hardness spec for cam plate & tappet

[petertha]

I finished the cam plates & tappets for my radial. The cams are 3.5mm thick A2 (air hardening) & tappets are 3mm diameter O1(oil hardening) tool steel.

I found a local knife maker guy that has the appropriate heat treat equipment. He said he heats A2 parts in foil bag with oxygen scavenger & clamps between 2 aluminum flats for quench & minimize distortion as much as possible.

My question is, what hardness target numbers should I specify? There is more work and material cost in the cam plates so I was thinking make them 5 points harder than the tappets. So maybe 55 for the cam & 50 for the tappets? Would it make any difference if I said 60 & 55? The only other consideration is the plates are the more critical pieces to minimize distortion, but I'm guessing the quench is where that mostly would occur anyways? Any experience or feedback appreciated.

 

[WOB]

I think RC55 is plenty hard for all parts and can be reached without scale formation since the temper temp is low. The operating loads are small and it's not like you are going for 1000's of hours of operation before overhaul.

I am curious as to why your knife maker friend quenches A2. It will reach max hardness by cooling in still air which eliminates distortion from a quench. That's why it's classified as an air hardening steel. What am I missing?

WOB

 

[petertha]

I might be using the wrong words myself. I think A2 parts are typically raised to appropriate temperature/soak time in the HT oven. Then removed & allowed to cool in ambient air. That's what I am calling quench. But he said for knife blades which are thin / delicate / potentially non-symmetrical, the parts are sandwiched between aluminum plates vs. still air, the idea being to minimize distortion or at least confine it to a more desirable plane. But the plates themselves are at room temperature, no external cooling or heating AFAIK.

Specific to my cams plates its the potato chip effect I'm trying to avoid. Its not particularly un-symmetrical but does have holes & lobes & such. What I don't know is if the aluminum itself is having an effect on temperature decay. He said no/minimal, but I guess I'm trusting (hoping) that knife blades might be more of a challenge & that's why they do this. Once this procedure is done, then comes tempering at much lower temperature to achieve target hardness.

The tappets are O2 so its the oil quench treatment. Here I can foresee if they just get dropped in the bath randomly they could distort in the long axis. These must slide in bronze inserts to a certain fit. Fingers crossed they wont banana too much. What can he do dunk each one axially with tweezers? (Might be pushing my luck -LOL).

 

[WOB]

The tappets can be rotated axially at a 100-200 hundred RPM in a drill press chuck as they are lowered into the oil. The rotation agitates the oil so that the vapor film is dissipated and max hardness is achieved. It also insures that the temp gradient is equalized radially as the tappet is submerged, minimizing any tendency to banana. If the whole tappet has to be hardened, you will need to make a holder that fits in the chuck and also exposes the entire length of the tappet to the torch flame. But that is the hard way. Pun intended.

I would not attempt to harden the entire length of the tappet, but just do the cam end. Then you can grip the other end in a drill chuck or even pliers with no chance of banana. The softer body inside the bronze insert will not wear significantly over the life of the engine.

WOB

 

[petertha]

Yeah, I'm not sure on these tiny tappets there will be much opportunity for any gradational hardness along the length. They are only 3mm diameter & 13mm long. So heat is going to transmit very quickly. When I did a 4mm diameter O1 shaft with a torch myself, the whole length was basically one uniform color, both in heating & tempering. In fact the straw to purple came so fast I basically missed it. I was thinking of just dipping the ball end into 1/4" depth of oil & just use the HT guy to temper (since that is outside my mini oven range). But the other consideration is the opposing end of tappet that rides the cam is a female socket where the pushrod ball end engages. I think I want the same hardness on the whole tappet so the parts alternate.

 

[mayhugh1]

Peter,
I think you're over-thinking the tappets. Just drop them in oil and then temper them.
My Hodgson cam rings had thinner cross sections and looked more prone to warping than yours. They were machined from O2, and so I just quickly dropped them in a half gallon of old transmission fluid and was surprised to find they didn't warp to any measurable degree. I tried to temper them at about 50F lower than the tappets so they would be somewhat harder than the tappets if I remember correctly. I have no experience with air cooled tool steel which is designed for a slow air quench. Although clamping them between two plates sounds like it might minimize warpage, it will also hasten the quench which will affect the hardness and maybe even damage them (as in crack them). Your sections aren't as thin as your knife-maker buddy's, and so I'd think twice about copying his technique. I think your only choice at this point is to go for it. If it were me I'd just let them cool in air. If you preheat the clamp plates you're talking about, that will slow the quench and affect the hardness in some (?) way.
The designer of the engine may have been familiar with the issues involve and selected A2 as a result. If you think too far outside his box your results may not be as good as his.
Be sure to let us know what you do and how it turns out. - Terry

 

[petertha]

Really good points, Terry. I have a spare, slightly boo-boo'd cam plate that I will be including for him to practice on. The aluminum sandwich plate thing has been nagging at me too from standpoint of increased heat dissipation. I've seen several pictures of blade makers doing it but difficulty finding temperature effect data. Plus a 'little' bit of distortion is only relative to how much they are dressing off in final grind which is kind of interpretive to me as a non-blademaker.

The whole reason for going A2 was to minimize quench distortion. I believe still air is the norm from what I've read. So I think I'll say still air, evaluate the tester result & go from there. I also reduced the size of lightening holes from plans & made the pattern a bit more symmetrical. The only reason I didn't eliminate them altogether for the 'who cares' weight savings is they assisted as a machining step.

I've also included a few spare tappets & sample tappet guide. Hopefully this story will end well. I'll keep you all posted.