Kerr Stewart 4420 - Live steam and 3D printing

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Kerr Stewart 4420 - Live steam and 3D printing

Post by Trevor Thompson » Fri Nov 26, 2021 10:35 am

I have been thinking about how I might be able to combine plastic 3D printing with live steam. Not that I expect to make cylinders and pistons from plastic, but building a steam motor and boiler in the normal way and fitting them into a locomotive where all the "bodywork" is 3 D printed. There are a number of prototypes which lend themselves to this idea - basically separating the boiler and the cylinder from everything else so that heat transmission to the plastic parts can be controlled enough to prevent things softening and distorting.

So I have done preliminary work on a shay, and then I found the book on the geared steam locomotives that used the same chassis as 4415 - the early British diesel loco. I have made a 3 D printed version of this - so it was the obvious one to experiment with.

So to set the ball rolling here is a screen shot showing the sketchup file so far:
Screen Shot 2021-11-23 at 20.37.52.png
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It is the same chassis as that for 4415, except that it is backwards. As in the gearbox is at the front on 4415, and at the back on 4420. Both are 6 wheeled and chain driven. So the steam motor is going to be behind the cab. I think that is far enough from any plastic bodywork for it to work. The boxes in the foreground are a mockup of the bodywork which goes over the boiler, and there is a first stab at a boiler in place. I want to allow loads of space for insulation - and it looks like I can get a reasonable sized boiler in here. The obvious boiler is a standard locomotive boiler with a single 15mm tube and a poker burner. The original was a marine type boiler, with water filled tubes and it didn't work very well - tempting to follow that in some form!

The only difference on the printed chassis is that I had to fill the cab step with a separate section. Here is the assembled chassis ready to take the steam motor:
IMG_1942.jpg
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I will shortly add the files for making the chassis to the 3D printing section, in case there is any interest. At least one member of this forum is going to attempt to make one as well - so it seems fair to share it openly rather than with only one person.

I will describe the chassis in the next post - and in particular how I have made 6 flanged wheels go around tight bends, using ball races.

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Re: Kerr Stewart 4420 - Live steam and 3D printing

Post by Trevor Thompson » Sat Nov 27, 2021 9:42 am

The chassis that I have created for these locos has an inner chassis and an outer chassis. The inner part has the axles and motor attached to it, and the outer has the footplate and buffer beams attached to it. The main component of the inner chassis:
Screen Shot 2021-11-26 at 17.33.52.png
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The holes for the axles are 10mm diameter, and although they print correctly I have just run a 10mm drill into the holes on a drill bench. I have pressed shielded ball races 10mm OD, 3mm ID and 4mm wide, into the 3 holes. The second part is a mirror image of this and prepared in the same way. I print all of the wheels which I use, and I mount them in the lathe and ream the holes 3mm before mounting them onto axles - to make sure they run true (I am getting about an 80% success rate). The axles are made from 3mm diameter stainless rod. I often use locktight or superglue on the wheels to ensure they don't move on the axles. The outer axles are long enough to go through the bearings, and the centre axle is about 4mm longer. I am using chain and gears obtained from "Technobots". 16 tooth chain sprockets part no 4601-002 for the axles, and 4601-001 for the steam motor. That is a 2:1 reduction already. The metal chain to go with the sprockets is from the same place. The plating on the links tends to lock the chain into a solid line, and even after loosening it manually I find it needs re-tensioning every so often (after fitting) until it is totally free:
IMG_1962.jpg
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When assembling the chassis I found the 3mm axles a sliding fit in the bearings so I used a minute amount of locktight to stop the outer axles floating freely in the bearings, and allowed the centre axle to freely slide from side to side. The chassis runs well and seems to go round curves well. The grey components in the photo are the inner chassis, and the black ones the outer chassis.

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Re: Kerr Stewart 4420 - Live steam and 3D printing

Post by Trevor Thompson » Sun Nov 28, 2021 9:21 am

I found the plans for Tony Birds steam motor on the model steam loco builders forum, and that seems to fit in the cab fairly well - without modifying the design.

Of course when I looked more closely modifications were going to be needed - for example the design calls for the cylinder bore to be 6.4mm (1/4") diameter and I have a 7mm reamer not a 1/4" one! Anyway I'm making it:
IMG_1961.jpg
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Firstly the cylinders, from 18mm OD brass. Straight forward boring job, finishing with a machine reamer. Then mounted in a rotary table so that the flat outer faces can be milled:
IMG_1963.jpg
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and the drillings for the centre pivot, and the spring retainers, at the same time.

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Re: Kerr Stewart 4420 - Live steam and 3D printing

Post by Trevor Thompson » Sun Nov 28, 2021 9:24 am

The finished cylinders:
IMG_1964.jpg
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Lastly I mounted the 4 jaw chuck and turned those slight recesses in the face which rotates against the port block. The steam ports will be bored later. I watched one of Tony's videos - so I know how to make all those ports line up properly!

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Re: Kerr Stewart 4420 - Live steam and 3D printing

Post by Trevor Thompson » Sun Nov 28, 2021 5:58 pm

Cylinder end caps turned, and bored and reamed where relevant.
I have always fitted o ring seals to the shaft end on my previous models, but this design calls for just a long bored hole. I will be interested to see if that leaks steam!
IMG_1968.jpg
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and the reversing mechanism:
IMG_1967.jpg
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The rotary table does make it easy to create the steam passages, in this case 1.5mm wide and 1.2mm deep.

I await 1/16" rod, and 7mm OD O rings so I will be making the main chassis / port block next.

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Re: Kerr Stewart 4420 - Live steam and 3D printing

Post by Keith S » Sun Nov 28, 2021 6:13 pm

This is pretty interesting. I've tried to find a picture of the subject locomotive online, but although I've found descriptions of it, I've not been able to find any photographs. What did it look like?

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Re: Kerr Stewart 4420 - Live steam and 3D printing

Post by Trevor Thompson » Sun Nov 28, 2021 9:03 pm

Keith S wrote: Sun Nov 28, 2021 6:13 pm This is pretty interesting. I've tried to find a picture of the subject locomotive online, but although I've found descriptions of it, I've not been able to find any photographs. What did it look like?
Fairly unusual. This should give you some idea:
Screen Shot 2021-11-28 at 21.01.30.png
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The source I am using is the booklet "Kerr Stuart water tube boiler gear drive steam locomotives" by Allan C Baker published by the Industrial Railway Society which I got from Camden .


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Re: Kerr Stewart 4420 - Live steam and 3D printing

Post by Keith S » Mon Nov 29, 2021 6:12 pm

Thanks! I enjoy watching these locomotive-building videos more when I can visualize what is being built.

It must have been a strange little engine. One doesn't often see water-tube boilers on locomotives, either.

Your last engine build was very interesting to watch; I'm looking forward to seeing more of this one, especially with the innovative 3-d printed parts.

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Re: Kerr Stewart 4420 - Live steam and 3D printing

Post by Trevor Thompson » Thu Dec 02, 2021 8:28 pm

I'm working on the main frame / port block.
It is quite a difficult part to make - all the holes are so close together that there is very little room for error. A hole slightly out of position and it broaches a steam port. So far I have made two attempts at this, and have eventually recovered from each error. So now I have 2 port blocks!
IMG_1972.jpg
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I have been trying NOT to modify the design as I build it, particularly wanting to make it as designed first so I fully understand it. The left hand block in the photo is the one I messed up first. You can see that one of the holes in the edge of the block has been filled. That is where I broke a No 1 centre drill. I managed to remove the broken end (its very tiny) by drilling it out oversize,. The hole was tapped 8BA, and a length of 8BA studding created and fixed into the hole using bearing retainer. So I will try re-drilling that one later.

The right hand block has had all the steam passages bored, the cylinder pivot drilled and reamed, and the reverser ports drilled. I have also checked that the passages pass air, and don't let air seep into holes where they should not reach. No point in taking it further if it isn't working correctly!

So then I bonded two legs onto the block, which will eventually take the bearings for the crank shaft. I drilled and reamed the holes for the locating pins, drilled and tapped al of the fixing holes. Then I tried to drill the bearing holes, which should be reamed 8mm. I progressively increased the drill size and did it gently - but the lower leg moved as I was drilling.

This is where my resolve faltered. Those legs are very thin - there isn't much bearing surface area there. So I have thought about it overnight, and I have decided to modify the design. I am not criticising the design - it is clear to me that it is intended to be able to fit between the wheels in an inside framed 32mm locomotive. I don't need that in this case. However modifying it has knock on effects. So if I make the legs 3mm thick, that enables me to use ball races for the bearings (I already have them. They are 8mm OD, 3mm ID and 3mm thick). The corollary of that is that I will have to modify the cylinders I have already made. If the legs are 1.5mm thicker than intended then the cylinders need to have their inner faces 1.5mm thicker. That isn't a problem - I can soft solder 1.5mm plate onto those faces, after all I have not bored the steam passages yet.

Firstly I will make new legs 3mm thick. So that is the task for tomorrow.

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Re: Kerr Stewart 4420 - Live steam and 3D printing

Post by Trevor Thompson » Sat Dec 04, 2021 12:33 pm

I have made the new cheek blocks - or legs as I called them last time- and fitted them to the port block:
IMG_1979.jpg
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I am much happier with these as they are much more robust and stay firmly in place. The bottom of the port block has also been cut away and the recess for the gear created. I have tried the ball races in the holes and they fit freely. I can hold them in due course with lock tight - but I am keeping them in the bag they came in until everything is machined and all of the swarf removed.

So cranks and crank shaft next.

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Re: Kerr Stewart 4420 - Live steam and 3D printing

Post by Trevor Thompson » Mon Dec 06, 2021 11:51 am

The gears which I have found have a 4mm bore, and bosses incorporating twin grubscrews:
IMG_1987.jpg
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I made a shaft the correct length from 4mm stainless rod and turned down the ends to 3mm for the bearings and cranks. Then I bonded the smaller gear to the 4mm section of the shaft with bearing lock. After 24 hours to let it set properly I then turned off the boss from the gear.

The cranks were a simple turning job 3mm wide lengths of 18mm diameter brass, with a 3mm reamed hole in the centre and another offset by 6.5mm. It has all gone together properly, with the ball races held into the cheek plates with locktight. I have yet to add the tapped holes in the cranks for grubscrews, but otherwise that is the crank shaft and cranks are ready for final assembly:
IMG_1986.jpg
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Next big ends incorporating ball races.

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Re: Kerr Stewart 4420 - Live steam and 3D printing

Post by Trevor Thompson » Wed Dec 08, 2021 6:29 pm

The big ends are now complete. I have changed the design again to incorporate ball races in the big ends. The first stage of making the big ends:
IMG_1990.jpg
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I turned down a length of 15mm bar to 12mm for 12mm, and then to 3mm for 6mm. Then drilled the centre of the 3mm section 1.5mm, to take the 1.6mm connecting rod shaft (and reamed it). That can be seen on the right.
Transferring this to the rotating table and converting the machine to mill, I held the part made component in the table chuck by its 3mm diameter end. I milled a flat on the top until I was left with 7.8mm of metal, turned it through 180 degrees and milled the bottom face until what was left was 3.6mm thick.
That flat face was then drilled progressively until I had a 8mm diameter hole centrally in the big end. The end result:
IMG_1993.jpg
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The end result works, the bearing fits on the pin and in the hole, and when the cylinder is held in place the piston can make its full travel.

I have made temporary pistons out of PTFE rod. They are a nice sliding fit at the moment - but that won't last. They are bound to seize in the bores when they are introduced to steam. But they will do to test it all on air.

My next task is to add 2.6mm to the inner faces of each cylinder.

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Re: Kerr Stewart 4420 - Live steam and 3D printing

Post by -steves- » Thu Dec 09, 2021 3:01 pm

Trevor, your machining skills never cease to amaze me, great work :thumbup:
The buck stops here .......

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Re: Kerr Stewart 4420 - Live steam and 3D printing

Post by ge_rik » Thu Dec 09, 2021 4:37 pm

I think I would struggle to get that level of precision when working in plastic - but in metal.... no chance!
I'm awestruck!

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Re: Kerr Stewart 4420 - Live steam and 3D printing

Post by Trevor Thompson » Sun Dec 12, 2021 8:30 am

Thanks for the kind words - I'm not sure that they are justified. Some of the other contributors to this forum are much more capable than I am!

However I keep trying, and get more experience as I go along.

PTFE isn't difficult to machine, but you have to take bold cuts or it just bends slightly instead of cutting.

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Re: Kerr Stewart 4420 - Live steam and 3D printing

Post by Trevor Thompson » Tue Dec 14, 2021 6:10 pm

Having finished all of the basic machining I have made a jig to make sure that all of the steam ports in the cylinders and port block align properly. It is basically a dummy cylinder and connecting rod. There are holes along the centre line for the pin on which the cylinders rock, and a hole for the crank pin to fit through. There is also a hole for each of the steam ports - each 9mm from the pivot pin hole, and again on the same central line:
IMG_1995.jpg
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You can see it here as it is intended to be used to drill two of the holes in the port block. I have made the sides of the jig the same width as the cylinder so I can use the same jig to drill the corresponding holes in the cylinder.

I did make a mistake of course - I read the wrong dimension from the drawing and drilled the holes for the ports in the wrong place before realising. So I tapped the holes 10BA and filled them with 10BA brass studding. Then I drilled them in the right place!

If all goes to plan the holes in the port block should align with the holes in the cylinder perfectly.

(It isn't my idea by the way - I got the idea from watching one of Tony Bird's videos).

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Re: Kerr Stewart 4420 - Live steam and 3D printing

Post by Trevor Thompson » Tue Dec 14, 2021 6:43 pm

So drilling the steam ports in the cylinders:
IMG_1996.jpg
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I fitted the jig on the pivot pin and lined it up parallel to the edge of the cylinder. Then drilled one hole as in the photo, removed it all from the vice, put a clamp on the other end before removing the first clamp, putting it back in the vice and drilling the other end.

That was repeated for both cylinders. Transferring the jig to the port block and drilling the first pair of holes in that:
IMG_1997.jpg
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Having drilled 2 holes the jig was moved to the other extreme of the cylinders movement, with the crank pin moved through 180 degrees, and the other 2 were drilled.

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Re: Kerr Stewart 4420 - Live steam and 3D printing

Post by Trevor Thompson » Tue Dec 14, 2021 6:51 pm

So the result with the jig removed:
IMG_1998.jpg
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The cylinders needed the recess in their face recutting because it had been lost in making the inner faces of the cylinders thicker. So that is the cylinders ready to assemble and test.
IMG_2003.jpg
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I went back to the port block to look at what else needed doing to it - such as making the holes to mount it onto the locomotive. I tried compressed air down all of the ports partly to clear out swarf, but also to try to detect air coming out of the correct holes. When drilling the ports I could feel the difference in pressure on the drill feed, on three of them at least. One of the bottom steam holes didn't seem to hit the steam ways as I drilled it, so I was trying to work out if it has connected or not. I have come to the conclusion that it isn't connected - because I didn't drill that steam way deeply enough. Something to sort out another day!

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Re: Kerr Stewart 4420 - Live steam and 3D printing

Post by Trevor Thompson » Thu Dec 16, 2021 8:43 pm

I have been thinking about the boiler (well the steam motor is almost finished), and of course that led me to the bodywork which is going to hide it all.

So this is a the outline of the bodywork in place on the chassis. The detail is not added yet, and I was contemplating adding the panels and rivets onto the surface, as separate pieces printed on the resin printer (to get the surface finish). Anyway the body:
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and with the boiler cover removed:
Screen Shot 2021-12-16 at 20.28.17.png
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So the boiler. The plan so far is to use 2 pieces of 11/4 inch OD copper pipe one on top of the other. Put slots in the bottom of the top one and in the top of the bottom one (where the cross stays will go). Add flat plate sides to give the outline in the photograph. Fit flanged ends front and back, with a 15mm tube running the length of the boiler to take a poker burner. All of this from 1.6mm thick copper. This would be strong enough for a boiler pressure in excess of the 40 psi I will use.

I calculate that 3mm diameter copper stays will be needed, on 20mm centres. Hence all the stays in the sides and ends.

Perhaps there isn't as much water volume as I would like but it comes down to what diameter pipe is available.

I have also thought about a vertical boiler and even a rectangular boiler - but simple is probably best.

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Re: Kerr Stewart 4420 - Live steam and 3D printing

Post by Trevor Thompson » Tue Dec 21, 2021 4:42 pm

I left the progress on the steam motor with the thought that a steamway wasn't connected to the cylinder ports. I mounted the port block in the drill and open out the plug in the end of the drilling. Re-drilling the hole established that it had been left just a mm too short. So with that corrected the surfaces of the cylinders and the port block were polished using wet and dry paper on the lathe bed (as a flat surface).

In the meantime the 7mm O rings had arrived so I made brass pistons with a groove for the O ring:
IMG_2025.jpg
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The pistons have ended up as an almost perfect fit. They slide freely in the bores yet when I cover either of the ports the piston acts as if it was an air spring - you can feel the pressure coming back as you move the crankshaft, and it springs back when you let it go.

Trial assembly identified that the crank shaft was touching the cylinder gland when the piston was at the upper end of its travel, stopping the crank rotating fully. So the cylinder was disassembled again and a millimetre taken off the top of the bearing housing on the crank. The same on both sides.

So I now need to take the piston off one shaft and shorten the shaft slightly. But we are getting closer to a running motor.

Trial assembly:
IMG_2026.jpg
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