Center column and canvas supports
I feel like I'm in the home stretch now.
Today I assembled the canvas supports, cut the center column to length, and got the canvas supports fitted to the center column. Then I set the center column in place temporarily while I checked operation.
Top Canvas support:
A general view of the front of the top canvas support
This is the back. There is an inch and a half wide dado in the center section. That was drilled for a studded knob and a blind nut was recessed into the dado for the knob that goes on the front. The piece of 1-1/2 inch wide aluminum flat stock is 1/8 inch thick. The dado is just slightly deeper than the 1/8 inch.
The aluminum flat stock is trapped in the dado by the top and bottom trim pieces. In this case they're walnut. The aluminum stock will get pressed against the center column to stop the upper canvas support from moving.
We need a few more pieces …
Here is a view of the side guides that will clamp to the center column when the knob is tightened. In the center, between the guides is a piece of the center column (clamped to the center of the back of the support) that I cut off when final sizing the center column. It will serve as the template for setting the side guides. The center column has a 1/8 inch groove cut into both sides. When I made the center column I also milled another board exactly the same thickness and with a groove in just one edge using the same setup as the one used to put the groove in the center column. This way I know the grooves in the guides will line up exactly with the grooves in the center column. I've added a double thickness of painters tape to the edges of the piece of center column to make sure I have clearance for this all to slide when it's supposed to.
Here the side guides are installed and the template piece of center column is removed. Pieces of 1/8 inch flat aluminum, 3/4 inch wide, will be going in the grooves of the guide blocks and they'll ride in the grooves in the center column, but we have to do something to keep them in the blocks so I glued 1/8 inch thick strips of cherry to the bottoms of the blocks and and for the tops I also used 1/8 inch thick strips of cherry, but I countersunk some small flathead wood screws. This get installed by sliding it down from the top of the center column
The lower canvas support is simpler in terms of mounting. There are no grooves in the guide blocks. I extended the guide blocks above the top of the support and put a big radius on them. For this one the guide block thickness has to be sightly less than the thickness of the center column. This is so that we can put a clamping board across the back, The 2 knobs on the lower support, when tightened, pull the rear clamping board in tight and traps the center column which secures the lower support from moving.
Why are the top and bottom canvas supports mounting to the center column in different ways?
support may have to slide past frame members when raising and lowering the inner frame. The bottom support does not slide past any interfering members. Also, using the clamping board in back gives a much tighter grip on the center column in case I want to paint on something heavier than canvas.
So the center column parts are nearly complete. I still have to put the radius on the upper corners of the top canvas support, and also at the very top of the center column.
I have to trim, shape, and polish the aluminum flat stock at the top of the outer frame and secure them with a screw. Then it all comes apart for finishing and waxing of the parts that slide.
I'll get on with building and installing the counterweight system.
Right now, it's starting to look more like an easel…