I want to make a couple molds for casting concrete pillar caps.
Each cap is square in plan, 32 x 32, and with a low-rise (4″) pyramidal top. My plywood molds will have 4 sections at the bottoms, all identical, triangular in shape, each bottom edge 32 inches across.
I can model this thing in 3D CAD to resolve the angles of cuts at each side, and the equal-leg dimensions.
I have a couple sidewinder “skilsaws” and a good table saw. Describe to me what your method would be to cut these triangular parts of 3/4 ply, and get them identical, with precision beveled edges, so they fit together cleanly.
Replies
One way to go would be to make a male mold first, and from that make a female mold to cast your concrete. Get rigid foam insulation board from a building store, or rigid foam flotation billets from a boating store. Laminate if necessary to get enough thickness. Carve everything away that isn't what you want. You can easily carve rigid plastic foam with wood tools, or with a hot knife. You can also plaster it with plaster or drywall mud, and sand that to a fine polish. Then make the female mold with casting rubber, fiberglass, or some other waterproof material. A waterproof mold is a good thing around concrete. Also, with a polished surface on the male mold, you should get a polished surface on the concrete right out of the mold. Vibrate the concrete while it is fluid to avoid bubbles.
With your cad, you can get measurements for each triangle.
Then you have to fgure in the blade angle of the cut, which you should also be able to get from the cad.
There are reference tables for this, but you'd have to do a search for compund angles.
As far as actualy cutting it with a skillsaw, you can use a straight edge to guide the saw, and tilt the blade the amount needed. Work off a large piece.
Sometimes it's easier to use a bandsaw, if you have one, and you are able to make a fair cut. For concrete, it would be fine, since you will have to smooth off the cast surface afterwords, anyway.
Edited 1/6/2005 5:40 pm ET by DAVE HEINLEIN
I thought one of you sharpies would have some nifty trick, better than the one I used last time I did a plywood pyramid.
Here is what I did last time.
Use CAD to define the triangle geometry, including all dimensions and bevel angles. Using CAD still, snap parallel offsets outboard each edge, about 2 inches, and dimension the whole thing, then print drawings.
Lay out and cut all four triangles, made to the larger offset lines. Use a skilsaw, and carefully cut outside the scribed lines.
Set the jointer for a teensie skim, and use the jointer to pare the triangles right to the lines. Ensure all triangles stack up identical like playing cards.
Now go to the table saw. Set the bevel to the up-the-pyramid bevel, tune in the fence to the right offset, and rip off the two equal sides of each triangle.
Do the same for the base leg cuts.
I'll do the CAD work on the whole mold and post an exploded view of the parts here, in a couple days. Skiing will be too good to miss tomorrow a.m.
The quick way is to forego the blade angles all together, cut the triangles square-edged, and "stitch" them together with battens on the outside. "Tape" the inside corners with a strong masking tape to keep the mud in. Perfect corners.
Gene
Instead of beveled or mitered corners why not use simple butt joints. Should be a lot easier cause then the saw is left at 90°. Should be easy to get your dimensions from your CAD file to figure the rest. This way you can run two sides past the other two which gives you room to fasten some cleats to make the whole mold stronger.
Rich
Just dawned on me that you would have a compound angle on the sides of two pieces but still this should be easier and stronger.
The Professional Termite
Edited 1/6/2005 5:56 pm ET by Trialnut
Gene, I would look to my framing square for some of the info. Mine is out on a job-site, so I don't have it here to look at now, but 4" rise in 16" would be 3 -12 pitch, so 14 degrees around the outside, and you could use the hip/valley # to figure the length of the other two sides.
I would use melamine instead of plywood because of the slick finish. Clear packing tape for the hip seams, and silicone caulk for the corners.
If you use a vibrator to get the bubbles out, you will be amazed at what a good finish this produces. Also you may want to think about adding a dye for another touch of class.
This forum post is now archived. Commenting has been disabled