Author Topic: machining valve cages  (Read 447 times)

Offline Mike R

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machining valve cages
« on: March 25, 2020, 11:36:42 PM »
So before I go back and remachine or lap all 18 valves for the P&W Wasp Jr. I want to make the valve cages.  By design they are meant to be a light press or slip fit and held by chemical (i.e. high temperature loctite or equivalent) in the aluminum cylinder head.  They are basically 3/8" (9.5mm) OD and have a 3/32" hole for the valve stem.  I have the material (bearing bronze) but its far more pricey than that of the valves so I would like to experiment as little as possible.  :help:


My question is how to machine the valve end of the cage? The spring end is less concerning, as long as I make it so the spring fits I'm good and if I figure out the valve end the same tool will likely work at the spring end.  At the valve end the internal geometry is quite small and relatively deep compared to the section and I want to not restrict flow as much as possible so I'd like to keep to the as drawn profile.  I'm guessing I need to shape my own tool, and I'm looking for advice from those that have been there and done that.
Thanks,
Mike



Online Brian Rupnow

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Re: machining valve cages
« Reply #1 on: March 26, 2020, 12:16:02 AM »
You need a tool that centers on the valve stem portion of the valve cage so as to keep perfect concentricity between the valve seat and the valve stem. This tool was originally designed by George Britnell, and I have found it to be an absolute life saver. The hole for the valve stem is machined using a reamer in the lathe, and then this tool is turned by hand to give a valve seat area of about 0.025" to 0.035"---Brian

Offline Mike R

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Re: machining valve cages
« Reply #2 on: March 26, 2020, 12:44:28 AM »
Hi Brian,

Thanks for the tip on valve seat cutting, I've seen that before and will remember it.  I'm still concerned with the recessed geometry and how to get down in there.  If I grind a form tool I'm thinking it will break due to its long narrow profile and it can't have extra material being inside a bore.
See picture with circled area.


Mike

Online Jasonb

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Re: machining valve cages
« Reply #3 on: March 26, 2020, 07:06:38 AM »
I think I would put the reaming size drill in for the 3/32" hole in first then plunge with a slightly undersize 3-flute milling cutter down to the end of the valve guide to get most of the waste removed before following up with a form tool to do the recess around the guide.

Rather that thinking of a turning tool as the form tool you may be better with one turned from silver steel and one or two teeth cut on the end, giving the swarf somewhere to go will be the main issue.

After that ream and turn the valve seat, I never use a seating tool if I can get the part into the lathe to turn the seat and don't have to spend much more than 60second lapping in the valves and not motoring the engine to bed things in and always seem to get plenty of compression.
« Last Edit: March 26, 2020, 07:09:46 AM by Jasonb »

Offline Allen Smithee

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Re: machining valve cages
« Reply #4 on: March 26, 2020, 11:04:53 AM »
It might be fiddly in these sizes, but have you considered milling the annular feature using a rotary table rather than boring it? It gives you more control over cutting tool pressure, speeds etc

0.0000007 supplied,

AS
Quidquid latine dictum sit altum sonatur

Offline Mike R

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Re: machining valve cages
« Reply #5 on: March 28, 2020, 02:46:04 PM »
I think I would put the reaming size drill in for the 3/32" hole in first then plunge with a slightly undersize 3-flute milling cutter down to the end of the valve guide to get most of the waste removed before following up with a form tool to do the recess around the guide.

Rather that thinking of a turning tool as the form tool you may be better with one turned from silver steel and one or two teeth cut on the end, giving the swarf somewhere to go will be the main issue.

After that ream and turn the valve seat, I never use a seating tool if I can get the part into the lathe to turn the seat and don't have to spend much more than 60second lapping in the valves and not motoring the engine to bed things in and always seem to get plenty of compression.

Thanks Jason,

After I posted I did think about it more and realized that I could remove much of the waste with "normal" tooling - I'll need to see what kind of end mills I have that would be appropriate. 
BTW - can you say why do you recommend a 3 flute end mill over either a 2 or 4 flute?  3 flutes are rare as hens teeth in my shop and the (cheap) places I source from.
I guess this is still going to be a case of making the tool to make the tool to make the part, just not a very complicated set of tools.  I don't have any current tool that will form the 3 degree inner cone that will need to be formed on the silver steel form tool.  Nothing to do but get up and go do it. 

Thanks,
Mike

Offline Mike R

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Re: machining valve cages
« Reply #6 on: March 28, 2020, 02:55:42 PM »
It might be fiddly in these sizes, but have you considered milling the annular feature using a rotary table rather than boring it? It gives you more control over cutting tool pressure, speeds etc

0.0000007 supplied,

AS
Not considered at all - at least in my case,  others may well consider it.  My mill is CNC'd by me, and although its relatively capable, it does have some backlash (+/- 0.002") and too slow a spindle for small cutters.  I've learned, from hard experience, that it eats tiny cutters due to that backlash.  Larger cutters can tolerate a momentary 1-2 thou increase in chip load, but a small cutter that has a chip load of less than a thou just breaks when you give it 3 times its normal load.  I'm also not really interested in chasing the backlash, as its one of those rabbit holes people seem to go down and never come out of. 

Online Jasonb

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Re: machining valve cages
« Reply #7 on: March 28, 2020, 04:09:57 PM »
I find the 3-flute ones drill a bit better than 2-flute and I have more of them in the 10mm or less sizes than 4-flute.

These are mostly the FC-3 "disposable" type cutters commonly available upto 1/4" or 6mm but you can also get them upto 3/8 or 10mm There are not that expensive in the 6mm shank sizes and not worth sharpening anyway.

Regarding cutting the tapered hole in the form tool after initial drilling out you may be able to hold say an 1/8" milling cutter in the toolpost and use that as a lathe tool with the topslide set to the required angle.