In my last column, I promised to write about some uncommon drill bits this time, so here goes.

The first is called an underdigger or eccentric bit, also sometimes referred to as an underdog. (How it got this name is beyond me, and really makes no difference.) The cross section of an underdigger bit looks more like a T than the bent H section of a regular bit. You only dress it on the wide side of the T, and the bottom of the tall side is finished at an angle of around 30 to 45 degrees. This bit, by design, goes down through casing and, when it encounters drillable formations, the dressed-out portion is forced into the wall of the drill hole. If properly dressed, it makes a hole larger than the diameter of the couplings and drive shoe on the casing.

This bit is designed to run below the casing and will not increase the size of an already drilled hole. If you’ve already drilled some hole below the drive shoe, you’ll need to backfill with hard material, like gravel or perhaps coal, so that the pilot or angle side of the bit will push out to full diameter. Due to their construction, these bits weigh a little less than a regular drill bit. They range in size from 4- to 12-inch. At least one manufacturer in catalogs I’ve seen lists only one length: 6 feet.

In hard going, I understand this bit drills about half as fast as a regular bit due to its having only one cutting surface. You can heat and forge an underdigger to keep gauge, but would probably find it easier to use build-up rod and hard-facing with a welder than to forge the bit out. I have never run one of these bits, but people tell me it should never, ever be used inside the casing. The cutting edge could get forced into the wall of the casing, especially the joint where the casing and drive shoe come together. I can visualize this happening and see how it could cause major damage to the lower portion of the casing.

 While I never ran one of these bits, my father and I did modify a regular bit to make it an underdigger-like bit. We simply cut off the expanded or gauged-out part of the bit on one side so the blade ran against the casing or the wall of the hole. We dressed out the opposite side to about double the regular amount, so it would still go down the casing but would tend to enlarge the hole below it. This was somewhat effective, but not nearly as much as a true underdigger. We didn’t use this bit very much, but modifying a regular bit was a cheap way to make our own version for when we did need it.

 Next, I want to talk about the solid reamer — sometimes called a round reamer. While not technically a bit, it gets manufactured much like a regular bit but with two round sides and a very small water course. This “bit” straightens out flat or crooked portions of an already drilled hole, removing projections into the wall. It can also ream a hole from top to bottom to increase its size. (I believe this would only work well in non-caving conditions.) One manufacturer recommends using drilling jars above the drill stem when running this solid reamer. These reamers range from 4- to 10-inch (and perhaps larger) and are considerably heavier than a regular drill bit. I have never run one of these, but I can recall occasions where it would have proved useful. I understand the cutting edge or face of the solid reamer is difficult to forge and keep sharp.

 I know of another type of reamer called a hollow reamer. Like its cousin the solid reamer, the hollow reamer can enlarge a hole and also straighten tools leaning against the side of a large hole. I have never used one of these — or even seen one — so perhaps some of you readers know more about this than I do. It see it listed in some drill tool catalogs.

 Lastly, I want to talk about the fabricated star bit. The cross section of this bit looks like the four-wing bit I wrote about last time, but the wings are much longer. These bits drill really large holes, and I have heard of these bits as large as 60-inches in diameter. I have seen fabricated star bits 36-inches in diameter in the yard of a contractor friend of mine. These bits are fabricated with the wings welded to a center round mandrel. Sometimes, they might have a pilot section below the cutting faces to enlarge an already drilled hole. Due to their weight, these bits are made to order to the capacity of the drill rig on which they will run. They do have a regular tapered API joint to attach to the drill stem. Again, I have never run one of these but they certainly qualify as a very special bit.

I believe I have covered every type from the common to the unusual, so this ends my discussion of drill bits. Next time, I will discuss other tools you need in a cable-tool operation — and they are as numerous as they are necessary. I may even get into fishing tools later on (and they are, sadly, very necessary).

I write this in early January 2022 and hope you have had a successful holiday season and not caught the dreaded Covid-19. Like a typical January in Michigan, we have some snow on the ground and everything is white. Temperatures have ranged from 3 degrees Fahrenheit to near 32 degrees. I expect more snow on the way. I hope you don’t get blasted with some of these heavy storms we seem to be having in the U.S. By the time you read this, 2022 will be well underway and I hope your operation is going well. ’Til next time, work hard, by all means work safe, and remember to enjoy life on occasion.

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For more John Schmitt columns, visit www.thedriller.com/schmitt.