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Bungartz T7 narrow gauge tractor Oldtimer. Trying to model the thing.

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So with our 3d makerspace open I'll post a long-term project where I try to model a very special and kind of rare tractor. This is me trying to get to grips with freeform modelling and sharing my findings along the way. All pictures will be loaded to this gallery: https://www.behance.net/gallery/101740501/Developing-a-Bungartz-T7-Model-for-3d-printing



A very small oldtimer Tractor of German origin, originally made by Bungartz & Peschke, this particular one was probably built in 1965. I'll try to model the T7 variant, which I have direct access to as my father owns one. These tractors are pretty rare, despite being exported all over europe and even US models available. It is believed only 2800 Bungartz tractors were made, including all models and markets. All Bungartz models are very small, some offerings made to be small enough to pass simple barn doors by having a width of as little as 0,7m! The tractors were intended and usually used in wineyards and on steeper fields. They also were marketed for use in greenhouses. For these purposes the tractors have some special features that are worth noting:

  • adjustable track width of the rear wheels via fixed axle adaptors or even via adjustable adaptors as special option. This allows for very narrow width were needed and for trespassing along a steep hill by pulling out the lower wheel to prevent tipping over.
  • 90° steering angle (!!!). This allows for a very small turning radius, ideal for indoor jobs in greenhouses and the likes.
  • individual manual brakes for the rear wheels to assist turning on steep hills and allowing the inner wheel to stand still while turning the tractor around it! This is quite a sight.

I have copies of the user manual and spares catalogue for the tractor and for its engine, a Hatz Diesel AKD 10 Z. Our example has a fixed axle and is 0,9m wide, which should be standard for the T7. Here's a teaser of how small this tractor is. To the left is an already small  Porsche Junior Diesel, but the bicycle puts it really to scale:






  • Fusion360 will be my main tool. I want to get to grips with it and especially the sculpting environment.
  • Blender may see some use. Maybe I will try doing parts in both, to see what I get the most of.



  • I plan to build a full 1:24 model of this tractor over time. This is the long-term goal.
  • The main goal is to learn freeform modelling and combining of software with the intent of doing a complex model instead of parts only.
  • This is not a real tutorial, as I may go wrong at any given point, lose interest or hit a wall. But I encourage everyone interested to follow along to learn as well and share their knowledge and ideas. Please shout at me if you see me doing something stupid. I might have an advantage regarding strictly regular geometric patterns, but with freeform modelling there may very well be some of you around here knowing far more than I do.
  • I plan to do a full 3d model of the tractor in 1:1 scale, no matter how the parts are produced later. Most parts should be 3d printable in the end. I will offer to print these on demand, should I succeed. I don't expect too much interest, knowing this is a very nieche corner of our hobby.
  • I plan to do a 1:87 (H0) print as a gift for my dad to add to his model railway layout.


The first steps have already been made. I'll share these in the following posts along with more pictures.




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So how to start such a project?



I need to know what makes my subject special, as these things should be present in the later model. It can be a generic tractor, or it can be a strange little tractor that the expert loves because he finds the detail setting this one apart from others. To find out what is special I dig through articles and books on the subject, advertising and photographs, if available. For this one there are very little online ressources. But some information on Bungartz in general can be found. I have copies of user manuals and parts catalogues at hand through my dad, which helps. Also I have access to the real thing, which is pure gold.

  • In my case the most obvious prominent thing is the 90° steering angle with the double steering linkage. If this was a model I'd expect the wheels to be able to reach at least nearly 90° when built up. So I'll have to ensure this will work out.
  • The front wheels sport a quite visible angle.
  • The front axle wears lots of additional weight to keep the wheels on the ground. These weights have a distinctive form that is easily seen and should be represented in the model. Luckily this part is quite simple to model.
  • The huge badge on the front of the hood and the decorative aluminium stripe all over the hood are very prominent and add very much to the characteristics. The badge might be large enough to be 3d printed at 1/24th. In smaller scales this would probably have to be a PE part.

So these things I should absolutely get right to make it a Bungartz in the eye of an expert.



I have easy access to this exact model, so I can get away with measuring whatever I need to. I take reference fotos of all the detail, trying to catch everything. Of course you'll always miss quite a bit. It helps mentally breaking the thing down into groups of parts that need to be modelled together and try to get these groups documented completely before moving to the next. All groups need a reference part that has known dimensions to model the others to. So place a known object, a ruler or something else in your pictures if you don't have the dimensions or take them manually and write them down. Also, and that is most important if there are no blueprints available you need straight on pictures from all sides to model the whole thing to.



All known dimensions are gold! Tyre sizes, track width, height and length of the overall subject help to dial in all other parts. Wheelbase is important. Everything that can be measured gives you confidence and reference in your design which is really important.

I am lucky to have the manual which gives me the main dimensions I need to know.



With all that stuff you collected it is a good idea to at least group and sort pictures on a high level so you don't have to wade through hundrets of pictures to find that single detail you need to see right now. Throw out all bad pictures and duplicates. Retouch, Crop and resize the straight on pictures, correct the angle if needed. A Foto editor of your choice can be used for this. My goto solution is Gimp which is free and powerful.


More to come - stay tuned!


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This is what I do



Notice I use reference lines to scale and align my pictures. These also help to transform distorted images. You'll get no perfect result if your subject stands on a slope as mine did, you didn't take your pictures straight on and also got the height from an angle. Using a highly distorting wide angle mobile phone as camera doesn't help with quality as well. But for my purpose it'll be enough.

To get reference lines in Gimp simply click and hold the ruler on top and pull down for horizontal or use the ruler on the left and pull to the right for vertical lines.

I create a large empty canvas and drop my pictures in seperate layers which allows easy activation and distortion of the single pictures. In the end the layers should all show an equally sized subject. They can then be exported as .jpg or .png file to use in Fusion 360.

Gimp can also be used to cut out unused backgrounds, to recolor or sharpen or to create an outline-only picture of your subject. For me all I really need is correctly sized pictures.



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Finally we arrive at



I start with some preparation steps first. I build some sketches on the origin planes that contain known dimensions. Most important is the wheelbase as this is the easiest thing to scale the reference pictures to. So I set this up first. Also I do rectangles on all 3 planes to give the overall size of the subject plus a 3d box. Everything I model must stay within these constraints. Make sure all sketches are fully defined. Under no circumstances would you want undefined moving sections. This is key to professional CAD work. Last thing I'll do is placing the pictures of my subject. I like to push them away from the origin planes, but I know some folks do keep them there. It comes down to personal preferences.




Next is blocking in known pieces. This usually means the wheels - the diametre is typically well known as is the width. Placing is also defined by the wheelbase and width of the subject. So here we go:



Note that for every part I create a new component. Also I do work with linked copies if I need multiples of a part: I copy the component and simply insert it. Then place it where it belongs in the design. This creates a copy of the original part, that immediately changes if you edit something on any of the copies. If you don't want that behaviour you have to explicitly tell Fusion to break the link.


With all easy to do parts placed it is time for the next step - working with the imported pictures. Every major geometry is blocked in. To do this use the straight on views, this makes sure your sketches end up where you place them.









The trick (to me) is not to get lost in detail. This stage is all about getting the basic geometry right. Nothing else is important here. All parts can be refined at later stages. All I want for now is ensuring the overall shape is correct and working.


Sometimes there is a point where you can't get any further with simple sketches and extrusions. This is what is hard for me: Working with sculpting tools and or splines. Fusion gives you two alternative ways to deal with freeform parts: Define surfaces by surrounding splines. This gives maximum control but is hard if you are not working from blueprints. Or work in the sculpting environment, which I did here to achieve the hood. It is a simple form, so probably was a good starting point to learn. It still took me two days and several attempts to get a clean mesh and smooth surfaces that at least look somewhat like the original. In the end I started from a Quadball primitive that I pulled and extruded until I got to the point you see here:



And this is the structure I ended up with. Nice and clean, as I'd expect it. The form needs to be refined and the front part needs to be stretched, but we will get there.




This is where I stand at the moment. Everything from now on will be slower progressing as I can only work on this in my spare time which is basically nonexistent at the moment.

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