Digger

Firstly I must say sorry to all those who eagarly clicked on the thread title that were looking forward to another off topic gardening thread

This one is actually about building "Digger" a slide valve oscillating steam engine from a set of castings that were at one time offered by B & R Tite. This seems to be a very rare engine the only previous mention of it on the net was buy the owner of these castings asking for information about it and a small entry in one of the "World of Model Enginnering" mags. I was contacted by said owner recently asking if I would consider building it for him as he felt it was beyond his ability and being so rare a mucked up casting would not be replacable particularly as he only wanted a limited amount of paint so any substitute materials would also have stood out out and he even hoped I'd be able to make an engine that ran well

The only image of a completed model was on the rather poor photocopied detail sheet and I was unable to track down the example in the Science Museum that it is said to be based on. If anyone has an image of the full size engine I'd love to see it.



I asked the owner for some images of the drawings and picture sof the castings and after looking through them and seeing that the castings looked to be of good quality I decided to take on the challenge. They are all Gun Metal which is not my favorite visually and it can also be a bit soft depending on what was boiled up in the pot. The one proviso for taking it on was that I could substitute metric fasteners in place of the BA ones shown as I prefer my spiral flute taps, stub length tapping size drills and it's easy to use cap head screws to hold it together during construction.





While waiting for the castings to arrive I quickly drew up the engine in Alibre to my often used 24mm bore which was an easy conversion from the models 3/4" bore using the ratio of 1/32" = 1mm and was able to determine a couple of missing dimensions and check that it all went round and round as it should. I may make one of my own at some time but will probably go for 18 or 20mm bore as the 24mm is a bit big unless I want to start adding more detail.

An interesting design ! What made it better than a normal slide valve, other than not needing an ecccentric ? Noel.

I'm not sure it is any better than an eccentric driven slide valve, if anything it is a bit more restricting as you can't alter the timing by moving the eccentric on the crankshaft. There are quite a few different arrangements for operating the slide valve with the links in various places relative to the cylinder's point of rotation but I've not seen many with this particular one.

I would think it is better than an oscillating engine where the valve arrangement is done by positioning holes in the two faces where cylinder meets trunions as it is easier to seal the shaft of the trunion and not have to worry about the flat faces.

As oscillator is also generally a shorter engine than one with trunk or bar guides for the cross head and a separate conrod but a bit harder to construct as the trunions need to be perfectly square to the cylinder/piston rod.

Edited By JasonB on 26/09/2023 18:59:59

Had a squirrel and they were still going in May 1993

Same address

Edited By Ady1 on 26/09/2023 20:00:28

Thanks ADY1 so thats a third engine as I had seen mention of "Hoppy" a hit and miss engine kit they dis that sounds like it may have needed some crankshaft balancing. Though Henry would be the only ready machined of the three.

I decided to start with the two spacers that go between the side plates to get a feel for what the grade of Gunmetal was on some parts that were going to mostly be out of sight. I started by just taking skim cuts off to remove the quite steep draft angles thus giving me some flat surfaces to hold while the 4 long edges were brought down to final size taking similar amounts off opposite faces so that the inverted "T" section stayed central. For almost all the "flat" milling on this engine even the quite small parts I used a 6 insert facemill with APKT inserts which are made for use on non ferrious metals. Most other milling was done with cutters intended for use on Aluminium but they also work well on other non ferrious metal, here you can see a two flute cutter has been used to square up the ends.



After a quick tickle of the remaining cast surfaces with a carbide burr in the Dremel the various holes were drilled and tapped M2.5 with a spiral flute tap



I was not that happy with the varying thicknesses of the end plated even after averaging out their thicknesses so decided to clean things up a bit more with a 4mm milling cutter which evened things up.



Using the face mill again I skimmed the top and bottom flanges of the side plates so that they could better be held in the mill vice to take off just enough material to clean up the flange edge. In most cases the castings were quite generous with machining allowances with the average being around 2.5mm (3/32" )



The casting was then flipped over to do the flat inner face and then a bit more off the top and bottom edges to bring the castings to the final height



The 2 flute 10mm cutter was put into use again to finish the ends with full depth passes of the 1/2" thickness, again taking a bit off each to leave as similar a flange thickness as possible each end.



Clearance holes were drilled and then spotfaced for the fixings into the spacers



The parts where then screwed together with temporary cap head screws, the assembly centred up in the mill vice and the holes for the trunion and crankshaft bearing blocks drilled and taped. The drawings also call for a couple of notches to be milled out to give clearance for the plates where the trunions bolt to the cylinder otherwise they would collide as the cylinder oscillates.

The cylinder pivots in two trunion bearings, I made a start by skimming the top, bottom and two side to get rid of the draft angle and was then able to close up the chuck jaws on these now flat faces, the sides needed a packer as the foot would have got in the way of the jaws otherwise. The 3/4" dia x 1/4" long outer spigot was turned, a 1/4" hole reamed right through and then counterbored 3/8" for the packing gland. Just like the milling I'm using CCGT polished and ground inserts for non ferrous metal here and did for just about everything lathe operation on this engine.



I made up an expanding mandrel so that the casting could be held the other way around, brought to width and a shorter 3/4" dia spigot formed and faced on the inner side.



Over to the mill a piece of 1/4" Silver steel (drill rod) was used to support the work so the bottom could be milled to the correct distance relative to the bore. Face mill gives a good finish and minimal burrs which can be an ussue with gun metal unless the tools are good and sharp.



After bringing the castings down to finished height they were again slipped onto the piece of silver steel to mill the sides and finish the feet to thickness and overall length.



The crankshaft bearing supports are similar shaped castings, after some initial cleanup i decided to use the CNC to tidy the cast tops and at the same time incorporated a small raised boss to take the oil pot rather than just filing the top and having the pot screw into a convex surface. After bringing them down to final size the foot holes were drilled 2.5mm and the stud holes drilled and tapped M2.5 before opening up the ends of the holes to 2.5mm. Red and blue Sharpie marks ensured I knew my right from left before sawing off the caps with a slitting saw.



After cleaning up the sawn surfaces the caps were screwed into place so that the hole could be drilled and then reamed 1/4"

Rather then do all the big bits and then be left with the small fiddly ones at the end I decided to do a few of those. First was the slide valve for which a small casting is supplied, this was cleaned up all over and then the slot for the valve nut milled. Unlike a lot of valves which have a second slot for the valve rod this one has a hole which is a bit larger than the rod to allow the valve to lift if needed.



The cast in cavity was tidied up with a 1.5mm milling cutter



The valve rod gland was turned and threaded then the slots for adjustment cut using the 5C indexer to space them out.



There are two fairly similar brackets for the valve linkages so I machined these at the same time so that the vice stop could be used which saves time centering up parts twice, the one with the forked middle fits onto the cylinder cover and the one with the single blade goes ontop of the spacing block under the cylinder.



The vertical valve link was drilled and a slot milled before using the CNC to give it it's final shape which is with full 1/4" dia bosses around the holes/slot and than thining to 3/16" between. A piece of thicker steel as also prepared and shaped on the CNC to give the rounded ends.



A quick test fit of the various parts using cap head screws which will be replaced with fitted bolts once the engine has been test run.



The big end casting was cleaned up, drilled and tapped and then screwed back together for final shaping and reaming of teh big end. I then made use of teh split arbor that I made for the trunion bearings to hold it while each side was turned down to leave a 1mm boss. An insert with a 0.8mm tip radius leaves a nice fillet in the corner.



I did not take a photo of the three glands that compress the packing around the trunions and piston rod as they are little more than top hat bushes with three 2mm holes in their flanges but there were machined at this time too.