Mc Donald Model tractor

[Four stroke FredParticipant @fourstrokefred]

The photographs show the shock absorbing tow bar and the structure that supports the platform on which the driver stands. The second photo shows the component parts for the rear wheel hubs. The parts at the front have all be cut by laser having drawn them on Fusion 360. The scalloped grooves on the left hand hub are for short rods that will be silver soldered in place and will be later drilled for wheel retaining bolts. The central hub parts have a taper for a collet but still require shaping externally and after the spoke plates have been bent to the correct angle the total hub can then be assembled. It sounds easy when you rewrite it down and I hope it goes together as planned!

Fred.

[Howard LewisParticipant @howardlewis46836]

FANTASTIC work!

Howard

[Howard LewisParticipant @howardlewis46836]

Without wanting to hi jack Fred's thread, have been thinking about Roger's model of a Marshall, and Tim's comments.

Tim is right that the early two strokes relied upon a "Harmonic" exhaust system to scavenge the combustion products from the cylinder, and to partially charge it.

Because this relied upon the exhaust system being in resonance, the engines were virtually constant speed units, to stay close to the resonant frequency. Anyone who has heard a Marshall two stroke running will testify to this

A model of a Marshall, running at 2,000 rpm, will sound anything but prototypical!

The dynamic magnifier curve is extremely "Peaky", so that a comparatively small deviation from the resonant condition can result in the loss of the advantage. What works well at 2,000 rpm may do nothing at 1950, or 2050 rpm. Witness the need for speedway and motorcycle racers to stay "on the meg".

Similarly, at 1,000 or 4,00 rpm the system may do just the reverse of what is needed, as the engine and exhaust pulse are then completely out of phase.

The resonance depends upon the speed of sound under the temperature conditions prevailing in the system, but that, unlike the components, cannot be scaled.

So to have an engine that runs in any way like the prototype, there will be the need to depart from the original.

It may be possible to lengthen the exhaust tract, and yet apparently stay with a prototypical looking "silencer".

A 1/6 scale model would need the exhaust system to be six times as long, as scale, to reproduce the conditions of the original.

It MAY be possible, but not easy, to fold the exhaust pipe into an extended coil. and site it within the "silencer" outline, so that actual length is possibly three or four times the apparent length, whilst keeping the divergent- convergent external form.

Just a suggestion..

Howard.

[Roger BParticipant @rogerb61624]

Splendid work on the Mc Donald

Howard, all the work on the diesel engine as well as petrol and diesel fuel injection systems is on Model Engine Maker. So as not to take over this thread I am happy to continue discussions on there.

https://www.modelenginemaker.com/index.php

You need to be a member to see the pictures. I initially ran the engine as a petrol engine with a different cylinder head to prove the porting would work. In this form it would run reasonably slowly but I don't have enough space for a big enough flywheel to allow the diesel to run at low speeds, the compression pressure is around 30 bar.

**LINK**

[Four stroke FredParticipant @fourstrokefred]

The exhaust system on the Mc Donald that my model is based on has a straight through pipe but I have seen a spark arrester fitted to another. The full sized engine ran at about 500rpm and I hope to keep the revs on the model low, helped by a heavy flywheel. I am looking forward to making and developing the engine and hearing the exhaust beat. My other interest is motorcycles and their development and I can remember being in the pits at Brands Hatch in the 1960s and seeing the East German MZ with Walter Kadem their design engineer. These machines used a resonant exhaust system to very good effect and was later taken up by the Japanese companies. On my model I will use the straight through pipe as am not after maximum power and will very very happy to just hear and see it move and also try to keep as it was on the original. Thanks for the comments.

Fred.

[Four stroke FredParticipant @fourstrokefred]

Over the weekend I had a visitor to the workshop and on seeing the tractors, boats, I.C and steam engines on display asked ” “where do you get the instructions from to make these ?” I had to inform him that they were home designed and made and that I had spent 17 hours so far photographing and measuring up the Mc Donald tractor and as for instruction you make it up as you go along. As you work on one part you are thinking of the next part and how they will relate to each other. The rear wheel hubs have now been turned down to size and will soon be ready for silver soldering. I have now checked the angle that the spokes have to be bent to and will make a jig for this repetitive exercise. Photo shows milling the pockets in the rear hub – as it was not a full circle great care was needed!

Fred.

[Four stroke FredParticipant @fourstrokefred]

[Adrian R2Participant @adrianr2]

Looking forward to seeing this on its wheels.

I remembered where I'd come across a vertical split gearbox before. My grandfather owned a Turner Yeoman of England with a similar arrangement. It was a v4 diesel if you fancy building another oddball.

Edited By Adrian R2 on 02/03/2021 11:12:05

[Four stroke FredParticipant @fourstrokefred]

The photo shows one of the rear wheels under construction and also the hub parts for the second wheel. I made up a wooden jig on which to build the unit as this enables me to centralise the hub to the rim and also give the correct offset from the outside of the wheel. It’s the sort of job where you need octopus hands and you need to plan out the order of work and assembly. The hub was silver soldered up to hold the six pins and one of the spoke plates in place with the second spoke plate being added later. The collet was more difficult to make than expected and again it was planning the order of the processes that was important ( holding the work for slitting and making sure both the taper and parallel bores were accurate). I will drill all the rivet holes before the final assembly and hope that it will all line up and be concentric – may even have to call in my wife to give me a hand and I know she will be thrilled with that!

Fred

[Four stroke FredParticipant @fourstrokefred]

[Michael GilliganParticipant @michaelgilligan61133]

It just keeps getting better …

Lovely work Fred

MichaelG.

[Four stroke FredParticipant @fourstrokefred]

One down and one to go – rear wheels that is. The job proved to be more difficult than I anticipated as trying to keep every thing concentric and to size was not easy. The second wheel should not take as long as I have now work out the order of work and made the various jigs and tools. The tractor on which the model is based does not have any form of tread but could be fitted with “ spade lugs” but I am contemplating fitting rubber blocks to offer some form of grip. As autumn is here I have to spend some time preparing the veg garden ready for sowing and as a consequence the second wheel will have to be put on hold for a couple of days. Plenty to think about.

Fred.

[Mark Williams 14Participant @markwilliams14]

George, wonderful work !!

whereabouts in Queensland are you? Currently up here from Tassie

regards

Mark

[Adrian R2Participant @adrianr2]

Some traction engine type strakes wouldn't look out of place, or there is one here that has been fitted with solid tyres.

https://cobahcastiron.weebly.com/mcdonald-tractor-twb542.html

[Four stroke FredParticipant @fourstrokefred]

Mark if you are in the Bundaberg area give me a call ( I am in the phone book) and we can arrange a meet.

Regards,

”Fred”

( George Punter).

[Mark Williams 14Participant @markwilliams14]

George, thanks for the kind offer to catch up! Not sure if I will get that far up, currently in Caloundra at my daughters place doing all the jobs she wants doing!! Looks to be 3+ hours from here to Bundaberg ??
regards

Mark

[Four stroke FredParticipant @fourstrokefred]

Progress on the Mc Donald came to halt this last week as I was preparing for a model railway and other hobbies exhibition in town. I have just returned from the last day of the show feeling rather tired but satisfied having met and chatted to many of the public and hopefully inspired then to take up a practical hobby. There were traders, many train layouts in Z, N and HO scale, model boats, planes and model engineering. The photographs show our effort ! Next week it will be back in the workshop building the second rear wheel and thinking about the front wheels.

Fred

[Four stroke FredParticipant @fourstrokefred]

[Four stroke FredParticipant @fourstrokefred]

It’s good to be back in the workshop and making bits for the Mc Donald. The show was very good but tiring on the legs and voice. The photographs show progress on the seat and the conical spring that is fitted underneath to give the driver some degree of comfort! The seat was beaten out of 1.0mm steel. The shape was photocopied to scale from a photo and then the slots were cut out with a piercing saw. The shape was beaten down into the wooden block and the raised centre section on the aluminium block with the groove machined into it. The red jig at the front was used to bend down the outer edges before planishing on a stake.The conical spring is made from 2.0mm fence wire (that gives just the right amount of spring) and wound on the tapered mandrel shown. 3.0 mm copper wire is used as a spacer and later peeled off. I had intended to make the second rear wheel but the laser cut parts have not yet arrived. There is still plenty to think about and do.

Fred.

[Four stroke FredParticipant @fourstrokefred]

It doesn’t look much for the days of effort that went into making it! The photo shows the small casting and pattern that was used to make it. The finished casting is bolted to the right hand mudguard to hold the seat in position. The seat can be swung through 90 deg to give easier access to the rear of the tractor. There were many “ funny” angles and curves in the shape and I had to make another visit to the tractor to check that the details were correct. At least at the end of the day I achieved what I had set out to do but just as well it wasn’t being costed by the hour! I still haven’t received the laser cut parts for the mudguards but the type of jobs we ask the laser cutter to do are probably more of a nuisance so I do appreciate that fact that he will actually do the work. If any of the readers of this project have any question I will do my best to answer them.

Fred

[Dominic BramleyParticipant @dominicbramley60728]

No questions Fred – but I am certainly loving this build. Fantastic attention to detail and a fascinating array of processes being employed.

Being a helpful forum I think a lot of members save their comments for when they feel they have useful advice to offer the builder. This has the unfortunate side effect of the most accomplished builds having relatively few comments. Rest assured though, there will be an awful lot of people following this keenly!

Regards

Dom

[Four stroke FredParticipant @fourstrokefred]

[Four stroke FredParticipant @fourstrokefred]

These photographs show the front wheel full size.

[Oven ManParticipant @ovenman]

Posted by Dominic Bramley on 31/03/2021 12:22:46:

No questions Fred – but I am certainly loving this build. Fantastic attention to detail and a fascinating array of processes being employed.

Being a helpful forum I think a lot of members save their comments for when they feel they have useful advice to offer the builder. This has the unfortunate side effect of the most accomplished builds having relatively few comments. Rest assured though, there will be an awful lot of people following this keenly!

Regards

Dom

Yes, indeed. I am in awe of the attention to detail and the quality of the workmanship. I look forward to seeing each posting.

Peter

[A SmithParticipant @asmith78105]

Fred, following closley, terrific work. looking forward to the next installment.

Andy

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