Acrylics cracking

Signed-In Members Don't See This Ad

Tiger

Member
Joined
Feb 15, 2009
Messages
245
Location
Melbourne, AUSTRALIA
I have cracked 2 acrylic blanks in the last week. One was a hole of 35/64 inches, the other was 37/64 inches. In both cases i sequentially drilled up to these sizes, so i start with a smaller drill bit and work my way up. One crack occurred in the drill press, the other in the lathe. Speed was 500 rpm, feed was slow. In both cases the drill bit seemed to grab the blank and cause the crack. Bits were sharp, only thing i can think of is to turn the blank round so there's more support, any suggestions? In one of the cases mentioned above, the proper size hole was 33/64 but with painting inside the blank and also painting the brass tube i needed to open it up, tried sanding the hole but progress was very slow.
 
Signed-In Members Don't See This Ad

Crashmph

Member
Joined
Dec 15, 2008
Messages
1,515
Location
South Riding, VA
Agree with what Ed said. I have had that problem before as well. Simpler to not drill all the way through the blank but deep enough for the tube to fit. Then cut off the excess.
 

Jim Burr

Banned
Joined
Feb 23, 2010
Messages
3,060
Location
Reno, Nv
Never, never, never drill through the end of an acrylic blank. Cut long, drill to depth and saw the remainder off. Your blank will never crack this way.
 

knowltoh

Member
Joined
Aug 16, 2008
Messages
423
Location
Alpena, MI 49707
If I have no leeway on blank that is very close to the tube length, I will turn round, use a collet chuck, drill
half way or so, then turn blank around and drill from the other end.

If the blank is long enough, I stop drilling before exicting the end as already suggested.
 

turncrazy43

Member
Joined
Apr 22, 2012
Messages
1,104
Location
Marietta, GA
I agree with what Ed said. Also, Heat is your enemy with acrylics. Use a spray bottle with water to quench the hole and drill bit, drill slowly and clear the debris often. I too have had blow outs at the end where the bit is exiting the material. When you can't long cut your blank and then cut off the end as suggested, I drill slowly until the tip of the brad point bit just begins to exit the blank or the tip of the other type bits begins to break through. At that point I stop the drill movement, shut off the lathe, withdraw the bit and spray with water to cool the blank and bit. Wait until the bit is cool to the touch and then ever so slowly begin to advance the drill bit until it completely exits the blank. This method has all but eliminated blow outs for me. Just MHO.
___________________________________________
Everyday I'm vertical is a great day
 

Tiger

Member
Joined
Feb 15, 2009
Messages
245
Location
Melbourne, AUSTRALIA
Blanks cracked before getting to the end, i always cut long, the flutes grabbed the blank and then the crack, the walls of the blank were pretty thin.
 

ed4copies

Local Chapter Manager
Joined
Mar 25, 2005
Messages
24,523
Location
Racine, WI, USA.
If you know the walls will be very thin, there is another approach (it takes time, so the price of the pen goes up accordingly!!):

Wrap the blank with gauze (like you use to wrap a wound). Then pour thin CA on the gauze--let sit for 2 days or more to harden. REALLY EXPENSIVE blank??? Add popscicle sticks (splints) to the outside of the blank, BEFORE putting on the gauze.

Of course, you will turn all of this off, but by then you have the brass tube supporting the inside.

By the time you get all of this done, you better hope you are working on a pen that will retail for $250 or more===you have invested a lot of time and effort.

FWIW,
Ed
 

gbpens

Member
Joined
Jul 1, 2011
Messages
821
Location
Homer Glen, IL
You mentioned drill speed but not the pressure used on the bit. As you approach the end of the blank ease up on the pressure. Back up the blank with scrap and you should not have a crack. With very thin walls use Ed's idea of wrapping or taping the blank. This is especially effective on segmented pieces.
 

KenV

Member
Joined
Oct 28, 2005
Messages
4,720
Location
Juneau, Alaska.
P.S. Need very sharp bits --- sharpen the bits or find a source for bit sharpening.

Did I note that you need really sharp bits to reduce the heat.
 

Wingdoctor

Member
Joined
Jul 8, 2011
Messages
272
Location
Columbus, OH
Perhaps the swarf (drill shavings) plugged the hole and bound the bit. I always retract the bit to clean out the every half inch or so.
 

RKB

Member
Joined
Mar 17, 2014
Messages
729
Location
Apollo, PA
I have been using canned air to blow out the blank holes. This has been working quite nicely and cools the blank down some too. :biggrin:
 

Tiger

Member
Joined
Feb 15, 2009
Messages
245
Location
Melbourne, AUSTRALIA
I think we can rule out pressure and heat, i was drilling slowly with light feed, also backing the bit out every few mm. I've tried to re-trace my steps and i do recall that the blank was wobbling a little in both the drill press and the lathe. That slight wobble and the drill catches and i think that is what causes the crack. The big drill bits are difficult to re-align once you take them out of the drill press or lathe. Bits are sharp and i've never cracked a blank less than 1/2 inch and i've done a few of those. I may investigate a different grind on the drill bit, something less aggressive on these acrylics?
 

Quality Pen

Member
Joined
Feb 2, 2014
Messages
1,403
Location
Lumberton, Texas
There are a lot of techniques, but I have good luck simply going slow and backing out frequently to clean the bit and cool it with water. Not a good idea for some things like wood, but if it's a standard polyresin then go for it. I even drill through the backs of inlace acrylester with very infrequent mess ups even when I "forget" I was about to break through.

Another simple solution, already mentioned, is just make the blank longer and saw off the end.
 

Tiger

Member
Joined
Feb 15, 2009
Messages
245
Location
Melbourne, AUSTRALIA
Gave it another go tonight, this time i turned the blank round so it was better supported by pin jaws in a chuck, went slow and got the job done but it is hard going when the blank has thin walls. Thanks to all, Amos will give you a call soon, was doing an Empress which as you know requires a 37/64 inch hole.
 

Sawdust1825

Member
Joined
May 5, 2013
Messages
83
Location
Illinois
If your going to use a pilot drill try only going from the starter bit to full size in one step. Such as 1/4" to the finish size. You didn't say what you left for the final bit but any drill bit when there is little stock to remove has a tendency to want to grab and or self feed and neither is a good thing. I don't know that this is what is happening but just throwing it out there. Good luck.
 

Tiger

Member
Joined
Feb 15, 2009
Messages
245
Location
Melbourne, AUSTRALIA
If your going to use a pilot drill try only going from the starter bit to full size in one step. Such as 1/4" to the finish size. You didn't say what you left for the final bit but any drill bit when there is little stock to remove has a tendency to want to grab and or self feed and neither is a good thing. I don't know that this is what is happening but just throwing it out there. Good luck.
Good point, I used a 25/64 bit first and then 37/64. The grabbing/self feeding is at the heart of the problem and the thin walls just aren't strong enough.
 

Ratimus

Member
Joined
Aug 14, 2014
Messages
13
Location
Warshington
Brad point or twist point? The reason I ask is because brad point drill bits like to grab the inside of the hole you are drilling, causing the blank explode.
 
Top Bottom