Mc Donald Model tractor

Fred,

It will be an awe inspiring exhibit when it comes.!

Do let us know when that will come about.

Howard

This is engine part number one. The piston is a flat topped deflector ( if there is such a thing) with the two cutouts being the deflectors for the gasses. Although I have the laser cut connecting rod I am going to try casting one in aluminium in order to reduce the weight factor. I am also giving some thought to the reed valve induction set up. I do have a an old Frog 149 cc model diesel engine with what was called “vibramatic” induction and will have a good look at that for ideas.

Fred.

I'm still following and enjoying

I decided to reduce the doubt for my two stroke diesel by using a commercial reed valve from a moped/scooter. This was not expensive however if you have a dealer/workshop near you you could see the various sizes and shapes available.

Good Day Roger,

Thanks for the email. I did visit my local motorcycle shop and after explaining what I was after and what it would be used for they donated me a set of small Boyesen reed valves. What capacity is your engine? The tractor engine will be 79.5cc and hopefully the revs will be quite low as it has a heavy flywheel to carry the moment through.

Fred

My engine is only 20cc so those valves are a bit big but that will hopefully help the volumetric efficiency.

This is the cast aluminium connecting rod and the piston. I recycled an old Briggs and Stratton con rod and hopefully by not over heating it in the crucible I may have retained the metals characteristics. The big end bearing is a needle roller running on a hardened and ground sleeve and the small end also uses a needle roller but it runs on a heat treated silver steel gudgeon pin. The pattern for the crankcase /cylinder is in 8 parts and when made will be glued together to form 4 quarters. Each of the 8 parts will take 6+ hours to print on the 3D printer but that’s next weeks project.

Fred.

This is the result of 50+ hours of 3D printing in PLA. The 8 parts are held temporarily held together with dowel pins, hence the gaps. Each quarter will be tested in oil pound sand before parts are glued together. If all is well then chemical sand will be used to make “ bricks” that can be later glued together ready for the casting process. I still have to finalise the core shape as this does present a real problem in both the making/ assembly and the casting. I may even have to end up with a half wet half dry liner - more thinking required !

Fred.

Over the last couple of weeks I have been refining the patterns and core boxes for the engine. While the 3D printer has been working overtime I have also been making the crankshaft. The crank disc are made from 20mm thick mild steel plate. The oversized blanks were a donation from the local laser cutter and had been cut to form holes in some one else’s work and as a result each had a “ start cut” abut 6mm in from the outer edge. By offsetting each disc in a four jaw chuck I was able to machine up two 82mm blanks which after cleaning up were soft soldered together and the machined to the 80mm size required.The two discs ,now soldered together ,were marked out for the crank pin and crankshaft holes and bored on the lathe using a four jaw chuck as a means of holding the work. Photographs of the finished crankshaft to follow later. The crankcase/ cylinder casting has caused much thinking and screen time and has taken longer than anticipated but it will be made!
Fred.

Super work Fred, please keep it coming. Out of my league - I'm following progress in stunned silence!

Dave

From one silly old duffer to another thanks for the comments. It appears that progress has slowed down but all the work completed so far has been the type of work that I have experienced before but this engine is a real brain scratcher. It’s trying to fit all the internals into the scale outside shape and above all to have it working and producing a reasonably characteristic sound. This type of engine is new to me and really stretches my capabilities, experience and equipment and even after all the planning there is no guarantee it will work! It’s the challenge and vision of the finished tractor that keeps me going.

Fred.

The top photo shows the parts of the 3D printed core box for the crankcase. The top part is in two pieces but the bottom is one piece and took 34 hours to print. I still have to print the cylinder core boxes and then check that it all fits together and can be cast. During the ,day while the printer was working ,I machined up the the crankshaft from blank mild steel discs and 4140 steel shafts. I still have to machine the second shaft of the crankshaft - this is the one that has the clutch and drive attached to it and the design is still being finalised. The crank is carried on four ball races to spread the load and give stability and has oil seals on the outside as the crankcase is a pressure chamber on a two stroke engine. The crankshaft has been balanced in the same way as described in the Edgar Westbury book on engine design and manufacture ,a book that I value and purchased in Bristol in 1962.The drilled balance holes have been filled with cork to keep the volume as high as possible.

Fred.

Work continues on the development and 3D printing of the core boxes for the cylinder/ crankcase unit. Before this part is cast I need to make sure that I have provided all the lugs and attachment point for the various parts that help to make this tractor engine work. The photographs show the CAD drawing / assembly ( complete with strobe effect) of the clutch and brake assembly. The second photo shows the thrust collar that forces the clutch plates together. On this tractor the clutch lever also applies the brake pad to the outer face of the pulley after it has disengaged the single plate clutch. This is the only brake on the tractor but later editions had an extended shaft from the gearbox and this shaft carried a drum and band brake assembly with the brake being applied by a lever. These parts have yet to be made in metal and will probably be prototyped with the help of the 3D printer to check that all is well.

Fred

So it does have a brake after all. That makes a bit more sense than relying on the operator always having to find somewhere flat to stop. Does the lever have a means of holding it in the brake position?

I suppose the next question is how is it started, both in real life and model form? I found one video on Youtube that appears to show heating the hot bulb and then inserting a peg into a hole in the flywheel to turn it over.

This is the assembled core box for the inside of the tractor engine with a match box for size comparison. For those not into casting this box makes the sand shape that makes the inside of the engine hollow. I still have to 3D print the core box for the water passage and hope to finish that later today or tomorrow. With reference to the “braking system” I am sure the brake will only slow down the engine via the pulley as when the lever engages the brake it disengages the clutch and as this is connected to the gearbox no braking of the tractor is applied. I will check this out next time when I visit the tractor and owner.

Fred

I have checked in the owners hand book for this tractor and can now see that the brake does slow the machine down as the pulley is connected to the gearbox by the chain and sprockets. As long as the tractor is in gear the application of the brake pad to the pulley and disengagement of the clutch should slow progress!

Fred.

I would guess (by extrapolation from my uncle's Field Marshall) that on the full size one just reducing the throttle to minimum would slow the whole contraption down to quite close to a standstill, then the final declutch and push the lever further to bring to a halt with the transmission brake if needed - on the soft it'll probably dig in anyway, but hard packed ground or a slope might be tricky. Stop engine and release clutch as a parking brake. Actually much the same as a single pedal ride on lawnmower nowadays thinking about it.

Thanks for the scaling factor - on several of your pictures it's not obvious what size the parts are which is a compliment to your work.

Back in action after a short holiday and this is the result of today’s efforts. This casting (and pattern) is for the engine drive side bearing and has the clutch/ brake mechanism attached to the flat area on the side. This is a prelude to attempting the main casting in the not to distant future , something that I am bit apprehensive about ! I have spent quite some time on the designing of the mechanisms relating to the clutch and injector cam as these are attached to the crankcase/ shaft and I want to make sure all is well before casting the crankcase.

Fred

Back in action after a short holiday and this is the result of today’s efforts. This casting (and pattern) is for the engine drive side bearing and has the clutch/ brake mechanism attached to the flat area on the side. This is a prelude to attempting the main casting in the not to distant future , something that I am bit apprehensive about ! I have spent quite some time on the designing of the mechanisms relating to the clutch and injector cam as these are attached to the crankcase/ shaft and I want to make sure all is well before casting the crankcase.

Fred