How to make a fair curve?

didn't you notice my post an hour an a half ago where I agreed with you about using the TS? Or am I not part of "you guys"?

dave

Sw> Jeeez, you guys sound like you'd walk around the block to go next door. Any

The teeth of a pattern maker's rasp are randomly placed on the surface and not in rows, like the usual wood rasp. It makes for an exceptionally smooth cut ... there is no comparison to a cheap rasp for curves and rounding a plane. I've not used a "micro-plane", simply because I own a couple of pattern maker's rasps, so I can't vouch for how they work.

IIRC, Highland or Rocklers are more expensive than Lee Valley ... there are also some European rasps which are very similar, and cheaper, but I have no idea where to find them.

Oscillating Spindle Sander.

Use a French curve (or a Freedom Curve if you prefer). If you don't have some, go buy a set of small plastic ones. Then photocopy them at several enlargements and scroll saw out some big MDF or clear Perspex versions.

The trick to using a French curve is to remember that every point where two curves meet should be a smooth join, i.e. their tangents are on the same line. This implies that the perpendicular to both curves should be on the same line, which is why some curves have perpendiculars marked at intervals.

French curves - hard to make your own shape (you can, if you Google) but useful and worth having around.

At ease, Dave ... you were excluded in this one instance, but don't let it go to your head! :)

However, I did think it was notable that a table saw would be an afterthought for the task to more than one woodworker ...

They're worth it, but not needed for last-making.

Rasps all used to be made by hand, but most are now machine-made. Machine made are definitely not so good - the teeth are formed at a regular spacing, so they tend to form "tram lines" when you use them. A hand-cut rasp has randomly spaced teeth, to they don't all fall into line like this. They're expensive, but definitely worth it if you're making cabriole legs.

Round here, hand-cut Italian rasps are about $50. I'm no big fan of buying old files and cleaning of sharpening them, but I do pick up old rasps when I see them around - and unlike files, you can't easily chemically sharpen a rasp (the tooth tip rounds off).

There's a description of rasp and file cutting in Bealer

Bent strips are OK for symmetrical curves (I use fibreglass skite spar), but they don't work so well for an asymmetric curve.

If you can find one in an antique store a float does a nice job of wood removal as well. Much faster than a rasp and not as groved a surface. It's like a strip of small plane blades rather than teeth. I love mine. let me know if you need a pic. Dave in Fairfax

but look at if from my perspective: I had an 1/8" piece that almost fit the bill. I didn't think of anything more immediate than to plane it down to to make it more pliable. Had I been staring at a piece of lumber, then I would have gone over to the TS... see? :)

dave

Sw>>about using the TS? Or am I not part of "you guys"?

I used this approach to apply a curve to some table base aprons. I bent a thin (1/8") strip of wood and clamped the ends. I cut it out with a jigsaw and used the first piece as a pattern for the second. I'm guessing that doing it the way I did, I ended up with a parabolic curve.

todd

Okay Andy, it's my turn to be the village idiot. I must have failed cut 'n' paste in kindergarten...

I don't "get" French Curves. Assuming there _is_ something to "get," what do they _do?_

I see the pretty curves, do I just shift it around on the drawing until I see something I like, or is there more to it than that? Am I supposed to just pick a section of a curve and use that? Which one? How do I know?

You're entire statement about perpendiculars and tangents tells me there's a lot more to this than I see by just looking at one.

Man, I don't even know what I don't know with these things... "When your only tool's a T-Square, every problem's just another crooked line." ;>

Michael

basically, yes.

how I use 'em is first I sketch in freehand with a light loose line the shape I want. then I shuffle the french curve around on it until I find a section of it that makes the line (or usually part of it) that I want. scribe that segment, find the next, repeat.

it can be a bit of a trick to get the segments to fair out right. one method is to shift it around until you get 3 intersections. pick the line up from the second.

that stuff is for convenience. it helps, but don't let it run ya.

If the strip is of uniform width and thickness, I think the curve is called a catenary. It's the same curve a chain makes when you suspend it from its ends. It's like a parabola, except that the apex is flatter.

I don't think you'd really like aprons with a true parabolic curve.

You've already had your chance in the dust collector static thread.

No problem - two hundred years of draughting just had it wrong. Thanks for setting them straight.

Some more comments on smooth curves and French curves.

A "smooth" curve is one that avoids corners. A "corner" is a point where there's a discontinuity in the tangent - i.e. a tangent drawn just a tiny fraction to one side of the corner wouldn't line up with a similar tangent drawn to the curve just the other side of the corner. By implication, if we can align the tangents of two curves, then we can avoid the corner.

Now the fun bit - this even works with two different curves. If we can just get those tangents to line up, then the transition between them will be smooth - no matter how much of a change in curvature is involved.

Aligning tangents is one approach. But if tangents align, then so must the perpendiculars align. Now for a simple arc, the perpendicular is a radius, which suggests a useful special case.

When joining a circular arc onto a straight line, the centre used by the compasses to strike the arc must be on a line perpendicular to the straight line, and crossing it at the point where the curves join. This sounds awkward, but it's pretty obvious in pictures. It's also the principle behind constructing the range of Gothic arches:

A French curve is a curved stencil whose shape is a mathematically somewhat complex curve that changes its radius of curvature along its length. There are several sorts, and those with access to Mathematica

or other drawing tools can probably generate their own. A useful source for some basic curves of this family are the Archimedian and logarithmic spirals, which can be constructed with simple paper and pencil methods.

A French curve is a curve that changes along its length. So you should be able to roughly eyeball what you need, then find a curve you already have that offers roughly the right curvature at two places roughly the right distance apart (the previous trick about making enlarged MDF duplicates by photocopying is worth it, if you need big furnituremaker's curves.)

To use a French curve, first choose the best fit you have. The place it roughly in place so that it touches the existing curve at the right places. Now slide it from side to side, keeping it aligned against those end points, until the tangents are a good fit. You're done !

If it doesn't work, then try different curves. They'll all fit and give a smooth curve, but they might not offer quite the radius you're after. _Don't_ use French curve by picking the radii you want, plonking it down and then trying to "smooth out" the corners. They just don't work that way. Each given curve will only fit at one place

- if you want different combinations of radii, then you need a different curve (but photocopiers with fine enlargement control are your friend).

Avoid Bezier curves (offered by any CAD program) for freehand drawing work. Although they can be used, it's all too easy to make a spline curve look ugly in a purely aesthetic sense (and the maths of exactly _why_ is both horrible, and does actually exist). However today is Valentine's Day, and I have taken a solemn oath to forswear all aesthetics.

Speaking of "cut and paste", you will save yourself a lot of trouble by doing exactly that.

Describe the curved line in a drawing program and print it out full size.

Spray adhesive.

Bandsaur.

...and it's PURPLE !

Thomas J. Watson-Cabinetmaker (ret) Real Email is: tjwatson1ATcomcastDOTnet Website:

???

Andy, did I piss you off in a previous life or something? You seemed to know about FCs, so I thought I'd ask. Whatever it was I said that prompted your last crack, I apologize.

Michael

Better still ,french curves being short, try ships curves instead. Of course the idea of halving the distance between the length of the curve and drawing one half freehand and then flipping it and tracing the other has to be way too simple dunnit. mjh

On Sat, 14 Feb 2004 19:20:45 -0500, Tom Watson brought forth from the murky depths:

Do you pick up the mouse and speak into it like Scotty did in "Star Trek IV: The Voyage Home"? Or how does the computer program understand your description?

Poor Mikey prolly didn't "get" Art Nouveau furniture, either. 'Tis a shame, wot?

So put a bandage on it and put the Playboys away, silly savage. It'll be pink again in a week. DAMHIKT

They might if you taper the strip.