Blowouts

[oakham]

Hi,
I know this question has probably been asked a thousand times, but how do you stop acrylic blanks from cracking during the drilling out stage?
I have tried slow feeds, drilling from both ends and meeting in the middle, retracting the bit to remove all the "swarf" but still they crack.
I feel like unless I can master this, there is no point in buying the more expensive blanks as I end up throwing them away.
Any advice would be most helpful.

 

[WriteON]

When I switched to lathe drilling the blowouts stopped. Proper speed & bit are needed.

 

[volozhin]

I soak the blank in boiling water to soften it and then drill and retract the bit very often. Works for me.

 

[builtbybill]

I have had pretty good luck using the DeWalt pilot point drill bits. As soon as I use a standard twist drill bit, even the ones sold by pen supply companies, I get blowouts even if I drill a Pilot hole.

I have used the pilot points on regular cheap acrylic, Alumalite and Lucite with great success.

 

[Cmiles1985]

Are you drilling on the lathe? On a drill press? What type of bits are you using? Are they sharp?

The best answer I can give is: make sure your bits are sharp! Turn slowly (~500 revs/min). Advance slowly, clear award often (every 1/8 - 1/4").

If you're blowing out the ends, cut the blank long. Don't drill all the way through, and cut off the end.

 

[Joey-Nieves]

I use the lathe, slow speed and a sharp drill bit. I start with the centering bit, I've found that I can the center of the blank beter, specialy with the harder woods. I drill very slow and I remove the waste constantly, somtimes I even use the blow gun in order to keep the bit cool.
I've found 3 things causes blow outs, drilling to aggressively with the tail stock, to much heat from the bit and the biggest one for me is when you close to the end of the blank and you have to much waste on the bit it actually expands with the heat and breaks the blank or causes a blow out. The waste expands inside the blank and seals the air and gases inside the blank and causes the end to pop out. You guys may think I' pulling your leg but I have this happen to more often than I would like. Last week while drilling a piece of Balata (Ausubo) It poped so hard that I jump off the dead man switch and ran for cover(being the brave man I am, it's better to say here he ran, than here he died!)

Hope this helps
Joey

 

[ctubbs]

Try a coolant. If drilling on a DP, water works well, on the lathe compressed air shot down the hole. Air also works on the DP. Again sharp bits, not just new but really sharp freswh off the Drill Doctor or if you are really good, off the grinder. I am not that good.

Charles

 

[ed4copies]

Don't drill through.

Cut your blanks a little long, drill until you are about an eighth inch from the end, then cut off the end, exposing the hole---you will have ZERO blowouts.

 

[Paul in OKC]

The cut long drill short method is probably the best. I drill on a drill press, and use a waste block under the blank. Blow outs occur when the bit breaks through. As the point comes out, the pressure on the bit drilling is released and it 'jumps' through the end causing it to grab the bottom of the blank and break.

 

[mike4066]

Adding more info on how your drilling the blanks may help.


Here are a couple things that helped me.

Make sure your piece is secured well. I used to put mine into a wood clamp which held it from two sides and I'd blow out a lot of blanks. I cut 90 degree notches so that the blank held by the 4 flat sides instead of two and it's much better.


Practice on small scrap pieces. I cut scrap pieces of hardwood down to 1/2" x 1/2" x 2.5" and use them to practice. I cut them small so the wall would be thin and break if I messed up. This helped me understand the setup and mechanics without it costing me $5 each time. Sure its wood and not acrylic, but you have to make sure the basics are right every time or you won't be consistent.

 

[oakham]

To clarify, I drill on the lathe at a slow speed, I usually sharpen my bits before drilling, and I clear the waste regularly, especially when close to the end of the hole.
It's not so much the blow out, more the acrylic blank splits along its length. The last time it was hairline and didn't show itself until I'd finished the initial turning.
If I had hair, I'd be pulling it out by now!

 

[Edgar]

Here's my method - it works for me and I haven't had a single blowout with acrylic or wood since I've started doing it this way. It's basically a variation on the things pointed out by others in this thread.

I use a drill press set to its slowest speed & I don't pre-cut the blanks. I put the entire blank into a blank vice & start drilling.

I only drill about 1/4" (or a little less) at a time, remove the bit & clear the hole. After 3 or 4 passes, I walk away from the DP & do something else for a few minutes while I let the blank & bit cool down. I don't use any coolant, but that could speed the cooling time. Heat buildup is not a problem with most woods if you go slow, but it is with some very hard woods and it certainly is with acrylics.

After drilling deep enough for the tube, I remove the blank from the DP and cut it to the desired length on my bandsaw. If it's a 2-tube kit, or if I'm making two single-tube pens at the same time, then I put the remainder of the blank back in the vice (making sure to keep the same orientation), raise the table as needed, and drill for the next tube, again making sure to only drill deep enough for the tube. Then back to the bandsaw and cut off the undrilled end.

If it's a 2-tube kit with different diameter tubes, I drill the smaller diameter hole first, then the larger one.

I tried using a waste block and it does help, but I still had an occasional blowout or crack even when I tried to be careful & really go slow as I got to the end. As Ed noted, I have had zero problems since drilling short and trimming off the ends.

 

[Edgar]

To clarify, I drill on the lathe at a slow speed, I usually sharpen my bits before drilling, and I clear the waste regularly, especially when close to the end of the hole.
It's not so much the blow out, more the acrylic blank splits along its length. The last time it was hairline and didn't show itself until I'd finished the initial turning.
If I had hair, I'd be pulling it out by now!

The chuck is probably keeping it from fully blowing out, but the issue is the same with the bit exiting the end of the blank. Some acrylics & some woods are just more prone to that problem than others.

Drill short and trim off the undrilled end - that should solve your cracking/blowout problem.

 

[eranox]

I'm another disciple of the cut long/drill short method. Like you, I drill on the lathe at slow-ish speeds, and I've also experienced cracks down the whole blank, regardless of how thick the walls are around the hole.

My bits are mostly cheap HF nitride coated ones, and I frequently have problems with little bits of plastic sticking to the outside of the flutes. The plastic rubs against the inside of the hole and causes excessive heat, regardless of how slowly I go or how often I back out and clear shavings. I fixed the problem by lubricating the hole once or twice during drilling.

Somebody recommended spraying water into the hole, but that didn't help. I use a spray lubricant called Jig-a-Loo (similar to WD-40), and it solved the problem of cracked blanks, and mostly prevents the plastic from sticking to the bits. You would think that an oil-based lubricant would prevent the brass tubes from sticking, but I haven't experienced that. If you try it, let us know if it works for you. I use this on Tru Stone and resinous wood blanks as well.

 

[jsolie]

I have a little spray bottle with water that I'll use when drilling plastics -- both on the lathe and drill press.

When drilling on my lathe I keep the speed at the lathe's slowest: 550 RPM. I also use a centering bit to get things started. When I get to actually drilling the hole, I start off with maybe 6-8 turns of the tailstock wheel. By then it's time to clear swarf and spray down both the bit and the spinning blank (I don't turn my lathe off--just keep it running and keep an eye on the knuckle busters on my chuck). As I get deeper into the blank, I'll get down to maybe 3-4 turns towards the end. I haven't had a bad blowout with an acrylic/PR/other resin blank in a long time.

When drilling plastic on the drill press, it's slow speed, clearing often and spraying both bit and blank with every pass. I've had one blank blow out in the past year, and I wasn't the one operating the drill press.

It should go without saying that sharp bits are a must. Dull bits just generate heat as they grind away material.

 

[nativewooder]

My blow-outs and cracked blanks stopped when I began drilling on the lathe, Jet 1236, with Colt Five Star HSS-M2 Pen Bit Set.

 

[jsolie]

...Jet 1236...

LOL! I have the same lathe!

 

[mutley]

To clarify, I drill on the lathe at a slow speed, I usually sharpen my bits before drilling, and I clear the waste regularly, especially when close to the end of the hole.
It's not so much the blow out, more the acrylic blank splits along its length. The last time it was hairline and didn't show itself until I'd finished the initial turning.
If I had hair, I'd be pulling it out by now!

Cracking along the length. That sounds like it's more lightly that the drill bit is not aligned perfectly central to the chuck, than a dull drill bit / speed / heat problem. Have you checked the alignment of the tail stock (with 2 centers or equiv) recently?

 

[oakham]

I haven't checked the alignment recently, but I will do to eliminate that possible cause.

 

[Tom T]

Here's what was happening to me. I was clamping the pen vice to tight. Some when the tube was hollowed out all the way it would crack. Acrylic for sure some times wood. Also put the blank flat on a thin board on the bottom of the pen vice. Do not let it get hot.

 

[toddlajoie]

I'm with Tom.

You have stated:

• You are drilling at a slow speed
• Your drill bits are sharp
• You are clearing chips away.

If the blank is cracking down the length of it, that is not what people typically would call a "blowout", which would be more of an irregular twisting break of the blank. If you crank down your chuck too hard, it will hold nice and tight when the blank is solid, but once it has a hole in it, you will be putting too much pressure on it from the contact points of the chuck. This will deform the blank, and depending on the amount of flexibility in the material, will either flex, crack, or crush inward.

The lathe chuck needs to just hold the blank enough that it doesn't move. If you are drilling square blanks, it needs VERY little actual pressure on the blank. just enough to give it enough friction so the bit can be pulled out without bringing the blank with it. Round blanks need a LITTLE bit more pressure to keep them from spinning.