January 6, 2015

CNC cut Espalier Apple Trellis

One of the central elements that persisted throughout our many iterations of designs for the back yard was apple trees espaliered on the back fence. Having expended a lot of money and time to finally arrive at 7 baby apple trees planted in a strip of soil behind a retaining wall, positioned in front of a hopefully long lasting fence, it was time to build a trellis to train them along.

Past time, actually. I didn't reach a point where I could get the trellis done until late summer. By then, some of the shoots coming off the trees were going in an undesirable direction, and the leaders were not growing ramrod straight as I had drawn them in my CAD orchard design. Weird, right? I can't believe the trees didn't pay more attention to my drawing. Some of them even had the nerve to not sprout limbs in the right places at all for my design. I'll have to speak to them about that during their next performance review (coming up in the spring, when I'll have my grafting knife in hand...).

I spent plenty of hours while commuting on the bikepath or laying in bed at night thinking about how to build the trellis to economize on expensive materials, be relatively quick and easy to put up, and be strong enough to do the job while looking elegant and fabulous. I designed this trellis to be based around 8 upright standards: 2 ends, plus one between each pair of trees.

The profile for the support brackets was mostly done in Inkscape. The profiles were sized to put the cables about 300mm out from the fence, and such that two brackets would nest well in the 240mm wide boards I had bought for the purpose.

I eventually got the CNC router to the point where it could do a real project, and this was the first one. The inkscape file was brought into Pycam and fiddled for a good many iterations.

Once I was happy with things, I cut out 32 of these pieces. I needed 8 supports, each with two brackets, each bracket being formed from two bracket profiles. For clamping stock during cutting, I just used screws and washers down into the plywood table of the machine.

Since then, I figured out how to do support tabs in Pycam, but at the time I just tried to set the home for the Z axis so it would leave the parts lightly connected to the rest of the stock, then crack them out and sand the edges later. This proved to be moderately successful, but the red cedar did tend to sometimes peel back grain from within the outline of the part when I was freeing it from the waste.

I hadn't exactly decided on where the screw holes were going to be or for what size screws, so they didn't get cut on the CNC. Easy enough to add after the fact.

Some extra 25mm thick wood was used to make filler pieces for the sandwich of each bracket. For assembly,  I used west systems epoxy and stainless screws. When the epoxy was set, I used a 3mm roundover bit in the router to ease the exterior edges of each assembly, then sanded them all with a palm sander. I also rounded over selected edges of the two vertical members.

Aluminum bar stock from Onlinemetals was cut and drilled to act as a big washer to hold the bracket to the vertical standard and the fence behind it.

The wood elements are all made from clear red cedar, which I ordered along with all the wood to make the fence panels. The vertical member which actually holds the cables and the longer vertical piece that is screwed to the fence are both made from 25mm wood, while the supports which stretch between those elements are robotically cut from 19mm stock. I used aluminum bar stock to clamp the cables.

Each vertical trellis support was put together and then screwed to the fence with stainless lags. I put screws in specifically where the cross rails of the fence panels run, plus a few other places to keep things flat. I ended up flipping the support brackets around from the original design in order to get the top cable up higher.

I tested each one by hanging with my full 80+kg mass from the top bracket, though I didn't really try to break it by bouncing. I figure if an adult should try to climb the trellis it would be nice if it could withstand the experience. Hopefully I have accomplished that here, but I can't help but worry that maybe I should have made it a bit stronger.

The horizontal tree supporting elements are 7x7 strand 3.1mm 316 stainless steel cables from ebay, and there are 5 of them, to provide 5 rungs of limbs on the espaliered trees. About the best looking reasonable cost end fittings I could come up with were from SC&R, and I ordered a hydraulic crimper and sufficient fittings for the job.

This might seem like a gratuitous investment for putting up five pieces of cable, and I'm sure my wife would agree! But I do plan on making a number of other cable trellises around the place, and I can always sell it when I am done (though I probably won't, if we are being honest). The other obvious choice would have been big thimbles to double the cable back at the ends and direct it into screw or crimp clamps. The crimper for that wouldn't be that much cheaper though, and the screw clamps didn't appeal to my sense of style. Plus, even wrapping the cable around the radius of a typical thimble compromises the strength significantly and I would have needed slightly more complicated hardware at each termination. If you go by the specs of the arrangement I ended up using, the assembled cables can each take up to 1000kg tensile load. I'm still not convinced this is high enough and probably something would fail below that load anyway, but it may be enough.

The brackets have little strength laterally, so to withstand the tensile load from the cables at the terminal braces, I made up and installed stainless straps at each cable end. These are 1.9mm thick, 25mm wide, and 43cm long 304 stainless, ordered precut from OnlineMetals. I drilled and bent them at work one day after hours.

Each strap acts as a washer for the crimped on M6 threaded fitting at the end of each cable. The other side of each strap is screwed to the terminal fence post with two stainless lag screws.

So when all five cables are in it looks like this.

Of course this will introduce a torque on the fence post, which is a bit concerning, but I'm hoping it will be ok. The post is braced against this torque in a few ways:

  • Through bolted aluminum plate in front and channel behind (this transfers the torque to the concrete though, which is not the ideal type of stress to put on concrete...)
  • Pins of the back fence panel mortised in on one side
  • Future side fence panels (would replace the diagonal brace visible below)

I couldn't quite decide whether the cable should be clamped at each vertical support, or allowed to slip a bit to accommodate thermal expansion or other stress. In the end I decided it was too much trouble to make 40 really secure cable clamps, so I did something easy which sort of lightly clamps the cable at each crossing point. There are two stainless screws on each side of every cable crossing in the vertical aluminum bar clamps; these push the cable into a square slot in the wood member.

The trellis was put up while most of the huge Hope Black Dye sunflowers I grew in the micro-orchard bed this year were still standing, which made things a little more challenging.

Once the trellis was entirely erected, Buster and I went around and strapped bamboo canes and corresponding tree limbs to the cables.

This made it more obvious how I should have done my dormant pruning after planting differently... ah well live and learn. The trees look cute strapped to their forms. I hope they grow more next season.

So now we are in good shape to get the espaliers under better training next year.


Alex Carr said...


jengod said...

Terrific! Love espalier for small yards.

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