vdubbs wrote:
Gary Dean wrote:
I have just been reading a tool catalogue and came across an advert forthis
device which the makers, Trend, claims to be the worlds fastest profile
scriber. It is basically a disk with a hole on the middle into which you
insert a pencil. You then, and I quote, "run the wheeled scribe along an
irregular feature and the pencil will accurately copy the profile directly
onto the work piece to be installed. It can then be cut and fitted resulting
in a perfect butt".

Anyone with only a rudimentary knowledge of geometry will know why thisdoes
not work.

This tool costs £23.95 in UK, I guess thats about $40 in the US. Trend make
excellent routers, trust me, I have one, I cannot believe that Trend,a
company I am sure you will know in US, could possibly have got this wrong,
so please guys tell me what I am missing.

I don't follow you on why it would not work. If the wheel follows the
surface and fits around the pencil, then the pencil would be held
parrallel to the surface you are needing to fit too. As long as the
pencil stayed perpendicular to the surface you are scribing on, you
should get a very close fit as long as you allow for any inside
corners.

BTW, I pretty much failed geometry, so feel free to correct my error on
this as I don't see where the problem is except for the price.

The device adds its radius to the radius of convex curves on
the original to produce the convex curves of the copy and it
subtracts its radius from the radius of the concave curves
of the original to produce the concave curves of the copy.

If you 'copy' the new profile(s) recursively on each new copy
the convex curves are broader while the concave curves are
narrower. The resulting profiles approach a straight line.
From then on the device will work fine. For profiles that are
smooth with long radiused curves the device will work 'good
enough' for some range of values of 'good enough'.

--

FF