Monday, August 8, 2011

The Center Line Challenge

Recently, I had a brain storm about finding the straight axis of a bent chair part. In my recent work, being able to register the axis of a bent piece has become increasingly important. So to ease my way, I thought of a method for marking the center of a bent piece, quickly and accurately.

As you can see below, when you look at the  piece so that the line is closest to you, the piece is split evenly and looks straight.


But as you see here, the piece has a curve.


By aligning this line with my sight line, I can use it as a reference to get the angle correct and very importantly, the rotation. Unlike straight parts, the rotation plays a huge role in positioning a bent part.

Of course a chair like the one below has oodles of complex reaming and positioning. I hope to go into this more later.


So here's the challenge. Before I tell you my idea, I want yours. How would you go about accurately marking the center axis of a bent piece? Remember, some pieces are bent in more than one direction like the one below (but only in one plane, for now!).


So think it over, stay up all night, drink 12 cups of coffee, climb a mountain and meditate...Whatever it takes, who knows, your idea might be a lot better than mine! I'll post the top three (IMHO) methods.

24 comments:

Mike Hamilton said...

I have visions of poppets holding the part off the work bench using the lathe center marks. Rotating the part, combined with eyeballing finds the straight line. Marking is like leveling legs, with the pencil at the height of the poppets...
Vision is fading, wish I liked coffee.....

Caleb James said...

Well the simplest one that comes to mind is lay the bend on a flat work surface and use a compass, as is often used to mark the cut length off for a leg, and set the height to the center of the piece and slide it along the piece while keeping the compass flush to the flat work surface. Of course this works in theory if the bend is uniform in thickness along its length. If it isn't then you would need to align it so that the center is at a uniform height from the work surface. I guess there in lyes the problem.

I have seen this concept used to make the center line of a piece while still chucked in a lathe and it works well but then again that wouldn't work well for a bent part.

That is the best suggestion I have for now. Could use some work.

Caleb James said...

UPDATE: If you had a donut shaped part that is the major diameter of your bend and with a center the diameter for either end you could use those to hold the ends of the bend at a consistent height off a flat work surface. So then when you laid it down on the flat work surface its center would be a uniform height from the work surface making the marking easy along the center.

johnjoiner said...

As long as the pieces are bent in only one plane, the laser levels will show a nice straight line the length of the piece. I've no idea how to easily mark that line though. Can you turn up the power on the laser so it burns the line in just enough to see, but easily sand off? ;)

Wallis said...

Cut a scrap of 3/4 X 3/4 inch thick wood about 1 1/2 longer than the bent piece is thick. Drill 1/2 inch holes about 5/8 inch deep in each end and glue a 1/2 inch dowel into the holes leaving about 1 1/2 inches sticking out. Drill a hole all the way through in the EXACT center between the two dowels so that a pencil fits SNUG. Set this jig over the bent leg until the pencil point touches and rotate clockwise or counterclockwise until both dowels are touching the bent piece. Slide down the length and there you will have a centered line down the length.

Wallis said...

Update --

After playing with my idea, and depending on the radius of bend, you may want to mark the piece as stated and then reverse the rotation of engaging the jig to the workpiece and run another line in the opposite direction of pull from they way you marked the first. If there is in fact 2 lines in the bends, the middle of the two is the center. Obviously the piece needs to remain flat and secure to your bench while marking.

Peter Galbert said...

Thanks for the responses! I am starting to see some constants!

Anonymous said...

OK, I'll bite. Roll the piece like you would a bent pool cue on your workbench to determine the actual plane of the curve. Fix it so the plane of the curve is parallel to the bench top(jorganson clamp?) measure the distance to the center of the fattest part and shim up the thinnest end so the center line is off the bench the same amount as the center of the fattest part and do the compass thing. I'm going out to try it.

Pablo Riviera said...

The most difficult part of this challenge is to express geometrical ideas in words. The problem is a complicated one, not just because of the compound bends, but also because of the varying diameters of the components. My suggestion would be to make a set of adjustible vee blocks and an adustible height gauge. Blocks of wood with vees cut and another with a hole to clamp a pencil using shims to adjust the height might doo the job. Set your component onto the vee blocks along the section you want to mark. Adjust the vee block heights so that the section centres are equidistant from your flat work surface. Set your pencil marking gauge to the appropriate centre height and mark a line on the component buy running you height gauge along the flat work surface.

Anonymous said...

1. Put the piece loosely back on your lathe. Gravity should pull the piece to align along one axis. Mark the "12" o'clock position on both sides of the piece. Remove from lathe. These marks will be for the "short side" of the piece. If you want the long side, mark by using the short side mark and the lathe center marks.

2. Using the v blocks you use to clamp and drill stretchers, place piece in vise and clamp.

3. Select a straight piece of pine about 1" x 3" x length of piece you are marking. The size shouldn't matter too much, but it will need to be reasonably close to the diameter of the bend of the piece.

4. Set your pine straight edge on your flat bench in front of the clamped piece. Using a Peter Galbert mirror, align the marks on the ends of the piece to the top of your pine straight edge.

5. Use a flat pencil and scribe the line. You may need to set the piece slightly above center in the mirror to allow room for the pencil.

That is a lot harder to write than it is to visualize.

Patrick

Anonymous said...

Put the piece back in the bending jig,lay it on the bench, make a pencil jig, mark the line. I need more coffee!

Greg Pennington said...

With the two back post mounted in the bending form run an aluminum straight edge over both pieces to mark the high points of the radius.
or
mark center before you steam bend then align the center on the bend form.

Anonymous said...

If it is only bent in one plane, you need only sandwish the piece between two boards, then make a block that matches the width of the piece and has a pencil pushed through a drilled hole at the center line. Run the block down the opening between the two boards letting the pencil mark the center.

Peter Galbert said...

I'm seeing some great ideas, keep em coming!! Boy, I don't know why I didn't do this sooner, it's like a chairmaking think tank!

Anonymous said...

Give us a hint, Pete!

Peter Galbert said...

I am seeing lots of interesting ideas, so I'm loathe to offer hints to my solution, after all, it's only one way to do it and I'd hate to stifle the creative flow! I'll post my solution soon.

Anonymous said...

When you build your bending jig, space the v-cut plywood pieces 5/16ths apart(or the thickness of a flat carpenters pencil) using periodic spacers. While it's still in the jig, slide the pencil in between and mark the center line. Next idea, rather than add mayonnaise to tuna fish, feed mayonnaise directly to the tuna....

Pablo Riviera said...
This comment has been removed by the author.
Pablo Riviera said...
This comment has been removed by the author.
Pablo Riviera said...

Forget the shims and height adjustment.

Cut six vee blocks from scrap 4x2 so that the vee bottom is 2 inches from a reference face. Mark three support positions on the work piece and clamp a pair of vee's either side so that the reference face of the vee's can be placed on a flat surface and the work piece centre line will be held at a height of 2 inches irrespective of the diameter. Use your height gauge which has been preset to 2 inches to mark the line.

I have posted a sketch
http://licensed2tinker.yolasite.com/blog/the-centreline-challenge

Anonymous said...

OK! Third morning trying to think about the thing. I'm sure Peter has an elegant(though maybe not pretty) solution. My mind takes me away from trying to scribe a line and on to the real problem(as I see it), which is determining the plane that the curve is bent on. I remember painting the waterline on a sailboat one time using spotlights reflecting a shadow on to a undulating surface. Perhaps light can be used in some way to deliniate the tangent point and thus the plane of the curve.
Bill

flyingshavings said...

I am missing something, or isn't this what a chalk line is for? Find the centre at each end and the middle, apply line, snap it - hety presto a line down the centre (I think?)

chrisvanaar said...

Hmm.... That's quite a knotty one, not only because of the challenge but also to explain it in an foreign language.
Ok, I would make a clamping jig, out of 2 staight and even plywood boards. It would look like big wooden hand clamps, but with 2 'pulling' and 1 'pushing' handles, in an triangle. Put the bent piece between the boards, next to the pullers. With trail and error, the piece can be clamped completely snug between the boards, so there would be no light visible between the piece and the boards. Then make a marking jig, with 2 dowels and a pencil, like the one Wallis mentioned, and now jou can scribe the center line. Let the each dowel run the outside of a board and there you are..... (At least I hope you can make something of this) Cheers, a wiskey made the differance,
Draaierchris

Andrew Jack said...

There is no spoon.