Sunday, August 2, 2020

Spot The Difference

Alongside building the tiny OO9 model of the 10hp Baguley I've been looking through the parts for a much larger 16mm scale kit I'm going to build shortly. I'm not intending to give any hints as to what the kit builds into as yet, but instead I want to talk about bolt detail.

The kit is mostly laser cur MDF parts and the bolt detail (which is quite prominent) on the prototype is represented by circles etched into the surface of the part. Clearly this doesn't give any relief meaning that the details will mostly disappear under a layer of paint. Now there are obviously many ways you could add more detail. Some people use dots of glue, others use brass rivets, and you can of course buy plastic nuts and bolts that can be glued in place. All of those are probably easier than what I choose to do, but then I own a 3D printer.

Whilst many of the parts are too complex to replicate in their entirety and will need individual bolts adding as details, there are four pieces that fit at the corners of the chassis. These are just 18mm by 10mm and have five bolts on each. These seemed like a sensible starting point to check things like the size of the bolt head etc.

It didn't take long to draw these up and print out some test pieces. The first print I made is on the left. I printed this at the normal settings I use, which means each layer of the print is 0.05mm thick. Unfortunately as you can see (especially if you click on the image for a full size version) the layering is very pronounced on each of the bolts and it is very clear that they are made from multiple layers. To the naked eye the layering is less obvious but I fear it would be highlighted by painting and weathering.

So the next print I did is the one on the right, which was printed using layers just 0.02mm thick. The layering is still just about visible in the photos but I can't see it with just my eyes. The problem of course is that printing at 0.02mm instead of 0.05mm takes a lot longer. On a piece this small, just 2.175mm thick, the print time is increaed from roughly 12 minutes to around 27 minutes. While that's not a huge problem, if I was printing a much bigger model the difference would get quite significant.

I was especially annoyed with the longer print time in this case as the 0.05mm layers are perfectly acceptable for the main rectangular piece which takes up most of the depth. When printed at 0.05mm there are 43 layers in total with 32 of them being for the slab and 11 for the bolt heads. When we print the same piece with 0.02mm layers then there are 109 layers with 80 for the slab and 29 for the bolts.

Some of you may remember that back in April of last year, I discovered that the layer height specified in the files the printer uses is actually ignored and each layer is instead given it's absolute position above the bottom of the resin vat. On that occassion I used this to essentially double expose a single layer of the print in order to improve the result. Having moved on to using a different resin (specifically the Elegoo water washable black resin) I've not needed to continue using this trick, but I decided that the use of absolute positioning rather than layer thickness meant I might be able to speed up printing parts like these.

So I took the two files I'd produced and merged them. This resulted in a file with 61 layers where the first 32 layers had a thickness of 0.05mm and the final 29 layers had a thickness of 0.02mm. It took roughly 16 minutes for this file to print and the result is in the middle. Even under a magnifying glass this print is indestinguishable from the one on the right where all the layers were printed at 0.02mm but took ten minutes less time to print. I'm calling this a success.

The same trick should make it quicker to print some other pieces in the future where I only care about the layer thickness in certain areas of the model. For instance, any model printed with supports doesn't need the supports printing at thinner layer thickness, so the print time could be reduced by using 0.05mm for the supports and 0.02mm for the first layer of model upwards.


  1. These look more like rivet heads to me. This printer is very impressive. No appreciable slump on the early layers. What is the maximum size it will print?

    1. Yes, not sure if they are rivets or round headed bolts if I'm honest; for some reason I just refer to everything as bolts to save me working out exactly what they are from a photo or drawing!

      The printer has a build volume of 115mm x 65mm x 155mm. Of course each layer (115mm x 65mm) is printed in one go (takes 8s with the current resin) but the 155mm takes a lot lot longer if you want to print something tall.

    2. I should have added that there is no slump on the early layers as these were printed flat on the build plate so I didn't need any supports. I did print a thin raft around the parts to make it easier to get them off, which I just cut away with a knife.

    3. Interesting, I should take some close up photos of the printed details on 7/8ths wagons I've been building. They are OK but I don't think would stand up to magnification to that size, or if they do it is because the material is quite grainy

    4. and if you think they look good wait until you see some of the other details I've been playing around with printing out!