Confused about drilling.....

[MatthewZS]

So I've got a brand shiney new collet chuck and a drill chuck for my tail stock and I make my first few tries at drilling a blank on the lathe. I'm running into one bit of strangeness that I can't quite put my finger on however:

The final hole thru the blank ends up flared like a trumpet horn. Not much, but enough so the brass tube fits well along it's whole length except for a few millimeters right near the end. Suffice to say this causes a few difficulties the details of which I won't bore you with unless you want.

1) I am using a center bit to start a PRECISELY centered hole.

2) I've tried different bits, they all seem to do this.

3) Tailstock and headstock appear lined up correctly to one another.

4) If the drill bit is wobbling, I can't see it.


Mind you the hole isn't out of round, it appears round as best I can tell.

Any ideas?

Thanks.

 

[jttheclockman]

Did you tighten the tailstock???? Is the blank running true in the collet chuck???

 

[rherrell]

If you've never used a collet chuck before, make sure you seat the collet in the collet ring BEFORE you screw the ring on to the body. If it's seated correctly it won't fall out if you turn it upside down.

Other than that you can always cut the blank a little long and trim it after drilling.

 

[leestoresund]

I often have the same problem.
You said the "final hole" so I surmise that you have drilled one or two smaller diameter holes. Thus there is less support for the point of the bit on the last attempt.
Can I tell you why the bit wobbles at that point - only less support.

My solution? Make sure the blank is about 1/4" longer than needed so you can cut off the wider section.

Just my way.

 

[hewunch]

what kind of bit are you using for your final diameter?

 

[Atherton Pens]

I suggest you measure the length of the blank and place a mark on the bit at the exact length of the blank. When you approach the mark, use very little pressure as you drill through the end of the blank. Too much pressure may cause the end of the blank to blow out as the main portion of the bit exits.
Your results may vary.

 

[studioso]

You'll be surprised to know thAt a good part of brand new bits are crooked ( thank you china).
Roll yur bit over your table saw while placing a bit of pressure with your finger. See if they wobble.

 

[Mac]

THIS IS VERY IMPORTANT!!!!! which end the((( enter))) or((( exit)))

 

[workinforwood]

Make blank slightly longer. After u glue in tubes you can re chuck and use parting tool to trim and square ends.

 

[txbob]

Here's my best guess at what's happening.....

The flaring is happening when you remove and reinsert the bit when clearing chips. The simple solution is turn off the lathe and wait for it to stop completely before removing the bit. Clear the chips from the bit, reinsert it, then turn on the lathe. DAMHIKT

txbob

 

[randyrls]

Mathew; As no doubt you have read, there is a whole laundry list of possibilities that can cause this. The underlying problem is likely that the lathe is not accurately aligned. The tailstock and headstock center lines must be exactly on the same line. Notice I said line, not point. Bringing the points of the head and tail stocks together only proves they are at the same height and width.

You can check for the lathe's alignment using this method.

http://www.penturners.org/wiki/Lathe_Alignment

 

[Chasper]

What diameter bit are you having the problem with? Is the hole flared out at the entry or exit?

My first guess is that you are using a 7mm bit and not clearing it as often as needed. The flutes fill up, there is no where for the chips to go so they get pushed around the bit, since it is thin it starts wobbling a little, causing the hole to wallow around slightly and flare out the entry hole. It that is the problem, clean the flutes more often.

 

[Chris Bar]

He [randyrls] beat me to it...head stock centerline and tailstock center line MUST be co-linear....(have the same centerline). There is no wobble to observe but it causes the tool to cut at an angle...thus the flare. Had this problem on a PM3520B until I raised the headstock outboard side (simple lifting pressure) before locking it down. Other causes exist but they may also been addressed above.

 

[KenV]

I am only apprentice level machinist -- but one of the first learning is that you cannot drill a precise hole with a drill bit. You may get a drilled hole that is "good-nuf" but it will not be the same size as the drill bit. You can reduce the errors caused by runout, dull/wear on the bits, bad grinds, etc -- but you will need to use some different techniques to get and keep precision -- as the hole will be circular, straight and consistent.

The irregular hole shape at the end will vary with the kind of bit geometry, the load in the flutes, the stiffness of the bit, the dullness, and the speed/force it is moved forward. Some look very close, but measurents will show the errors tend to accumulate and the exit hole has the maximum opportunity for accumulated errors, material problems, and transitions.

If you have a well aligned path -- the bit is parallel to the axis of the lathe; and the bit is stiff, and the bit is very sharp; and you are keeping the flutes working and removing swarf, the last piece of drilling where you move back to cutting air is your most vulnerable point.

There is where the material being drilled is being pushed forward into the air and is unsupported by material behind it (air offers little to no support for wood or acrylics).

Wood working techniques for some centuries have recognized that the bit going through breaks out the wood. The flat work technique is to get close and then go the the back side and counter bore. The machinist under drills and bores or reams to get the final hole at the point where the drill bit comes out. All deburr the hole because there is unsupported bits pushed out at the back of the hole.

The comments above all are good suggestions to help minimize the chipping/breakout effects of drill breakthrough. Slower feeds with less force forward, and clear flutes with a super sharp high quality bit will help.

The practice of putting your trim waste all at the back side of the bit (drill from the center on two piece blanks and set the pieces up with minimal trim at the middle) will put the max trim at the point were the greatest drilling error is likely to occur.

You can minimize the effect, but there are a lot of reasons in the tooling and gear why you will not be successful in getting perfect holes with a drill bit.

The good news is that the drill press is lots less precise and less controlled than a lathe.

A good rule of thumb is that price and precison are directly related. If you want precision to increase, price will go up fast too. As precision increases, time to achieve the precision also increases. Machinists use process controls to get better precision.

Note that the potential for precision goes down as the accuracy the tools and tooling decreases. A typical wood turners cheap jacobs drill chuck is not a high accuracy piece of tooling. We use it because it is quick and handy -- and generally "good-nuf" - We then add process controls to keep the outcomes in the range of acceptable.