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I bought the oak dowels supplied as 25mm x 300mm to fix through three oak sleepers total height 300mm.

I then bought a 25mm auger bit.

When I just checked the dowels with a calibrated digital caliper, the diameter came up as 25.4mm. Is that normal? Would I be able to hammer them in a 25mm diameter hole?

gnicko
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PeterM
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1 Answers1

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On a small scale wood is highly compressible, so the basic answer here is yes. As a joke goes, you might just need a bigger hammer ^_^

Is that normal?

Yes it's quite normal for shop-bought dowel to measure slightly off its nominal size.

In fact it might be accurate to say that dowel is rarely the stated diameter. It can be larger or smaller. Occasionally you'll even see dowel for sale that has already gone slightly ovoid — so, over dimension in one axis, and under dimension at right angles to this. Although this generally takes some time to happen (more than a few seasons) stock turnaround might be low enough that you'll see it on 'new' material.

Would I be able to hammer them in a 25mm diameter hole?

The basic answer is yes as stated at the outset. However, a further surprise might be in wait for you when you check the auger. Even drill bits are frequently not exactly the size stated!

If the auger does happen to measure as exactly 25mm, do note that drilled holes are always larger than the bit that made them. This is true even in hard materials like steel (!) and even when made in a very rigid setup like on a drill press/bench drill.

So, assuming your bit did happen to be exactly 25mm, the thing you think you're facing — hammering in a dowel to an undersize hole — won't actually be an issue.

Regardless of the exact fit, you do need to drive the dowel very deeply here, much deeper than would be commonly needed. And if the fit is tight the friction will get higher and higher the deeper you pound the dowel in. It is possible that you will even get to the point that you won't be able to drive the dowel in any further.

Glue?
If you intend to glue, note that some glues increase friction and others do not.

Foaming poly and epoxy are highly lubricative and don't produce 'grab'. They're also reliably 100% waterproof, which argues in their favour anyway.

PVA on the other hand can massively increase the friction, due in part to its high water content, and as a result could easily result in the dowel becoming impossible to insert further once in a certain depth.

No glue
If the plan is not to use glue, you can increase your chances of success by lubricating the dowel. You can do this with wax or oil.

Graphus
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    Timber framers will often put a handle on a chunk of log (10-12” diameter) and call it something burly like “The Persuader”. – Aloysius Defenestrate Oct 01 '23 at 14:32
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    @AloysiusDefenestrate, I was going to mention big mallets and even mauls were used in timber framing for driving in trunnels but it didn't make it through writing and editing ^_^ – Graphus Oct 01 '23 at 17:42
  • Would chilling the dowel help? – John Gordon Oct 02 '23 at 01:41
  • @JohnGordon, no, thermal expansion/contraction of wood is negligible. You can do a similar trick with wood using moisture content (MC). Wood shrinks as it dries, so you can in theory dry it further and then as it naturally absorbs water and its MC returns to normal it would go from a sliding fit to something much tighter. On a practical level one has to be careful with Dominos not getting damp for this reason, they can become hard (and according to some reports, impossible) to insert after a waterbased glue is applied because they're so close a fit to the matching-size mortise. – Graphus Oct 02 '23 at 04:54
  • @Graphus thermal expansion of wood is greater than that of steel (link includes oak), so it's not really α that's the issue. I've done this with steel. The issue is that the temperature range for wood is much more limited. We chilled a steel tube in liquid nitrogen to insert it into an undersize bore for a vacuum-tight seal; it still needed a big mallet and a strong arm. Such deep freezing isn't likely to do much good to wood, and you'd get loads of condensation on it, so getting damp as soon as you start. – Chris H Oct 02 '23 at 10:39
  • What drill power do I need to drill through 300mm (3 x 100mm) of oak sleepers with a silverline 25mm x 450mm auger bit? – PeterM Oct 02 '23 at 13:00
  • You'll have to try it and see. The bit might not be up to it for starters (maximise your chances by going slow, don't force it, evacuate chips regularly, and if necessary cool it in water periodically). But I certainly wouldn't try doing this with a cordless drill unless it was a particularly good one. FWIW if the sleepers are good and solid I would personally be doing this sort of drilling with a brace, or possibly even a Scotch auger since I happen to own a couple, having picked them up by accident along the way! [Cos, 'unlimited' torque.] – Graphus Oct 02 '23 at 18:25
  • @ChrisH, well in context steel's thermal expansion is also negligible, since we're not talking tolerances of ten thou or even a couple of thou :-) Wood's thermal movement is effectively taken to be zero virtually always, discounted entirely on the scale of normal work (same as longitudinal expansion/contraction due to changes in MC, even though it is actually not zero). – Graphus Oct 02 '23 at 18:34
  • @Graphus, well, having used the effect in steel I was curious (it's also well-established using heat in steel). I don't think it could be made to work in wood, even with the precision lathe we used to make the steel block, despite only needing around 1/5 the temperature difference with wood as we used with steel. And certainly not with a simple drill – Chris H Oct 02 '23 at 19:25
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    @ChrisH: Some people say the opposite: "While concrete and steel buildings are typically designed with expansion joints to account for thermal movement due to environmental temperature fluctuations, wood has a significantly lower coefficient of thermal expansion.". – David Cary Oct 20 '23 at 21:01
  • @DavidCary they're solid values, but how materials behave, particularly in combination, goes beyond just a number. Steel and concrete expand very differently; when they're fixed together, something has to give. You also need to consider thermal conductivity - steel heats through its volume much more readily than concrete or wood. Wood expands at different rates with/across the grain. This has implications for wooden construction & especially engineered wood products. – Chris H Oct 22 '23 at 20:15
  • ... Oddly enough, I'm currently in my van. When it gets hot (not now) gaps open between the MDF lining panels. If I fill those gaps with caulk on a hot day, the panels deform in the cold and strain at their screws – Chris H Oct 22 '23 at 20:17