Making a custom cutter for a Parisian-eye ring

A necessary piece of equipment to tackle an uncommon problem in bow repair

Bow maker based in Boston, MA, US

TRADE SECRETS

Makers reveal their special techniques

Replacing worn-out mother-of-pearl eyes on old frogs is a near-daily task for many bow makers. But the replacement of just the metal ring when the pearl eye is intact is a rarer operation. The ring often needs replacing because its channel in the ebony is too shallow to secure the ring in place properly. Degradation of the glue joint, rigorous handling and seasonal shifting of the ebony work the ring loose, and it is often lost completely. In the frog shown to the right, half the ring is paper-thin and lifted out of the channel. We have to deepen the shallow end of the ring channel to accept a replacement ring of sufficient height. To do this, we have to make a bespoke cutter that will cut the bottom of the channel deeper but not touch the sides, which would alter the eye’s overall aesthetic.

Diagram of Parisian eye dimensions

Samples of tubing

ALL PHOTOS ERIC LANE

1Before I begin, I take accurate measurements of the ring’s outside diameter (OD), inside diameter (ID), and the metal’s wall thickness. Digital callipers make quick work of measuring ID and OD, and various-sized micro-drill bits, inserted shank-end into the channel, can help determine wall thickness in the absence of a measurable ring.

I then check my stock of brass tubing to see if I have a match with either the ring’s OD or ID.

Hopefully I will have a tube with at least one of these dimensions spot on. Then I can adjust the tube where it differs to achieve the proper cut. For example, a tube with an extra-thick wall can be machined on the lathe to a proper OD and/or ID, and a thin wall can have the set of the teeth adjusted to make a wider cut. The chosen tube is then face-cut (the tube’s end is cut on a lathe to be completely square to the tube body) and a one-inch section of the faced end is removed and deburred.

Facing and cutting

2 I begin forming the teeth by securing the cutter, faced-end out, in a drill chuck and then clamping the chuck in a vice. I proceed to make a series of cuts, which angle slightly leftward, into the wall of the tube at regular intervals. The particulars of the cut aren’t critical; around 2mm deep at intervals of about 2–3mm is adequate. Here, I use a jeweller’s saw fitted with a fine 4/0 blade, and apply very little downward pressure as I cut. Too coarse a blade or excessive pressure will likely snag the tube and distort it.

The tubing with the first saw cut...

...and with saw cuts all round

3 Once I’ve made my way all around the tube, I form the teeth using a triangular needle file. Lightness of hand is even more important on this step. I leave a little excess material at the tooth point for now.

Forming the teeth

4The triangular needle file is used again to clean up the cutting edge of the teeth. The rough edge of the filing is quite sharp enough for our purpose. I remove any internal and external burrs the filing has created, being mindful not to deform the delicate teeth. I then test the cutter in a piece of soft wood (to limit blunting) and compare this ring channel to the original.

Filing done

Cutter deburred

5 If the prior machining didn’t quite get me to the final dimensions, I may need to adjust the ‘set’ (angle) of the teeth to achieve the proper cut. The purpose of a saw’s set is to create clearance for the saw body as the teeth cut into a material; a channel cut slightly wider than the saw body is less likely to bind. I can use the set of my cutter’s teeth to adjust ID, OD and even wall thickness. In this case, I was luckier than usual in that I found a brass tube with a perfect OD dimension, an ID only 0.1mm larger than the ring’s, and a slightly thin wall thickness. So I decided to apply some inward set on every other tooth to lower the ID and increase wall thickness, but keep the tube’s original OD.

Bending the cutter teeth in the lathe

The cutter is tested in soft wood

6 The most consistent way to alter the set is to secure the cutter in my lathe chuck and use an old lathe bit in the cross slide to bend each tooth the same amount using the dial as a reference. Teeth can be bent in either direction this way. I always re-test the cutter in soft wood after each alteration. The second photo shows the change to the cut after adding the inward set.

The cutter and frog in the mill

7 I use my bench-top mill to deepen the ring channel as it offers me both secure holding of the frog and the precise positioning of the frog relative to the cutter. Remember, I only want to cut the shallow bottom of the channel. The cutter contacting the side walls of the channel will alter the dimensions of the eye and increase the possibility of ebony chip-out during the cut.

I angle my mill head to match the angle of the frog cheek at the eye. This will allow the cutter to deepen only the shallow portion of the ring channel.

The cutter has performed the task

8 Once I’ve triple-checked the positioning, I’m ready to cut. I am careful not to cut through to the mortise as this compromises the stability of the entire eye. A depth of 0.8mm is sufficient, but 1mm is better. Brushing clean the teeth of the cutter between shallow cuts is a wise approach.

With the ring channel sufficiently deep, I’m ready to go about fashioning the replacement ring from scratch. This process will be described in detail in a future Trade Secrets article.