On the minus side its more CNC cutter time - a tight joint will require a slower moving small cutter head. -> higher cost to cut

On the neutral side the outside is going to be clad in glass so there is unlikely to be a strength issue or anything to do with leaks

On the plus side you will save some assembly time BUT

most importantly its really hard to misalign a puzzle joint, and this SHOULD lead to much more accurately aligned parts and the benefits that ensue.

Does anyone else have opinions on this ?

Anonymous wrote:

Provided that I've got everything right – which is why I'm itching to get back home and order a set of parts in nominal 2mm birch plywood at 1:4.25 scale (which would equate to 9mm plywood at full size), as this is the best way to check that I've got all the sizes right, and that everything will slot together as planned.

I do think you will find more places to put tabs once you knock up a model, the cockpit seat backs spring to mind, and a few more in the topsides would make getting them on almost a one person job, but I gather you need to be back on dry land to implement that.

I do still wonder about puzzle joints as well. I note that Chesapeake Light Craft have been using them without significant issues for a long time. Here are some thoughts here:

https://www.clcboats.com/forum/5/thread/28425.html 

Its not so much that they are stronger but they are supposed to be idiot proof. Speaking as a certified idiot who has misaligned scarfs before it just seems that using the CNC machine to do one more task makes sense.

I can see myself doing another model once you have polished things. Its the only way to be certain.

David Tyler wrote:

Interesting, and encouraging, Gary. I'm having difficulty spotting which errors are due to the plywood being thicker (the difference between 9mm and 12mm at full size), which due to the CNC cutter not using the latest drawing, and which due to you having to enlarge slots and ease the fit of the tabs by hand. 

How should we proceed? Perhaps if you send me the dxf file back, with the points that you think need adjusting ringed or marked up in some way; and a sketch of the tab and slot arrangement that you think we should use, with sizes?

At this point I suspect the majority of the issues are down to the CNC folk, well except for the ply thickness of course. I think there are issues where they have just busked it. 

They said something about trying to get the tabs to fit which makes me think they knew what was up.

Clearly they did not just copy the stuff you sent or there would not have been 2 station 2s !! 

Having said that the structure is around 90 % spot on so far. 

SO I am quite pleased so far but there are a few points of issue. 

But of course thats why we did this. 

Will send you full details direct - BUT what we have now looks really good. 

A very fine effort on your behalf.

Hopefully we can tune this so it becomes idiot proof 

(as a certified idiot I hope so :-) )

Well not everything went as smoothly as it might, for which I take full responsibility.

Firstly I managed to get the cut plywood wet - this required trying to dry it without getting inordinately warped. It was still a bit wobbly but I decided it was time for a fitting exercise. This was short lived as I discovered that the 1.5 mm ply I bought sight unseen and had shipped direct to the CNC folk was in fact about 2.11 mm thick. So nothing would fit in the slots. Fortunately I had precisely the right dremmel bit to sort the slots - you can see a half cut slot in the middle.

Of course the tabs were now too short with only a tiny amount of tab hole poking through which meant my wife got her cocktail sticks back ..... but lost some dressmaking pins :-(

Assembly did allow us to spot a few errors such as station 4 which required its slot enlarged

Also discovered that most of the interlocking tabs required trimming 

There were also a few cuts missing such as this one for the cockpit seat Ooops. Easily fixed with the dremmel. 

The astute boatbuilders here will note that at this stage the structure is not attached to its jig. I was really only checking fit for which the flexibility helped lot.

What was interesting to feel was that the hull is almost self jigging. The strength that appeared once the bunk assembly was in place was quite striking. 

The good news is that once we have ironed out all these little errors, those who follow should be able to progress much more quickly, and with more confidence. 

So we are learning some lessons about CNC. One is that for sure the CNC folk have messed about with things. For instance I got two station 2s. And some issues which had been corrected early on became magically uncorrected. 

I also feel we need a different type of tab - square section like the original mirror dinghy tabs with angled wedges that could be cut from the spaces between stations. Tabs through the bottoms and topsides will also prove extremely useful, especially if hull planking is to be a single person job. I think that tolerances for tabs could also be readily increased. 

Thats what I think so far. I will let you know how things progress.

In the UK, the leader in the field of production of plywood kit boats appears to be Jordan Boats, in Somerset. They will take any design, and produce a kit of the plywood parts. So, when we're a little bit further on in proving the design by CNC-cutting a model, this would probably be the place to go, for UK builders,  as they already have a lot of expertise.