The acrylic and plastic parts we use for our projects are very weak, and I’m sure it happened to anyone to have some important pieces broken because of dropping on concrete or unintentional force applied on them and you can’t find a missing part anymore. This happened to me as well with the electrical panel. I was having it in my hands and opssss… it slipped away and I found a corner missing in it I wasn’t able to find the missing piece anymore… so now what? Make such a complex part again would have took me at least 2 days of work.

I then looked around for a good way to repair it and found out that the polyurethane resin used for casting parts in silicon molds is just perfect for the job. What to do then?

1. In the first image you can clearly see the broken corner of my elec panel. If the part has been already painted remove the paint a bit around the junction point with the specific remover and sand a bit to help the resin to attach to the acrylic piece. I had it painted only on front and I did it on that side. Clean thereafter with acetone or generic home usage alcohol.
2. You need to create a frame that will reproduce approximately the missing piece. In my own case as you can see in the image the part to restore was plain and squared, so using Lego blocks I made a wall around the corner where the resin will be casted. Be sure that the base is closed (use some adeshive attached on bottom to prevent flooding). Keep everything ready for cast the resin, because it usually dry very fast (less then 5 minutes) and you don’t want to ruin the part even more.
3. Mix the polyurethane resin. This one I have is 1:1 mix so easy to make in very small doses. I used a syringe to get 1ml of each component and mixed in a glass with a tipped stick. Cast the resin in the frame slowly until it reach the level needed to reproduce the missing part. Don’t mind if you put a bit more, you can sand it later before repaint the part.
4. Wait for it to be dry a bit (5-10 min) and remove the frame. The resin attaches perfectly to the acrylic by actually gluing to it. The specific strength is a bit less of the polycarbonate or cast acrylic but for this job is just perfect.
5. You can now remove the bulk extra with a cutter while it’s still slightly soft and then wait at least 30 minutes so that the resin will reach the final state. During this period pressing with force on it will leave your fingerprints on the part so use with care. Once rock solid sand the new reproduced part to match the old one.
6. Finally paint again following your usual way. To prevent paint signs don’t cover the remaining part of the piece by sticking adhesives on it else paint signs will be seen from different paint timings and thickness. Just put a newspaper sheet covering the part you want to care of and paint with the gun or the spray can  at a fair distance. That will spread the paint enough to give a good result.

Operation completed.

written by l45 captain \\ tags: acrylic, cast, plastic, reconstruct, repair, resin

I know I didn’t update the blog a lot during past months, but don’t think I gave up the project..! In truth I’ve worked a lot on many different parts of it but simply haven’t had the chance to make one of my usual build stories because none of the pieces were really complete. Actually I’m working at the same time on different jobs:

The avionics set is about complete. All the parts have been milled, painted and engraved. PCBs for the Display Units are ready as well and I managed to have the RMU ones (that are very complex) fabricated in a professional center. Those should arrive during this week, so I hope I can complete them very soon. Also a lot of time was needed because those sets have been made in multiples because some will be sold to finance other parts of the cockpit (this is a very very time and money consuming hobby…)

I’m also hardly working on the lower panels. The elec panel is complete so as the Gear and Lights panel that only misses the gear handle (I will receive one soon from Ron of hangar45.us). In this moment those parts are having a small makeup to allow a different backlighting way. In the early periods I chose to use wheat of bulbs to backlight the panels. Though this is the way the panels are backlit, for our purposes the power drain and hot that they produce can become an issue, so I gave a shot to both SMD and 3mm high intensity leds and both gave me the sensation that they can substitute bulbs still giving a very good overall eyecandy.

Also the small details of the cockpit are in production. Six full sets of MIP and glareshield knobs have been created and shipped oversea past Fall to take part to other fellow builders projects. Now the Lower MIP and pedestal knobs are being developed. It will still take about 3-4 months to complete them, but I can say that this other part of the building has had a start as well.

I leave you with some spare images took here and there from the various progresses:

written by l45 captain

I’m making most of the knobs of the Center MIP and glareshield in this days. This include the RMU dual concentric knobs, all the DU knobs, the FGC ones and the EFIS controller’s ones. In total those are 23 pieces and I managed to make 6 full set that will be sold as well to some friends that are enjoying the very same project. Among other things I had to solve the problem to engrave some of them in series to obtain an overall same looking for all of them. I then managed to make my very first CNC automated Jig and wrote the gcode script to make the job I needed. Nice to say it has been a great success! I think that I will use such a way also for other things that need a replicated approach. Here are some images of a part of them and a video showing the Jig in action!

written by l45 captain \\ tags: buy, engraving, knobs, learjet 45, set

Trying to find a cheap and effective way to dim my panels, I made a lot of research around to web to find the right solution. Finally I found out some nice voltage regulators that can be used with a very basic circuit and make an excellent job. The LM338 is a 5A variable regulator that can manage any voltage among 3 and 36V and that with a simple trimmer/pot and a resistance drop down the voltage up to a defined value. It’s very cheap (2 Euros or less) and with as much as 10 euros of components you can make a board that can support up to 250 leds or 50 grain of wheat bulbs. Downside is that the much power it has to dissipate the much it will get hot, so an heatsink is required when you use it with 9V or up supply.

The circuit is basic. The LM338 in the TO220 package just has 3 pins, Input, Output and regulation. the output pin is connected to a 120ohm resistor as feedback and tied together to the output pin of the pot. The regulation pin is connected to the pot to variate the voltage down. There are many calculators around the web to choice the right pot max resistance in relation to the needed application.

A smaller version with 1.5A is also available, the LM317. But because of the very cheap price I would anyway go for the LM338.

You can have further information about here.

written by l45 captain \\ tags: board, bulbs, circuit, dimming, leds, pcb