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Post by keith1500 on Feb 22, 2009 22:47:56 GMT
I am making a piston valve with bronze piston rings to suit gunmetal liner. (exisitng loco)
I have made a steel test bore and form tool upon which the rings are mounted. The idea being they will be lapped in the steel bore before being fitted to the loco.
I found that fine ginding paste creates an tight interference fit and doesn't aid movement to lap. Therefore any ideas on a fine grinding medium that will do this job?
Thanks Keith
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Post by welshy on Feb 22, 2009 23:53:22 GMT
Brasso may do the job
Mike
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Post by Tel on Feb 23, 2009 1:28:49 GMT
.... or toothpaste.
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Post by albert on Feb 23, 2009 9:29:31 GMT
Hello,
I understand what Keith is doing ,but do not under stand why.I thought that you lapped the two parts that run in each other together?
If the rings and bore are machined good to start with,running the engine will wear the two parts to each other.
Or am I barking up the wrong tree???
I feel sure I am about to add to my knowledge!!!
Albert.
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Post by Tel on Feb 23, 2009 10:36:11 GMT
It's purely in the interest of getting a free-running engine to start off with Albert, otherwise you could spend several hours running it in at 785 psi
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Post by ettingtonliam on Feb 23, 2009 10:49:20 GMT
I seem to have a vague memory of reading that lapping bronze items wasn't a good idea because the lapping material embedded itself into the (relatively) soft metal, and couldn't easily be got out, so you would end up with piston rings which acted like a lap on the gunmetal liner, causing rapid wear. Or am I wrong?
Regards
Richard
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steam4ian
Elder Statesman
One good turn deserves another
Posts: 2,069
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Post by steam4ian on Feb 23, 2009 11:09:43 GMT
G'day Richard
I recall the same thing about grinding pastes and soft metals to the extent that you could used a brass rod and some diamond paste to "drill" through very tough material. That said I think that toothpaste and Brasso may be OK. Fine pumice could also be used. I remember using pumice to polish the skin off dc generator commutators, we finished off with chalk.
There used to be a rhyme about one brand of toothpaste which went "You'll wonder where the enamel went when you clean your teeth with Pepsodent."
Regards to all
Ian
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abby
Statesman
Posts: 942
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Post by abby on Feb 23, 2009 13:33:59 GMT
Metal polish will work fine as will toothpaste cerium oxide , jewelers rouge or diamond paste if you can get it , remember the golden rule for any lapping , lots of lubricant and very little abrasive.
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Post by keith1500 on Feb 23, 2009 13:33:58 GMT
Apparently the principle of lapping is the abrasive will embed in to the softer material and therefore wear the harder. So when i lapped the slide valves on my OS Rocket I used a piece of aluminum to get the port face flat and then lapped the valve to that. - it worked surprising well. In this case I should have also used ali. Didn't really give it any thought. I used a lump steel as it was conveniently to hand. The ring(s) is 2 thou over size and is sprung in the normal way. The lapping should leave the ring at the right diameter all around. Any further wear will ensure it fits the bore snuggly. I have held the valve a brass test bore which is 2 thou over size and tried to blowing passed it - i was quite surprised how well it was sealing. I'll try the tooth paste or brasso tonight. Good suggestion and easily to hand  |
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Post by keith1500 on Feb 27, 2009 8:45:24 GMT
I have used the Brasso metal polish for the final lapping of the piston rings and found it very good.
Once the piston rings had been heat treated to give them a gap and springiness I lapped them using some fine grinding paste in a bore two though over size using a simple jig.
The rings were then transfered to the piston valve and lapped in a bore matching the piston valve liner. The Brasso was particularly good as it is mainly liquid and therefore lubricates and allows for easy lapping. The rings appear to have a nice uniform matt pink surface to them so i guess fitting is next.
Would using engineers blue give an indication how well they fit or should i not worry and let them bed in naturally?
Keith
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Post by drjohn on Feb 27, 2009 11:10:40 GMT
The problem with mild abrasives like brasso and Tel's toothpaste (when did you last use that on your teeth Tel? ;D) is they take the sharp edge off the ring.
Don Young used to suggest coating the piston with molybdenum grease and thumping it back and forward through the bore - albeit with cast iron, but I don't see why it wouldn't work with bronze.
DJ
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Post by Tel on Feb 27, 2009 12:19:31 GMT
The problem with mild abrasives like brasso and Tel's toothpaste (when did you last use that on your teeth Tel? ;D) is they take the sharp edge off the ring. Don Young used to suggest coating the piston with molybdenum grease and thumping it back and forward through the bore - albeit with cast iron, but I don't see why it wouldn't work with bronze. DJ TEETH? Wot teeth? |
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Post by chris vine on Feb 28, 2009 0:16:32 GMT
There used to be a running in abrasive called something like "Time Saver".
The idea was to assemble the machine with some time saver in the tight bits and then run it in. The abrasive was a crumbly type which cut for a while and then dissintegrated so that it does no further wearing.
Has anyone else heard of this or used it??
Chris.
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Post by baggo on Feb 28, 2009 1:02:40 GMT
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