1/4 scale Porsche 917 180 degree V12 Build

[steamer]

What Mike says. Mostly. However I would say that for this sort of size of model a valve clearance of 0.010" is more than necessary or desirable.
Firstly, if the valve clearances are not accurately set, then your acceleration curves and such are thrown to the wind. Accurate setting means you do not really need a lot of clearance.
Secondly, a large clearance on the base circle radius makes it difficult to design the ramps between the clearance and the bottom of the flank. At least not if you want a cam that looks as though it has a continuous smooth surface.
As I ground the cams for my Seagull, I decided I could halve ET Westury's recommendation, and went for 2-1/2 thou, and that seems to be working nicely.
I designed them with ramps in the form of a radius connected to the relieved base radius by short tangents, like this exaggerated diagram:

Yup   Similar approach to what Mike is talking about,       And from the perspective of lash, the key will still be making all the part as identical as possible....   the better I do there, the less adjustment I'll need to do later.

Dave

[crueby]

On my DOHC Halfa build, I adjusted the valve clearance mostly by shaving bits off the valve stems, rather than the buckets. I did do the shell game with the buckets initially to get the best starting points.

I'm going to start by making the parts as identical as I can....it will be a touchy process as I started with some prototype parts this past weekend.   boring the two parts will be the way forward.  I need to hold the depths of the bores tight on both of these parts.    So I'll need to make up a nice boring bar for this....Also I need to confirm the buckets don't go "Potatoe chip" on me when I harden them.    Well  At least no too much

Dave

What would cause parts to warp in hardening? Is it evenness of heating, or something in the quench itself?


I didn't  get all the subtleties in  the cam discussion,  but still wowed by the model!

 

[steamer]

On my DOHC Halfa build, I adjusted the valve clearance mostly by shaving bits off the valve stems, rather than the buckets. I did do the shell game with the buckets initially to get the best starting points.

I'm going to start by making the parts as identical as I can....it will be a touchy process as I started with some prototype parts this past weekend.   boring the two parts will be the way forward.  I need to hold the depths of the bores tight on both of these parts.    So I'll need to make up a nice boring bar for this....Also I need to confirm the buckets don't go "Potatoe chip" on me when I harden them.    Well  At least no too much

Dave

What would cause parts to warp in hardening? Is it evenness of heating, or something in the quench itself?


I didn't  get all the subtleties in  the cam discussion,  but still wowed by the model!

 

Heating up thin walled parts to cherry red and quenching in oil....If I can fixture it so I can quench it symmetrically,  I might stand a chance.     It all takes some thought.....but who am I to tell Crueby about planning ahead!   

[crueby]

On my DOHC Halfa build, I adjusted the valve clearance mostly by shaving bits off the valve stems, rather than the buckets. I did do the shell game with the buckets initially to get the best starting points.

I'm going to start by making the parts as identical as I can....it will be a touchy process as I started with some prototype parts this past weekend.   boring the two parts will be the way forward.  I need to hold the depths of the bores tight on both of these parts.    So I'll need to make up a nice boring bar for this....Also I need to confirm the buckets don't go "Potatoe chip" on me when I harden them.    Well  At least no too much

Dave

What would cause parts to warp in hardening? Is it evenness of heating, or something in the quench itself?


I didn't  get all the subtleties in  the cam discussion,  but still wowed by the model!

 

Heating up thin walled parts to cherry red and quenching in oil....If I can fixture it so I can quench it symmetrically,  I might stand a chance.     It all takes some thought.....but who am I to tell Crueby about planning ahead!   

Yeah, well, I DO tend to do a lot of thinking and planning ahead, making notes, sketches, later on tearing them up and drawing more while imagining I can hear the machining gods laughing so hard they fall off their shop stools...      Many times it all comes down to, "Doh - Put a rivet over there to hold it!"

Doing some tests on a process before assembly line making dozens of complex parts its WELL worth it!
 

[Charles Lamont]

What would cause parts to warp in hardening? Is it evenness of heating, or something in the quench itself?

Another likely cause is the shape of the part. As hardening causes changes to the crystalline structure of the metal, it can also cause dimensional changes. Silver steel (drill rod to you) usually grows a bit. A bucket tappet is likely to end up tapered because the base disc and cylindrical walls behave differently. There is also likely to be some inconsistency in the quench, in getting liquid down the hole. I think this is what causes ovality. I had a right game with the Seagull tappets (http://www.charleslamont.me.uk/Seagull/tappets.html), with eleven attempts needed to make four. On 9/32" diameter, after hardening (in water) they were typically at least 0.001" out in combined taper and ovality. Lapping them true and to size was not as difficult as I feared.

[steamer]

Today's effort.   Fixture build time.

A 4th axis trunnion fixture for the cam box on one side and the cylinder heads on the other, specifically the features for the valves and valve buckets.   

[Vixen]

Impressive fixture. Does your CAM sort out the trunion offsets?

Mike

[steamer]

Impressive fixture. Does your CAM sort out the trunion offsets?

Mike

Hi Mike,
No  I worked that out.   1.096 offset on one side and 0.210 on the other for the cam box and cylinder head respectively.    I'm making up the trunnions   Obviously, there is one on the other side...though it's not shown.    I'll have a plate with a bearing to support the far end..

I'll do the center sections in the vise, and the fins in a seperate fixture to get 5 sides on the cylinder head..    The trunnion is just for the inclined surfaces for the valves, cam tunnels ect.

Dave

[steamer]

Mike

Also, the cam box is biased slightly to one side as the angle for the valve features is 30 degrees on one side and 35 degrees on the other.    Where these two features cross needs to be at the center of the trunnion.  then it's easy to pick up and machine.

Dave

[Vixen]

   The trunnion is just for the inclined surfaces for the valves, cam tunnels ect.

Dave

Ah! now I understand.  The trunnion is being used as a very accurate rotary table controlled angle plate. So each separate face can be treated as normal 2.5 D geometry with a little mandraulic input for the lateral offsets. That's still tricky work but should make for good repeatability for machining the valves etc.
I thought for a moment you had won the lottery and now into 4/5 axis machining.

[Vixen]

Hello Dave,

Here is a 'happy camper' at Crossthwaite and Gardner With a full size W125 cylinder head/ block set up on a big 4th axis trunnion to do the same work as you are planning for the 917.



Things would be a lot easier if we both had a 'walk in', ' sit on' Haas milling machine to play with.

Cheers   

Mike

[steamer]

Yup!   Exactly   nice bit of kit though mine will be manual.

Dave

[steamer]

Made a lot of chips!    buried Samantha Bell....I'll get them cleaned up and drill and tap some holes tomorrow.

The dividing head needs to go to the bed, so I'll probably need to machine the base of the DH so the key matches the Tormach T slot.

Dave

[Art K]

Dave and all interested parties,
Just thought I'd throw this out here. I happen to follow Barn Finds and this came in my email today!
https://barnfinds.com/last-one-made-1981-porsche-917-k-81/?utm_campaign=Newsletter&utm_source=Newsletter+(Daily)&utm_medium=email#google_vignette
If nothing else it'll make good reading!
Art

[steamer]

Alright....Summer distractions are about done now....back to the Porsche!

I'll complete the drive line parts and we'll see about the blocks.

Dave