PM #1 steam ports

HenryBanjo · Feb 1, 2023

Hello all,
I'm building a PM#1 steam engine, and am soon going to start on the cylinder casting. When looking at the drawing, I noticed that the slots for the steam ports are 3/32 & 3/16. I do not have a slot drill or end mill in either size, however, I do have a 3mm end mill and a 5mm. Will the 3mm do for 3/32 (2.38mm), and 5mm for the 3/16 (4.76)? I have attached drawings for the valve, cylinder and eccentric which i belive are all the relevant ones.

Zeb · Feb 2, 2023

Hi Henry,
2 possible options. The more you deviate from values the less margin for error you'll have setting up the timing. Others could chime in.

I might use your 3mm to carefully mill out the 3/16" (4.76mm). If you can find a 2mm end mill, you could drill out the smaller 3/32 with several holes then run a clean up pass. You might be able to deviate slightly by going slightly big on the slots, but I wouldn't push your luck. The smaller radii at the slot ends shouldn't matter. Might make it run happier.
A more complicated but completely doable process would be to match your cam displacement (how far it moves back and forth) and machine a new valve from scrap. The port slots as machined would drive your .375 valve pocket dimension and your .188 eccentric dimension.

Going under on the .188 eccentric dimension will make the valve travel less distance fore and aft. Going over will increase.
Going high on the .375 valve pocket dimension might place pressure on both ports at the same time (not good). Going under may not get you any pressure in one or both ports.

Not having both the smaller ports an even distance from the large port will make things even more difficult, but the nice thing is PM designed it to be somewhat forgiving.

Hopefully my logic this evening checks out ok. I'm a bit sleepy. Wait and have someone else chime in before making chips.

Nikhil Bhale · Feb 2, 2023

Hi,

When I built my PM #1 engine I had the same problem. I had all metric tooling. I think I used a 2.5mm endmill and 4mm endmill for the slots. I don't remember if I changed centerline distance for the slots or the pocket dimensions inside the slide valve.

The engine is a runner, so I had done something right.

Regards
Nikhil

sawyer massey · Feb 2, 2023

Correct for the errors. With the length of the valve .....and the pocket for the exhaust

Steamchick · Feb 3, 2023

Hi, my opinion, may not be everyone's opinion. Make it right as the width and spacing of these ports is critical to the correct performance functioning of the engine.
Narrower slots will run, but can limit power from the engine at higher speeds.
Wider slots will leak steam due to "overlap" as the valve slides back and fro. That means a loss of power.
Of course you can re-design the valve and stroke to suit your tools?
But the real options are to chain drill the valve ports with slightly undersize drills, the join the holes with a smaller milling cutter, milling the sides back to the correct dimensions. As mentioned already.
That is the challenge of making stuff properly. It is not about quick metal removal, but correct metal removal. Not "making chips" but "making the right parts".
Also I would never mill a slot to size with a cutter of the same size. I would always use a smaller tool, to make the slot a slot, then open out either side to the correct dimension. That means your finishng cuts are smaller, more precise , and you are only cutting on the correct edge of the tool for the travel direction. You must never cut to drag the material into the cutter, but always so the cutter is pushing against the material. Otherwise you can induce chatter, tool breakage, or other issues. Sometimes you can get away with bad or wrong practice, but one day it will catch you out.
Enjoy
K2