My Little Donkey

[Jasonb]

I first saw one of these engines on e-bay 3-4 years ago, liked the look of it so copied the images to my "Future Projects" file though I did not know exactly what it was. Over the years a couple more have cropped up on the same site and one of those included a copy of a page from an early Bassett-Lowke catalogue which gave a couple of sizes as well as a description



Using one of the photos that had a reasonably square on side shot I imported the image into Alibre and scaled it to suit. I could then take sizes off of that to produce a replica, this time in metric and I kept it about the same size with a 16mm bore and 15mm stroke.



It was done using a mixture of silver soldered fabrications and cutting from solid. Mostly on the manual machines though I did use the CNC to cut a couple of parts that form the frame and also do the flywheel from a slice of 80mm CI bar.

It's the first engine I've done with a Scotch Yoke so that's another notch on the workshop door frame. Seems to run quite well on a few psi and even pumps though I think some house training is in order.



<a href="https://www.youtube.com/watch?v=T871NGafEAo" target="_blank">http://www.youtube.com/watch?v=T871NGafEAo</a>

[Jo]

Cute little engine  Is this actually going to be a build or another showcase of your completed build Jason?

If it is not a build it needs to be put into the showcase area.

Jo

[Jasonb]

Who knows how the thread may develop, if someone askes me about how I did a certain part I'd like to be able to answer here.

Besides it does not mention "build" for the own design topic

[Jo]

For the avoidance of doubt the engine section is intended for members to post their build logs.

The showcase section is to enable people to show off the models they have built without detailing the build. If other members enquire about aspects of the build of a completed model that can also be included in the showcase area.

Jo

[Vixen]

Jason,
This is a fine looking engine and beautifully presented; no matter which category they eventually decide to place it in.
Mike

[Jo]

To avoid other people getting as confused as you seem to be Jason I have updated the forum layout to include the words" Post your build logs in here" 

Jo

[Bearcar1]

Ouch! Jason, "Mammie Yokem has spoken" (from an American western movie) ... Anyway, that is one heck of a cool looking engine, and I good runner. Would/could you share the drawing sets or roughly dimensioned drawings with us?


BC1
Jim

[Jasonb]

Thanks Mike and Bob.

I'm not sure you would really want my "drawing set" as I tend to pick what I need off the 3D parts and take a rough sketch to the workshop, this from another of the small engines



Even my rough initial pencil sketch has had changes made as the design developed



But if you can open current Alibre files or pick the parts out of a STEP file of the assembly then drop me an PM with e-mail address. Having said that I nay do some proper drawings as I have also been asked elsewhere for them. I also want to stretch it a bit as the crankshaft could go a bit higher to make a bit more room for the pump.

Either way it would probably help anyone wanting to build it for me to describe at least the major parts so looks like Mammie will have to leave the post here.

[Jo]

We are looking forward to hearing all about the build 

Jo

[Dave Otto]

Nice little engine and pump Jason!
Another project well done!

Dave

[Jasonb]

We are looking forward to hearing all about the build 

Jo

Could it please be moved back to the build section of the forum then.

I won't do a bit by bit build description but will try to cover the main parts subject to what photos I took

The frame seemed the most logical place to start as that was the most complex so any deviations from design that may result could be gotten out of the way and  the design of other parts altered to suit rather than having to remake or modify them, In the end that was not really needed. I tend to design my fabrications as a single 3D part and then pull off the details of the various sketches and extrusions that make up the part as individual pieces.

Starting from the bottom a piece of 5mm sheet was thinned down to 4mm, taking equal amounts off both sides, then milled to the required length and width. To help locate the other components some 2.5mm  x 1mm deep slots were milled and the holes for the mounting screws and pump ram body drilled



Although they would not be too hard to shape by hand I made use of the CNC to cut the shape of the 3 pieces of 2.5mm steel that make up the majority of the frame. Once cut I put some more slots in on the manual mill and also squared up the internal fillets left by the 3mm milling cutter on the CNC with a file so they would drop into the slots. Bending was done after the machining was complete, nothing fancy just a piece of round rod in the bench vice to give the desired radius and bent by hand.



The cylinder mounting flange was just simple turning and I left it over length and over diameter so it could be finish machined after assembly. A piece of Culphos had a spigot turned on one end to locate in a hole in the vertical leg of the frame and was thinned in the middle. This thinner area will get milled away but it helps to keep the two bearing pieces lined up if they are soldered as one piece.



The parts all slotted together quite well and I only used one loop of soft iron wire to hold a couple of bits together though I did decide to solder the cylinder flange on as a second heating.

Here is the rest of the soldered frame having the location radius for the flange machined as it was easier to locate at that stage than when all the bits were separate. The main clamping is down to the mill table but I added an angle plate and a clamp to reduce the chance of any chatter while plunge cutting to form the radius.



After soldering on the flange a similar setup was used so that the OD of the flange could be "turned" concentric to the hole for the pump ram body using the boring head and the thickness brought down to finish size ensuring it was at right angles to the vertical axis of the engine and the hole for the piston rod gland drilled and reamed true to that. Cylinder stud holes were the last things to do to the flange. I also used a long series non ferrous specific milling cutter to face the ends of the bearings to length.



The final job on the frame was to set it up on it's back so that the two bearings could be drilled 4.8mm and then reamed out to 5mm diameter.

[Vixen]

Jason,

Some very interesting fabrication techniques in use. I love fabricated 'castings'.

Much more satisfying to make than simply buying a a ready made casting.

I needed to make a lot of this type of fabricated 'castings' for my Fowler PE.

Mike

[Admiral_dk]

Fasinating ways 'to Divide the Part up' in several pieces 

Hopefully inspirational to others 

Per

[Jasonb]

Thanks for the interest guys.

Some very interesting fabrication techniques in use. I love fabricated 'castings'.

Don't let Surus hear you saying that, you may get excommunicated

As you say there is a lot of satisfaction to be had from designing the "casting" allowing for how you will fabricate it as well as the actual fabrication. You also get a lot more workshop input per part which stops the completed models piling up on the shelf at too fast a rate. Take that frame as an example, if a casting it would just have wanted it's bottom smoothed, bit of work to the flange and then face and ream the bearings. Doing the fabrication probably took at least 10 times longer  and was a lot more enjoyable and challenging.

I doubt I will get the interest back to build up the Superba that I have castings for but if I di I know a lot of those castings would not now be used. Even Jo started to build up her bearing and ploughing shaft brackets but unlike me she got lured back to the casting mountain from where I fear she will never escape

[Jo]

Good start to explaining the build JB. Looking forward to hearing more about it - like you can explain things like the Scotch yoke design to the members as they may not be aware of its simplicity.

And like Uncle Perce said to me every year the new comers to our hobby are starting from scratch so you have to explain everything to them from how to correctly open the packet of seeds to digging over the vegetable patch - it is all obvious to old hands but even they can learn a new trick or two if things are explained in detail.


Yes I started putting together the pair of brackets for my BB1 but fabrication takes ages and on really complicated pieces made up of a dozen parts can be soul destroying. Alternatives like CNC from solid are available.

Jo

[Laurentic]

Loved the way you fabricated that 'casting' Jason, I admit I had to read it a couple of times to see excatly what you had done and how it was made up, but having done that I was even more impressed.

Nice and unusual looking model, ususual meaning different, good one, liked it!

Chris

[Jasonb]

Thanks, I do seem to be attracted to the models that are a bit different from the usual subjects.

Jo, although CNC cutting from the solid would be an option that is now available to me I think I would more likely go for a fabricated construction for those parts. Possibly cutting at least the more curved pieces on the CNC or maybe sending out for laser cutting. But I doubt it will see the light of day, however there are two Stuart subjects drawn up and I recently got the materials to revisit one of Anthony's engines that never ran so watch this space.

[Jasonb]

I came across this little gem today, would make a good companion for the donkey. Pity there is no illustration of the pump but I expect it to be very similar to the Bassett-Lowke one if not the same.

[Jasonb]

Those that have Been paying attention will no doubt have noticed that the hole in the cylinder mounting flange for the piston rod is quite large and that there is no short spigot to locate the bore of the cylinder. The reasons for this are 1. it would be tricky to counterbore and thread a larger hole than the 3mm piston rod required to take the gland from the underside. 2. forming the spigot to locate th ecylinder would also be tricky though could be done by mounting the frame on the rotary table or using a boring & facing head if I had one.

To overcome these two issues I turned up a separate piece from Cast Iron bar and loctited it into place, this section of the drawing should show how it works.



The crankshaft is made up from a length of 5mm PGMS with the over thick crank disc loctited into place and a silver steel crank pin retained with a CSK screw. Here you can see the turned and reamed crank disc has been screwed to a scrap block, its ctr location found and then offset to drill and CSK the hole for the pin retaining screw. After that it was loctited to the shaft and once set the shaft was held in a collet and the disc faced to the required thickness which ensures the face is perpendicular to the shafts axis.



A quick circle drawn onto the 3D model of the cylinder showed that it could be carved out of  apiece of 40mm dia CI bar so a piece was sawn off and then faced both ends leaving it a little over length. I then milled down 3 sides, one to form the port face and the other two to make it easier to hold in the 4-jaw before ctr drilling to mark the location of the bore. Withe the block back in the 4-jaw and the ctr drilled hole set to run true the cylinder could be bored and the end faced off true to the bore while bringing the cylinder to final length.



Moving back to the mill with the port face upwards in the vice the ctr was found and the inlet and exhaust port holes drilled as well as drilling and tapping for the valve chest studs.



It was then a case of standing the cylinder on either end to drill the steam passage down to the port and mill a slot to connect that drilling to the end of the bore. The cover stud holes were also drilled and tapped M2 on their PCD at the same set up. Vice stop in use here to save having to re find the bores ctr when it is turned up the other way.



I could have used the CHC to profile the cylinder with it stood vertically be decided on the manual method of firstly reducing the width either side to that required to match the valve chest using full height passes the swarf was coming of in nice long ribbons.



The last bit of machining on the cylinder was done with it mounted on a mandrel and held in the spin indexer to make a series of cuts to form the "rounded" part of the cylinder. After machining these were just blended into one another by filing.

[Jo]

Love the use of 5C collets in a Spin indexer to enable the outside profile of the cylinder to be machined round.

Jo

[Jasonb]

Bet you wish you had kept it now

It's quicker and easier than using a bit of bar across the vice jaws method as you don't have to keep stopping the spindle to move to the next position and easy to get all the facets the same which makes for easy filing. You can also offset either side of the ctr line when milling upto the edge where the valve chest edge is.


One of these various "little" engines would be a nice winter project for you as it could all be done with sexy and his new friend so you can do it in the warm and if you need a break after 20mins you can just leave it and come back in a while.

[vtsteam]

Beautiful model again!
Jason do the drilled steam ports look like a right angle inside the cylinder metal, or do they rise obliquely from the cylinder end to the steam chest?

[Jasonb]

On these smaller engines I tend do drill them parallel to the bore, it's not as though I'm after every last bit of performance where the slightly smoother flow of angled drillings would perhaps offer a tiny percentage advantage. They do end in a 118deg drilled hole not the flat bottomed one drawn, 2mm dia in this case

[vtsteam]

Thanks! That looks a lot easier for a small engine.

[Jasonb]

Two blocks of cast iron were milled up to the overall sizes needed for the valve chest & cover and a couple of ctr drill holes added to aid clocking them in the 4-jaw.



First the valve chest was turned to form the spigot for the gland, final cuts were done with a 1mm radius tool to leave a small fillet where the spigot meets the chest.



The same methods were used on the chest cover to form the boss where in inlet pipe screws in, this was tapped M4 x 0.5 metric fine. I've taken to using my old Unimat 3 four jaw chuck on a 1/2" parallel arbor for smaller items as that can be slipped into the 5C chuck and saves having to change to the larger 4-jaw.



Then back into the mill vice to drill the stud clearance holes and mill out the cavity in the chest which was roughed out with a couple of drill holes then a few laps around the edge with a 4mm dia cutter to form the rectangular cavity.



Progress so far





The valve was machined on the end of some 8mm square brass bar, first the cavity



Then the slots for the nut and rod

[vtsteam]

That looks great, Jason!

[Jasonb]

Thought I better finish off the details of the last few bits.

The eccentric strap was made from two pieces of 4mm thick brass, I milled steps in the sides and drilled and tapped so they could be held together with M2 fixings while the main hole was started with a drill and then completed with the boring head.



Due to the compact design which the scotch yoke allows there is no room for an eccentric rod so the end of the strap was formed and drilled to fit directly into the fork of the valve rod. Final shaping was done by filing



A groove was turned around some 16mm steel bar and sized to suit the strap, this bar was then offset in the 4-jaw and drilled and reamed to fit the crankshaft. At the same setting the boss was turned and after parting off the boss was drilled & tapped for a M3 grub screw.



I started the cross head by turning the two smaller diameter lengths either side of the larger diameter central section then moved over to the spin indexer to add an 8mm counterbore and 5mm through hole.



After silver soldering in a piece 8mm diameter brass that had the end reduced to 5mm into those holes it was back into the lathe to drill and ream 6mm for the slipper. With that done it was back to the indexer to tap the larger boss M5 x 0.5 for the pump ram and the smaller one was tapped M3 x 0.5 for the valve rod. The last job before parting off from the parent bar was to mill as slot to clear the crank pin.



The pump body was constructed in much the same way - Turn the outside, add some locating pockets, solder in some stubs and then finally machine.



The last bit of machining I have a photo of is the cross head slipper that was turned to a sliding fit in the yoke's 6mm bore and then cross drilled and reamed for the 4mm crank pin.



With all the bits roughly assembled I set the eccentric by eye  and gave it a try, after increasing the pressure a bit it was running but not as smoothly as I would have likes but good enough to strip down and then sort out the bugs on final assembly. I actually found one of them as I took it apart, on setting the eccentric I had got it a bit tight to the bearing and this was causing some drag. These are all the bits.



I put it back together carefully checking for smooth rotation as each piece was added and found that the end of the crank pin was just touching the outer edge of the hole in the yoke which was putting sideways load onto the piston rod and pump plunger. With a little machined off the offending pin all went together well and on trying it on air again I could now run at less than 5psi and get down to a nice slow tick over.

<a href="https://www.youtube.com/watch?v=T871NGafEAo" target="_blank">http://www.youtube.com/watch?v=T871NGafEAo</a>

[steamer]

What a sweet little pump!     It would be at home on a steam launch at twice size.

Nice!

Dave

[Twizseven]

Jason,

You certainly achieve some amazing results with your fabrication skills.

Colin

[Admiral_dk]

Great looking, running + pumping     

What is not to like 

Per

[vtsteam]

Beautiful little engine and pump! 

[Bearcar1]

WOW!, I turn my head for a little while and when I look back, you have completed the project. That trulu is an absolutely stunning piece of fabrication and I also commend you for such great set-up photos that document your journey. Well done and BRAVO!!    Now if yas can just get some drawings put together and share with us common folk. 


BC1
Jim

[RReid]

That looks and runs just sweet. Congratulations on another job well done, Jason!

[Kim]

Beautiful little engine, Jason!
Thanks for posting your build explanation, I find your fabrication very interesting.
Kim

[Zephyrin]

lovely little pump, with all the appealing of its model, congratulations !
the mastering of fabricating the parts is fantastic, as ever  !
I thought that the name of "donkey pump" was used only for the pumps without crankshaft, (as were the pumps drew by LBSC  for its locos), with a steam valve operated by a lever on the piston rod itself...

[Jasonb]

Thanks for all the comments

Busy couple of weeks but may see if I can get some working drawings out over Christmas, the biggest problem is that once I have finished an engine the next one(s) takes all my attention

I have generally thought of a donkey pump as being a separate pump rather than one driven of the main engine's own crankshaft or axle in the case of a loco - usually by an eccentric. I which case Donkey could be used for both pumps like this with a rotative action and also the weir type linear pumps.

[Don1966]

Nice work Jason your fab skills are a plus……


 
Don