How To Build And Use A Simple Caul

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How To Build And Use A Simple Caul

toxophileken
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toxophileken
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April 27th, 2007, 2:25 am #1

Way back last year when I started posting photos on PaleoPlanet, I did a thread about steam bending. I was excited about the results I was getting, and I wanted to share the techniques I had been taught and learned with others, hoping they would be helpful.

"How I Steambend" Thread

Last August, at Phil's cabin in Big Bear, Sal showed me his caul, and how to straighten a stave with a caul, some clamps, and a heat gun. Sal got his caul design from someone else, and mine is very similar.

A caul is useful for inducing initial reflex, side to side correction of straightness (from big moves to minor ones just to correct string tracking), correcting some amount of limb twist, and for belly tempering a stave into reflex (different from initial reflexing, as it is done later in the process, to counteract any set).

The big advantages of using a heat gun and caul for straightening (as opposed to using steam), is that it is faster, more efficient, and easier. You can work on one section of the stave at a time, without worrying about hurrying before the stave loses its heat from the hour plus long steam bath. Also, the bend seems to stay in better. I have had staves that have been steam straightened lose the correction upon reheating. Those done with a heat gun seem to be more stable. Maybe I am getting them hotter, I don't know.

At any rate, I used jury-rigged cauls for some time with the heat gun before going ahead and making a dedicated caul. I would recommend that you just go for it. The caul is very easy to use, and not hard to make. Should take only an afternoon or at most two.

How to Make the Caul

First, you must decide on the shape of the caul, and its length. I made mine a few inches longer than the longest bow I expected to make.

You will be laying the stave in the caul belly up, back down. I recommend a soft wood, so it won't damage the back of your bow. I ended up using redwood and pine. The redwood is more expensive, but it is dry (so it won't warp on you), and much lighter. It has proved strong enough, which I had been concerned about. It is soft enough that the bow wood dents it easily (which doesn't affect its function).

My caul gives about 2 1/2" to 3" of reflex, depending on the length of the bow. Keep in mind you can increase the deflex of your caul temporarily with shims, if you need to for some bows.

A good method for laying out the curve of your caul was taught to me by the late Bill Price. You take a thin lathe of wood, that is long, straight, and flexible. Lay it on the wood of your caul (or cardboard, to use as a template later), and bend it until the shape is what you want in your finished bow stave. Keep in mind that most staves will lose just a touch of reflex when you take them out of the caul (which is why when straightening you usually take it a bit farther than you want it to end up). Draw your line along the piece of lathe, and make sure you like it. Once you are satisfied, it is an easy matter to draw another line exactly matching the first, about 3" over, using a compass or block of wood, etc.

If you take the extra step of drawing the layout on cardboard or paper, you can more easily line it up on your wood and avoid any knots. Some people like to draw one half on a folded piece of paper, and get a mirror perfect caul template when they unfold it. Not really necessary with stave bows, in my opinion, though. Another advantage to making a template is that you can take it to the lumber yard and make sure you buy wood that is wide enough for your caul. If your reflex is extreme and your caul long, you might find you need a wider board (more expensive). There is no reason you cannot make your caul in two halves, and connect it in the middle, though (and if you use a hinge, and an arrangement to make the caul one rigid piece, you could easily make the thing more portable, or at least more easily stored).

My caul is made from three pieces of wood. I settled on about 2" or more of width for the main portion the bow to ride on. Wider bows should be able to hang over a bit, with little trouble. To get the width, I used a 2x6 of redwood (nominal 1 1/2" width) and a 1x6 of pine (nominal 3/4" width). Firsrt, I drew the outline of the caul on one of the boards, then I screwed these two boards together, after smearing the inside surfaces with wood glue. I placed the screws well inside the outline of the caul, so as not to have screws hitting my saw blade. I would have preferred to use all redwood, but they didn't have 1x6 redwood, and I didn't feel like ripping a 2x6 to dimension. As it has turned out, the different color has given me a useful line down the center of my caul for reference during straightening.


The caul design drawn out on the boards, which have been screwed together for correct width.





Closeup, showing screw placement well within caul, and how I laid it out to avoid knots.





Next, I cut to just outside the lines, using a bandsaw. A jig saw or even an axe could be used (see the axeing tutorial).








Here is the rough edge left even from using a bandsaw.











At this point, I cleaned up the top face of the caul with a belt sander and orbital sander. You don't have to get really technical, especially on the bottom. If you want it smooth, wait until you attach the side piece and sand the whole bottom flush. The top needs to be cleaned up to where you want it, before attaching the side piece.

You might be tempted to leave the bottom of the caul square, as opposed to having it follow the shape of the caul face. This will make it rather inconvenient when you use your clamps... Better to have a parallel surface to clamp to. And the caul will be lighter, in case you have to move it much.


Sanding the caul














Next a side piece is attached. This is not a step I would skip, as the ability to put clamps on the stave from the side is what makes this caul so very convenient.

You may need a wider board if your call is very reflexed. I don't think I needed any wider than 5 1/2" nominal width. It is an easy matter to take your half finished caul with you to the lumber yard at this point to select the right size piece of wood (if you didn't use a template already). I chose to use another 2x6 redwood board, but Sal's caul was 3/4" wide pine, and it seemed plenty strong.

I took my caul main section and layed it on top of the board for the side piece, and drew the layout, avoiding knots where possible. The side piece sits higher than the main body of the caul. I made it about 1 1/2" higher than the top, with the bottoms flush. 2" high wouldn't have been bad, but 1 1/2" is usually fine. If you want to correct tracking on any straight (no reflex) staves, a higher side piece will help. You may even decide not to cut the top to the shape of the caul, but leave it straight.

So, I drew the line for one edge on the board for the side piece, then moved the caul over to draw the next line. I kept them parallel to each other, and made the side piece about 4 1/2" wide. Next, I cut to the lines with the bandsaw.


The side piece and main caul body before they are glued and screwed together.





I used clamps to hold the pieces in place while I screwed them together. Self explanatory, but the next two photos show the caul in its pretty much final state (other than the clamps).








Using the Caul

Once you have your caul done, you don't really need all that many clamps to use it. Since you will be heating up only relatively small areas with your heat gun, you can clamp them in place as they become hot enough to be plastic, and move on to another section. You can do one limb at a time, or even only a short section at a time. The same applies when belly tempering.

In the photos below, you will see I used quite a few clamps. Partly this is because I am Captain Overkill, and partly it is because on the particular stave shown, I was also removing limb twist. Limb twist is more difficult to correct than side to side straightness, but this caul certainly makes it doable. Often, shims are placed under one side of the limb, to allow the other side to be taken slightly past flat, so it comes back to flat once out of the caul.

One caution: If you use shims that come into contact with the bows back, make sure they are of softer wood than the bow and/or cushion them with leather or something soft. You get a lot of pressure and leverage if you aren't careful, and you don't want to dent the back. Also good to remember is that you don't need to crush the bow into the caul with the clamps- just get it into contact. Additional pressure once the back is against the caul surface is not necessary, or much advisable...

The following few photos show a black locust stave I reflexed and took some limb twist out in one operation (I straightened it side to side later, but forgot to take photos). As mentioned, this many clamps are not necessary, but I left them on to more readily show what I had done. For each approximately 4" section to be straightened, I hit it with a heat gun for about four or five minutes. Basically much like belly tempering, but a twice as large section hit by the heat. Then a small wooden block was put on to protect the stave, and a clamp positioned. I was free to walk away and come back at any time in the process...

Avoid scorching the wood. Too little heat is better than too much, as long as you don't try and force a big bend without enough heat. You can always do the correction again later, if it doesn't take or go far enough the first time. Not so if you break the stave (Yes, I've done it several times). Practice on hopeless staves first and get the process dialed in, before you take heat to your best staves.

Note the osage shim under one side of the tip, and how the tip has been lined up with the line where the redwood meets the pine in the next photo.








The black locust stave, width and side profiles out of the caul





Note I got some darkening (not quite scorching) on the stave's back during the process... I will address how I learned to avoid this, a little later in the post while discussing belly tempering.


Side to Side Straightening

Perfect straightness isn't necessary in a bow. All you need is for the string to track through the handle. A caul makes it easy to get the stave to where the srring will track where you like it. You may also be able to salvage a crooked stave that would otherwise be beyond your abilities to make a bow from.

You can easily get too obsessed with straightness, and end up removing all the character from your stave...

Regardless, this caul is the easiest and safest (for the stave) way to correct side to side straightness I have used, on average.

Here's how I do it. Clamp the stave into the caul in at least two places. Usually the handle is one. This keeps it from moving when you apply side to side clamping pressure. Position the stave so that the correction takes place towards the caul, so you can pull it the correct direction with the clamp (obviously). Sight down the stave and find out where the first bend that you need to correct is. Mark it with a pencil, so you can find it when you are looking down at it.

You need part of the stave up against the side of the caul for leverage. Often, I need to place a shim, to get the amount of travel needed to make the correction. Other times, there is a swell in the stave that acts as the fulcrum.

The following photos are simulations (sorry). The hickory stave didn't need any straightening (it was just there when I went out to grab the photos). The osage had been straightended, much as shown.

Note that often even fewer clamps are needed than when reflexing, if the bend is localized.








In this next photo (of an osage bow) I have placed a shim to show how even just a little bit of extra leverage is enough for many corrections. If you look at the blocks sitting above the shim, they outline the approximate area I would hit with the heat gun. Once the wood is hot enough, the clamp on the far left would be positioned and tightened. The limb tip would be taken slightly past where I wanted it to end up. Osage and black locust tend to creep back the least of any woods I have tried. Let's not even discuss juniper. If you look closely, you will see the line in the caul created by the differing colors of the pine and redwood. I use this line as a reference when measuring how far to move the wood. Another way is to measure the tips and see if they are the same distance from the edge of the caul as the middle of the handle.





Here's that black locust stave again, just out of the caul. Belly up on the right, back up on the left. Again, getting the string to line up is the thing, not super technical straightness. Compare this photo set to the one above, with the same stave leaning up against the door. Looks a lot better when you don't get up close and sight down it...





Belly Tempering

Belly tempering is a subject for a different thread entirely. Simply put, the stave is clamped into reflex, and the belly is heated, even beyond what is necessary for heat bending, to make it take a more permanent bend. The reflex I put in the stave initially, while straightening, often pulls out while even just floor bending, or at some point in getting the bow to full draw. Some bowyers temper before that stage, and some after the stave has started to lose reflex. Some do both...

Essentially, the tempering seems to make the bow stronger, as the induced reflex adds to the force needed to bend the bow. I'm sure I am not explaining it very well... The practical result is: You can strengthen specific areas of your bow to correct tiller, or you can reduce localized or overall set.

One problem that I ran into with this caul while belly tempering and some straightening procedures was that I was getting the back of the bow scorched. I couldn't see it, as it was against the caul, facing away from me. A nasty surprise when you take the stave or bow out of the clamps, let me tell you! I finally figured out that the hot air was flowing around the sides of the stave and being concentrated on the back of the bow. The random pattern of scorching brought me to the conclusion that the dips and humps of the back gave different spacing for the hot air to flow around.

I corrected the problem by using aluminum foil as a baffle on both sides of the stave to block the air flow. No scorching of the back since then... I highly recommend the method.

In the following photo, you can see on the far limb where the foil is folded over and laid down under the limb. On the near limb, I have folded it into place to block the air flow from going under the stave and scorching the back. Also note the bottle of vegetable oil in the upper right corner. It is sometimes a good idea to rub the area to be heated with oil, to keep the wood from drying out.





Here's a closer view. Many ways to do this same thing... You get the idea.





Views of the finished caul (after about 5 or six months of use)





I know I made it appear way more complicated than it really is. I talked to several experienced bowyers who don't have a caul like this, and it surprised me, as it is so convenient. I hope this post makes it easy for some of you to try, yourselves.

Ken

(Edited to correct a photo. Ken)
Last edited by toxophileken on December 29th, 2011, 10:40 pm, edited 1 time in total.
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Salvador6
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April 27th, 2007, 3:14 am #2

Ken, the original design for that caul belongs to Gary Davis. Gary is one of the best guys to have around when making a bow, he goes out of his way to help everybody. Gary uses it to straighten and put reflex, I don't think he heat tempers.
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toxophileken
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April 27th, 2007, 4:31 am #3

Sal, I was hoping you would chime in with that. My thanks to Gary and yourself!

Ken
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French Crow
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April 27th, 2007, 4:49 am #4

Very useful and documented, very nice work. Thanks for sharing that, Ken !
Bruno
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Hin Kraka
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April 27th, 2007, 8:15 am #5

I know we have a lot of Gurus and wizzes around here, but when it comes to photos and tutorials you are plainly the man Ken! It is so amazing how well you can both show and describe in words! Thanks a million!
//Jakob - too little time for all interest and hobbies!
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tqpotia
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April 27th, 2007, 8:47 am #6

Great, Ken !

Some questions .
1. What is the curve of the caul - is it a part of a circle ?
2. How did you draw part of a circle with such a big radius ? Nail in the neighbour`s yard and a rope for compass - LOL ? Seriously - I can not imagine. The only method I can figure out is to take a thin slat with absolutely same thickness and to bend it for patern. But it is a bad idea because it will bend more in the center...

Thanks !

Kosio
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toxophileken
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April 27th, 2007, 4:43 pm #7

Thanks guys, very much.

Kosio, I personally didn't get that technical. Using the flexible piece of wood, as I described, I found the curve that looked best to me. You can place nails to hold the lathe in place while you measure the curve to see if the ends are the same distance from the center, etc. My caul is a little flat in the middle... Again, if I want more reflex in the handle, or overall, I can use blocks to shim the stave and thus temporarily change the shape of the form.

Another way to get the shape of your form is to trace the shape of a bow having the same reflex you like. Or, as you suggested, string up a piece of lathe to get a uniform bend... To avoid the middle bending too much, glue in thin pieces of wood where necessary and "tiller" the lathe to get the best reflex. Someone showed me how to draw arcs of circles with a big compass, but I forget how right now. If you have a slide projector, overhead projector, or opaque projector, you can draw your shape on paper or acetate and/or photograph it, and project the drawing to life size and trace it. Many ways to skin the cat here...

Remember, the caul won't make your stave perfectly straight like a board, and that a perfectly matching reflex isn't super important, either. If the caul looks good to a critical eye, I would think it is enough. After all, the wood will be tillered, and varying amounts of set will occur... "Wood happens".

Bill Price used to forgo the use of a form when he glued up his bamboo backed longbows. He would just place nails in his bench, and place the laminations on edge between the nails. There were only about six or eight nails, on alternating sides, to hold the curve. When he liked how it looked, he would drive the nails in a little deeper, and remove the slats, then smear on the epoxy, and place the glue up back between the nails. Then he would apply clamps, which would hold the reflex while the epoxy was heat cured. Eventually he was able to just use old nail holes... He placed wax paper down before putting in the nails, to protect the bench from the glue.

Ken
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badger5149
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April 27th, 2007, 5:02 pm #8

Good thread Ken, A lot of otherwise trash wood can be turned into premium bows with the use of a caul. I am working on a large steam chamber that will hold a caul and a bow togehter, I notmally use just dry heat but I can see on badly twisted wood steam and slow twist might come in handy. Steve
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DCM
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April 27th, 2007, 6:11 pm #9

Ken I think you might have made yourself understood better with some pictures and discussion. LOL

Caul is one of the most fundamental bowyer tools for me. Very rarely do I make a bow that I don't use my caul, even if just for little adjustments here or there.
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Marc St Louis
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April 28th, 2007, 1:02 am #10

Ken
Odd that you would be posting this and using the word "caul". The meaning of the word, as per dictionary is used to describe a thin membrane that covers the head of a newborn baby as it comes out of the womb and does not fit that of a piece of wood used for shaping. I have been looking for a word to use to fit that purpose and "caul" does not fit the bill. Could you suggest another? Marc
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PaleoAleo
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April 28th, 2007, 1:29 am #11

I'm putting a copy of this into the Reference Area Ken. Nice job (as usual) on the photos, etc.

Tom
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toxophileken
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April 28th, 2007, 10:02 am #12

Thanks again...

Steve, your idea is interesting. I want to hear more about it...

Hope it will do some good there, Tom. Thanks.

Marc, that is the primary definition... But here is a quote from the Webster's Encyclopedic Unabridged Dictionary of the English Language (1989):
Quote:
Caul (2): n. a form or plate for pressing a veneer or veneers being glued to a backing or to each other. [< F cale shim < G Keil wedge]


Would seem appropriate to laminated bows, at the least...

I have also heard it called a "form", but this is usually in the context of laminated bows, both natural and of modern materials.

I have heard the term "caul" used in wood working shows for the blocks of wood used between a clamp and a piece of work, to avoid marring it. Note the French and German words of possible origin in the above definition...

For bowmaking purposes, I would propose the following simple definitions, which reflect current use in "bowyerese", as far as I can tell:

Form: A piece of wood or other sturdy material shaped depending on desired bow profile so that laminations of wood and/or modern materials may be clamped into or against it, in order to give the shape of the form to the glued up bow blank.

Caul: A piece of wood or other sturdy material shaped in such a way that a bow or bow stave can be clamped against or into it during the heat straightening or belly tempering process, in order to impart the desired shape, and/or correct crookedness or twist.

Ken

[Edited to put quote in quotation bars. Ken]
[Edited to add missing word. Ken]
Last edited by toxophileken on February 7th, 2008, 6:58 am, edited 1 time in total.
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Marc St Louis
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April 29th, 2007, 3:06 pm #13

Thank you Ken. That is excellentMarc
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censu
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April 29th, 2007, 6:47 pm #14

Great post Ken, a useful tool as I venture into the deeper waters of bow making I must make one




Cheers




Censu "The Matchstick Maker"
Censu "The Matchstick Maker"
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Juri
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April 30th, 2007, 9:34 am #15

Thank you very much Ken!!
The side piece is is a really good idea.
Using aluminium foil is a genius innovation, worth of a Nobel price at least! I have also burned a back of a good bow. Masking tape protects also pretty good, but the sticky glue melts on the wood and its a nightmare to clean off.
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James Carmichael
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April 30th, 2007, 9:01 pm #16

Not to be a nit-picker, but I would say that device is more accurately described as a form than a caul.
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toxophileken
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May 1st, 2007, 3:16 am #17

Yes, but the common usage hereabouts has prompted my attempt at a new definition for both form and caul, as regards to bowmaking (above).

Both are forms... Certainly. Perhaps it should be noted that:

"A Caul is a variation of a form, where the shape of a stave is modified with heat, rather than glued in."

Language is living, and changes with use (or misuse).

I thought it useful to have two words - one for forms used in lamination, and a different word used for forms used for heat correction. The fact is, both words could be used interchangeably (going by the secondary definition from Webster's quoted above, and the French and German origin words).

A separate definition is probably only useful to us bowyers...

Ken
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tqpotia
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May 2nd, 2007, 8:01 am #18

Thanks, Ken for explainations and for definitions.

Steve, your steam chamber sounds interesting. I`m sure Ken would be happy to help you to make a tutorial about it.

Regards,
Kosio
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toxophileken
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March 1st, 2016, 4:43 am #19

Apologies that the photos have disappeared. The photo host, ImageShack, seems to be down or to have lost the photos, or both. Not sure when I'll have time to put the photos back up.

Ken
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