Gallery, Projects and General > How do I??

Engine question.... bore finish.

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kvom:
Once you have a tight fit, then abrasive paper and/or scotchbrite takes off very little at a time.

miken:

--- Quote from: John Hill on July 04, 2009, 05:09:59 AM ---John, I really dont yet have the knack of turning to a tight tolerance, it seems taking off just a shaving makes a tight fit sloppy. :scratch:   Any tips?

--- End quote ---

I once read in the Model Engineer (but ive not tried it).if you want say a 3/4" dia bore .You machine it to .748"/.749" then obtain a dia 3/4" steel ball and press it straight through and out the other end. As it passes through it burnishes and produces a parallel finished bore . Anyone tried it?
Mike

Bluechip:
John

http://www.pollymodelengineering.co.uk/

If you download the 'Bruce Cat.' from here and look at the cylinder hones, there is, I think, a pic.
also a device (castings for) honing pistons. I have the cyl. hone, and they work well.. depends on the abrasive. They have stones in them, but if you put some other, finer, abrasive in the bore, (I have some Cerium Oxide, used by lapidary-ists (?) for stone polishing,) it polishes very nicely. Then wash the stones off with Ultraclene or similar.

I once tried the ball-bearing method on a 0.5" bore donkeys years ago, with a spectacular lack of success. Did the ME article give any idea of the press force to get .002" out of a 3/4" bore ??  I'd like to see it done. Not with a bench vice, I think.

Had a look, pics are there ...

pdf is 3.8Mb ...search 'cylinder hone' & 'external hone'

Dave

NickG:
John,

On my stirling engine I left the finish as it was straight from the lathe on the piston, the bore was reamed. I used to have the problem you do, seemed that I always messed things up on my final cut! I was simply to impatient and took too much off. Remember the cut you take off on the lathe doubles off the diameter, so you really have to take a tiny fraction off at a time as you approach the size, a fraction of a thou. In my experience I've never had the need to lap anything yet, providing you get a good surface finish from your machine you certainly won't need to for a steam or air powered engine. An i.c. engine may be a different matter although I was talking to a fellow model engine who's building a twin cylinder aircraft engine over the weekend and he said he is going to leave his pistons on that as from the machine.

The sprung cylinder hones look good for larger bores as they should keep it nice and parallel. I've got some brake cylinder hones but never tried them as not made an engine with a big enough bore yet!

Nick

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