Thread: help with a profile ?

Brisand, half way on the 100mm ,draw a vertical line ,up and below the base line. Draw a line between the end of the base line and the the25 mm height on both sides. Bisect these lines , a 90degree angle will give you a point on the centre line which will be the centre point for your compass.
hope this is clear enough, nine fingers

I may not have made myself clear on what I was trying to achieve, with the curve, I am hoping somone can tell me a method how I can cut the profile along the length of a 100x25 piece of timber

You could draw the profile on the ends of the timber, then cut up to the line with the tablesaw in increments, chisel out the waste, sand to final shape with a peice of sandpaper glued on a matching profiled peice of wood. I suppose it would depend on what tools youhave and how long the timber was. You could trim close to the line with a planer, then use spokeshaves and files.

Donna

What's the number of lineal metres you want to produce, Brisand?

The length will define the process used.

Cheers,

eddie

Originally Posted by brisand

I may not have made myself clear on what I was trying to achieve, with the curve, I am hoping somone can tell me a method how I can cut the profile along the length of a 100x25 piece of timber

There are a few ways to do it.

1. Spindle moulder with a table edge profile.

2. Router bit setup in a router table and use CMT bit 856-601-11 table edge bit. $115.00 from Carba-tec. Or similar.

3. Mark the profile on each end of your length of timber. Set your table saw blade to cut up to the line and make passes from each side starting with the blade just protruding from the table top and fence set at about 45mm, then raise the blade up a millimeter or so and move the fence in the thickness of the blade, pass from each edge and repeat until you have about a 4 millimeters left on the outer edge of the board, each edge to stay to support the round. Like the shape of the Sydney harbour bridge upside down with a bit still 4 mm on each edge for support. If you continue to form the round right to the edge it will wobble and you will lose a finger or two. After the round is formed, set the fence a few mm from the blade, drop the blade right down and cut the support bits off with the flat bottom of the rounded piece against the fence. You will have a stepped round shape that then needs planing to the line with a plane or rough it down with a belt sander then fine sand with a random orbit. It is hard to describe without sketching for you on paper but if the above is not clear I can do a sketch and email it to you. Or you can drive to Hoppers Crossing and I will do it on my machine.

I used to be the woodwork teacher up at Lavers Hill just up the hill from you at Colac.

Col.

I think the fastest method would be running the wood on edge for two passes with a shaper (spindle moulder) set up with appropriately sized quarter round cutters -- might be easier than cutting it in one go with a half round cutter, and the quarter round cutters might be easier to find.

Since the final shape wouldn't be stable (the flat would be cut off), you should probably laminate another thick piece to the 100mm face, maybe with brown paper in between the layers to ease removal after the profile is cut.

It took some head scratching, and correction of my initial arithmetic, to sort this out.

For such a cross-section, there's a formula for the geometry:
With:
b = rise (25mm)
c = chord (100mm)
Then R = (4b^2 + c^2)/8b
This gives R = 62.5mm (I think! - check your own arithmetic.)

Depending on the length of the workpiece, one way to do it, with a router table, could be similar to the process I used here: http://www.woodworkforums.ubeaut.com...ad.php?t=57412
Build a couple "bookends" for dead centres and attach a stout plank vertically on the back of the workpiece to engage the centres. The vertical size of the stiffening beam should be consistent with the length of the workpiece; a ratio of about 1:24 is often used in preliminary design of highway bridges. Attach the bookends to another plank, and add a couple runners at the upper corners of the bookends. Flip the whole assembly over, and work back and forth along a square-cutting bit, increasing the bit extension until you achieve the final profile. This can be quite tedious, and best left for one-off production, although the length could be almost unlimited, because a very broad stiffenning beam won't interfere with the swing.

Substantially simpler, for production in multiples of four, attach each workpiece to a block 100mm x 100mm and mount on a lathe; precision centering is mandatory. Use kraft paper and glue for final removal. Turn to the indicated profile. The length must of course be limited to the lathe's capacity. You might need to contact for help on this.

Joe