Dog hole shrinkage

[mouppe]

I cannot believe what I discovered yesterday. I built a workbench over the summer, the top consists of a 1 1/2" thick maple board laminate. I drilled 3/4" dog holes with a new forstner bit in a 6" row along each side of the bench, including the ends. The holes are perfectly clean with no chipping. Yesterday I tried to use my new brass bench dogs from Lee Valley...and they would not fit! Using digital calipers I ascertained that the holes have shrunk in diameter by roughly 1 millimetre or 1/32" which is amazing. This is akin to a 4% shrink...but across a hole!

I used the same bit on an MDF panel and the dogs work fine.

I am resolving the problem by gluing a piece of sandpaper to a dowel and inserting it in my drill, thereby enlarging the hole.

But can seasonal changes really move wood this much?

 

[peter halle]

The short answer is yes.  You may also find that your hole is actually not circular anymore - it will be more of an oval.

 

[mouppe]

Well thanks, Peter, you live and learn.

 

[Nigel]

Can you not just run the drill bit through again?

 

[AlexThePalex]

But can seasonal changes really move wood this much?

The short answer: yes. 1 mm is nothing. Considering it was a hole so the change came from two sides making the wood expansion just 0,5 mm which is even half of nothing.

 

[mouppe]

Nigel, I cannot run the Forstner bit through again as there is nothing for it to centre on.

I built tolerances into the workbench everywhere else, but I just did not anticipate the dog holes shrinking. I have not read of this anywhere before and I really tried to read up on building workbenches before I designed and built mine.

Attached is a picture of the shrinkage along the side of the bench with two quarters resting in the slot. I took this expansion and contraction into account when drilling holes for the bench bolts.

Well I am relatively new to woodwork and am learning all the time. this has been a valuable lesson with no real harm done.

Thanks for the responses.

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[Nigel]

Hi Mouppe,
                The centre point on a Forstner is only to locate it.The bit is guided by the outer rim.So if you can locate the bit on the hole you may be able to drill.The problem with this bit is that due to the rim guide it can wander.Best used in a pillar drill if possible to bore  perfectly straight holes.You would have to hand drill but it may work ok.Good luck.Nigel.

 

[danjames]

Mouppe,

To redrill the holes with the fostner bit simply drill the hole in a bit of plywood or something first then double side tape or clamp it with your hole in your ply directly over the hole that you want to adjust, this will act as a guide, and your good to go.

I use this method also when i need to take a wooden plug out and replace it accuratly.

Thanks - Dan James.