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Fine finish on alloys

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Steve White20/02/2011 21:38:15
11 forum posts
2 photos
Hello there, has anyone experience of achieving a fine finish on aluminium or light alloy?
I am renovating a small airbrush type compressor and the valve seats, which are recessed and cast into the head are corroded. I can achieve a flat finish on my vertical mill but the seats need to have a fairly good finish to make a seal with the metal shim "flapper" type valves and I am unable to get a surface totally free of machining marks.
Is there some way to grind or polish the alloy to a smooth finish?
I wonder if I could lap them flat with some grinding paste?
would high or low speed be best?
Any suggestions would be gratefully received.
 
(I am aware of the dangers of grinding aluminium having searched the net for some info- this is a small area about 10mm x 20mm).
 
thanks and regards
Steve W
Ramon Wilson20/02/2011 22:10:44
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1655 forum posts
617 photos
Steve, take a small block of flat steel preferably ground like gauge plate and stick some wet and dry abrasive paper to to it with double sided tape. Start with about 320 /400 grit. Use Parafin as a lubricant and lap in circular/figure of eight motions - by reducing the grit to 600 /800 you will achieve the finish you desire I'm sure - this is tried and tested
 
Hope that helps - Ramon
Steve White22/02/2011 23:20:25
11 forum posts
2 photos
Thanks Ramon, you gave me some ideas...
After trying various methods, including abrasive paper, in the end, I used a piece of metal rod, faced the end dead flat, polished it and put it in the chuck of my drill press on the lowest speed. With a small weight on the end of the drill down feed to give constant pressure and some fine grinding paste, I achieved a very good finish on the valve faces and the compressor works like new.
Something else learnt.
Steve
 
Ramon Wilson23/02/2011 10:06:00
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1655 forum posts
617 photos
Welcome to the 'Guild of Cat Skinners' Steve
 
Good to hear of a successful outcome
 
Regards - Ramon
The Merry Miller23/02/2011 10:52:36
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484 forum posts
97 photos
Steve,
 
Something else to remember for the future.
 
I used to lap in the seats of gate valves many years ago in a power station. The technique used at that time was almost identical but the lap was a small disc of cast iron I seem to remember.
 
The difference was this:
A small dimple was put in the top face of the lap with a slocumbe drill, the lap itself being about 1" thick.
The last 2" of the spindlle that was held in the drill chuck was machined 1/8" eccentric and and had a 60deg point machined at the end (as a conventional lathe centre).
This gave a nice occillating motion to the lap when engaged and also prevented the valve seat from getting a slight convexity.
mgj23/02/2011 17:44:12
1017 forum posts
14 photos
Interesting. I always thought lapping ali and stuff like that was a bit of a no no, (depending on later use of course)
 
The soft metal, while it can pick up a good finish also loads with abrasive and becomes a lap in itsself. Doesn't apply to the harder metals of course, like iron or steel.
 
So the theory goes.
 
Anyway, one thing is for sure - if you want to make a good lap, just take a bit of ali, brass or the like, roll it in abrasive powder, pressing a little firmly of course, and away you go.
 
So, for instance doing the portfaces for a model, I don't mind lapping the iron face, but the bronze valve - that is left to wear in from a highly finish machined surface. Thats a sliding surface so therein may lie the a difference.

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