Engine question.... bore finish.

[John Hill]

I am making my little air/steam engine and I have a piston that is a slide fit in the cylinder bore.  The piston is smooth and somewhat polished (because that was easy to do) but the cylinder is straight from the lathe and not exactly rough but not smooth and shiney either.

My inclination is to leave the bore as is for better oil holding. However if smooth on smooth is better I will need to remake the cylinder (no big deal) undersize a little and polish it out somehow.

[John Rudd]

I thought the usual way was to make a lap tool for both piston and cylinder?

Using lapping compound to achieve the desired finish....

[John Hill]

Err, what does a 'lap tool' look like? 

[Darren]

Bit of dowel with some grinding paste,

Or soft metal, plastic etc, make it a close fit....

[kvom]

If the engine works as-is, leave it alone.  No one can see the piston or cylinder anyway.

What I have done the past few times is turn the piston to a tight fit and then use it as a lap with toothpaste as the lapping compound.

[John Hill]

Umm.... if you can lap the two together with toothpaste it maybe that mine are not really close enough, even after second attempt at the piston.

[bogstandard]

John,

If you are only going to be running it on air as a demonstration model, and because it is only single acting, if you can get it to within about 0.002" (0.05mm), then it should run pefectly well. The average film of oil will easily fill the gap I have mentioned.

It is only when you start to use hot gasses (steam or I.C.) that problems occur, mainly with expansion of dissimilar metals. Only then do you require closer running tolerances.

John

[John Hill]

Thanks John, I am not sure if it is a close as .002 and I would have to stop and think how I could measure that however it does make a nice 'pop' when I pull the piston out of the cylinder (dry).

[bogstandard]

That should be fine John.

A lot of people go to great lengths to obtain superfine clearances on model engine bores, but in reality, near enough is usually good enough for what we do.

The exceptions are as I have said, engines that have heat in them, steam, IC, sterling, flame gulpers etc.

But there is nothing stopping you going for fine finishes and fits on air driven models, it doesn't do any harm.

BTW, you can measure your bore by turning up a tight fitting plug, and measuring the OD of the plug, you should be able to gauge it to within 0.0005", which in your situation would be more than close enough.

I have had working steam engines thru my hands with bores and pistons worn to more than 0.010" gap, and they still run satisfactorily, though not efficiently.


John

[John Hill]

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.    Any tips?

[kvom]

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

[miken]

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.    Any tips?

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