I said I'd have a go at explaining how I fit the back bars.
After the bars have been glued up, the first step is to clean up one edge and then plane them to 30mm width. Cleaning up hardened UF glue is not very easy. My latest approach is to deal with the worst of it using a belt sander before using a scraper to deal with the rest before planing.
I then start fitting the bars into the seat rail. This step is easier than fitting the top end, because the bars are independent of each other and can be fitted individually. Addressing gaps at the other end usually requires adjusting more than one bar. I'll cover that in a separate post.
The mortises I'm fitting the bars into were cut with a domino machine meaning they have rounded ends:
My initial plan had been to square off these mortises and make square tenons on the bars. However, I thought I'd have a go at making rounded tenons on the bars and have ended up doing it that way. I've found that it's just as easy to do it that way and, more importantly, it gives me more scope to round over the edges of the bars without exposing the mortise.
The final point to note about the mortise/tenon is that they are bare-faced with only a small shoulder at the edges. The mortises were cut with a 10mm domino cutter and I had to thickness the bar laminations to give me a 10mm thick bar (5 layers a hair over 2mm thick).
The layout of the tenons looks like this:
I've slightly guesstimated the angles of the shoulder lines doing the middle two bars at 90 deg, the next two at 91 deg and the outer pair at 92 deg. I should probably angle the tenon at 90 deg to the shoulder line, but forgot(!) and subsequently decided it wasn't really necessary (although see below - I may have changed my mind). I do that for the top end of the bars but the angles are more severe there.
The next steps are to saw the edges of the tenon (I don't go right down to the shoulder line yet - leaving a bit of waste adds another step, but helps protect the shoulders when doing the next steps) and then chisel close to the round line:
Finally, I use this nice little file to finish the shape. It has a safe edge which makes it perfect for this:
I do a quick trial fit at this point (to make sure the tenon fits in the mortise) before chiselling the shoulder down to the knifed lines.
It's then a case of testing and adjusting each bar to get a nice gap-free fit. To make sure the vertical alignment of the bars is where it should be, I made this dummy crest rail which has slots for the bars to bit into:
Sometimes there are gaps which need adjusting. This was the worst one (both outer bars were like this so I think I need to revist my 92 deg estimated angle and/or think about angling the tenon slightly to see if I can get a better fit first time):
To correct that, I use a feeler gauge which is same thickness as the biggest gap (0.8mm in this case - they're usually much closer than that!) to scribe around the shoulders - running a knife along the feeler gauge:
I also made this clamping arrangement (you might have seen Custard posting pictures of this sort of thing) which allows the bars to be pulled down snugly into the seat rail):
All of the above was an afternoon's work.
I'll take some photos when I fit the top end of the bars and will write that up in another post.
.
