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Lapping valve tapered plug

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tractionengine4219/09/2009 09:19:49
377 forum posts
110 photos
Hi everyone
I am building a 3" scale Allchin, I am looking at making the draincocks for the cylinders which have a tapered plug. The drawing says the valve body and plug tapers should be ground in. I am concerned that if I lap/grind them in using a diamond or similar grinding paste then the abrasive is going to become embeded in the parts. (The drawing suggests bronze for both parts). I was just wondering what procedure is normaly used, is there a method of lapping in the tapers without impregnating the parts with abrasive? At this scale the plug will be 1/4" diameter.
Many Thanks
mgj19/09/2009 11:13:20
1008 forum posts
14 photos
I had the same thought on a boiler feed pump tapered valve. Not on an Allchin, but same idea.
I turned the taper on the plug, and the taper for the body reamer at the same setting using a nose radiused cutter - so the finish was pretty glossy. Then I just added a dab of silicone grease.
I havent tried it in service with water, only air, but that holds air without leaking at 150psi.  If it won't hold water properly I shall just add a small O ring this side of the hole in the taper.
tractionengine4224/09/2009 14:42:55
377 forum posts
110 photos

Hi Meyrick>>

Thanks for your reply>>

I was being led by the drawing saying grind in the valve plug but what you say sound good to me. Basically making the cutter/reamer and plugs at the same setting and keeping the quality good means there should be no or little problem.>>


Unfortunately I work overseas most of the year which some what slows progress on my Allchin. However, I have a small Sherline that I use in my overseas apartment so progress can be made on smaller parts. I will make these parts following your advice but will not be able to test them until I get back to the UK in December.>>


Again Thanks>>




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