Thursday, March 28, 2019

Grey Resin

I've not had much time recently for modelling but today I set another test print running in my new 3D printer, not only to try it on a more detailed model but also to try a different resin. The default transparent green resin worked well but looks odd so I thought I'd try a more normal looking grey resin.

As you can see I printed the Sand Hutton Wagon which stalled because I couldn't get decent prints from Shapeways. On an initial look at the prints I was blown away. The crispness of the detail and the smoothness of the surfaces easily beats any prints of this model I've ever had from Shapeways. The underside of each part is deliberately less detailed due to the way they are supported while printing, but I can probably live with that. There are issues though.

First up, the floor of the wagon is missing plank lines. I'm guessing this is because the lines are quite thin, and unlike the sides of the wagon the whole floor surface is cured in one go, and the UV light has bled through enough to fill in the gaps. This could be easily fixed by widening the gaps ever so slightly.

A bigger issue though is the underside of the parts which is frankly horrid. It's all thick and bumpy. My feeling is that this is a combination of the grey resin being a little thiker so it doesn't run off as well when the layer is lifted, and that the part was printed parallel to the build plate. This means that when the layer lifted it held on to lots of the resin which cured slightly when the next layer was printed. It may be that the green resin would give better results as it seems to pour easier, but also tilting the model slightly so that the floor is printed in stages might also help.

So more experimentation is needed, but I'm getting there, and looking at the detail on the wagon sides, if I can iron out the teething problems the printer is going to be fantastic.

Sunday, March 17, 2019

405 Nanometres

As most of you will know I like 3D printing, I've been a convert since I got my first print back in August 2012. One thing I worked out quite quickly was that while I really enjoyed designing the parts and receiving the prints (usually from Shapeways) I had no interest in having a printer of my own.

The main reason for not wanting my own printer was a simple cost versus quality trade off. All the printers I could contemplate affording couldn't get anywhere near the quality of the prints I could buy via Shapeways who could afford to buy very very expensive printers. Over the last year or so a couple of things have changed that caused me to rethink my stance.

Firstly Shapeways are no longer the friendly, open, and helpful startup they once were. Don't get me wrong they are still producing high quality prints, but they've made a number of changes to their platform that make it a lot less fun to use. First they hid the pricing formula and a lot of the related details which made it difficult to print items without them becoming more expensive than necessary. Then they increased the prices, with some models seeing price rises of over 30% and they doubled the shipping costs. Added to that they changed the website layout making it so much harder to simply order a print.

At the same time as Shapeways were doing their best to alienate their customers there was a significant change in the home 3D printer market. Until fairly recently almost all the printers that I'd consider affordable for home use were based around Fused Deposition Modelling (FDM); essentially a nozzle moving in three dimensions depositing melted plastic. Whilst these printers can be used to produce great models, they tend to be quite large, and the quality, especially on small details, doesn't match up with what I'm used to getting via Shapeways (the process there is also FDM but again it's the cost/quality trade off issue). An alternative approach to printing is to use light to cure a resin, often referred to as stereolithography. Traditionally this involved a laser and mirrors and was an expensive approach, but there are other ways of achieving the same result. One such way is often refereed to as Digital Light Procssing (DLP) printing, where the light for a single layer is projected onto the resin. Depending how this is achieved it can be done relatively cheaply while still producing high quality prints. More importantly it can be done cheap enough for home use.

As you've probably guessed by now I've gone and bought myself a 3D printer. Specifically I've bought an Anycubic Photon (best price I've found for a UK seller is Amazon, and they seem to have fairly regular offers on, I got mine for quite a bit less than the current list price in one of Amazon's "Lightning Deals"). The printer has a relatively small build volume (i.e. the maximum size of object you can print) of 115mm x 115mm x 65mm but that's plenty big enough for the models I usually print. The small size is actually an advantage as it takes up very little space (22cm x 20cm) on my desk. It works by using an LCD screen, at the bottom of the resin vat, to act as a mask for a UV light. After each layer has cured the build plate is moved up a tiny amount before the next layer is made.

This means that essentially the printer boils down to a UV light (with a wavelength of 405 nanometre), an LCD screen, and a single stepper motor (plus the control electronics obviously) making it fairly cheap to produce. Of course the approach does have some limitations. For example, the XY resolution is dependent on the resolution of the LCD screen, and the smallest detail you can produce is therefore the size of a pixel -- 47 microns in this case. Also with the LCD essentially acting as a mask there will be some bleeding of light which can make producing small holes an issue as they tend to close up slightly. So the question becomes how good can printers like this be for the price.

Well there is really only one way to find out, and that's to print something!

Okay I didn't just print one thing, but three things. Specifically I started by printing a sprue of 40 L&B roof finials as they didn't need any supports adding. I used the default settings with a layer height of 0.05mm and the finials look perfect; no obvious print lines and nice crisp details. Of course they are not the most complex or detailed parts so next up was the 6.5mm gauge chassis for the peat wagon I'm currently working on. This was a bit more involved as it required supports to be added. Even so it came out really well. A few lines on the print (possibly because of the orientation) and the fit of the wheels is ever so slightly tighter than I'd like but usable as they are. The final test print that I've done so far was the kit of parts for the K12 diesel loco.

This is a much more complex model with lots of parts requiring supports adding and a number of small holes and tight clearances (as it was designed to minimise the build volume to keep the price on Shapeways down). I altered some of the automatically generated supports which caused a few problems; the bottom of the buffer beams drooped downwards and half the power retaining clip is missing. Also the small slots for the etched parts to fit into are a little undersized. This seems to be a known issue with the printer and is probably a combination of light leakage on the LCD and the viscosity of the resin. There are a number of ways to work around the issue so it's not the end of the world. More importantly, I gave the part a quick waft of primer to get a better look at the details; I made no attempt to smooth the surface all I did was remove the supports.

Now this does highlight the drooping issue with the buffer beams but, look at the axle box detail, and how smooth the sides are! In this area, the print is better than the last print of this I got from Shapeways where I had to do lots of careful sanding around the axle boxes to remove the print lines.

All in all, I'm exceptionally happy with the printer so far. Yes I have work to do in order to fix a few issues (mostly around small holes and sorting out the best way of putting supports etc.) but I don't think it will be too hard to move to the point where I'm happy to use the prints for my own modelling. It might even be that I start printing parts for some of my kits as well. This isn't to say I'll never use Shapeways again as I'm sure I'll want to print in other materials (like brass and stainless steel) or I might decide that in some cases the prints from Shapeways are better for some reason (maybe something is difficult to support and print) but if nothing else I should be able to prototype new models a lot quicker and cheaper than before, and if nothing else I'm going to have lots of fun.

Tuesday, March 12, 2019

Alan Keef K12 Diesel: Origami

The K12 diesel loco features the most complex etch parts I've ever drawn up. The first version was complex enough, but once I'd realised that I'd got the bonnet side panels wrong they become even more complex. The problem is that there are two many layers involved to be able to etch the bonnet as once simple piece. What I eventually came up with is this odd shaped piece.

What you are looking at on that side of the part is the inside of the bonnet front, but the outside of the side panels. You can see two etched rebates down the sides of the bonnet front into which the detailed etched side panels will fit so that they correctly fit behind the front wrap-around.

To bend this part to shape we start by doing something odd; folding the part the wrong way. Usually you fold a part so that the half-etched line is on the inside of the fold. In this case though, to make sure the parts will lay flat against each other, we bend it so that the etched line is on the outside of the fold. Here you can see this half way through being folded flat fro both sides; note I've coloured in the rebates using a permanent marker to both highlight them and to stop glue filling them in.

Adding a small amount of glue next to the rebates and into the fold line, and completing the fold to squash the parts flat results in this slightly more normal looking piece, where the outside of both the front and sides are now on the same side of the part.

What you'll notice though is that there is metal behind the peak of the bonnet front which is clearly wrong. If we flip the part over it's easy to fold this part and snap it off; it was a sacrificial piece designed to strengthen the part during the initial folds and to ensure both sides were folded equally.

We now have an almost normal looking part which simply folds along the sides of the bonnet front.

After assembling the cab and attaching the bonnet to it, the detailed side panels then simply slot into the half etched rebates behind the front wrap-around and glued into place.

As I said, it's by far the single most complex etched part I've ever drawn, but it's both easy to assemble and allows me to accurately represent the prototype so I think that was a success, even if it did take me three attempts to get it right!

Tuesday, March 5, 2019

Alan Keef K12 Diesel: Third Time's A Charm

So late last week the third version of the body etches for the K12 arrived back. If you remember I needed a third go at the body etches having messed up the bonnet side panels (specifically how the fit against the wrap around of the bonnet front) in different ways on the first two attempts. Happy to say that this time I've nailed it.

Still got to add the details and paint it, but I'm really happy with how it's looking now.