Article 44 – ‘Expansion clamp design and construction’


In the article ‘Wiring practices part 2B’ it was mentioned that the twin trunks of my mountain ash/rowan, Sorbus aucuparia were trying to fuse together, which would have spoilt the overall design. (image a) Hence a 3cm block of wood was wedged in place to keep them apart, purely as a temporary measure. (image b)

Rowan red wires & wood

Having given some thought to the problem 2 options came to mind, (1) to use 4 heavy guy wires (red lines) to keep the trunks apart, but where to attach them – they cannot be attached to the plastic container, because (a) the amount of tension and force required would distort it. (b) The present angle is too acute and the wires would slip down as soon as tension is applied damaging the bark.

Alternatively build a large box around the container and attach the wires to that, this was also rejected, because to reduce the steep angle the wires would require a minimum extension of 45cm on either side resulting in an overall measurement of 135cm container 45 + 45 + 45. Moreover, using wires to keep the trunks apart creates tension in the length of the trunks as opposed to force centred at one small area, hence the decision was option (2) design and construct a small expansion clamp.

The following image shows the ‘new’ expansion clamp in situ, a device that can be adjusted by periodically turning the handles giving equal force to both trunks. Below this are two more images showing plan A & B followed by a tutorial on how to make this clamp


Expasion clamp in situ

Expansion clamp A

Plan B. overview

Expansion clamp construction

Before we begin the tutorial, Europe uses the metric system as are the dimensions given here however, there are countries that use the imperial format, the reason for this is because threaded bars have different threads for example, UNC (Unified National Coarse Thread) and UNF (Unified National Fine Thread) therefore, when making threads they have to correspond to the type of threaded bar being used.

The tools required for the project include: a drill press as it will give accurate alignment when drilling; if access to such is not available a cordless drill can be used, but ensure that all components are aligned properly and the drill bit is perpendicular to the worked object.

Drill bits 3mm for pilot holes and holes in the back plates, 4mm drill for the 4 x 5mm threaded bars, 5mm drill for the 6mm bars, 6mm drill for plate B. A file, Phillips (star pointed) screw driver, punch, ruler, marker, masking tape, hammer, vice, electrical tape, plastic shrink wrap (optional), hacksaw, 4mm & 6mm taps + holder.

Materials are: 30cm x 4mm x 3 cm aluminium flat bar, (steel can be used)

3 x 12cm x 5mm and 24 cm x 6mm threaded bar, soft rubber foam and adhesive,

2 x 6mm hexagonal threaded barrel nuts and 8 x 1.5 x 3mm bolts with nuts + washers. Note: (threaded bar is usually sold in 1metre lengths) all these materials are available from supermarkets and hardware stores.

Step 1. Cut the aluminium bar into two 12cm lengths and the remaining 6cm in half (3cm) and file away all rough edges. Tape the two 12cm lengths together ensuring they are properly aligned, measure, mark and centre punch the holes as shown above, then drill out the 4 holes using the 3mm pilot drill. Separate the 2 x 12cm bars and select one to be plate A the other to be plate B. Tape the 2 x 3cm back plates to plate B ensuring alignment is correct then measure mark and centre punch 4 holes and drill through using the 3mm pilot drill as shown below.

Back plates

Separate plate B from the back plates making sure you mark which back plate is on the left and which is on the right and the way they were first fitted, this important because if they are incorrectly placed the holes will not be in unison and the plates when bolted on will be out of alignment.

Step 2. Re-assemble plates A and B and ensure alignment is correct which is easily attained by inserting the 3mm pilot drill into the holes, then tape them together. Using the 4mm drill bit, drill through the inner holes in both plates, with the 5mm drill repeat the process on the outer holes. Separate the two plates and on plate B only use the 6mm drill to widen the 2 outer holes, this is to allow the 6mm threaded bars to fit snugly and turn in the recess provided by the back plates.

Step 3. Using the 5mm tap carefully thread the inner holes in bars A and B, then cut one length of the 12 cm x 5mm threaded bars into 4 equal lengths (3cm) and file off all rough edges, test fit by screwing these bars into the holes of the plates to check for alignment if all is is fine remove and set aside. Using the 6mm tap cut a thread in the outer holes of bar A ensuring that the tap is straight and perpendicular, again this is important because if these threaded bars are out of alignment the clamp will not function properly.

Step 4. The 2 x 12cm x 6mm threaded bars need one end on each to be filed down so that the bar is able to turn easily without scarring the recess, regardless of whether one uses aluminium or steel. Remove any rough edges on the other end and insert it/them into the hexagonal barrel nuts so that the ends are flush with the outer surface of the nut.

Measure and mark the exact centre on the surface of the barrel nut and make an indentation with the punch, check the alignment, if it is out use another side of the nut. Wind some masking tape on the protruding end of the bar close to the barrel nut as this will stop it from turning when you drill through. Using the 3mm pilot drill carefully drill through the nut and inserted bar, change drill bit to the 4mm and widen the hole then repeat with the 5mm; remove all burs. Insert the 2 x 12cm x 5mm threaded bar into the barrel nuts and wind electrical tape around the bare threaded ends.

Step 5. Assembly, bolt on the back plates, insert the 4 x 3.5cm x 5mm bars or lugs into A and B ensuring they face in opposite directions, wind electrical tape over the threads and cover with shrink wrap, to stop any hard contacted with the trees’s bark. Shrink wrap although optional is perfect for this kind of project. Finally, cut some foam rubber and glue it using contact adhesive to the areas between the lugs as a cushion for the 2 trunks.


finished clamp

This expansion clamp was specifically designed for my Sorbus aucuparia having two trunks each 3.25 cm in diameter, but can be adapted for larger or thinner trunks by widening or closing the gap between blue lugs as the case may be. Of course new holes will have to drilled and tapped to accommodate any alterations.

The advantage of this clamp is that it is adjustable able to do more than a static block of wood can. The clamp will stay on the tree for a period of two more growing seasons, periodically turning the handles to increase the gap between the two trunks.

Regarding maintenance concerning the bare threaded bars, these were sprayed with WD40 and molybdenum grease was inserted into the back plate recesses to reduce wear and tear; alternatively petroleum jelly (vaseline) can be used if the former is not to hand.

You are free to use my design or in part thereof should you wish to make this clamp, the materials aluminium flat bar, hexagonal barrel nuts, 5mm and 6mm threaded bars cost under 20€; the 3mm bolts, nuts, and washers had been purchased previously. (1,50€) Naturally making only one clamp does leave surplus material, but fret not, it can be used for other projects for example, bending clamps as described in the article ‘Making bonsai clamps’. (8th October 2016) Until next time, BW, N.



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