A Tandem Compound Mill Engine

[Admiral_dk]

Great to see you making parts again David     

Per

[deltatango]

Thanks Per, it's good to hear from you!
I think that I've got a few other bits of life sorted (famous last words, of course) and I should be able to make more time for the workshop. I may be about half way (??) with the big engine just have to stay focused.

David

[MJM460]

Hi David, good to see a couple of tricky little parts nicely done.  Definitely good progress.

MJM460

[deltatango]

MJM, its good to hear from you again!

The HP cylinder block now has more holes and most have threads tapped in them. The valve end covers and valve bonnets (four of each, two steam and two exhaust) are each held on with six 8BA studs, the steam and exhaust chest covers each have 12 at 6BA. There are four more 6BA for the dashpot body and four 5BA for the steam lever trunnion:



so far no broken taps (whew)! However, if I'm going to attach the cladding with fasteners anything like scale size there will be a bunch of holes for 12BA screws to tap into cast iron, those can wait - and maybe there will be a way to avoid the problem (can't find a chicken emoji)...

The valve bonnets and other protuberances were all turned from the solid. The shapes are either small and fiddley or quite complicated to make with shallow external tapers and the machining sequences needed a lot of thought. I started with the end covers which were made in pairs on stubs of steel:



with the six holes drilled deep enough for both bits to be parted off:



and after parting super glued to a fixture for finishing the outer sides:



The valve bonnets were tackled similarly:



but with a seating formed to take an O ring to seal on the valve spindles. The little "boring bar" here is a 5/32" D bit which worked well:



The original drawings show the steel cladding butting up to the outsides of each of these pieces, getting that to look good sounded difficult. A pause for thought and I remembered taking a lot of pictures of the Stott engine at the Anson Engine Museum and this one shows that there the bonnets etc. are rebated to go over the edges of the cladding and over the brass edging:



Problem solved - I hope. All the bits had a small recess turned in them for this:



Whilst the exterior bits were still cylindrical and easy to hold the stud holes were drilled:



and then (finally!) the parts were glued to the fixture and turned to a slight taper for appearance:



With some studs made and temporary screws elsewhere the little family now looks like this from the valve gear side:



and, from the other:



At a bit of a guess this assembly is going to take a quarter to a third of the total build time, which may explain the length of this post and the length of time since the last one. I hope you're all still with me!

Regards, David

[Michael S.]

Hello David,
the cylinder unit looks very good.
I have never built this type of steam engine. Maybe I'll get a blueprint like that later.
Good luck with building.

Michael

[Kim]

David, I'm definitely still with you!  Great work on the cylinder block and the valve covers!  They look great! 

Kim

[Roger B]

Looks good  A lot of complex little pieces

[Chipmaster]

First class 

Andy

[cnr6400]

 

[deltatango]

Thanks everyone!

With the parts so far looking good it was time to start putting some of the bits together from the pile accumulating on the bench. Thinking this through (this can pay off sometimes  ) brought to mind that I hadn't looked at making the cladding sheets and brass angles. On the original drawings the sheets are shown as outlines for square ended cylinders with notes about the 12BA screws and "trim where needed to clear cylinder covers". The brass angles are drawn with single mitres at the corners which would actually have to be double mitres to work. Avoiding having to cut those, and have them look good, was one reason for making the HP cylinder with curved top ends. Of course having curved ends requires bending the angle, something I put off thinking about until now. It also seemed reasonable to assume that small brass angle would be easy to buy in, h'mm... When I went to look, any angle that was available looked like it had been folded from sheet and had very curved corners that didn't look the part at all. What I did find was 3/16" square x 0.014" brass tube from K&S Metals that had nice sharp corners and looked rather like four lengths of 3/32" angle joined edge to edge. "All" that had to be done was to cut them apart.

This required a fixture for holding the tube while a fine slitting saw went to work. This fixture was a 3/16" x 3/32" groove on one corner of a lump of stock from the hoard:



and the tube was super-glued into this and attacked with a 0.01" slitting saw (something I hadn't used at all until now):



This left two pieces of channel which, thank goodness, stayed straight after cutting off. The fixture was then turned to present one of the channels to the saw:



and you can see that the saw has gained some big washers as cheek pieces to keep it in line. The first cut didn't go well with the saw wandering wildly and wasting material. I'm learning all the time.

Now to make the 90° bends at accurate distances apart, time for more thought (and a couple of dismal failures) and the realisation that yet another jig would be needed. Fortunately someone had in fact done most of the design work for me and, a long time ago, I'd made a pipe bending jig to the design of Derek Brown [Model Engineer's Workshop, Feb/March 2000, issue #64, pp37-40]. I borrowed some of the bits from this and made up bending formers that would grab hold of the little angle and form it into 1.020" bends (nominal 1" but allowing for cladding thickness):





The bends start at a reproducible fixed place on the fixed former (blue line) and I annealed the angle and blew on a bit of powdered graphite to help with smooth movement:



and the second bend went well:



Most of the bits of the jig are loose pieces and removing the bent angle is easy. There was a lot of messing around but the final results are worth it:



David

[cnr6400]

 

[RReid]

Lot's of good ideas there. Thanks for showing your process for turning the tubing into angle stock in such detail. Beautiful work on the engine overall, I'm enjoying following your progress!

[Admiral_dk]

Lot's of good ideas there. Thanks for showing your process for turning the tubing into angle stock in such detail. Beautiful work on the engine overall, I'm enjoying following your progress!
 

I can only agree + great to see you back on track so to speak 

Per

[deltatango]

CNR, Ron and Per.

Thanks for the comments, it's good to know you are following.

The brass edging took a long time, but I think it was worth while. The cladding sheets are on the go now, getting the steel to the right shade of blue and even all over is proving difficult. Gun "blue" left it a rather streaky brown and so far I haven't got the setup quite right for heat bluing. I haven't really been "off track" but lots of family things (e.g. new granddaughter) have had priority.

David

[Charles Lamont]

Gun "blue" left it a rather streaky brown and so far I haven't got the setup quite right for heat bluing.

ISTR a heated sand bath is often recommended.