From my model making experience both in steam and I.C. combined with a manufacturing background I have found that good performance and longevity go hand in hand with effective sealing of components and in particular where pressure is present.
The subject described relates to piston rings.
I see that the rings either in steam or IC applications are of major importance and deserve to be made with a degree of excellence. I also hold the view that pistons must have clearance but they are acting as carriers for the piston rings that are doing the actual sealing against pressure.
The rings I have chosen to describe have a diameter of 0.875” [bore size] Inside diam. of 0.799” with a thickness of 0.038”. The ring section is a 0.038” square. A word about material. I use S.G. Cast iron [spheroidal graphite.] This a ductile close-grained iron and takes a good finish. This leads to less chance of breakages when installing over the piston. [Surmans supply a range of sizes] There have been numerous articles written about ring manufacturing in the modelling world but the method described is based on Professor Chaddock`s and relates closely to full size manufacturing methods.
There is a degree of tooling required which may be deterrent, but the end result is justification. e.g. A round ring with a degree of wall pressure.
Initial operation is to machine the OD of the ring to a plus size of + 0.008/10”
Turn a length which will give a number of spares. Important is to make this roughing size a recorded dimension as it will be used later. Also make this section parallel. Drill and bore the inside diam.to the finished size e.g. 0.799”. A good finish will avoid stress fractures.
Part the rings off with a narrow parting tool to lessen burrs to 0.001/2” wider than the finished ring width. The ring is then placed into a recessed boss which is bored to take the ring OD with the recess to the desired thickness.
The faces of the rings are lapped on fine emery until the finished thickness.
To create the gap in the ring I have made a cleaving tool. Consisting of 2 opposed sections of HSS sharpened to a chisel point. These are brought together with the ring In between. The resultant breakage is difficult to detect.
The wall tension of the ring is created by opening up the gap to a predetermined size, in my case 0.131” [tables are available]. Tooling consists of a shouldered mandrel with a peg over which the ring is spread. [see photo] While the rings are spread, they are heated together with the mandrel to a good blood red [550-600 c] and held at temperature for 8- 10 minutes. I turn off the lights in the shed so you see the heat colour better. Allow to cool in air whilst still clamped.
Upon removal the rings should hold the preset gap. If not repeat the treatment.
Scaling is minimal and a light brushing will brighten then up.
The ring gap faces are lightly dressed to a clean-up condition and the rings are lightly deburred with wet and dry.
A stepped mandrel is required with ends centred and a convenient thread section machined on one end. A section is turned down to the original inside diam minus 0.002. A sleeve is bored out to the original turned OD diam. In which a number, generally 2 rings are pushed. The sleeve with the rings enclosed is slid on to the inside diam section of the ring and the ring clamped with a washer which is slightly smaller than the sliding sleeve. Having tightened the nut, the sleeve is then removed, leaving the rings clamped in a true running condition. The rings are now turned down to bore size. This finishing operation is important. The diam. required is to the finished size of your cylinder bore as precisely as possible.[target plus/minus zero]. Making your cylinder bores all the same size means all rings can be machined at once. Varying bore sizes means making one ring at a time to suit individual bores.
After finish sizing the nut is released and the rings removed for a light deburr on the corners.
All the above involves a degree of tooling but the end result is a ring that is round, and has a degree of wall pressure ensuring a good seal without the need for long term bedding in. Parting of a ring of cast iron and cutting a gap in it is a poor option for all the other fine work you put into your project.
Next operation is to spring the ring into the recess which is the depth [thickness] of the finished ring. Ring is sized to thickness on emery paper.
Mandrel for ring heat treatment. Ring is spread over the peg and clamped.
Mandrel for machining OD. Ring is pushed into the large sleeve. The section next to the threads is a clamping sleeve. When removed the ring is up against the small shoulder. Ring is clamped with the nut. The large sleeve is slid off, leaving the ring firmly clamped ready for final OD machining.
Over the range of models I have made I have sets of piston ring tooling that may be suitable tor sizes you may require.
For info I centre the turning mandrels to allow them to be removed and reloaded with rings for the final operation. I have the facility to final grind to size.