Two Stage Bevel

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How and Why To Use a Two-Stage, or "Stair-step" Bevel
By Jess Nicholas

I am just a hobbyist driller, but I'm also a hobbyist driller with a long-standing issue concerning thumb taper and release comfort. Just recently, I have found a way to correct issues concerning hang-up at the top of the thumbhole (base of the thumb), which allows a bowler to gain more confidence in delivery shot-to-shot, as well as giving him or her some mistake room if they happen to knuckle or squeeze the ball too much on a particular shot.

The reason we would use this technique is twofold: It allows us to remove the 90-degree angle between the inside of the thumbhole and the shell of the ball, but also does so without running the risk of beveling the entire thumbhole into the shape of one large tapered cone. Bevel sanders can "cone out" the top of the hole if used improperly, while hand-filing the hole can do the same thing.
Using the two-stage or stair-step method, the pilot hole remains a perfect cylinder all the way to the bottom of the hole, while the very top of the hole gets a beveled gap so that the thumb can get a clean, somewhat early exit.

Here are the steps to do this:

1) Choose a pilot hole size. This is the size your thumbhole will be all the way to the bottom of the hole. It's probably what you're already using. For this particular example, we'll choose a 15/16 pilot. Drill the pilot hole to normal depth and at your normal pitches.
Drilling the pilot hole, 15/16 inches

2) If sticking is an issue, consider a vent hole. Vent holes are typically very small-diameter holes (5/32 is a common size) that help get rid of the vacuum in the thumbhole. It takes some getting used to, but if all equipment is drilled with vent holes, there is no longer the possibility that one ball will have a vacuum issue while another ball might not. A bowler can then use shape tape to get a snug fit. To drill a vent hole, I suggest going an inch to the right of the thumbhole between 3 o'clock and 4 o'clock, then setting the press at 7/8 forward pitch and rotating the ball in the jig so that we're drilling directly into the thumbhole. The vent hole will penetrate the thumb hole somewhere near a depth of 1.5 to 2 inches. You will need to acquire a long 5/32 (or comparable size) bit from a home improvement or hardware store.
5/32-inch vent hole. These drill bits are available at most hardware stores.

3) Once these two holes are drilled, remove the ball from the press and take it to the bench. There, you'll need at least one of two tools to help shape the thumb -- either a rasp, or a half-round rat-tail file. Many bowling pro shops and supply houses sell rasps (Master makes a very nice one), while woodworking supply houses sell files. Rasps will do the job more quickly, but they also can take too much material out of a ball if the user is not cautious. For that same reason, an inexperienced user can also unwittingly "cone" the thumb hole, taking out more material at the top of the hole than deep inside it. Also, a rasp can leave grooves inside the thumbhole that can be felt even after finishing with a bevel sander. For these reasons, a rat-tail file is recommended. For roughness, choose bastard cut or finer. Fine-tooth files can be useful as finishing tools if the user chooses to start with the rasp. Use these tools to oval the thumbhole to mimic your own thumb shape. Given hand angles, a righthander will typically be concentrating his work at 10 o'clock and 4 o'clock, but it varies bowler to bowler.
Half-round rat-tail file in bastard cut, available where woodworking tools are sold.

4) Now, take the ball back to the drill press. This is where we will make our second cut into the thumbhole. Replace the pilot hole bit with a bit at least 3/16 inch larger. For the purposes of this example, a 1 1/8-inch bit is used. Match the X and Y coordinates on the jig to the original pitches of the pilot hole.
Preparing to make the second cut

5) Center the bit exactly in the middle of the hole. Manually turning the drill shank should reveal that the bit will take the same amount of material away from the edge of the thumbhole on all sides. Finally, turn on the drill and go down into the ball anywhere from 1/16 to 1/4 inch. A depth of 1/8 inch seems to be preferable. Be careful not to cut too deep. In the second photo here, a ball-point pen is used to illustrate the depth of the second cut.
The second cut is done
A ball-point pen is used to illustrate the depth of the second cut.

6) Remove the ball from the press and return it to the bench, where a bevel sander will be used to finish the project. The product of choice for this job is an Ebonite product called an Abranet Cross. It resembles a sanding disc somewhat, but instead of a round disc, the cross is cut into four squares not unlike a weathervane or propeller. The material is also not sandpaper, but rather a material akin to drywall finishing screen. Turn on the bevel sander and de-burr the inside of the pilot hole to smooth out any ridges made by the rasp while ovaling the hole. What makes this part of the process work for us, however, is the 1/8-inch deep hole we cut with the larger drill bit. It creates a shelf of sorts where the Abranet Cross will ride. The end result is that the Abranet will take the edges off the two drill cuts without coning the top of the hole. Be careful not to spend too much time here -- a fresh Abranet Cross will take care of this job in just a second or two. If you feel the edges of the holes are still too sharp, hit them with the Abranet until things feel right to you.
An Ebonite Abranet Cross.
Using a bevel sander and Abranet Cross to complete the job.

What you're after here is the feel of a smooth "step" under the base of your thumb when the ball is situated on your hand. It will feel somewhat strange holding the ball, but when throwing the ball, the bowler shouldn't feel anything uncomfortable. The 3/16 inch of material we removed from the top of the hole will serve to quicken the release by keeping ball material away from the base of the thumb, further helping to reduce any vacuum that might exist there, but the pilot hole diameter remains unmolested.
The finished product.