Stuart S50

[WolfieParticipant @wolfie]

Hi chaps, now that North Yorkshires winter has finally let me back into the workshop, I’ve done a bit more. And I’m dead chuffed with this even if it is small stuff to you lot, so be gentle.

 

I was worried about tackling the cylinder casting as I really wasn’t sure how to hold it to begin with, but after talking to someone at PEEMS (http://www.nrmodeltruck.co.uk/peems.html) at the Feb meeting I bit the bullet and went for it.

 

Using lathe tools and a reamer I reckon this is a success! The piston fits beautifully (although there doesn’t seem to be much in the way of compression) and it moves up and down spot on even with the covers attached. It does bind slightly when I tighten the gland up, but I can open up the hole in the top of the gland a dab as there will be an O ring in the bottom.

 

I have to tell you its great when things like this come off, I have new confidence and all sorts!

 

Be gentle

 

 

 

 

 

Edited By Wolfie on 23/02/2011 22:09:07

Edited By Wolfie on 23/02/2011 22:10:28

Edited By Wolfie on 23/02/2011 22:12:58

[John OlsenParticipant @johnolsen79199]

Hi Wolfie,

 

Looking good there. Don’t worry about the compression, you are not trying to make one of those model aeroplane engines where the fit of the piston is all that seals the compression pressure. Stream engines will run fine with steam leaking past the piston, they just get even less efficient. You can groove the piston for some soft packing, or for an O ring and get a good seal that way, but if it is a nice sliding fit it would probably run fine without. You may notice that the Mamod style of engine has no attempt at sealing on the piston. (You can also make proper piston rings, but that would probably be a bit ambitious at this stage of things.)

 

regards

John

[Ian S CParticipant @iansc]

Your problem with compression may be because you reamed the cylinderuntil the piston fitted, with a reamer the cylinder will be parallel, but may have corrigations. You should stop say.ooo5 before the piston dia is reached, then lap it the rest of the way (well thats what I would do, except I would not be using a reamer, but boring in the lathe), my expiriance is with hot air engines, not steam so I’m not sure what sort of fit is required. Ian S C

[WolfieParticipant @wolfie]

Whats lapping??

 

I made the piston first as it happens and then made the cylinder bore to the plan as I only had the one reamer (5/8″)

 

I may yet make another piston as the rod is a tad short too.

Edited By Wolfie on 24/02/2011 15:03:35

[TerrydParticipant @terryd72465]

Hi Wolfie,

 

‘Lapping’ is done using an abrasive to finish the bore of the cylinder. It is done with a lapping tool which is a simple piece of softer material e.g. in this case, brass or aluminium or even plastic or wood, which fits quite tightly into the bore. It is made so that it can be expanded with a simple wedge device or similar and is loaded with abrasive such as grinding paste and then introduced into the bore of the cylinder which is rotating slowly in the lathe chuck. the ‘lap’ is fed in and out rhythmically so that the whole bore is lapped evenly. This is continued, renewing the abrasive and expanding the lap as needed from time to time, until the piston is a close sliding fit in the bore. It can also be done on a drilling machine by holding the cylinder in a vice and the lap in the chuck and move the quill up and down. Far quicker to do than describe.

 

Finer abrasives are used as the bore gets to a close fit similar to lapping valves in an i.c. engine. I often end with Brasso on the piston to get a good sliding finish. It is quite rewarding to see a piston and it’s rod slowly slide down a finished bore under it’s own weight.

 

I do hope that is clear,

 

Best regards

 

Terry

 

By the way it is usual in engineering to make a hole first and then make the other component (in this case a piston) to fit. This is because it is easier to turn a piston to a high degree of accuracy than it is a hole, especially a deep one.

Edited By Terryd on 24/02/2011 16:06:31

[Richard ParsonsParticipant @richardparsons61721]

Wolfi Terry missed out some things when lapping. They are these: –

1. Always try to use a different pattern of lapping. This will let you see when the finer grit has rubbed out the scratches left by the coarser grit.

2. Never proceed to a finer grit than the one you are using until you have removed All the scratches from the previous grit.

3 Never let the lap become dry. In the UK I used a mixture of oil and paraffin (the stuff you buy for stoves (NOT liquid paraffin which is very moving). In Hungary I cannot get paraffin I use either very thin oil or a mixture of cooking oil with a little meths added to thin it.

4 I charge my lap by mixing the grit with the oil on a bit of glass then quite firmly roll the lap in the mixture. I use thrown away ice popsicle/lolly sticks to mix the grit. (I used to live near a several schools so I have a huge box of them). Watch out SWAMBO may nick them for her plants.

5 This is the first bigee before start lapping, cover your machine over with disposable plastic sheeting (Black dustbin bags are just right for the Myford and you can turn then inside out for SWAMBO to use.

6 Finally for success always scrub every trace of the previous grit from everything, every nook and cranny (except the machine cover) – the lap, the work piece, the glass, and your hands before starting to use a new finer grit. Use washing up liquid, suitable brushes (in your case old tooth brushes) and lots of running water. You must even wash the brushes before you use them to remove the next grit. 

It is a good idea to run the lap slowly holding it in the chuck and hold the work piece in your (gloved) hand. You can then ‘feel’ how the job is going and when to add more lubricant etc.

 

Watch out with brasso it often contains quit large grains. 

Edited By Richard Parsons on 24/02/2011 17:38:02

Edited By Richard Parsons on 24/02/2011 17:42:36

[John OlsenParticipant @johnolsen79199]

While this is all excellent advice on lapping, let us remember that this is a steam engine, not an IC engine, and further it is his first attempt, so we don’t want to make things too hard.

 

My Dad, as a young fellow in a rural village in depression era New Zealand made a few small steam engines without access to a lathe or any power tools. He made pistons for them by casting lead into the well greased bore.

 

regards

John

[TerrydParticipant @terryd72465]

Hi John,

 

I agree with you wholeheartedly. Wolfie simply asked what ‘Lapping’ was, not how to do it with loads of inconsequential detail. I tried to inform him in as simple a way as I could avoiding excess detail and complications which can be found on the internet or in countless books.

 

If you want to see a simple steam engine I suggest you look here. It is eye opening for those who insist that toolmakers lathes and super accurate micrometers and dust free environments etc are essential for simple working models.

 

Best regards

 

Terry

Edited By Terryd on 24/02/2011 21:47:08

[Ian S CParticipant @iansc]

Back in the 1930s and before, lead was used quite a bit for making pistons, and flywhels.

I should have explained lapping, but thats been covered better than I could have done. also the point of making the hole first, then making the piston to fit.

For a lead flywheel, it may be cast in a wooden mold, and a brass bush can be cast into the center. Heat and pour the lead out side, if you can find lead these days. I’m OK for it I’v got about 200kg of clean lead. Ian S C

 

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