Joseph Paris
gantry construction (6)

Original page created on 31/01/2020.

Cab, hoist trolleys, hook blocks in 3D printing

Although the choice was made to build this gantry in photo-etched metal, there are still some parts easier to make in 3D printing. However, this technique still has drawbacks, such as relative fragility, perfectible accuracy and a difficulty in properly cleaning the parts.

For this last point, see the few following photos, knowing that cleaning is the provider’s responsibility, even if all claim that it is not possible to ensure a perfect cleaning. At Shapeways, I didn’t encounter this problem, but at Drim 3D, I did. For example, here is the amount of support material removed from the inside of a hoist trolley, though not very small. And there is still some.

In other cases, this material, a supposedly soft support wax, is almost as hard as the resin itself and cannot be easily removed from small corners. I tried different methods: ultrasound, with pure water or with the addition of household cleaner, cold or hot (50° C is the maximum allowable), acetone, with very mixed results, acetone being totally ineffective. I still have to try other products: vinegar, isopropyl alcohol, etc. All this means that I will return to Shapeways for the future version. Their service is not without fault, but it is half the price, for almost identical quality, and with parts delivered almost clean.

Here is for example a spreader trolley (I will return to this later) before and after cleaning with the methods indicated. The difference is not obvious! The relatively large surfaces are fairly well cleaned, but not the nooks.

The photos above, of poor quality, were taken with a USB / Wi-Fi “microscope”, on which I writed an article. I put quotes because this device is closer to a magnifying glass than a microscope.

Cab

The cab has fairly thin uprights, which provide housing for the windows. The main difficulty is to insert these windows and glue them from the inside. I actually started, but I can’t finish until I have painted the cab. This operation will wait for warmer days.

For cutting the windows, I have provided a photo-etched template. In the photo below, a strip of clear polystyrene is first taped to the cutting mat. The template is taped over. Obviously, it must be prevented from moving during the entire operation. It may be possible to use a cutting machine. For that, I propose a dxf file, which might need modifications to make it compatible with the machine (the drawing units are in millimetres).

The new template will have notches to facilitate the cutting of the corners.

It would be much easier to insert the windows into the cab if its back was open. This is what I did on the advice of my provider. But he failed to print the cab in two parts. He didn’t give me the reason, but I guess the overhang at the bottom of the sides is the cause. This is why, in the future version, I have planned a separate back, but with a reinforcement at the bottom back of the main part, which should solve this problem.

View of a window put on the cab door. Okay, we don’t see much because of the translucency of the parts…

Click on the image to — try to — get a closer look at the window.

A stencil is also provided to paint the glass joints in black, but I could not test it, because I was wrong in the dimensions (I took those of the windows without taking the joints into account, precisely…).

Hoist trolleys

The trolleys consist of two main parts, the chassis and the bonnet, and details such as photo-etched interior beams and a synchronization arm. The bonnet is simply fitted onto the chassis and remains removable, which makes it easier to adjust the length of the ropes. Here too, I had concerns, although I planned functional clearance: the chassis is wider than the bonnet! Barely a tenth of a millimetre, but that’s enough to prevent assembly.

So, I really have to mill the frame, on my mini drill, to remove only the material just necessary. Obviously, the vice tightening must be moderate. Holes can be seen inside the chassis that will receive the ropes and allow to block them.

Hook blocks

Just a tricky point, the rope path “channel” clearance. To do this, the best is to pass a sufficiently flexible, curved metal rod, then, if that’s not enough, an interdental brush! After which the rope (a very flexible elastic 0.5 mm thread) slides easily.

Of course, we’ll have to avoid filling these channels when painting.