|
|
Post by johnharkness on Jun 4, 2005 19:47:10 GMT
Hello there,
Has anyone heard of making piston valves out of PTFE, and can it be machined to size on the lathe?.
John.
|
|
|
|
Post by greasemonkey on Jun 4, 2005 22:31:35 GMT
Hi John
I haven't done it myself yet but it can be done and I have heard of people others doing it. The trick seems to be get the cold clearance right as PTFE expands quite a bit when hot. Shouldnt be any problem machining on the lathe, make sure the tool is sharp though.
Andy
|
|
|
|
Post by alanstepney on Jun 5, 2005 2:15:10 GMT
BUT, do watch the safety precautions. The main one is do not smoke whilst working with PTFE, and avoid getting any of the dust / shaving hot enough to burn, or igniting the waste.
|
|
Garry
Active Member
Posts: 45
|
Post by Garry on Jun 5, 2005 15:01:24 GMT
Hello John, I have used PTFE for the piston valves on two locomotives to replace the very poor quality bronze ones supplied with my locos, the first was a 3 cylinder A4 which has steamed succesfully on a few occasions and the second is not yet ready to steam but has had the chassis working well on air. At the moment I do not forsee any problems and I have heard of quite a few people using PTFE. On both my locos PTFE is like a ring about 10mm wide on each end of the bobbins which were supplied with the locos. Each ring had 3 or 4 small grooves to form what I think is called a Labyrinth seal. It is good to machine but you do get a lot of swarf everywhere and do take note of Alans comments. Mine were made slightly oversize and a press fit onto the bobbin which were then put in a spilt bush to machine to the correct O/D. These were then checked with the valve liners before removing from the lathe.
Garry
|
|
|
|
Post by Steven Robson on Jul 2, 2005 12:42:38 GMT
You could try PEEK bearing grade available from RS. I have made the pistons and slide valves for a 0 gauge loco. from it. It was very easy to machine, and looks to be wearing well. The downside is, as so often, the cost.
|
|
|
|
Post by ridleyian on Jul 2, 2005 18:39:00 GMT
I made a solid bobbin from filled PTFE (from D Hewson) for the inside cylinder valve of my enterprise as the cast iron one with cast iron rings didn't seal very well. I was recommended to do this by a chap at the last Halton show who had a very nice B1.
The snag is that the difference in expansion of a solid bobbin means that it has to be about 8 thou undersize at ambient which means that you can't run the chassis on air. however it dit run very sweetly on my 'test boiler' ( a pressure cooker with the 15 psi weight on!)
I have since made a three piece bobbin with rings of the same PTFE ( about one eighth square) with three sretched O rings underneath to take up the expansion. This runs very sweetly on air but I haven't had a chance yet to try it with the old 'prestige.
|
|
