Valve cage design

[petertha]

I’m drawing up my design that assumes a valve group like the sketch. After searching some posts here, the general consensus was to keep the 45 deg valve face itself relatively thin for good seal, like 0.010 – 0.015”range.

Visualizing how (a newbie like me) would actually machine this, I’m guessing: set the compound to 45 deg, reference ‘zero’ off the corner with the cutting tool, then come back a fixed amount, in this picture example removing 0.010” yields a face of 0.014”.

But would one risk being ‘off’ by a couple thou just looking the corner reference? Say you were over +0.002” on one cup because there was a slight burr, then under 0.002” on another by misjudging a hair thin shaving. Now this error progresses to a proportionately varying seat dimension by adding 0.010”? Normally one can check how dimensions are progressing against an actual measurement, but how would you accomplish this on a relatively tiny, internal circular face like this?

This leads to another question. I’ve also read that you should maintain the same compound angle for all the valve seat faces & the valves themselves to ensure a good fit. Maybe the actual set angle is 44.5 deg vs. target 45.0 deg, but at least everything is 44.5 deg & matches. So I’m thinking maybe if one had a ‘stop’ point so the cage extends outside the collet to a fixed position & then you are always lopping off the same (0.010”) amount they would be the same & bypasses the corner searching issue?

Am I over-thinking the problem, or maybe there is some setup advice I’ve missed? I would have to make 10 cages + 10 valves + spares so its worth hearing advice!

 

[stevehuckss396]

You might be a bit more worried than you need to be. If you miss by .002 there should be no trouble. Shoot for a .012 seat and if you miss by a little, no biggie. I made 40 of them last year and they go pretty fast.

Valve cages start at post 280 followed by the valves. Get a good finish with a sharp tool and you shouldn't need to lap them in.

http://www.homemodelenginemachinist.com/index.php?topic=9609.270

 

[mu38&Bg#]

Maybe I'm not sure what the issue is. This is how I do my chamfers on ID's.

Have your compound set to the angle you want during machining. This isn't necessary however.
After making the final ID pass with your boring tool, or just touch off if you're not boring the cage, note the reading on the cross slide.
Touch off the tool on end face of your cage and leave the carriage there.
Dial the cross slide back inside the bore radius.
Advance the compound toward the center of the part.
Bring the cross slide back to the previously noted number, plus how much material you need to remove.
Feed the compound out to make the cut.
To be accurate on size you'll have to take tool radius into account. You could start the seat cut with the tool outside, and advance it inward to leave the burr on the other side.

To accurately set the compound angle you can use a dial indicator.

Greg

 

[gbritnell]

Peter,
Don't worry about the size of the seat. Ideally it should be small as in just breaking the corner. If you go to the following link you will see a diagram of a 3 angle seat configuration on a full sized head. The idea behind this is to first produce the seat angle of 45* and then back cut it to obtain the desired width, in this case .049-.061.
http://www.team-integra.net/forum/b...eat-angles-should-i-do-3-angle-valve-job.html
So if you take the diameter of a full sized valve and reduce it down proportionally to what we are generally using for our model engines you can see where we came up with .015-.020 for the width.
That's not to say that a wider seat won't work. What happens if you make it wider is that if your valve angle and your seat angle are just a tiny bit off (.002 for example) then you will have a bit of a job lapping the valve to the seat to get a good seal whereas the smaller the seat the smaller the area that needs to be matched.
When a valve job is done on a full sized head the valve and the seat are cut with tools that already have the proper angles ground onto them so when they are ready to be assembled there shouldn't be a need for lapping.
gbritnell

 

[deverett]

The wider the seat, the more difficult it is to seal the valves.

What some people do is to have the valve seat at (say) 45 deg and the valve machined to an angle of 46deg. The theory is that there will be line contact between the two surfaces and line contact is much easier to seal than a wide land. Theory, mind you.

Dave
The Emerald Isle

 

[Russel]

Back in when I worked in a small engine shop (chain saws, lawnmowers, etc.) The rule of thumb that we used was to make the seat contact about half of the valve face. The valve face being the part of the valve that's at a 45 degree angle and contacts the valve seat. To do this we used valve seat cutters with 31, 46, and 60 degree angle cutters so that you could under, or over cut the valve seat to center the contact point on the valve face and make the contact between the valve and seat about 50% of the valve face. The seat was cut to 46 degrees, the valve face ground to 45 degrees, then the two were lapped together to form a good seal. After a little lapping it was easy to check the valve face for proper seat placement, as well as full contact on the valve face and valve seat. We obtained the best results from always grinding the valve face until it was completely smooth (even with new valves, they aren't always straight) and at least lapping the seat, but more often then not, cutting, then lapping, the valve seat (even with new valve seats.)

 

[petertha]

Thanks everyone, those are great links & suggestions!

Part of the reason I was asking asking about how to make the (simpler) 45-deg example & the corner reference issue was - the multi-faceted faced valve cage would appear be even finer tolerances. For that I was visualizing turning the 45 deg face like suggested, but then some sort of tool that made the 30 deg & then 60 deg angles by 'stopping' to the base of valve cage so the amounts were consistent.

..am I over-complicating again? ;D

 

[Russel]

I don't think that you're overthinking it. It just depends on how much work you want to put into it. Sometimes I won't bother with too much because I just want something to run. But, other times I've gone to much more trouble if I plan to run something more than occationally.

For small engines we used Neway valve seat cutters. http://www.newayts.com.au/seatguiderest.htm

The cutter body doesn't look that hard to make, but the cutters would be hard to produce. I think you can get the cutters fairly inexpensively. But, then there is the question of size. Even the small engine cutters might be way too big. Anyway, I hope this gives you some ideas that help.

 

[petertha]

I was visualizing turning the 45 deg face like suggested, but then some sort of tool that made the 30 deg & then 60 deg angles by 'stopping' to the base of valve cage so the amounts were consistent.

I was thinking something like this, a D-bit profile with the relief angle. When it 'stops' at the base of the cage, the depth is set. And then a similar tool for the 2nd relief cut to yield the final 45 deg seat width.

<edit>hmm.... that lower angle looks hard to do with a cutter now that I look at it...

 

[Dave G]

Petetha, Here is a pic of the seat cutters I use. I made these from removable pilot counterbores. These allow you to use different size pilots for different size guides. Since I started using these I no longer have to lap the seats.

Heres a pic of how I lap the seat face on the valve. The toolpost is set at 45 degress and is used as a guide. I start with a
fine india stone and finish with 600 grit paper wrapped around a piece of square keystock. I have tried many ways to smooth the surface of the valve and this way works best for me.

The seat cutters are ground on an OD grinder with the wheel dressed at the desired angle. After grinding, the secondary relief must be ground by hand. I then stone the cutting edge being careful not to disturb the cutting angle to get the primary relief. I think these could be made using a tool post grinder.


I have been thinking about contacting one of the chamber reamer manufacturers to see if they could possibly make these from scratch but I don't know if there would be a market for them. What does the forum think? Dave