The mystery of the expanding hole

[davidwalser]

I have a mystery I'm hoping you all will be able to help me solve. Yesterday evening, I prepped some bottle stopper blanks for turning. To do this I mount the square blank in my four jaw scroll chuck and drill a shallow recess in the tailstock end of the blank with a 1" Forstner bit held in a drill chuck mounted in the tailstock's quill. I then drill a 23/64" diameter hole about 5/8" deep. After preparing several blanks in this manner, I then mount my bottle stopper mandrel in the headstock, screw on one of the prepared blanks and start turning. (The mandrel I use is from Ruth Niles that incorporates a 3/8" X 16tpi tap for creating the threads. It's a great time saver.)

Anyway, here's the mystery: When I tried to mount one of the blanks I prepped last night on the mandrel, the mandrel's 3/8" stud/tap slipped loosely into the hole drilled by my 23/64" bit. The threads of the tap didn't even touch the sides of the hole when the blank was centered on the mandrel. Huh? This was the 2nd or 3rd blank I prepped out of five or six blanks. All the other blanks were drilled with the same bits (I never removed the bits from the drill chucks) and all the other blanks could be tapped properly. So, what happened? If it matters, the blank in question was African Blackwood (I think). The other blanks were from other exotics.

 

[Dale Miner]

One possibility, if your tailstock has some slop in the base that allows it to move from side to side, then perhaps the drill was not centered, and bored a hole with one side of the drill touching, the other side well away from the wall of the hole.

Another possbility, one of the flutes clogged (assuming twist drill), forcing the point of the drill to one side, same effect as above.

My last thought, the wood expanded from the heat of drilling, forcing itself into the hole, and the flutes of the drill kept reaming the expanding wood away as it expanded, creating more heat, expanding the wood into the hole, etc. After things cooled and perhaps dried some, the wood returned to the normal non expanded dimension, leaving a hole larger than the drill bit. If this is the cause, a sharp drill frequently cleared of shavings should reduce the oversize hole problem.

Later,
Dale M

 

[Bill Boehme]

The cause of your problem is a slight angular misalignment between the headstock centerline of rotation and the tailstock centerline. The remedy for the problem for this problem is to always drill from the headstock and not the tailstock.

Here is a brief explanation of the problem:
There is always some misalignment between the tailstock and headstock. That is a fact of life that can't be changed. The misalignment can be resolved into two components -- angular and radial. Radial misalignment is much easier to see -- it is what we address when we bring center points together from both the headstock and tailstock. However, we can have the center points touching perfectly, but not see the angular part of misalignment.

When angular misalignment is significantly bad, here are the two possible scenarios:

• The drill is mounted in in the tailstock and the wood is being held by a chuck in the headstock. The rotation axis of the wood is at an angle to the centerline of the drill and the resulting hole has a slight cone shape. A similar situation would occur if a bent drill were installed in a drill press and a hole were drilled into a clamped down piece of wood. Similarly, the same thing would occur if the drill was not properly aligned in the drill chuck before it was tightened down. This also explains why you will often hear a lot of squealing noise and smoke when drilling from the tailstock end with a large drill -- the drill is constantly binding and doing something that it is not intended to do - cut on the sidewalls of the drill.
• The drill is mounted in in the headstock and the wood is being held by a chuck in the tailstock. The rotation axis of the drill is at an angle to the centerline of the wood and the resulting hole is cylindrical, but at a slight angle to the centerline of the wood. The angular misalignment is very small and for woodturning it would be unnoticeable (precision metal machining would be a different matter).

Which is better? Most of the woodturners that I know are right brained do not appreciate the difference and think that left brained engineers are crazy. I would prefer to drill a cylindrical hole than a conical hole if it going to be tapped for screw threads. What I would actually prefer to do is fix the angular misalignment in my lathe, but having a lathe where both tailstock and headstock move and tolerances that are loosey-goosey, that is not much more than a pipe dream.

Unfortunately none of the woodturning lathes that I know of have a good alignment adjustment to permanently correct for either component of misalignment. There is at least the capability for some radial adjustment when both headstock and tailstock are locked down and the headstock and tailstock points are brought together. However, there are absolutely no provisions for angular misalignment corrections. The good news is that angular misalignment is not a problem except when drilling -- and when drilling that problem is minimized by drilling from the headstock end.

 

[MichaelMouse]

Wood contracts as it dries. Perhaps the inside of this dense stuff was still pretty wet. Given that others prepared similarly were fine, I would say to examine what made the one different. Setup produced so many suitable, I'm going to doubt it. Easy answer is moisture.

 

[john lucas]

Did you unpack them from a box. It's a well known fact that anything you unpack from a box will never go back into the same box. Maybe holes drilled in wood are the same thing.
Seriously though I like MM's suggestion. Many of the woods we buy online are wax covered and vary in moisture content a lot.

 

[Finn Posner]

The expanding hole

So Michael if you think it's better to drill from the headstock end, how do you hold the object on the tailstock?

 

[Paul M. Kaplowitz]

Using the lathe to drill

I have never liked using the lathe for drilling. I find I cannot get the precision I can with a drill press.

 

[Dave Kartzman]

I like Dale and Bill's explanations. I turn bottle stoppers every week to give to my clients when I visit. I use Ruth Nile's mandrel as well as her drill bit (jacob's chuck in the tailstock).

I know you had the problem with the Blackwood which is a good hardwood exotic. But I have run into the situation that you described when drilling the holes in softer hardwoods (walnut, cherry, etc.). Frequently, when screwing the wood into the mandrel, I couldn't tighten the wood because the threads created were too soft. It could also have been caused by the 'wobble' that Bill described.

At any rate, I fixed the problem with the softer exotics by drilling a 11/32" or21/64" hole. I then have no trouble screwing the wood into the mandrel and turning it.

When using nice dense hardwoods, I have not run into that problem so the 23/64" drillbit works great.

Is it possible that some of the wobble you may have encountered was caused by a little dirt in the tailstock hole where you added your drill chuck? I have to clean mine out frequently. I bought a taper cleaner to clean the headstock and tailstock taper, and I think it has helped.

 

[Bill Boehme]

So Michael if you think it's better to drill from the headstock end, how do you hold the object on the tailstock?

I'm not Michael, but I am not sure that he advocated drilling from the headstock since he did not say one way or the other. The way that I hold a chuck in the tailstock is with the Oneway live center and their chuck adapter to match the threads on the chuck. There is also THIS WAY.

Did you unpack them from a box. It's a well known fact that anything you unpack from a box will never go back into the same box. Maybe holes drilled in wood are the same thing.

I believe that is a corollary to Gay-Lussac's Law dealing with solid objects as influenced by the interaction of large bodies described by Kepler's Law. (in other words, a lot of hot air and the phase of the moon)

 

[MichaelMouse]

So Michael if you think it's better to drill from the headstock end, how do you hold the object on the tailstock?

You mean Bill, of course. Though you could actually buy a taper with your thread on the other end to mount your scroll chuck on the tail.

I bore from the tail end, and I let the drill seek center before tightening the tail to the ways and snugging the quill. Seems to work, especially as I use my steady to keep the main piece from deflecting under torque.

 

[Bill Boehme]

Wood contracts as it dries. Perhaps the inside of this dense stuff was still pretty wet. Given that others prepared similarly were fine, I would say to examine what made the one different. Setup produced so many suitable, I'm going to doubt it. Easy answer is moisture.

That also is a reasonable assumption and some woods just simply do not like being tapped. I ran into this with Ruth's bottle stopper design. When cutting threads, some types of wood tend to crumble. Thin CA will sometimes help. When it doesn't, I just put a small amount of epoxy on the metal threads and then gently screw it into the wood. I like her stoppers, but tapping weood can be problematic. Also, do not overtighten or the threads will surely fail.

 

[davidwalser]

Thanks everyone!

I appreciate all the insights that were shared. It's given me a lot to think about. In the end, I'll probably never know for sure what happened with this blank that has not happened with the dozens of others I've prepared using the same process. Perhaps, this one time, I forgot to lock down the tailstock before drilling. Or, perhaps, this particular blank is denser than the others and the grain caused the drill bit to wander (even though I'm using a short drill bit). Perhaps the this blank had a higher moisture content than the others and the drilling dried out the wood around the hole, causing the wood to shrink and enlarging the hole. Even though the blank has been sitting in my non-climate-controlled-very-dry-here-in-Arizona shop since last January, this is the explanation I'm leaning towards. When I was treating each of the holes with thin CA glue, I noticed the sides of the hole in this blank had some small hairline fissures in it that I thought was from the heat generated by the drill.