As indicated earlier I got 19 mm ( 3/4") tubes , in 5 metre lengths wholesale for a good price. I did not want to spend additionally for the smaller tube which would be the inserts.I choose the option of making inserts from the larger dia tube itself. More work , but that was fine. Click on the images so see the approach I took.
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General Concept
The outer dia (OD) of the 19 mm ( 3/4 ") OD aluminium tube is 19.2 mm and the inner dia (ID) is 16 mm.
This means the circumference of the inside of the tube is :
Pi x D = 3.142 x 16 mm => 50.3 mm approx.
Similarly the circumference of outside of the tube is:
3.142 x 19.2 mm =>60.3 mm approx.
So if the tube is slit with a width of slightly over 10 mm and then closed on itself to form a new tube again, it will slide into the original 19 mm ( 3/4") tube. -
The Set up to make inserts
This was my setup. An angle grinder with a thin kerf metal cut-off disc , a jig to hold the 15 cm (6 ") tube for slitting and some lubricant. I used SAE40 car engine oil. It was quite messy, I cleaned up the area for the photograph!!! -
Here is a close-up of the jig. 2 blocks of wood drilled with 19 mm holes so that the tube slides in. The holes are about 15 mm from the edge. A small wooden guide bar is screwed on . The guide bar is required to ensure you are slitting the tube in a straight line.
The far end has a mark. After the first slit the tube is rotated so the slit edge coincides with the mark, the second slit is now made ,this ensure the total slit "width" is a little over 10 mm.
To achieve a nice straight slit, make sure that the cut-off disk slides against the guide bar. I brushed on a lot of oil on the guide bar for smooth movement. -
Here is the operation to make the slit. Make sure the side of cut-off disc runs against the wooden guide bar. Do not apply too much side-wards thrust or the guide bar will be ground. I used plenty of oil on the guide bar side.
Expect a lot of aluminium swaf !!! Good idea to be dressed appropriately -- dust mask , and old cloths you dont mind getting splattered with oil.
I dressed in a painter's disposable coverall.
Really, it was'nt too bad. -
And here is a tube with the slot completed.
During the slitting operation the tube must be prevented from rotating else you will end up with a spriral slit. Notice the aluminium strip used as an "anti-rotation " tool. -
Close the tube by gently hammering with a wooden mallet or a make shift mallet and presto you have an insert for the 19 mm (3/4") keel, chines,gunwale and deck ridge stringers.
After some initial goofups, I made some 25 inserts in about 5 hrs . A coffee/ginger biscuit break was mandatory.