How to make your own molded servo covers
by Len Buffnton
You’ve done all the hard work, the servos mounted in the wings, the wiring is complete and the control linkages are in. Now what do we do about covers for the servo openings? In most cases, the kit or plane will come with molded servo covers.
In the event it doesn’t or if one gets lost or damaged, the following is a simple, step by step process for creating your own.
There are a couple different was to make fiberglass parts. You can make a plug (the part you are trying to replicate) and then make a mold ( the tool used to reproduce the part you are trying to make) off of it. That mold is what you then make your final parts from. The other method is to make a plug, and pull the parts right off of it, skipping the mold-making step.
The advantages of making a mold from a plug is, the parts you make from it are made IN the mold, allowing for a smoother surface. Making the parts over the plug is perfectly acceptable for this application though, because we plan to use a fine weave cloth to produce the same result.
Decide on the size you need by measuring the opening where the servo sits. Be sure to allow for at least a ¼ inch overlap around all the edges. I make these covers about ¾ inch bigger all the way around, this way the covers will fit many different size opening and I just trim off what I don’t need.
The covers we are going to make have the servo arm/linkage cover molded into them.
For this article we will make a plug and pull the parts directly off of it.
Making servos covers for you wings require you to make a left and a right, depending on how you mounted your servos. The process detailed below will need to be repeated for the other side. You can and should make both plugs as you go to speed up the process.
Building the Plug
Start with a piece of balsa or hardwood. Draw the shape of the arm cover on the wood. Rough cut the shape, then sand it to the final contour.
This shape will be glued to a piece of flat base plate material. The material can be plywood, hardwood, glass or even fiberglass sheet. (G-10)
Cut the sheet larger than the opening, as described above.
Glue the sanded block you made to the base plate. When doing this, you need to place it in the correct position. I line up the base material over the opening and mark where the servo arm will penetrate the wing.
Glue the arm cover onto the base plate. You can use super glue or epoxy.
Now you have the plug for your part. Prime it with some high fill primer, which allows you to sand it smooth.
You may need to apply a couple coats to get the plug smooth as glass, that’s the end result you want to achieve.
That was simple, the plugs are made, now’s the time to decide what material you want to use to make the covers.
The options are fiberglass, or plastic ABS, which would be vacuum formed.
For a fiberglass cover, you need to prepare the mold so you can get the cover to release once it cures. Start with some good mold release wax. The wax can be purchased at any fiberglass supply house. Apply at lease 3 or 4 coats to the plugs. The more the better. You can get away with just wax with small parts like these, but for long term health of the plug. Its best to use a release agent after the wax. Release agent is a liquid called PVA, Poly Vinyl Alcohol. PVA is typically a pink solution, which usually is applied by spraying.
Once you have the plug ready, we can get on with making the parts. These covers don’t need to be heavy duty, so we will plan on making them out of lightweight fiberglass cloth.
Start by cutting some fiberglass cloth larger than the plug, remember, the cloth needs to contour to the shape of the part. Cut all the cloth you need ahead of time, making enough for as many covers as you choose to produce. The layup schedule will be 2 layers of 2 oz cloth and 1 final layer of ¾ oz cloth. This last layer has very fine weave, which will provide a nice smooth surface for finishing.
**(A little trick here is to mix in some coloring with you resin when laying up the part. The coloring can be purchased at the same fiberglass supply house as well as some auto parts stores too. Mixing in the coloring will produce a ready to use part right from the plug.)
You need to also decide why type of resin you will use for making the parts. The choices are Polyester resin or Epoxy resin. The Polyester resin is the least costly resin to purchase; it cures very quickly and will produce a good part. The down side is it really, really smells bad. You will NOT want to use this resin inside your house.
The other choice is Epoxy resin. Epoxy is an excellent choice for many reasons. It’s a strong resin, it has very little smell, it gives you time to work with it and it can be used for any application. The down side is it’s a little expensive to purchase. I use West System epoxy. This epoxy with #205 hardener will cure enough to handle in about 6 hours.
Both of these resins can be tinted as described above.
Mix a small batch of resin; use a shallow dish to mix the resin vs a cup. The cup keeps the resin in a smaller area, allowing it to heat up quickly and start to gel. (Kicks off)
If you use a small dish, the resin is spread out and not as deep, keeping it from heating up and kicking off.
Paint the plug with a little resin, then lay your first piece of 2 oz cloth. Make sure the glass cloth turns clear, this is called “wetting out”. Once the piece is wet out, apply another layer of 2oz cloth, try to change the weave pattern if possible. Changing the pattern will make a stronger, stiffer piece in the end. Wet out that piece too. Next is the ¾ oz cloth, this is the final layer. Make it smooth, being sure to wet it out, but not so much it runs or looks like puddles are forming. It should be smooth.
Check a few times to make certain the cloth is staying in place, especially around the bump. Take your time and keep checking your work. Once you are completely satisfied, set the piece aside. It’s best to set it on top of a can or roll of tape, something that will allow the edges to droop a little and not push up. If you can lay it in the sun, it will cure quicker.
Once the piece is fully cured, carefully work the edges loose, a little at a time until it “pops” off the plug. Now, take a moment and admire your work.
Trim all the rough edges off the new cover, be careful, the glass edges will stick you and they’re like fishing barbs. Place the servo cover over the hole and mark the final size you want. Be sure to leave at least an 1/8th to ¼ inch around the entire opening. Most importantly, be sure the arm will travel fully in each direction with the cover held in place.
After trimming to shape, you can either use some silicon to hold it in place, or use some clear tape. I personally use tape to hold them on. If there is sufficient room, you might want to trim the cover a little wider, then use small screws to in the corners to hold them on.
Congratulations, you just made a custom molded, fiberglass servo cover. Now make the rest.
Plastic vacuum formed covers
The process is the same as far as making the plug for fiberglass, except you don’t need to wax the plug.
If you have the plug made, you’re ready to vacuum-form some parts off it.
Vacuum-forming is a process in which the parts are sucked down onto the plug by vacuum pressure. In order to do this, you need a vacuum former box. This box is simple to make with a piece of pegboard and some scrap wood. Check the Internet for directions or check out the video below.
The cover material is 1mm ABS white sheet. This sheet is typically available from hobby dealers. Make a small frame to hold the plastic, the directions for the vacuum box is here. http://www.youtube.com/watch?v=yhajk_IDTUo
Assuming you have the box and a vacuum, we can proceed with making the covers.
Place your plug (s) on the vacuum box and hook up the vacuum to it. Leave the vacuum off for now.
Place the ABS sheet in the frame and clamp it in. Pre heat your oven to 450 degrees and place the frame in it for a couple minutes. Remove the sheet, and immediately align the sheet over the vacuum box, turn on the vacuum and viola, the parts are sucked onto the plug. They will cool and harden pretty quickly, so if you want to make more, just pop the cover off the plug and repeat the process.
Align the molded cover over the servo bay, line up the servo arm, then make the final trim lines. Once cut to size, you can paint then if you want. The only thing left is to attach the cover in place and go fly.