Spontaneous expansion of seasoned hardwood

Factual Questions
1

Here’s something strange that happened to me today.
(I should start off by saying that I’m a reasonably experienced amateur woodworker).

I was making an object that required a long row of holes into which protruding dowels would be set. The workpiece was formed out of some nice pale pinkish-brown hardwood of unknown identity (reclaimed timber from a pallet carrying Japanese electronic goods).

Anyway, I drilled a row of 6mm holes, then gave them a very slight countersink, then spent about 5 minutes sanding the piece. When I came to insert the dowels, they were too big for the holes - in fact, their ends only just fit into the countersunk recess.
I thought I had used the wrong sized drill, but there it was, still on the bench, where I had placed it when I put in the countersink bit, and yes, it was a 6mm bit. It wouldn’t fit in the holes any more though - not even nearly. The holes were only about 4.5mm diameter.

Bereft of understanding, but needing to press on, I just fitted the 6mm drill again and drilled out the holes, again. The dril bit in sharply and removed little curly toenail-like shavings from each hole - there was hardly any countersink left now, but that wasn’t a problem.

So I started to fit the dowels - 42 of them in total - a fairly time-consuming task, as each one needed a thin smear of PVA glue before inserting and tapping home. It got harder and harder to insert each successive dowel, and toward the end of the job, I was having to apply such force that I feared splitting the timber and thought I might have to get the drill out again.

So here we seem to have a type of wood that expands measurably upon cutting. I’ve never heard of such a thing.

Some other pertinent facts:
-The dowels are precut manufactured things - they’re very uniform in size and they are indeed 6mm in diameter.
-I was using a proper 6mm dowelling drill with a lip and spur tip - designed to cut right to the edge of the hole, rather than push the fibres aside.
-There’s no chance I was mistaken about the initial hole size - I always test drill and dowel in a piece of scrap, also, it was the only drill bit on the bench.
-The timber should be quite stable - it’s been in storage in the corner of my (dry) garage for nearly a decade.
-Atmospheric conditions were not unusual at all - certainly not especially humid.

So… help me out here? has anyone heard of a hardwood that expands after cutting?

2

Oh, also, and most perplexingly, there was no obvious distortion of the holes - they didn’t shrink to ovals, they just got smaller.

3

Did the whole piece shrink? Or just the holes?

4

Difficult to say for sure, as the item itself wasn’t made to precise dimensions, but the rails in which the holes were drilled are only about 20mm wide by 15mm thick - there just doesn’t seem to be anywhere for a significant amount of material to have come from, to close the holes.

I do have some of this wood left though, and my daughter has proposed an experiment in which we drill holes and measure their diameter at intervals.

5

Post the answers…hubby is interested. He’s suggesting do some research to find out what kind of wood they were making crates out of then. But he’s mostly interested in why this is happening and to what degree.

6

I don’t think there’s any way I’d be able to research the source of the timber - too much time has passed and I doubt if records exist. I’m sure the species could be identified by an expert though, if necessary.

I’ll see if I can conduct the experiment tomorrow and document it with pictures - I’m thinking about drilling a 6mm hole and testing it by dropping in a very shallowly-tapered dowel to see where it sticks, then removing the dowel, waiting and trying again.

7

I’d also find out what kind of wood it was as well. That information is needed for the sake of thoroughness. Can you videotape the process so you can have more irrefutable proof of what happens?

8

Did the outside dimensions of your piece change as well, as if it was swelling up?

Also, how sharp is your bit, and how soft and fibrous is the wood? A dull bit *might * tend to push fibers into the hole wall instead of cutting them, and that the fibers then “decompressed” back out. If it’s a dense hardwood that you cut cleanly, I don’t get it.

9

I will document the experiment with photographs - I don’t have the facilities to make a good job of videotaping it.

10

I have never heard of something like this… What is the grain orientation to the face of the board? What is the humidity in your shop now? Are you near a black hole? :slight_smile:

11

Not noticeably, but as I say, the item was not manufactured to precise dimensions, so it could just have happened and escaped my attention.

It’s a lip and spur dowel drill bit - quite new and still quite sharp.

The wood is fairly soft (for a hardwood) and quite fibrous - quite prone to breakout when drilled right through, even when backed with a waste piece. - quite prone to splitting, therefore, I think. Possibly significant is that the volume of shavings produced when drilling seems greater than one might perhaps expect, but that’s a very subjective judgment and may not be for real.

I didn’t mention that the other thing I did with an offcut is to show my son how my small hand grooving plane worked - I cut a groove in one edge. The plane is now a tight fit in the slot it cut itself, but nowhere near the amount of expansion has happened in this case. Significant differences here are:
-OK, the plane isn’t a rotary tool, of course
-The plane cuts ‘cold’ - this wasn’t the case with the drill - the bit was too hot to touch after drilling a couple of holes
-The cut made by the plane is at 90 degrees axis to the drilled holes - the holes were drilled through the thickeness of the plank (at 90 degrees to the long grain); the slot was cut along the edge of the plank.

Here is a picture of a small piece of the timber - this piece is about 25mm wide. Colours not really accurate in the picture - IRL, it’s less yellowish and more pinky brown.

12

The grain is straight along the axis of the piece - viewing the endgrain, I can see that the board has been cut from the round so as to leave the growth rings running diagonally across the end. There is no discernable curvature to the growth rings, possibly indicating that this was cut from a log of large diameter, and nearer the outside than the centre.

13

One thing that comes to mind is that the board is somehow pre-stressed, So when you cut it you are relieving some of that.

14

I don’t know a lot about woodworking like do for metalworking, but I’ll give my 2 cents.

My best guess is that the wood underwent expansion around the holes due to residual (i.e. internal) stresses induced by the heat of drilling. I could find out more if I knew the type of wood in question.

15

Did the object you were making happen to see an attractive naked lady object at the time?

I only mention it because I’ve noticed that this sort of thing can cause my seasoned hardwood to expand spontaneously.

16

This is a possibility - on some of this reclaimed timber, it’s very hard to rip saw, because the kerf closes tight on the blade. Didn’t notice that with this piece though.

17

The wood kind of looks like lauan . It’s cheap, comes from the Philippines, and normally used for plywood.

18

Instead of me copying several paragraphs from a book I have, and assuming how much you really want to spend getting an answer to this question, I would suggest this very excellent book; “Wood Handbook”…Wood as an Engineering Material, from Algrove Publishing, ISBN 1-894572-54-8.
I use this as a reference book in the work I do. It is by far the most comprehensive book I have ever came across for its’ sheer amount of technical information about wood.
464 pages with a lot of technical information, which includes sections on; Shrinkage, Transverse and Volumetric, Longitudinal, and Moisture-Shrinkage Relationship".
I highly recommend this book to any woodworker!

19

My theory is the same as ElvisL1ves’. It seems very unusual to me that the drill bit would heat up that quickly. To me that suggests the bit is dull, or has been dulled by silica in the wood, which is typically higher in tropical hardwoods. If there’s crushing in the hole, rather than a clean slicing of the fibers, I can see the wood “rebounding” slightly. It wouldn’t take much rebound, on a percentage basis, before the hole was too tight for the dowel.

20

If you had a reliable supply of this wood it might be cool to use this characteristic to create tight joints.