Another Blow Out! Lesson Learned

[Fleabit]

The other day the wife picked out a piece of acrylic at woodcraft. "I want a baron from this". Ok...first time doing acrylic, so I read up on the forums to make sure I was doing it right. Drilled slow, cleared constantly and dripped water on the bit to keep it cool. First part of the blank drilled out fine, but the second one was the problem! Just as the bit was exiting the bottom of the blank.....BLAM! WHIRRRRRRR! and the bad sound of a drill press out of balance. The blank was wrapped up the drill bit and the broken piece was laying on the bench. Come to find out (here is the lesson learned) the bit grabbed as it was starting to exit the blank. New drill bit and sharp. In the future I will measure the blank longer and mark the drill bit for the exact length and finish the blank on the table saw. Torqued me good since it was for the wife picked by the wife!!

 

[RPM]

Been there, done that, thinking about having a t-shirt made.

Good luck on the next one. When ever I drill the large diameter holes in the acrylics I go into super bu++ pucker mode!

Richard

 

[Dario]

Sorry to hear that and thanks for sharing the "lesson"

 

[wayneis]

Drip water IN the hole, make sure that the blank is tight to the bottom of what ever you have it sitting on. The other thing that I do to keep the bit cool is to stick it inside the vacume hose when I withdraw it for a couple minutes and then drill some more.

Wayne

 

[woodscavenger]

Good tips wayne. I never thought of the vacumm cooling trick. I have had similar blowouts....always just as I am about to gloat over a perfect drill job then BAM!

 

[Fleabit]

Wayne: I was letting the water run down the drill flutes into the blank. The problem I had was that my drill table has a hole in it for the drill bit....I did not realize that the blank was centered over it...therefore, no support of the blank...as you said above "make sure that the blank is tight to the bottom". Never thought of the vacuum.

 

[pipemaker]

When drilling acrylic, a 50/50 mix of liquid soap and water helps not only to cool, but to lubricate,the bit and blank.

 

[Daniel]

Here's a little amature physics about the bit exiting the blank. so for those that are board by such things feel free to tune out now.
the drill press in pruducing X amount of energy. this energy must be converted or used in the process of drilling the hole. Energy is never lost it is simply converted to anouther form of energy. two energy transferes come to mind right off the bat for the presses of drilling. one is the energy transfered to the drill bit to make it spin and cut. the other energy trnsfer is Heat. this is why bits getting hot is a bad sign the more heat the less cutting that is going on.
for this discussion we are interested in looking carefully at the energy that goes in to cutting. the cutting edge of the bit is where this energy gets transfered. so for sake of having some numbers lets say the cutting edge is 1 inch long. big bit I know. lets say this edge has 1000 pounds per square inch of force in order to cut. this pressure is spread out over the entire length of the edge...until the bit starts to exit the wood. then this force is being applied to less and less edge. cause the pressure to increase very quickly. when half the edge is exposed to air the pressure at the cutting edge is now 2000 lbs per square inch. then 4000, then 8000. pay attention to just how little bit was still doing the cutting when they grab the way they do. I usually find it is about 1/4 or less of the edge.
so why is this important to know. If you think about it all you need to do is make sure the entire edge keeps cutting until you have passed through your material. !!Backer Block!!
and it doesn't even need to be glued on. but you do have to make sure it will not spin with the blade that would be the same as spinning in the air. anyway the idea is to keep the pressure at the cutting edge consistant.
More than you cared to hear. well sorry but sometimes I just can't help myself.

 

[JimGo]

Actually, very interesting explanation, Daniel! Thanks!

 

[wrightal3]

Thanks Danial, always helps to understand why. Maybe I'll be more diligent now that I understand.
By the way, haven't forgotten your offer to help on my twist mechanism issue, just too many irons in fire right now to get back to it.

 

[Old Griz]

Here is what I do... I have an old spray bottle from some accelerator that I keep filled with water.. I use a sharp brad point bit.. I liberally spray the blank and bit as I drill slowly, and keep removing the bit and spraying the hole every 3/8" or so... as I get to the end of the blank, I slow down my rate of feed and kind of feel the bit to near the end.. remove the bit, spray more water and then very very very slowly drill the rest of the blank... it takes a lot more time to write it than to do it.. but it works for me... NOW this is not the be all and do all way.. but it works for me...

 

[Rifleman1776]

Tom said he uses a brad point bit. My first reaction to reading your problem was that you were using a twist drill, not a brad point. Other contributing problems might be that your drill press is the teensiest bit out of true and the bit point is kinda wobbly as it exists. Or, you could have cut the blank over length and not drilled completely through. Or, you could do like I do, avoid plastics. [][]

 

[jdavis]

We do alot of arcylic pens. Brad point bit, water to cool, and slow in and outs. Have had good success after started using water to cool plastic blank.

 

[Mac In Oak Ridge]

Daniel,
I have a completely different theory of why things like this happen.

You put your pen blank in a vice. You clamp down on the blank. The blank is solid material. You then start drilling a hole. The blank starts getting weaker and weaker.
The bigger the drill bit the weaker the blank becomes.

Some wood is stiff and absorbs the pressure of the vice. Some wood is rubbery and acrylic falls in that same category.

The undrilled part of the blank resists the pressure of the vice. Once you start drilling through the bottom of the blank you loose that resistance. The pressure of the vice causes the hole you are drilling to go egg shape.

With some wood and with synthetic materials when the drill breaks through and the whole blank can go egg shape the drill bit catches on the side of the bit and grabs. That pulls on the blank and encourages it to go even more egg shape and then the vise looses it's pressure and the blank is torn apart.

 

[TheHoneymooners]

It's always that second half of the pen blank that explodes...ever wonder why? I use canned air to cool my blanks...expensive....nah..a guy at work gives it to me....lol. I've learned not to sweat the busted acrylics and celluloids...I just think of em as an endless supply of center band material

 

[elody21]

I don't usually have any problems with acrylic blow out. I use a bullet bit instead of a brad and do most of the stated things above. You could try storing your acrylic blanks in the freezer !? Maybe ?

 

[swm6500]

I use a brad point and just go slow and easy and keep clean and cool.

 

[53Jim]

I have had a few blow outs while turning "plastics". What I use to keep the bit cool are those things they give you in the hospital to moisten your lips. Little sponges on a plastic stick. I dip the sponge into water, and as I pull the bit out, I shut off the DP and slide the sponge down each flute, than I re dip the sponge and squeeze some water into the hole.

This is MY WAY of turning acrylics. It may not work for everyone.

 

[Daniel]

Originally posted by Mac In Oak Ridge
<br />Daniel,
I have a completely different theory of why things like this happen.

You put your pen blank in a vice. You clamp down on the blank. The blank is solid material. You then start drilling a hole. The blank starts getting weaker and weaker.
The bigger the drill bit the weaker the blank becomes.

Some wood is stiff and absorbs the pressure of the vice. Some wood is rubbery and acrylic falls in that same category.

The undrilled part of the blank resists the pressure of the vice. Once you start drilling through the bottom of the blank you loose that resistance. The pressure of the vice causes the hole you are drilling to go egg shape.

With some wood and with synthetic materials when the drill breaks through and the whole blank can go egg shape the drill bit catches on the side of the bit and grabs. That pulls on the blank and encourages it to go even more egg shape and then the vise looses it's pressure and the blank is torn apart.

Mac in consideration of the specifics of penturning I am sure this is true. one other symptom of this would be the squeeling you sometimes get while drilling. but I can't be totally sure of that. the tear out at the exit is a common problem for all woodworkers. for us it ends in destruction of the project.