Author Topic: Webster build issues  (Read 921 times)

Offline John Rudd

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Webster build issues
« on: August 30, 2016, 09:11:37 AM »
I have restarted the Webster build after a long spell.....
Got to the stage of making the rocker arm for the exhaust valve.....
Having measured and made all the necessary bits, I find that the centre of the bolt that operates the valve does not sit directly under the valve!
I made the spacer as described but the rocker doesnt sit right..... :scratch:
Anyone else have this problem? Have I done something wrong ?
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Offline cwelkie

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Re: Webster build issues
« Reply #1 on: August 30, 2016, 10:04:36 AM »
The (Webster) drawing (on page 8) isn't as clear as it could be ... there is an offset to the valve end of the rocker.  Even though each end is dimensioned at 0.25" wide, there is a 0.063" offset which means starting with 5/16" wide stock and most of it back along one side.

... then again that may not be what's going on here.
Charlie

Offline John Rudd

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Re: Webster build issues
« Reply #2 on: August 30, 2016, 10:18:55 AM »
Charlie,
Looks like I made a skoolboy error......
I started out with 1/4 sq stock and looking at the draŵing and how my bits fit, I think that is is the issue.....more scrap for the bin :dremel:

So start with another bit.....
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Offline John Rudd

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Re: Webster build issues
« Reply #3 on: September 08, 2016, 09:15:41 AM »
So how is the cam for the exhaust valve made?

I dont see it as a simple offset turning exercise?
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Offline DavidA

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Re: Webster build issues
« Reply #4 on: September 08, 2016, 09:51:52 AM »
I do recall that on a lot of early engines the cam follower was slightly offset to the valve stem to allow the valve to rotate a bit with each operation. This allowed for more even seating of the valve on the seat.

I don't know if this is related to your problem or not.

Dave.

Offline John Rudd

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Re: Webster build issues
« Reply #5 on: September 08, 2016, 09:57:18 AM »
David,
I understood what you said, but my issue is in creating the profile...
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Offline seadog

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Re: Webster build issues
« Reply #6 on: September 08, 2016, 10:06:14 AM »
John, the cam profile, as you say, is not a simple eccentric, so cannot be made on a lathe unless you have a pattern and a profile turning attachment. Normally it would be ground on a specialised machine. Alternatively it could be made on a CNC mill.

Offline Jasonb

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Re: Webster build issues
« Reply #7 on: September 08, 2016, 11:18:15 AM »
To make the cam turn the end of a bit of 3/4" bar down to 0.712" and drill & ream 0.375.

Set up the rotary table to hold the work, can be vertical or horizontal. Using say a 1/2" cutter bring it down until it touches the 0.712" dia and zero the Z axis handwheel or DRO.

Take a couple of passes at 0.025" depth of cut until you have removed 0.075" which will be the base radius of the cam and lock your head.

Now turn the rotary table say 2Deg (nothing too accurate 1/2 a turn of the handwheel) and make another pass over the cam blank

Keep rotating and cutting and you will slowly see the cam profile stary to develope

stop when you are left with 0.254" of the original diameter remaining.

See my post part way down this thread for the step by step as its the same sort of cam just in this case it is part of a gear

http://www.model-engineer.co.uk/forums/postings.asp?th=117930


Offline John Rudd

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Re: Webster build issues
« Reply #8 on: September 08, 2016, 12:15:21 PM »
Jason,
Thanks for the explanation.....  :bow:
I'll take a look at the link too.... :beer:
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Offline John Rudd

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Re: Webster build issues
« Reply #9 on: September 10, 2016, 07:24:36 AM »
.....and one finished cam ready for hardening... :dremel:
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