Tuesday, February 19, 2013

Lead Astray

Occasionally I delude myself that I've got a bit of a handle on this old tool business - and then I clean up some auger bits and confusion descends like a familiar cloud.

Auger bits are not simple. Auger bits are complicated little blighters. F'rinstance, early in your auger bit orientation you've probably come across the revelation that lead screws can come in coarse, medium and fine. You may have tripped and fallen in the quagmire that is the uncertainty over whether coarse lead screws are for soft woods and fine for hard woods, or... not. But it's not even as simply confusing as that. Take the lead screws on these two Cornelius Whitehouse and Sons Jennings pattern bits. At least I thought they were Jennings pattern, although they surely don't look like it in the picture... But that's not the point. #11 on the left, #10 on the right.

The lead screws appear to be much of a muchness, no? Yes, the one on the left needs a little attention, but that's not the point either. Well it is the point, but it's not the point.

Wrap a length of cotton thread round those screw points, why don't we? Top tip of the day there, incidentally, and possibly the only useful thing you may glean from this blog entry. Having wrestled and sworn over trying to trace the spirals with a marker in the past, it dawned on me that a piece of thread wrapped round the, um, thread, did the job in a fraction of the time and inconvenience. Unless it occurs to you that the nearest piece of thread is the loose one on your shirt and you start pulling, at which point your inconvenience quotient can increase dramatically. So don't do that. Anyway:

Can you see it? The cotton goes round every spiral on the left like a kiddie on a helter-skelter; it only goes round every other one on the right. Yes, the blighter on the right is a double thread. Cunning swine. Which means...

Well that's where I get confused. More confused. It means the thread pitch is coarser than it appears. I think. But the number of threads is doubled for better grip, maybe? If so, in what exactly? Which lead screw is preferred for what type of wood? Does it matter? Do I even need to know? Most books, however ancient, and the few catalogues I have are all oddly silent on lead screws and the differences in same, so maybe it's no big deal. But I can't help wanting to know, and so I wonder. In circles.

Around this point I feel a headache coming on and silliness ensues.

Oh come on, I can't have been the only one thinking it.


  1. These are the first auger bits I've seen without cutting spurs on them. How do they cut? In the picture it looks like the outside outside edge curve down. The double threads on the right maybe they were re-cut?

  2. Generally, I have heard to use the coarse pitch on softwoods and the fine on hardwoods.

  3. .

    I have accumulated a lot of Jennings pattern augers over the years and many of them are twin-threaded. They are predominantly the larger sizes, ½” and upwards. Having said that, none of my centre-bits are twins; they are all single, coarse thread for quick cutting softwood. I believe that twin threads were intended to give a straighter, more secure pulling action in hardwoods, less prone to mashing the entry point. They also cost more.... penny plain, tuppence fancy.

    R A Salaman notes the different types of auger points in his Dictionary, single, twin and brad point.

    As Ralph noted, the spurs seem to be absent from the pictured examples. If this is intentional, (rather than worn away or out of camera-view) then the augers are Scotch pattern. There should be a cutting face on the side of the auger flange in place of the spurs to define the sides of the hole – again I’m indebted to Salaman.

    All best from Wales

  4. They are Jennings pattern and their spur cutters are all present and correct - it's the camera angle wot is deceptive.

  5. The camera angle is a little deceiving, if you look at the lands at the cutting lip and again as the lands "rounding the corner" they're at equal thickness. At the base of the lead screw, the lands are wider. At first look, I also thought I didn't see the spurs.

    Clever photo, kept me awake! Thanks!

  6. The double thread causes the lead screw to turn twice as fast. It would take twice as many turns to make a hole of the same depth with the bit on the left as the one on the right. Drywall screws also use double threads to help installers be faster.

    1. If you're aiming for the same number of thread edges biting into the wood, then a double thread will take half as many turns to achieve, say, ten edges engaging in the wood, than a single thread. Viz: 5 vs. 10. Yes. But when just faster boring was the aim, manufacturers simply offered coarser threads, and even single spurs and cutting edges - such as the Irwin Speedbor. I suspect it's more of a juggle between speed, grip, and lack of clogging going on in lead screws.

  7. I've seen a lot of people pressing down on the palm button of their brace while trying to force the bit through the wood. Auger bits are designed to self feed. With a properly sharpened auger bit the palm button should only be needed to control the angle of the hole. I've found a few problems that stop the auger bit from self feeding.
    It has been my experience that the twin thread pilot screws tend to strip out easier than the single thread pilot screws. The twin thread screws have a relatively larger barrel which increases the loading on the thread. When drilling through narrower hardwood or near the end of a board I found drilling a pilot hole about half the diameter of the pilot screw lowers the load on the pilot. The wood doesn't split and the auger bit cuts easier.
    The most common problem I have found while rehabbing auger bits is the bit choking and pulling the pilot screw out of the wood. A splinter will wrap around the radial cutting edge. Acting like a wedge it will pull the pilot out of the wood. To prevent this make sure the pilot thread ends right at the inner edge of the radial cutting edge. The radial edge will then have a smooth transition for the chip to follow. The chip will be lifted and slide over the radial cutting edge without jamming.
    Lastly the auger bit may have been butchered. The bit may have been sharpened incorrectly by filing on the pilot side of the radial cutting edge. If the angle isn't maintained correctly in relation to the cutting edge it can end up too flat. The radial cutting edge is prevented from slicing through the wood and again jacks the pilot thread out of the wood. If the auger hasn't been completely ruined, judicious reshaping of the radial cutting edge can be done, making sure that the pilot screw to radial cutting edge timing is maintained.
    The metal that auger bits is made from is tough. The spurs can be rolled over by hitting a hard spot or stray bit of metal. Rolled over spurs can be cold formed with a light hammer and anvil. By tapping the edge the metal can be stretched and the spur brought back to near original shape.
    Only file the spur if necessary. A sharp spur cuts the outside edge of the wood grain and eases the work the radial cutting edge has to do. With the outside cut the chip will pop free and clear the hole much easier.


Owing to vast quantities of spam this blog is getting, I'm afraid only registered users can post. All comments are moderated before publication, so there may be some delay. My apologies.