Replacing a Broken Tongue on a Violin Bow Frog

This article describes the step-by-step pro-cedures we use in our shop to repair abroken tongue on a violin (family) bow

frog. An example is shown in Fig. 1. Our tech-nique will produce a new tongue as strong as theoriginal and the repair will be essentially invisi-ble. When doing this repair a new cutter is madefor every frog, exactly matching the individualferrule. The cutter pictured in this article wasmade to fit a ferrule that had a uniform radius asthe rounded portion. This is rarely the case, sousually a more involved template needs to bemade to fit the new tongue. This is a very trickyprocedure and should only be attempted by aqualified person.

MATERIALS REQU IRED

• Ebony for the replacement tongue. Theebony should be matched and split fromwell-seasoned wood.

• Ebony for the template. The approximatedimensions of the ebony should be 4 mm x15 mm x 50 mm.

• O-1 drill rod for making the cutter.• A good quality, non-stretching packing tape

to wrap the frog.• Oil for quenching the cutter. We use motor

oil.• Wood for making gluing cauls.

J. Violin Soc. Am.: VSA Papers • Summer 2007 • Vol. XXI, No. 1

AbstractIllustrated step-by-step procedures are described for precision fabrication and fitting of a new ebony tongue into a vio-lin bow frog whose original tongue has been broken.

Replacing a Broken Tongue on a Violin Bow Frog

JERRY PASEWICZ

4911 Waters Edge Dr., Suite 200 H, Raleigh, NC [email protected]

www.TriangleStrings.com, www.NetRehair.com

Figure 1. Violin bow frog with a missing tongue.

VSAP Pasewicz 5/11/08 2:00 PM Page 1

http://www.tristrings.com

http://www.netrehair.com

mailto:[email protected]

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REPAIR PROCEDURES

Initial StepsThe first step when starting this repair is to takemeasurements. It is important to measure thewidth of the rails right behind the ferrule, as wellas the height and weight of the frog. The ferrule’sinside width and inside height should be notedfor use in making the cutter, as well as the silverthickness for use during the cutter setup. It isalso a good idea to take photographs of the orig-inal, paying particular attention to any missingchips, distortion, cracks, or wear.

A single layer of non-stretching packing tapeis applied to the entire frog to protect the fragileedges of the repair area, as well as to protect thefrog as a whole from glue squeeze out. The pack-ing tape should be carefully trimmed so as not toget in the way later. At this time, any remnants ofthe old tongue are removed.

Making the Cutter The cutter for this project is made from O-1 drillrod, an oil-hardened, non-shrinking, general-purpose tool steel available from any industrial

supply company. The first step is to make ablank. After cutting the rod to length, the mate-rial is turned to the appropriate diameter to fitthe collet of the milling machine. Then the headof the cutter is shaped so that the head matchesthe inside ferrule height and width measure-ments, with an additional 0.1 mm to allow forferrule fitting later (Fig. 2). The neck should besmaller than the mortise for the pearl slide. Oncethe blank is made, the shaping of the cutterbegins (Fig. 3). The cutter used in this projectneeded to have an exact radius, so we were ableto use a radius cutter. When the cutter does nothave a fixed radius, the blank is colored with apermanent marker and the shaping is done withfiles, paying attention not to remove the last lit-tle black of the marker representing the heightand width. The ferrule is used as a guide by hold-ing it under the cutter blank and comparing it tothe work in progress.

Next, cut the teeth. A 0.5-in.-dia. 60° dove-tail cutter is mounted in the lathe, and the tonguecutter blank is mounted in a milling attachmenton the lathe compound. The cross slide is used toslowly feed the tongue cutter blank into the

Figure 2. A tongue cutter blank is machined from a drill rod in a lathe.


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J. Violin Soc. Amer.: VSA Papers • Summer 2007 • Vol. XXI, No. 1

dovetail cutter to the appropriate depth. Themilling attachment is then lowered to the appro-priate height and the carriage is slowly movedaway from the chuck, creating a cutting surfaceon the blank (Fig. 4).

This is repeated four times to make the fourteeth of the tongue cutter. The steel is then heat-ed to 1450-1500°F and quenched in oil. Oncethe teeth have been slid across a polish stone, thecutter is complete, as shown in Fig 5.

Setting Up and Removing Material toAccept the New TongueThe frog is now clamped into the milling visewith the use of cauls to begin the setup for cut-ting. It is very important to know where the cut-ter is going before it gets there. Therefore, a dialindicator is used on the top and the two sides ofthe rails to determine the position of the frog asshown in Fig. 6. The dial indicator is set to con-tact the rail and the cross slide is moved in orout, taking note of the amount of deflectionshown on the dial. (Re-position the frog andtake another measurement.) This process is

repeated until the top of the rails shows nodeflection, and the two sides of the rails have thesame amount of deflection. This ensures that thecutter goes down the center of the frog and par-allel to the top of the rails.

Once the frog is positioned correctly in thevise, the cutter is clamped into the milling headand centered on the frog (Fig. 7). At this pointthe cutter is raised so that the top of the cutter isat the same level as your rails. In other words, ifa straight edge is laid on the rail from back tofront, the straight edge would also be resting onthe top of the cutter. Using the silver thicknessmeasurement from earlier, the cutter is adjusteddown. Allow for the fact that the ferrules on old-er bows sit slightly above the rail surface. Thesetup is ready to cut.

The cutter is backed away from the frog andthe mill speed is set at ~800 rpm. Using the crossslide, the cutter is slowly moved into the frogwhile being careful to avoid heat buildup (Fig.8). The goal is to mill until the cutter touches theback of the mortise. As the cutter goes in, therails will tend to spring out. This is normal.

Figure 3. With the cutter blank in a lathe, a radius cutter is used to machine a rounded portion.


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Figure 4. A dovetail cutter is used to make teeth in the new tongue cutter.


Figure 5. Custom-manufactured tongue cutter.

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Figure 6. A dial indicator is used to position the frog.


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Figure 8. Cutting the frog to accept a new tongue.

Figure 7. The cutter is aligned to the frog ready for machining.


Making the Template and Fitting the New TongueWith the cutter still in the milling machine andthe frog removed from the vise, it is time to makea template to fit the tongue (Fig. 9). A notch ismade in the 5-mm piece of ebony to accept theneck of your tongue cutter. The ebony is placedinto the vise and the cutter is run through it,making an outline of the frog cut.

If the rounded part of the ferrule has a circu-lar curvature, this is the only template that isneeded. If, however, the rounded part of the fer-rule does not have a fixed radius, the templateshould have this cut, a cut of half of the profile,and a cut of just the rounded portion of the fer-rule up to the flat (Fig. 10).

This template (not the frog) will be used tofit the tongue. For the bow worked on in thisarticle, the tongue was spun down on the lathe tothe correct radius, the round at the end was cutwith the radius cutter, and the template was usedto gauge how much of the tongue blank was tobe planed away (Figs. 11 and 12).

When the rounded part of the ferrule doesnot have a radial profile, the tongue is construct-ed a little differently. The ebony is first planed tothe correct thickness, the thickness of the origi-nal tongue cutter blank that represents the insideof the ferrule +0.1mm. After the ebony is planedto thickness, it is planed to the width of the cut-ter blank. While referring to the template, aseries of even, parallel chamfers are planed intowhat will be the rounded portion of the tongue.Smaller and smaller chamfers are blended untilthe tongue blank fits through the template.Using the half profile portion of the template,the end of the tongue is fitted. Figure 13 showsthe new fabricated tongue as it is progressivelyfitted into place in the frog.

Gluing the New Tongue and FinishingWhen the tongue is shaped to exactly match thefrog, it is glued in place. A gluing caul will beneeded that matches the underslide of the frog,as well as a flat caul for the rail side of the frog.The flat caul will be used to wrap the new tongue

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Figure 9. Tongue-fitting template for use with fixed-radius tongues.

Figure 10. Tongue-fitting template for a tongue with non-constant radius of curvature.


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Figure 11. Ebony dowel prior to being formed into a new tongue.

Figure 12. Using a plane, the ebony dowel is formed to fit the template.


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Figure 14. The new tongue is glued into the frog while being held under pressure in an appropriate fixture.

Figures 13. Fitting the new tongue into the frog.


area, so it needs to be of appropriate length.The frog is dry-fitted, so be sure your mea-

surements are correct. The tongue is then gluedusing a good epoxy designed for wood. Thetongue is wrapped with a rubber band to applypressure to the graft area (Fig. 14). Once theepoxy is cured, the slide and ferrule are fitted asnormal, and the project is complete. Figure 15

shows the violin bow frog before and after fit-ting the new tongue into place.

ACKNOWLEDGMENTS

The work shown in the illustrations was per-formed by Tom Goering.

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J. Violin Soc. Amer.: VSA Papers • Summer 2007 • Vol. XXI, No. 1

Figure 15. Violin bow frog before and after replacing the missing tongue.


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Replacing a Broken Tongue on a Violin Bow Frog