A Chat about Chatter
(or, the divingboard problem)
Chatter is one of those mysterious subjects that gets skipped over in many books of metalsmithing. In its loosest form, ‘chatter’ simply refers to the way a cutting tool can skip or ‘chatter’ across a piece of metal if either the metal or the tool is not supported rigidly enough. The most familiar example to many metalsmiths is probably the way a coarse steel cutting burr can suddenly start vibrating when doing heavy cutting, and leave a cut mark with ‘waves’ in the pattern, or can then suddenly grab and go skating across the metal. Another common example is the way that sheet metal will vibrate violently during piercing with a jeweler’s saw. The farther away the metal is from the support of the bench pin, the worse the vibration becomes. These are both examples of ‘chatter’. There are many more, but these are probably the most familiar.
At base, they are all issues rising from the fact that even the hardest metals are slightly flexible. The farther away a cutting edge gets from rigid support, the more the tool will bend. The elastic bending will cause the cutting tool to chatter and grab, rather than cutting evenly. Think of it like a diving board: we’ve all had the experience of walking out on the board. The farther away we get from the support of the base, the more the board bounces as we move, even though we haven’t grown any heavier, or moved any more violently. The principle is exactly the same with metal, both cutting bits and metal being worked. If you’re trying to pierce or file a piece of metal, the farther the work area is from the support of the bench pin, the more it can flex, causing chatter and an uneven cut. With any kind of cutting bit, either a burr or a drillbit, the farther away the cutting edge is from the support of the chuck, the more it can flex, leading to either breakage or rough, inefficient cutting.
The cure is very simple: always minimize overhang, in anything. Support workpieces against a rigid support as closely to the work area as possible, and always grab burrs or drills as close to the working tip as possible. Always try to mount the handpiece or chuck as rigidly as possible. With practice, it will become second nature to contrive as much rigid support as possible for any process.
A couple of notes about drillbits and burrs:
Always grab drillbits as close to the tip as possible without grabbing the flutes. (the spirals) They will cause the chuck to grab unevenly. So grab right at the last bit of smooth shank before the spirals start. Believe it or not, drillbits come in several lengths. The bits one normally sees are called “Jobber’s length” bits. They’re a compromise length: long enough to do most jobs, but short enough to be faintly rigid. For jewelry work, we’re rarely drilling through inches of metal, so all we need are short bits. These are called “Screw Machine” bits. They’re a little harder to come by, but MSC and Enco both have them. They’re shorter than normal bits, and therefore, much more rigid. In the smaller sizes, this makes them much less likely to break. Definitely worth buying if you’re going to order bits. In a related issue: small bits are very flexible. This makes it very easy for them to start drilling off-center to the axis of the drillpress. This makes it imperative to make a centering mark (center punch) to start them, or they’re liable to wander across the surface until they pick their own place to start, making it very likely that they’ll break.
Three different 1/8” drill bits. The top one is an extra-long aircraft length drill, the middle is a standard jobber’s length bit, and the bottom is a screw machine bit. Which one is going to be the stiffest?
Flex shaft burrs are another issue: they all have a standard length regardless of the size of the cutter on the end, and they’re usually all 3/16” diameter shafts, again regardless of the nature of the cutter on the end. This means that with most handpieces, you can’t get the teeth of the chuck all the way up against the end of the cutter, leaving a fairly long section of unsupported shaft hanging out of the chuck to bounce around. The cure for this is to use a cutoff wheel, and trim away the bottom 1/2” or so of the bottom of the shaft, so that you can get the cutter down against the teeth of the chuck. There will still be plenty of shaft left over to allow you to stick the cutter out in front of the handpiece to any practical length.
A pair of large cylinder burrs for milling wax.
Note how small the shafts are in relation to the diameter of the cutter.
Used at their stock length, these burrs chatter like banshees.
Cut down to a size that allows better support of the cutter, they work beautifully.
The cylinder burrs mounted in a flex shaft.
Note how far the untrimmed burr sticks out from the jaws of the chuck, and the thin little shank.
Does it surprise you that the burr chatters? The trimmed bur can be supported much more rigidly in the chuck.
Note the widening of the shaft near the cutter head. Make sure not to grab that with the chuck. There’s not enough of it to hang onto securely.