Layout out a Radius

[Patrick F Graham]

Hello All,

For my first post here, an embarrassing question that I just can't figure out.

I'm working on a reproduction Morris chair and have to stem bend the back-slats. The direction for the radius of the back-slat is as follows:
The slats have a radius of 23-inch. To ben them, I sandwich them in a shop-built form made from two bandsaw blocks that mate to create a 22-inch radius which overbends the wood a little to allow for inevitable springback.

Dim: The length of the back-slate is 21-3/4".

I get the initial radius of 23-inch -

measure 21-3/4" our as two parallel lines (length of the back-slat),
Find a center point of the 21-3/4" and using a trammel or string or whatever, make an arch between the two parallel line 23-inches up from the center point.


I get that, all good.

It's the part about '22-inch radius which overbends' to reduce the spring back. Just shortening from 23-inch to 22-inch doesn't make your radius more 'tight', it just makes a closer concentric arch.

What the hell is he talking about - I don't get it. What is am I missing that reducing the radius makes a tighter radius?

Thanks - Patrick

[Kevin Jenness]

The smaller the radius the tighter the curve. Steambent wood often does spring back to a degree depending on the species, initial moisture content thickness and radius and the technique used, best determined experimentally before committing to production of any quantity. Failure rates can be lessened by using air dried, radial sawn wood with end blocks and a back strap. Here's a link to a primer you might find helpful https://assets.leevalley.com/Original/10105/31161-steam-bending-instruction-booklet-c-01-e.pdf

[Bill Dufour]

I think you have the center point wrong. As I read it the center point is on a line perpendicular to the parallel lines. Set it so the parallel lines are vertical. Say center is to the left. It should be out 22" from the right hand line. Run a radius from that point touching the right hand line and swing an arc up and down until it touches the left hand line. If they did the math correct it should touch at 11 inches up and down from the center line.
Bill D.

Reading again I am not sure which way the slats curve. Along the long distance or across the 3" width
Arc or segment is the search term to use.

[Patrick F Graham]

The smaller the radius the tighter the curve. Steambent wood often does spring back to a degree depending on the species, initial moisture content thickness and radius and the technique used, best determined experimentally before committing to production of any quantity. Failure rates can be lessened by using air dried, radial sawn wood with end blocks and a back strap. Here's a link to a primer you might find helpful https://assets.leevalley.com/Original/10105/31161-steam-bending-instruction-booklet-c-01-e.pdf


No Mate, I'm all good with how to bend wood (for now at least).

I don't understand the part about laying out, marking, doing a 'tighter' radius going from 23-inch radius to a 22-inch radius - it's that part I'm stuck on.

Patrick

[Patrick F Graham]

I think you have the center point wrong. As I read it the center point is on a line perpendicular to the parallel lines. Set it so the parallel lines are vertical. Say center is to the left. It should be out 22" from the right hand line. Run a radius from that point touching the right hand line and swing an arc up and down until it touches the left hand line. If they did the math correct it should touch at 11 inches up and down from the center line.
Bill D.

Reading again I am not sure which way the slats curve. Along the long distance or across the 3" width
Arc or segment is the search term to use.

No no Bill, I think you figured it out. Let me try that. Be back tonight.

Thanks - Patrick

[Doug Garson]

I don't understand the part about laying out, marking, doing a 'tighter' radius going from 23-inch radius to a 22-inch radius - it's that part I'm stuck on.

Patrick
What's your definition of a tighter radius if it is not a smaller radius?

[Rich Engelhardt]

I missed a part - sorry.

[Jim Becker]

22" radius circle is smaller than a 23" radius circle. Bending to the former while gluing up the lamination allows for it to relax slightly when you take it out of the clamps after the glue cures enough and it should end up at about the desired 23" radius, give or take. It will be close enough that when you assemble your project, fiting will be easy.

[Kevin Jenness]

Patrick, sorry I didn't answer your question. I'm not sure exactly what is unclear about laying out the bending form, so I will describe how I would go about it.

Assume the finished part has a chord length of 21 3/4" and an inside radius of 23". When steambending it is usually a good idea to make the blanks and form oversize in length to get rid of any checking, clamp marks or flat sections at the ends, so I would make the form parts 30" long. On a piece of plywood 30" x 24", mark a point 1/2" in from a 30" edge and 15" in from a 24" edge and describe a 23" radius arc from that point. Saw or rout the arc and cut an edge parallel to the arc's chord about 4" away from it- the form blank should be about 10" tall by my calculation. Duplicate that piece and screw 2x2's 12" long about 4" apart spaced evenly along the arcs between the form blanks and flush with the top edges, then screw a baseplate to the bottom edges of the form blanks. Now you have a form that you can test out your process on by clamping a steamed blank to the crosspieces.

If your test piece springs back significantly, you can take apart the form and cut a tighter arc (smaller radius) on the form blanks. You will have to estimate the change in radius to get the desired shape. I would not use a two piece (inner and outer) form for steambending as it will be harder to pull your blank into shape and the cooling/drying process will be slower. The crossbar design will allow for easy clamping and bending several blanks in one session.

Steambending can give varying results depending on numerous factors. One way to overcome this is to bend your parts with extra thickness and then machine them to your exact desired curve with a shaper or router jig.

A 23" radius seems pretty tight for a back slat. I would do a mockup to ensure it is a comfortable sit before proceeding too far.

[Edward Weber]

It's the part about '22-inch radius which overbends' to reduce the spring back. Just shortening from 23-inch to 22-inch doesn't make your radius more 'tight', it just makes a closer concentric arch.

What the hell is he talking about - I don't get it. What is am I missing that reducing the radius makes a tighter radius?

Thanks - Patrick

To be short and to the point, spring-back occurs along the length of the piece, not just at the ends. This changes the radius which you are trying to compensate for.

[Patrick F Graham]

I am going to start over - I don't know how many of you guys are on other (not necessarly wood working) sites / forums - this has to be the friendliest of them ALL - THANK you all for trying to help me out considering how little info I provided.

Starting over.

I'm building a series of 10 Stickley Drop Arm Morris Spindle Recliner Chairs, Model #369:
460442

About 15 years ago, I built a version I called 369-EZ - the 'EZ' being I removed all the complicated angles and steam-bending of the back slats etc. Again, a set of 10 to give to family and friends.

Now, I've undertaken to make exact reproductions of the 1909 version. There are about 4 sets of plans that I get most of my final measurements from: Fine Wood Working Mags (1993 - Craftsman-Style Comfort in a Morris Chair - Gene Lehnert), Bob Lang did one (you know, cause Bob likes to measure stuff), The Furniture of Gustav Stickley, (1982 - Joseph J. Bavaro & Thomas L. Mossman) and a couple others. Gene's is probably the closest to the original (mostly).

Done right, it's a long and involved project. If the Blacker Dining Room Chair is a 10 on the difficulty scale, this is a solid 7+. There are 15 spindles per side, each tenon cut on matching 2.3 Degree angles then finished by hand - 15 per side, 2 ends per, = 64 angled tenons, all finished by hand, etc......

So, I've finished all the detective work for the full sides less the drop arm, quick shop pic of the work (no clue why the pics are so small) :
460450

Now, I'm working on the back including the back-slats. Here's a drawing that should show the scale and curve / radius:
460451

And this is from the best of the plans showing the actual size of the back-slat:
460452

And the sketch of the two ways I have been trying to get to that curve / radius - incorporating what Bill said as Starting Point A and my original as Starting Point B:
460453


So, other then not knowing how to make the pics bigger, I still don't see how either of these methods give me the correct curve / radius for the back-slats.

First, how to reproduce the exact curve shown in the plans (two pics up). Then, once I can reproduce that, I'll then try to figure out how to reduce it to a 22-inch version to take into account the springback.

Final pic:
460454


Gene Lehnert's exact words to describe how to get the curve are:
Theslats have a radius of 23 in. To bend them, I sandwich them in ashop-built form made
from two bandsawn blocks that mate to create a22-in. radius, which overbends the wood
a little to allow forinevitable springback. Allow the wood to dry completely in the formto
prevent excessive springback.


AGAIN, thank you guys for jumping in to keep me from loosing my mind on what should be simple.

And, sorry for being so obtuse with my first post.

Patrick

[Patrick F Graham]

I don't know Bill, didn't match the full side drawing that one of the plans came with. Also, seems overly bent. Maybe I didn't understand your idea. Anyway, I wrote a more exhaustive description of the entire project on this thread.

Here is the drawing I did that shows your way of doing it (A) and what I originally thought (B):
460455

Just doesn't match this (half) actual size drawing:
460456

No idea why all my photos are so small - not on purpose.

Thanks - Patrick

[Prashun Patel]

We will get there.
Some dumb questions:
Are your measurements in mm or inches? Unless you have a giant pencil, the radius looks too tight.

A 22” radius may not appear visually more right than a 23” to the naked eye, but to the wood it will be.

[Patrick F Graham]

Oh man, Kevin, good reply. For those of you playing along, here's a high school flash back refresher of terms he's talking about:
460457 (no, I didn't make it small, I just don't know how to make it larger, sorry).

From Gene's original paragraph:
Theslats have a radius of 23 in. To bend them, I sandwich them in ashop-built form
made from two bandsawn blocks that mate to create a22-in. radius, which overbends
the wood a little to allow forinevitable springback. Allow the wood to dry completely
in the formto prevent excessive springback.

We know:
Radius (finished) is 23"
Cord is 21-3/4" (finished L of back-slat)

Given that, yes, your math is correct but, I'm not sure how I layout the arch based on your description.

"On a piece of plywood 30" x 24", mark a point 1/2" in from a 30" edge and 15" in from a 24" edge and scribe a 23" radius arc from that point." - Kevin

Isn't that what Starting Point A on this drawing is???
460459

[working on making just the drawing to get it to match the Actual Size sample here:]
460458

I can't get the drawing to match to the actual size drawing.

[Patrick F Graham]

Inches.

I agree, I can't get the radius I'm coming up with to match the scale drawing of the radius.

Hence, why I'm here - I know it's something so simple - I just can't see it.

I'm not worried about 22" v 23" radius as described. I can't figure out first and foremost how to draw the 23" final radius. Should look like this when done:
460461

Patrick

[Prashun Patel]

What am I missing? Cut your template longer than the finished piece. Cut a whole Semi circle if you wish. Cut your blanks over sized, Bend them in the form and then trim them to size after they come out.

[Patrick F Graham]

Just that I can't figure out how to layout the semi circle.

Cord 21-3/4"
Radius 23"

That's all I got.


I know how to cut long.

I know how to cut oversizeed.

Drawing the template is what I'm not getting. I'm sure it simple but when I try to match what's described, it get something very different then what the full size drawing shows.

Patrick

[Prashun Patel]

Forget the chord. The template will work regardless of whether it's just as long as the chord or longer.

Measure 22" on a string and tie it to a nail in your bench. Tie a pencil to the other side. Or make a trammel out of a piece of wood. Screw one end into the bench not too tight, and make a hole for the pencil 22" away.

Put your template blank under the pencil and scribe out an arc.

Use a ruler to measure the cord after the arc is scribed if you wish. But as long as you're longer it doesn't matter. The picture with the A and B marks you posted a couple times is definitely not a circle with a 23" radius. The pencil in the picture makes me think that radius is only about 6-7"

[Ralph Okonieski]

You have an actual (half) size drawing. Draw a chord on it, measure the rise and and chord length, use the mensuration formulae for circle to calculate the radius. It should be close enough that it provide a clue as to the radius you want.

[Patrick F Graham]

Ben, Kevin and Prashum,

It was where to start the mark for the radius. Following ya'll's similar solution, I made this and it comes pretty close to almost exactly what the full size drawing represents.
460473

Prashum, forget that 'scaled' drawing, dimensions were a guess, I used it as an example of A and B.

The line just inside the outer arch is the 22" radius (right side only). The outer is the 23" radius.

Pretty damn close.

Thanks all - so simple once you hear, see, read about it.

Time to fire up the steam bender!

Patrick

[Patrick F Graham]

For sure, that's one way but I was trying to have the most clean start rather then a copy of a copy of a copy.

To your second point, yes, 'close enough' is a problem I have - I'm working on it.....

Thanks Ralph.

Patrick

[Kevin Jenness]

Be sure to make the form and the blanks longer than the finished part dimension.

I have a hard time reconciling the photographed chair with the plans you are building from. The height of a 23" radius arc 21.75" long is 2.73", and the slats in the photo don't match that to my eye- must be the camera angle. I would do a mockup to be sure. I must say that doesn't look like a particularly comfortable chair, but what do I know. Have fun.

[Patrick F Graham]

You know it - I'm adding 2" to each end.

It's a crazy comfortable chair, trust me, I wouldn't make it otherwise.

And yes, I'm a mockup maven!

Gimmie some of the Doug-Fir, I can build anything!!!!

Thanks - Patrick