A guide to using pins for holding balsa during construction

The humble little pin, how could we possible not use it the best way?

For most balsa assembly projects you generally will be able to use the pins in almost any way and the finished product will still come out fine, however, if you want to ensure that your model comes out as strong and as accurate as possible then these couple of simple pinning techniques can make a difference.

Use the right pins.
Pins are just pins right? Wrong. Just as you wouldn't use 6" framing nails to put your photoframe together you can't just use any pins with balsa construction. Immediately there's three types of pins that you'll encounter.

Straight pins; Looking more like wire cutoffs (which they are), these pins are very cheap and available everywhere. The downside of these pins is that they're easy to lose, they don't have a lot of holding strength and they have the most incredible ability to find their way into your partner's foot (which is bad for your hobby!). The lack of a broad head on these pins can make them rather painful to insert and remove. It's good to have a packet of these on hand but in general avoid using them. Thumbnail of straight pins
Dressmaker pins; Slightly more expensive than the cheap straight pin, they differ by having a nice plastic bulb at the top rather than a miniscule flat head. These pins are actually substantially more dangerous during the insertion process, several times the plastic heads have cracked under pressure which then allows the applicants finger to proceed immediately onto an equally sharp end of the pin - expect much cursing and considerable staining of immediate benchspace. Thumbnail of dressmaker pins
T-pins; These are the best pins to get for modelling, they cost quite a bit more than standard pins but they excel in what they do and they're also a lot safer due to the unique folded head (hence the name "T-pin"). Invest in at least 100 of these pins and keep them in a dedicated container, preferrably with a magnet to hold them in. Thumbnail of T pins

Okay, enough about the basic pins, let's discuss some basic rules of pinning.

Do not put pins into your framework
This rule mostly comes from when you're constructing built up wings, fuselage sides and tail feathers. A lot of people push the pins straight through the wood stock being used to build up these parts, don't do it. The reason why it's something you should not do is that the pins crush and damage the wood, the very wood that you're relying on to stay together in flight. This is not to say that your plane will fall apart because you pinned through the framework, however if there's enough pin holes in the wood there is a chance at some point that the wood will break.
Balsa showing examples of damage from pin insertion
Use scrap balsa to hold the edge of balsa
Because a pin has a very small cross-sectional area (ie, the width of the pin shaft), there can be a considerable amount of pressure applied to the balsa that you're trying to hold in place over that small area, this will cause crushing which of course is undesired.
To resolve this problem, use small scraps of balsa that are thick enough to handle the item that you're trying to hold in place (if you have multiple items stacked then make sure your balsa scrap can restrain all them).
By using a scrap piece of balsa and pinning through it instead of your workpiece you will be able to restrain your job with no damage and more reliably due to the scrap balsa holding more area (think of trying to hold a door shut with a brick rather than a pencil).
Thumbnail of image showing how to pin parts using a scrap piece of balsa
Angle the pins so that the holding force causes the scrap to move down
A little hard to say but fairly simple to explain. Essentially, when you pin down your workpiece there will tend to be a pressure pushing against your piece of scrap balsa, trying to push it away. To make use of this force, tilt the pin with the head of the pin towards the workpiece (ie, the pin will rise upwards towards your pinned workpiece). What happens with this configuration is that the pressure against the scrap of balsa will force the scrap to run down the pin, which of course forces it harder into the workbench making for a more secure hold. It's a form of wedging.
Thumbnail of image showing how to angle a pin so that it forms a wedge lock

So there you have it, a basic introduction to using pins with building. There's a few more places where we can use pins (ie decking) but we'll cover those in a later article. For now, go out and get yourself a couple of packets of T-pins and start building better models.

If you wish to make a comment or suggestion, please send an email to pldaniels@gmail.com