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Cheval

kari
Author: Kari Azure

A cheval is a form of wood replacement. Often, due to a bow frog’s small size, an entire section of wood has to be replaced with new wood to reinforce the structure and to supplement missing wood. Traditionally, a cheval is performed by planing away original material and replacing it with new wood. A rounded cheval maximizes the surface area for gluing, minimizes stress by dispersing the seam of new to old wood, minimizes the amount of original material that needs to be removed by using the precision of a mill, and creates a feather edge that visually blends old and new wood.

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Original highly damaged frog.
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Two chevals in progress: one glued and one milled.
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Two chevals glued and in the process of being trimmed down.
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Two chevals along slide surface.
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Two finished chevals.

MATERIALS AND TOOLS

– a split out piece of replacement ebony with same qualities of color, porousness, and appropriate size
– high-quality calipers
– alcohol
– good-quality, non-stretching transparent packing tape
– dental tape if the metal parts have to be removed
– chalk, pencil, and scribe point
– loupe to see medullary rays in ebony
– various clamps and cauls
-lathe and mill
4 flute ball-end mill with a diameter appropriate for area needing replacement (10mm is probably the most used ball-end mill here at the shop)
– mineral spirits
– closed cell rubber
high-strength hide glue – 379 grade
– formaldehyde [1]
– safety measures for handling formaldehyde – full-face respirator mask, nitrile gloves, etc.
– q-tips
– carving tools, including a knife with a sharp tip, chisels, and files
– 220, 320, 600, 2000 grit sandpaper
– steel wool (extra fine and course #3)
– nitric acid
– dental compound – optional

ASSESSMENT

Two instances of chevals needed because of missing wood:

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Missing wood along underslide.
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If necessary, glue all available pieces back together.
Take detailed measurements. If necessary, research appropriate proportions with finished frog in mind.

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Identifying Medullary Rays and Preparing the Frog for Milling

Identify the orientation of the medullary rays. They look very similar to grain lines, and the wrong orientation can cause the cheval to separate from the original wood.

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Medullary rays running out on the thumb projection. This is a great area to look for orientation if it is available.

Use chalk to highlight area that absolutely needs to be removed to create a feather edge.

Cover the frog with packing tape. Especially the area where you will be removing wood. Trim the excess tape near the area to be cut so that it doesn’t get caught up in the mill which could tear off the feather edge. The tape should at least cover the damaged area to help maintain a feather edge when milling.

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Here you can see the faint chalk line underneath the tape that indicates the area that needs to be removed near the underslide.
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Find a ball-end mill that is as small in diameter to cover necessary area to maximize gluing surface while still getting damaged area. Set it aside. We will come to it after the frog is oriented. If you anticipate the original material to get really thin, as in the thumb projection in the photo below, add some dental compound to support the delicate area.

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Dental compound added to support a thin thumb projection.

Setting Up the Mill

The 3 way tilting machine vise is really great for this part as it has a full range of motion for frog. Support frog using curved wood cauls so underslide area isn’t crushed. Use a mill with a fine point (in this case a broken .5 mm end mill that works perfectly because of the precision) and without the machine running, run the frog back and forth until it is adjusted for a straight back, level (if that’s what you’re going for), and direct to the damaged area to maximize the feather edge.

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Checking the orientation of the frog.
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Another way to set up for a cheval is to use the lathe:
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Possible setup using the lathe.
Follow safety precautions by tying back hair, wearing safety glasses, and be alert.
Mill down in small increments (a good range to start with is 1/10 of a millimeter). Taking too deep of a cut might cause more damage. Keep an eye on the tape. Carefully trim off any excessive amounts with fine scissors or a sharp knife. Sometimes securing some of the tape back with masking tape will work. If there is a chalk line, watch for it to barely just disappear.
Making fresh glue about now would be good for time management.

Replacement Ebony Dowel

Use the lathe to turn down a dowel of matching wood. Use calipers to be sure the dowel is the same size as the ball-end mill  before taking ebony out of the lathe. Sometimes finishing it with a file and sandpaper is a little easier. Keep an eye on discrepancies in diameter of the dowel through the length of the dowel.
Mark out medullary rays on the end of the dowel.
Line up medullary rays by rotating dowel.
Put a mark or two on the dowel and frog to help with lining up when gluing.
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Medullary rays are visible on the surface my thumb is on.
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Medullary rays running almost parallel to slide and underslide surfaces marked out along with some lineup marks.
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Medullary rays running through the bottom of the cut.

Gluing the Cheval

Prepare the glue joints by carefully cleaning the dowel and original wood with a paper-towel wet with alcohol. Have a few clean and damp paper-towels on hand to clean up messes.

Do a dry run first to help figure out the best clamping situation, especially through supporting the feather edge with tape covered closed cell rubber. See photos below for clamping examples.

Use cauls to protect the original wood and/or underslide area if necessary. Stick the cauls to the frog with double-sided tape if it will help the gluing situation.
Spread glue on the old wood first, as it is less susceptible to changes due to moisture, and then put glue on the dowel and clamp the cheval to get the most structurally sound fit. Then clamp the closed cell rubber onto the feather edge for support .
Glue with high strength hide glue: 379.
A variety of clamping examples:
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Let the glue dry overnight.
Remove the clamps. Carefully remove the tape with mineral spirits.
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Treating the Glue Joints with Formaldehyde

Treat the glue joints with formaldehyde using a q-tip. It changes the cellular structure so that the hide glue isn’t water soluble. Use whatever safety precautions possible: ventilation hood, full respirator mask, and nitrile gloves.

Trimming Down the Cheval

Use the initial measurements and the metal pieces taken off the frog for reference to cut down dowel replacement. I like to wear nitrile gloves and a dust mask while dealing with the ebony that has been treated with formaldehyde.

Finish the piece with different types of abrasives. Sometimes applying a little nitric acid to the new piece can make the new wood match the old wood by adding a punkiness or softness to the wood. Applying shoe polish and/or a little magic marker can help transition the new wood to old by masking the change until a little wear takes care of the rest. Sometimes a grain line might run out in either the old or new piece and extending the line just a bit with a tip of a sharp knife can help lead the eye past the joint.

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Missing wood along underslide of frog – before.
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Cheval along underslide of frog – after.

 

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Cheval before trimming down.
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Cheval on thumb-side of frog along underslide – after.
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Wood selection can be your best friend or worst enemy.

 

1. It has been suggested to try Glutaraldehyde instead of formaldehyde. As of the writing of this article, we have yet to try it out.

Email Kari at kari@tristrings.com if you have any questions or comments.