact that the lap WAS often shorter
than the piston out of convenience with no thought beyond that simple
fact.
I won't go any further into the process of lapping as it is
well covered in many 'Model Engineer' articles over the years, several small
diesels are available in UK as plans and castings, the lapping of cylinders
and pistons being well covered. Never do I recall has the advantage of having
a short piston lap ever been mentioned but I may be wrong in that. I do not
pretend to have read every ME magazine published during its 100 odd years of
printing.
As a further point to free flight Aeromodellers a diesel
piston does not tolerate much wear before it is clapped out I reckon it is
less than 0.00005-0.0002" obviously the smaller the bore the less the
tolerable loss of diameter will be; 0.00005 (0.00254 mm)for those miniatures
of about 0.250" bore and perhaps up to 0.0002" or 3(0.00762 mm) for pistons
of 3-5cc engines whose bores are in excess of 0.5" (12.7 mm) diameter. I do
not suggest I can accuratly measure these sizes but it is certainly of that
order.
Model Diesels in long term storage
Have you ever noticed that a model diesel 'wears out' if
you store it for years and years, something you may not have heard before, it
obviously doesn't wear, it rusts. If you want to store a diesel for a long
time remove the cylinder, do not move the contra piston, and wash the piston
and cylinder bore first in petrol, methelated spirits or methanol to get rid
of the old oil, dry the spirit away then rinse it in clean water with a small
dash of washing up fluid, rinse very thoroughly and dry it all as quickly as
you can, ( I use a hot air blower and get the cylinder and piston to boiling
point) you are doing all this to remove the acid deposits of combustion. Then
lubricate the whole engine with new oil and replace the cylinder. I never
leave the piston at top dead centre as if it rusts at TDC it does the maximum
of damage, store it at BDC or just closing the ports but not at TDC. I stuff
soft loo roll wads into and around all the inlet and exhaust ports and oil
the wads in situ with the motor oil, then I seal it into a new polythene bag
and keep it in the dark. It takes maybe 30 minutes to do but it works. The
oil does not degenerate over many years in these conditions. I do the same if
they are in a fuselage, in my dark loft they look like they have been
bandaged up, I use cling film if it is in a model, they keep their
compression that's all that matters. I recently acquired a new boxed DC
Spitfire from an old Friends widow, unrun since an initial 'try out' after
purchase and well lubricated with engine oil as I suggest above and suggested
to my deceased collegue 30 years ago. That engine was run for an hour or so
on the bench all those years ago, it started third flick and restarts hot no
problem. These notes are only part complete as at mid Feb 2007 drawings and
update to follow on GEW. SUITABLE AIRCRAFT The Parker Comet engine was
designed as an integral part of the airframe and it would be difficult to
better the combination just as it is. I would substitute the thinnest litho
plate for paper or other lightweight model covering under the port wing above
the firehole door fire where it is above and adjacent to the worst fire
hazard on the whole model. For future work I will certainly use a pusher
layout to protect the engine from the worst effects of a seriously misjudged
landing. In the case of the Parker Groves hybrid illustrated earlier I think
that the Aeromodeller Plans Service F F desiun Ptishv Caf would be an ideal
place to start for inspiration. Ifthis desGn were to he used there \\oLLld
ha\e to he some weiuht redLtction done as the fuselaue isa sheet slab sided
affair and a close eve kept on the CG position and modifications to the
fuselaue design and v~inu mount to accommodate the large engine case As I
said before Pushy Cat is not a bad place to start hut it lv is only a start
If I do get to the point ~ here a real acrobatic performance becomes a
reality I will opt for a pusher twin boom layout.
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