Can somone explain this?

[campzeke]

After drilling a cocobolo blank with a 12.5mm drill bit, the tube will slide in about 1/2 way from one end and about 3/4 way from the opposite end. Since 12.5mm is a fairly large bit, I first drilled with a 7mm bit and chased that with a 3/8" bit and ended with the 12.5mm bit. This is the second cocobolo blank I have had the same problem with. After a round file around the inside of the blank the tube slides in fine. This afternoon I drilled another hardwood blank using the same process and the tube slipped in perfectly.

Any ideas????

 

[KenV]

Two reasons i have seen

1. Moisture is not stable in the wood and the drilling allowed moisture related movement

2. Stress in the wood was changed by the drilling and the wood moved.


The second is fairly common with reaction wood from branches.

 

[Jolly Red]

I have had similar results sometimes, and I think it is related to the seasoning of the blank. During drying, the blank is subjected to a number of stresses, and these can still be present when the blank is drilled, which can cause localized warping of the wood.
This could result in the hole becoming oval shaped in part of its length, which would give the results you have reported. A similar type of stress relief is sometimes seen while ripping a board, when the saw kerf will close up and pinch the blade. This is why table saws have a splitter to help prevent the binding.

Letting the blank sit for a few days after drilling, then re-drilling should make the blank usable.

This is my take on it, I am sure some one else will have a different idea.

Tom Wilson

 

[CREID]

Im with KenV

 

[Rolandranch]

It could also have to do with the diagonal grain of the wood (if it is diagonal cut). I have this problem a lot when doing layered segmenting.

 

[campzeke]

Thanks for the feedback. I had considered the stress and drying possibilities. I did buy both blanks from the same vendor at the same time but the confusing part is, the blanks were cut for a 2 piece pen and only the upper piece had this problem. The lower sections were fine. Oh well.... Stranger things have happened.

 

[More4dan]

It could be more heat generated in the larger hole expanding the wood while drilling and shrinking back after it cools. 2/3 in is where I sometimes have trouble with acrylics if I'm not careful. Sharpen the bit and reverse often works for me.

Danny

 

[br0warren]

A brad point bit is not necessarily the best bit for chasing a hole and can make egg shaped holes depending on the grain and density of the wood.

The brad point is what keeps the bit centered and the hole round. On subsequent passes, the point is floating, and the hole can drift depending on what density wood the nibs are hitting.

It may pay to use a nice sharp twist drill chucked up as short as possible to reduce bit flex. Also make sure all tailstock adjustments (if drilling on the lathe) is locked and the chuck is in snug. In a drill press, use a good holder or V block.. Align with the existing bit and hole, lock the work down, and then drill the enlarged hole.