New screen takes shape

img_0186It’s been a while since I posted and I thought I owed you a status update. The build of the new screen has been going slowly for reasons I’ll explain shortly, but it’s definitely progressing. Just today I put on the new screen surface. I hope to be painting by mid-week and mounting the new gantry next weekend. First light on the projectors maybe by the weekend after.

When I last left you, I was having the parts I’d drawn up in my CAD program to build the new screen frame cut by a local CNC house. (All credit to the good people at Aldworth James & Bond who did me a great deal including delivery to my door. If you’re in South London and need design consultancy, fabrication or CNC services, look them up.)



The parts turned up the Monday after I ordered them, nicely bubble-wrapped to avoid getting chipped. They sat on my table for the rest of the week, because I didn’t have the time to get into the build. I rarely get home from work before 6.30-7pm, and I don’t use power tools after 7pm because my build room is adjacent to where my neighbours young children sleep and they obviously go to bed early.

Come the weekend, I unwrapped the parts and got my first minor disappointment. I had obviously miscalculated somewhere, because the slots in the vertical members which should hold the circular ribs of the screen frame were slightly too long. The members should slot in and lie flush, but ended up about 10mm short. This wasn’t a disaster, as I could manually place them where I needed them during fixing, but it made assembly rather fiddly and deprived me of a little of the structural strength that comes from a tight joint. Never mind, lesson learned!


Actually assembling the parts was awkward. It’s a large structure to assemble in a room that’s only just big enough for it. Getting the ribs to sit in the slots and stay in place to be fixed down was a painful operation that definitely could have used two pairs of hands. After a while, I got into a rhythm. But, having assembled the first third of the frame, I realised that the plastic corner braces I was using to join the thing together just weren’t strong enough to keep the frame rigid, especially with the slot size problem. So I downed tools and ordered some sturdy steel brackets from Amazon (B&Q doesn’t stock enough in-store – I needed 40+ brackets) and that was another week to wait for my power tool window at the weekend.


To begin with, I re-did the joints in the first frame section using the new brackets. Much more rigid this time. I assembled the other two sections in rapid order. Then it was time to join the final frame together. To do this, I put a pair of legs on the middle section and one leg at the far side of each of the two side sections. I was able to manoeuvre the end sections up against the middle section, vertical frame member to vertical frame member, and clamp them together for fine adjustment. When everything was in place, I screwed the three sections together, and the whole construction stood on its feet, nice and stable.


While things were stable and reasonably rigid, the whole frame could still flex and move. For my purposes I need it to be anchored in place, so I made a set of braces to connect the frame to nearby door and window frames at the top and bottom, and secured the feet to the floor with brackets fastened to the floorboards or skirting board. This makes the whole assembly secure and very rigid. Random vibrations will not make this thing shake any more.

(As an aside – during this build I found out that at least one wall of the build room, which I had assumed was an original plaster-and-lath partition wall from when the house was built in the 1890s, is in fact a modern skimmed plasterboard construction and has obviously been rebuilt some time in the last 30-40 years. This I know because I managed to knock a couple of small holes in it, and the weight of the frame placed between the underlying studs has caused the plasterboard to crack underneath the wallpaper, so I have a couple of ‘soft’ spots in the wall. Oh well. When I come to re-decorate in a few years I’ll just have to repair them – or take down the plasterboard and re-build the wall. The joys of DIY. I shudder to think how many mummified rodents I’ll probably find in there.)


So, with the frame in place, it was time to attach the screen surface. As before, this is made from 3mm hardboard, which is a kind of MDF (actually, it’s technically high-density fibreboard or HDF, but we’ll gloss over that) but which is supremely bendy. As always when doing this, I had to cut it down freehand because the only space large enough to let me lay a full board on the floor is my living room. And as always, the cut was not perfect by any means. Not terrible, but not perfect. This time around, since I have a constant circular guide for the screen surface, I decided to use the full width of the sheet, which is 2.4m, and tack on small end pieces of about 300mm width each side. I actually ended up with about 350mm on one side and 250mm on the other as the central section is slightly off-centre (which matters not a jot since everything will be finished nice and smooth and you shouldn’t be able to see the joins, which is the whole reason for doing this new build in the first place). A little bit of jiggery-pokery ensued, as usual. I used flat-head nails this time instead of screws or panel pins, because I knew there would be a fair bit of tension at the nail point and I needed the strength of a large nail head, and screws are overkill for this job and leave lots of cross-heads to fill in.


I did have to remove and replace a few nails a few times to get the board lying flush against the circular ribs across the entire arc of the screen, so there are a few cosmetic issues that need to be addressed before I can paint, and there’s a little bit of trimming to do, but the end result is pretty good.

What has happened again, though, is flexing of the board at the edges where the joins happen. Pressed firmly against the circular surface of the frame ribs, the board has no choice but to adopt a smooth curve, and the ribs are close enough together that it cannot pull itself straight between them due to tension. But at the edges the board is able to flex a little more and will try to straighten out. The end result is a series of troughs and valleys along the vertical line where the board edge is unsupported between the ribs. This happened on my old screen design too, but in both directions rather than just the one this time. The solution to this is to brace behind the board by using a piece of flat wooden baton to support both sides of the join and hold it flush in between the ribs. The circular shape of the ribs and their constant presence underneath the board will take care of any flexing in the horizontal direction and make the surface transition as close to perfect as I can get. I’ll look at this during the week if I can get home early enough one evening to do some power tool work.

Once I have the joins properly supported, I can fill in the gaps between the joins. This varies from a hairline to 2-3mm along the length of the join. I’m going to use expanding foam filler for this as it will not crack under thermal expansion in the summer. Then I can apply a few coats of latex ceiling paint which will provide additional protection against cracking, and finally I’ll apply the Screen Goo paint that’s been waiting over a year now to get used. Fingers crossed it’s still good!

After that, I’ll re-build the gantry. I put some parts for this and the new projector mounts on the CNC cut list, but I seem to have doubled up on one set of parts when I should have doubled up on another, so I don’t have enough parts for both projector mounts. Not a problem, I can cut those parts manually easily enough. The gantry rails will be made from 25mm square-form steel tube this time around for rigidity, and the projector mounts will be made to slide along them and then bolt into place. This will let me adjust them in-situ with them turned on and visually ensure the images line up properly rather than the suck-it-and-see method I’ve been using up until now. I’ve done all the calculations and I know exactly where the projectors should be positioned in this new design.

If all of this works as intended, I should have a much more stable, less distorted, and easy-to-overlap image from the projectors, and a much better simulator experience. And I really, really hope that this is the last time I’ll have to build this screen, until I move the sim into its shed home finally and need to build a much bigger one, for which this one will be a prototype.

Expect some activity on my YouTube channel soon, too, as I begin re-mounting and testing the projectors.