Hot Tacks

This neat adaptation of a small soldering iron will allow you to tack covering films down in all those tricky tight spaces.
Mike Bell describes his DIY film tacking tool for difficult to access areas.

When it comes to film covering an airframe, I'm sure that, like me, you have come across loads of annoying areas where the covering iron just can't reach properly. Areas like air intakes and air exit slots, together with fillets around wings and fins, all immediately come to mind.

I was pondering this problem during a regular Friday afternoon coffee and natter with my mate, Richard, during which I suggested that it would be good to be able to attach a suitable mini shoe to a heat source like a soldering iron, in place of the soldering bit. The obvious problem with this idea is that the heat would be much too high for film and even with a temperature-controlled iron the lowest possible setting probably wouldn't be low enough. Anyway, I suspect that most of us only have a simple, non-adjustable type of soldering tool.

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Mike's low wattage iron after adding a dimmer switch and silver soldered hot shoe.

The conversation then moved on to what adjustable power supplies could be used to throttle the iron back a bit. There are circuits for such things on the internet and, at first, I was proposing to acquire the components for a simple-looking project before another modelling friend of ours came up with a simpler solution. He said,"You could always use a lighting dimmer switch." The idea was so simple it made me feel a bit daft not thinking of it myself!

By this time, we were already in Coronavirus lockdown so there was no chance of having a mooch round B&Q for a suitable switch, but Amazon came to the rescue. I was going to try a standard wall mounted dimmer switch but during my search I came across an in-line lamp dimmer that would be much neater, so that's the one that got clicked.

Hot Shoe
In the meantime, I needed a shoe assembly to fit on the iron. I came up with the idea of using the end of an old table knife for the shoe. Such knives are usually made of a type of stainless steel that has a quite hard, springy and smooth surface. The end of the blade is also a nice, narrow shape for poking into small spaces.

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You'll need a spare table knife and some copper rod or tube of the same diameter as your soldering tip.

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Dremel off the end of the knife using a cutting disk.

To make a soldering iron tool attachment like the one I came up with you will need:

A soldering iron (low wattage) with a detachable bit, preferably the type held in with a grub screw.

An old table knife.

Some copper rod or tube of the same diameter as your soldering tip. You could probably twist smaller diameter copper wires together to make up to the right size.

A silver soldering kit (or a friend who has one!)

A 100W dimmer switch. A higher wattage is okay but not necessary. This needs to be the type for incandescent bulbs, so don't buy the LED compatible ones (I'm not sure if they would work for this or not but they are more expensive anyway).

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Silver solder the flattened tube or rod to the knife end.

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Polish the sole with emery cloth.

Looking at the shoe first, find an old table knife and cut about 40mm off the end; I used a cutting disc in a Dremel. If the surface of the blade is a bit dull it should polish up nicely with some fine emery cloth. It will be easier to do this before you cut the end off. I bent a gentle curve on my blade to enable the shoe to contact the film at various angles depending on access. You could leave it flat and put an angle on the stem if you prefer but I think the curved shoe gives a better chance of an even contact.

A potential alternative to using a knife blade is to use some brass sheet, maybe 2mm thick. This would be a bit more work to make but would silver solder nicely and have the advantage of better heat conductivity and heat capacity - it's up to you and what you have to hand.

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A gentle curve in the shoe allows for more even contact with the film. It'll be less likely to dig in too!

Next, cut your copper rod or tube stem; mine was about 45mm long. Hammer a 10mm flat on one end, then clean it up ready for silver soldering. Silver solder the flattened end of the stem to the shoe. Please note that soft solder won't do. Even if you can get a good joint to the shoe you risk melting it if you have the iron on full blast at any time.

Dimmer Control
Turning to the dimmer switch heat control, if you use a standard wall mount job you will need to fit it to a suitable back box, making sure it is safe. If you have it in a box wired up to a wall plug on one side and a single socket on the other, you can just plug your iron into the socket with no mods required to the iron lead. If you are unsure about the required wiring be sure to get help from someone who knows what they are doing.

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Dimmer control sourced from a popular online superstore.

I went the in-line route, which is neater and a lot less work, but you will probably have to source one online. Mine is a MxFans 100W unit sourced from Amazon UK for around £7. Be aware that this unit comes with no connection instructions, which is pretty surprising these days and the connections you do make have to be soldered.

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There's no connection diagram so if you don't know what you're doing then pass the job onto a qualified electrician. Actually, these days our best H&S advice is to do that anyway!

When you open the case, you are confronted with a circuit board that has two pre-tinned holes at one end - these are fairly obviously where you connect your lead. These are not marked in or out, or positive or negative; like I say, pretty weird in these health and safety dominated days. It seemed clear that the unit has to be wired in series, so I split the positive wire in the flex to the iron and soldered the cut ends to the circuit board.

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Make sure all the cables are secure and inside the case before closing it up.

See the picture nearby showing the wiring on mine. With the negative and earth wires stuffed in as neatly as possible around the board the case can be re-closed, and we are ready to go. Again, if you want to go this route and are not confident with the wiring please outsource this bit to someone who is comfortable with it.

First Test
For the first test I switched mine on with the dial set to full and found that the iron heated up as normal. With the dial turned down to half I waited for a while and could sense that it was now cooler, but I had no means of measuring the actual temperature. I do have a non-contact temperature measuring device but be warned these are not accurate for measuring temperatures on shiny metal. This is due to a material property called emissivity. The devices are normally calibrated for duller surfaces, so when you point them at bare aluminium or steel, for example, the reading can be a very long way out. I used mine pointed at the dull part of the iron near to the heating element to get a reading, just to confirm that the iron was responding to different settings of the dial. If you have a contact thermocouple type of heat measuring device, you can set your tool up much easier. But don't worry, it's simple enough without one.

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If you have one use a non-contact temperature measuring device pointed at the dull part of the iron near to the heating element to confirm that the iron responds to different settings of the dial.

Unlike the pukka film covering irons this one is not thermostatically controlled. The iron will build heat according to the power setting and eventually achieve an equilibrium whereby there is a balance between electrical energy going in versus the energy needed to heat the iron, plus what is lost to radiation to the atmosphere. This means that you need to wait a couple of minutes to make sure the heat has stabilised before use. Unlike when soldering, the job is a relatively poor conductor of heat so it won't cool the iron too much on contact and the temperature of the iron shouldn't be affected that much.

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Perform some 'wrinkly film' tests to find the correct temperatures for various coverings.

Without a special thermometer the only way to find the right setting for film tacking is by trial and error. For my iron the setting was somewhere near mid way between max and off, so this is a good place to start. Try a scrap bit of your film on the shoe and see how it reacts; if it wrinkles gently you are somewhere near. Leave the iron to stabilise between setting adjustments before trying the film test again. Also be aware that the different films have different heat requirements so you will have to find the settings for the film you are using and retest if you change film type. I suggest you mark your switch with paint marks to indicate the working range.

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Mark up the control dial with the best settings.

Because the heat is being applied to only a very localised area of the shoe via the stem the heat will vary noticeably over the shoe surface. It will be hottest at the stem and cooler towards the tip. You can use the hot part to activate the film adhesive then drag the cooler part across the film to set it down. It is always best to prep the wood first with Balsaloc or Coverite, which will help the film stick firmly.

So, this little project will provide a useful device to help cover in those annoying inaccessible places that the standard tools can't reach. In no way is this a replacement for a proper covering iron for the rest of the model, but for the intended purpose it works well. You could experiment with different shaped shoes for different shaped cavities too.

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