Woodworking in a Tiny Shop: Centre Bits

At a recent show of the PAST Tool Collectors, I bought several centre bits. I'd been curious about these for a long time and wanted to see what they're about. And the price was right - 12 bits for $12!

Before anybody gets all worked up, I'm using the British spelling of "centre" just because it's how I've seen it written most of the time. And because it makes me giddy. I don't know this for sure, but I suspect that these bits were not used as much in America as they were in England. Please correct me if you know otherwise.

The line-up of centre bits.

Note that the holes that these bits cut are not (or are no longer) standard sizes. I wonder if they ever were. The sizes (in 16ths) and makers (with all markings) are, from left to right:

4 1/2 / 16 No markings
5 / 16 No markings
6.5 / 16 R.M. DIXON / HERMITAGE WORKS / SHEFFIELD
9+ / 16 W. CHANCE & SON
9.5 / 16 T.E. WELLS & CO. / CAST STEEL
13.5 / 16 E. COOPER / IRONMONGER / CHESTER
15.5 / 16 No markings
15.5 / 16 W. MARPLES & SONS (with flower? or clover? logo)
18 / 16 R.M. DIXON / HERMITAGE WORKS / SHEFFIELD
19 / 16 J. ASKHAM (this may be an owner's mark)
20+ / 16 No maker mark, but "AR" and "1 1/8" possibly stamped by owner
21 / 16 T.E. WELLS & CO. / CAST STEEL

In his writing for "The Woodworker", Charles Hayward notes that these bits are best suited for shallow holes. Especially when boring into end grain, the center point can follow the workpiece's grain, leading to a curved or crooked hole. That problem was solved when spiral auger bits came around, as once the perimeter of the spiral section enters the hole, it keeps the bit from wandering.

Shannon Rogers did a nice video on centre bits that tells a more complete story, but I'll mention a few things here. First, lets talk geometry. The following pic shows a typical bit.

Cutting lip at left, center point, and spur at right

For these bits to work properly, the cutting spur has to be longer (closer to the wood) than the cutting lip is so that it scores the wood before the cutting lip starts removing the bulk of the waste. In this picture, the spur is up to a line on the paper, but the lip is a few millimeters lower.

This pic shows a bit with cutting lip equal to the spur.
This bit will not cut a clean hole as is, so I had to file the lip down a bit.

Here, you can see the spur has scored the perimeter of the hole.

As the bit goes deeper, the cutting lip engages to remove the waste.

Sometimes you get a nice spiral shaving

It's easy to sharpen these bits, but it's also easy to sharpen these bits incorrectly. It seems like most bits in the wild had not been sharpened properly at some point in their lives. Lets start with the cutting lip.

The following picture shows a cutting lip from the side. There is quite an angle on the underside of the lip (top side in the orientation shown). This clearance angle assures that the leading edge of the lip is what cuts the wood. Many centre bits are sharpened incorrectly by filing the underside in such a way that the leading edge can't cut the wood. If you were to file in the direction my fingers are pointing, you might get a sharp edge, but you would also create a new bevel on the underside and the heel of that bevel would contact the wood before the cutting edge can, rendering the bit useless. One should only file the top side of the lip, with only slight work on the underside to remove a burr.

Note the angle on the cutting lip

The correct thing to do is to file the top of the lip.

Red marker on the top of the lip will help gauge progress

Red marker removed, feel for burr on underside,
remove the burr and the lip is sharp

Now take a look at the center point. It is actually a three sided awl, with each corner a cutting edge.

In this close-up, you can see two facets of the center point

In Shannon's post about these bits, Bob Rozieski commented that when these center points are not sharpened evenly on all facets, you can change the diameter of the hole that the bit cuts. If you think about it, it's the lateral distance between the center point and the cutting spur that determines the hole diameter. Some of mine were cutting holes pretty far off of standard sizes. I'll have to take a look at the center points to see if anything more can be done to get them dialed in to cut proper diameters.

Here is a picture of the other side of a bit, showing the third facet of the center point.

This third facet started out in plane with the rest of the back of the bit

I noticed that on most, if not all of my bits, this back facet is slightly convex along its length. That probably puts the very point more in line with the central axis through the bulk of the bit. My thinking is that would result in a truer hole.

The last part to sharpen is the cutting spur, seen at left in the previous picture. Its leading edge is towards us in the picture, and its top point needs to be filed to a sharp cutting geometry, without reducing its length. Too many bits have been ruined by sharpening the spur poorly. Never file the outside of the spur, as that will change the diameter that the bit cuts. Only file the inside concave curved area, but be careful not to go so far as to reduce the length of the spur. Small round files or sandpaper wrapped around a round (cylindrical) object work well for this.

One thing I'm not sure about is an aspect of the cutting lip. Shannon reports that the outside of the lip starts cutting before the inside does, thereby creating a slightly domed surface in the hole you're cutting. The bits I have are all over the map on this point.

Note the difference in angles of the cutting lips.

The bit at left would cut as Shannon states, with the outside of the lip cutting first. But the bit at the right would have the whole lip cutting at the same time, leaving a flat bottom. Did different manufacturers make them with different geometries? I don't know. Did they all start out looking like the one on the left and then some get filed improperly to look like the one on the right? I don't know.

That ends my treatise on centre bits. I don't know if I'll pick them up instead of my auger bits. But if I need a shallow hole and don't have to get an exact diameter, these might just be the ticket. I'll end with pictures of the maker stamps.