Hi folks,
Ok, make an octagon - should be an easy job - it isn’t, when the timber is 4 x 2 and 850 is the side width.
I cut slightly over length on the chopsaw at 22.5°/67.5° and did a fine trim cut on the table saw, having used the angle finder to set the sliding table. We had previously drawn a circle on some 18mm ply and divided it into eight, and drawn the outline of the octagon. The side length was measured off this. Having trimmed the first side until it was the correct length, we fitted a stop to the sliding table to enable repeat length accuracy.
That was the theory; in practice the ends didn’t meet, and some of the cuts although measuring correctly for length and angle, didn’t sit solid together. We then clamped one side down to the lines and added pieces slowly to see where the problem was. After three lengths the angle was opening out. They were checked again with the a/finder, and were spot on, so we have no idea what is causing the trouble. The t/s angle was adjusted to bring the side back to the line without changing the length. This took several times round the octagon before it was back to the line, and then the cuts were a little rocky so the faces were planed in the middle until they nestled together reasonably. We were shaving off .2 of a degree at a time, and on something this big it makes a noticeable difference. We still have no idea what is going wrong, so we will be destined to repeat this process for the other window. Has anyone any idea what is going wrong, because the angles measure seemingly correct?
The next job was to mark the wood for the domino err biscuits. This went well and all the parts fitted together.
To facilitate holding all the parts together, five sides had guides screwed to the ply, on the lines. then a sixth one was fitted on the angle, to allow a wedge to be tapped in to hold it all together. The dry run went well with no gaps showing, so we went for the glue up. There are a lot of parts to apply glue to, so we worked steadily, assembling the last three sides and then fitting them as one to the rest. This stopped us having opening angle issues. The glue inevitably changes how things go together, so we had a bit of a wrestle to close up all the gaps. Having driven in the wedge, some gaps were still apparent, so I used a long guide/clamp to pull one section in, retightened the wedge, and then clamped the opposite side, and again, tightened the wedge. This incremental approach squeezed more glue out until the sides were mated. At this point we beat a hasty retreat :smile:
See the finished job below.
Now repeat the whole process again....oh joy.
Regards...Dick.