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Mc Donald Model tractor

Single cylinder semi diesel

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Adrian R201/02/2021 10:16:48
138 forum posts
5 photos

Good stuff. I don't think I'd fancy furnace work in 30C, well, not unless you have one of those closed and insulated electric ones. On the plus side less chilling from the moulds I guess.

Four stroke Fred04/02/2021 08:11:24
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240 forum posts
229 photos

b191e157-1d85-40ea-93d2-b4e23f52ff5f.jpegThis week I cast the steering wheel in aluminium having made the pattern on the 3D printer. As the wheel is dished this caused some head scratching when casting. When the print was made I included a thin membrane to separate the two sand layers - this would also allow the metal to flow over the whole wheel and was cut out when the casting was made. The photo shows the steering wheel on the left, the pattern in the centre and a support cone that the 3D printer makes to support the print - this being used in the casting process.3ac6d528-d529-49f3-9c6b-9fa09bca1ecc.jpeg

Four stroke Fred08/02/2021 07:36:33
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240 forum posts
229 photos

Earlier on I said that I had found a worm gearing system that suited the the steering box well the truth is it was the correct size but unfortunately it was right handed and no good for the tractor - must have had a seniors moment, If I had used it the driver would have turned the steering wheel to the right but the tractor would have gone left ! After checking on the internet I discovered that most commercial worm wheel sets are right handed. The challenge was then to make my own that fitted the criteria and today I think I did just that. I still have to check and finish the matching pair but hopes are high. Yesterday I assembled the gearbox and joined it to the chassis, Gearbox is10kg and chassis 8kg.

Fred.

Henry Brown08/02/2021 10:20:31
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509 forum posts
106 photos

More great work Fred, really enjoying your adventures!

Tim Stevens08/02/2021 18:38:32
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1517 forum posts

I have a message for Roger - I hope he is still reading this story

I suspect that the Field Marshal is going to be a real problem to get running nicely. This is because its working depends on the shape and size of the exhaust system. As the exhaust port opens, the rush of gas is intended to overshoot, rather - drawing fresh air into the pipe as the 'bang' reaches the narrowing at the far end. This causes a reflection of a pulse back towards the engine, and this pushes the fresh air back into the cylinder, just in time to be trapped by the exhaust port closing, improving the compression and turbulence, for more power. This process needs to be 'timed' with the engine rpm, of course. But the timing of pressure reflection in the pipe depends on the length of the pipe. So a half-sized pipe on a half-sized model will need to rev at twice the RPM to work properly. So, your tick-over will not sound anything like the low beat of the original. If you can get it to work at all ...

If I can help explain further, please send me a Private Message and I will do my best.

Regards, Tim

Dominic Bramley08/02/2021 19:33:24
51 forum posts
1 photos

Looks great Fred!

I'm really enjoying following this build!

Cheers

Dom

Roger B08/02/2021 19:45:01
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172 forum posts
76 photos

Thank you Tim,

I fully understand the problems of scaling real life and am learning the additional problems that come with two strokes. I will be happy if I can get a operating range between 2000 and 4000 rpm. A 500rpm idle is out of the question. It sort of runs but the injector atomisation requires some more work.

**LINK**

Four stroke Fred08/02/2021 20:31:53
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240 forum posts
229 photos

Thanks for the comments and I am pleased that you enjoy reading about the construction of this model. For me it’s going into the unknown that is exciting as I enjoy the problem solving as well as the construction. The part I am working on is the model of the day and in the end they all add up to the completed model. I am looking forward to making the engine as I know there will be plenty of problem solving there.

Fred.

Four stroke Fred09/02/2021 07:59:02
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240 forum posts
229 photos

5cf8c32b-2909-463e-ac3f-91f431f29c4b.jpegToday I worked on the steering box gears including making the hob and hobbling the worm wheel. Photographs show progress. It was a steep learning curve but satisfying to think that I didn’t have to buy them after all. I may have to hob the gear wheel a little more but that’s for tomorrow

Fred.b6fb91af-c449-4bf8-9184-bce537b6eba1.jpeg

Four stroke Fred12/02/2021 07:46:16
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240 forum posts
229 photos

As you can see the steering box is now finished and now has its position on the side of the gearbox. I had to make the steering wheel shaft and temporarily fit it to the box as it I had to set the shaft at 66 degrees to the horizontal top of the main gearbox. The steering arm that fits onto the worm wheel shaft has been shaped but has yet to have the square hole made at its pivot point.

Fed.1c3a7f8a-d6ea-4913-9ef3-22bf57079ece.jpeg

Four stroke Fred14/02/2021 09:33:33
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240 forum posts
229 photos

aeba204a-2826-4573-8dad-e6f676eb3098.jpegJust to wet the appetite the photographs show progress so far ( but still a long way to go). The small part between the wheels is the steering arm that attaches to the steering gearbox. This part has a 6mm square hole in one end and it took most of a day to make as I made and used square broach but the end result was that the hole is a good fit on the square shaft and that was the aim of the day.

Fred.

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Four stroke Fred14/02/2021 09:35:07
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240 forum posts
229 photos

aeba204a-2826-4573-8dad-e6f676eb3098.jpegJust to wet the appetite the photographs show progress so far ( but still a long way to go). The small part between the wheels is the steering arm that attaches to the steering gearbox. This part has a 6mm square hole in one end and it took most of a day to make as I made and used square broach but the end result was that the hole is a good fit on the square shaft and that was the aim of the day.

Fred.

5dda2d3d-7b1c-4e02-86df-61eea1f40c8e.jpeg

Four stroke Fred18/02/2021 08:03:19
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240 forum posts
229 photos

This is this weeks”model”. It’s the gear change lever and as small as it is it’s fiddle and takes time. It’s a case of fingers too big and parts too 4669dca4-37ff-43fa-8212-f6bedd4d0174.jpegsmall! The next part to be made is the clutch lever and as I didn’t have all the dimensions I had to make a visit to the farm armed with camera, rule and sketch pad. I am lucky in that the property is only 15 km away.

Fred.

Four stroke Fred28/02/2021 07:07:38
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240 forum posts
229 photos

1858cc48-baa9-4a55-a3b2-3bcad16c11a8.jpeg8f2ca7b5-eebc-45a9-9afa-186f6ffc8870.jpegThe 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 Lewis28/02/2021 11:08:45
5750 forum posts
13 photos

FANTASTIC work!

Howard

Howard Lewis28/02/2021 14:01:06
5750 forum posts
13 photos

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 B28/02/2021 17:03:59
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172 forum posts
76 photos

Splendid work on the Mc Donald thumbs up thumbs up

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 Fred28/02/2021 21:44:54
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240 forum posts
229 photos

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 Fred01/03/2021 07:54:03
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240 forum posts
229 photos

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 Fred01/03/2021 07:57:59
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240 forum posts
229 photos

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