Redesigning Models for FDM Printing

FDM (Fused Deposition Modeling) 3d printing is a marvel, and it has revolutionised the way that I make the toys I push around the wargames table while making "Brrrrm brrrm BOOM!" noises. However, it is not without its limitations.

1:100 scale (15mm) Toldi II Hungarian WWII light tank
The major structural limitation lies in the fact that the strings of molten plastic can't be laid down in mid-air, and need some sort of support to lie on. The printer can make its own supports as it goes along, and usually they do a pretty good job at supporting, but the bottom surfaces, lying on those supports, can be pretty ugly. Often enough, that doesn't matter too much for my purposes, because those surfaces will seldom be seen.

Shown here to the right is a 1:100 scale Toldi II which has been printed sitting on its tracks on the bed of the printer. The bottom of the hull, and beneath the turret where it was raised up above the print bed by the turret-plug, show the characteristically loose fibrous-looking surface you get over supports. It's ugly, but you never see it, so it doesn't really matter. You can see that the bottom of the tracks, and the bottom of the turret plug, both of which were built up directly on the print bed, are much smoother.

Although the bottom surfaces aren't usually visually important on these vehicle models, there is a situation where the imprecision of a supported surface can matter. That is at the meeting between the turret and hull.

These two models (left) of the Vickers Light Mk.VIb were modeled differently. The one on the left had its turret printed in one piece, the same way as the Toldi II above, and you can see that the loose net-like structures left behind by the supports have somewhat rounded off the bottom edge of the turret, and they lift it away from the hull. The model on the right had its turret printed in two pieces, as shown below, so that the bottom edge of the turret itself was built up directly on the printer's glass bed, giving a very smooth surface and a clean, sharp edge. It sits right down in contact with the hull top.

The way I did this was to split up the turret into pieces. This picture (right) shows the whole model, laid out ready for printing.

When I was cutting the turrets up, at the same time I punched a hole through the turret plug, with a matching one in the bottom of the turret itself. The small cylinders you see here are used as locating pins, to ensure that the pieces get glued together accurately.

I also use the hole in the turret plug to house a 3x3mm cylindrical magnet, with a matching hole and magnet in the hull. The magnets are there to allow the turret to rotate freely without falling out and bouncing away under some furniture where it would never be found again.

You will observe that the hull is printed standing up on its tail. The reason for this is because this orientation is optimal for reducing visible layer lines on the vehicle's sloping panels, and it also gives me better, cleaner detail in the track run.

This turret for a British A10 cruiser tank (left) has been treated in the same way, except that the turret ring has also been detached from the turret and is printed along with the turret plug.

This arrangement maximises the cleanliness of the edges where it matters, where it will be seen, while keeping the number of individual parts to a reasonable minimum.

I could, if I so chose, split up the models into a multitude of component parts, each so arranged as to print to its best advantage. However, part of the appeal of a 3d printed model, for my purposes at any rate, is simplicity and speed of assembly. If I want to build a whole kitset model, I'll buy one. My aim is to find a happy medium between model quality and convenience, and this system achieves that for me.

All of the base models I've used as examples in this post have been by m_bergman (Toldi II, Vickers Mk.VIb) and TigerAce1945 (A10). Their models are available on Thingiverse, and I highly recommend them both.

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