Why did my pen blank crumble?

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ndep

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Jan 23, 2021
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exoticblanks_fail.jpg
 
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magpens

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Feb 2, 2011
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Coquitlam, BC, Canada
I would call that more a break than a crumble ... but whatever you call it, it is a very undesirable occurrence .

A contributing factor might be too much vise pressure .... ??? . I have one of those vises. . My feeling is that the jaws tend to compress the gripped material more strongly at the bottom, thus causing dangerous stresses down there on the blank being drilled.

Is it possible for you to do your blank drilling on the lathe ? .... preferably, but not necessarily, after the blank has been rounded ....
Why "preferably" ? ... I think with a rounded blank the vise pressure stresses are more uniformly spread out than with a still-square blank

I realize that may require additional expenditure for appropriate lathe chuck jaws to grip the blank, and a drill chuck for your tailstock.
The accuracy of the hole drilling will be improved ... plus the sideways stress on the blank during drilling should be considerably reduced.

I seriously recommend that you consider doing that. . I have been drilling on the lathe for many years with much improved results.
 
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egnald

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Jun 9, 2017
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Location
Columbus, Nebraska, USA
Greetings from Nebraska.

Although plastic blanks can be successfully drilled with standard high speed steel drill bits (120-degree grind) and brad point bits I suppose, however I prefer using bits made specifically for drilling plastics. The bits are ground so they bite into metal as they are pushed into it. In plastic this action results in chips and more friction which causes heat. Bits that are made for plastic have more of a sharp point (60 to 90-degrees) which allows for a more gradual penetration (less bite) that reduces chipping and reduces friction (less heat). This is something I learned from managing a fabrication shop for many years that worked with plastics and ceramics as well as common metals. If you are interested, I get all of my drill bits for plastic from McMaster-Carr.

For successful drilling into plastics with standard bits, take it very slow, more like a slow pecking action (drill a little then pull the bit out) so that the bit can cool down and the flutes can cleaned out. Material packed into the flutes causes more friction (heat) and pressure inside the blank. Some people use a lubricating coolant when drilling plastics (water, oils, etc.), but it can leave a little bit of a mess on your drill press or lathe if you are drilling on the lathe.

I just saw your response that you are asking about a reconstituted stone blank. Although reconstituted stone blanks, FauStone, TruSonte, SimStone, etc., typically have a plastic binder, they are mostly powdered stone that has been compressed. For these I use regular high speed steel bits. I have heard that some people pre-drill using a carbide, masonry type of bit and then use a standard bit to get the right diameter. I have splurged for carbide drills from McMaster-Carr for a couple of the pen types that I use frequently (10mm and 3/8-inch). These bits are harder, stronger, and more wear resistant than high speed steel or cobalt steel bits so they retain a sharp, hard edge even on abrasive materials. They are however very brittle so they need to be used in a drill press, lathe, or mill i.e. no handheld drilling! The same in-and-out, pecking, cooling, process is used to make sure that the flutes don't get packed and to allow the bit to cool.

Since standard drills usually cause more problems (grabbing) when exiting a hole, another great trick is to cut your blanks long, measure how deep you need to drill, drill, then cut the blanks to length.

Regards,
Dave
 

jttheclockman

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Feb 22, 2005
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NJ, USA.
Many factors could have come into play. I will list them. The blank was not secured to a base that is flat . You can not use those type jigs and have the blank suspended in mid air. Needs to be resting flat on the bottom. The speed of the drill press must be slow enough to match the material you are drilling which in this case around 500rpm. tops. The drill press quill must not be wobbly or out of round. You need to secure the jig so it does not move. You need to drill in steps and not all at once with a bit that size. You do need lubricant of some sort. You need to clear the swarf from the hole often. You need to keep the bit cool. I use DNA. Has worked well for me for many years. The jig needs to hold the blank straight and steady and not too much crushing pressure. That blanks gets thinner the bigger the bit and the remaining material becomes weaker.

I too prefer to drill on the lathe. Better control and more accurate. Good luck.
 

renichols

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Oct 9, 2005
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Bellevue, Nebraska, USA. 68123-2302
A couple of things I like to do when drilling with my vice is to set the blank on a waste block. I also start out with a smaller bit and work my way up to the finished size taking my time drilling clearing the bit frequently. Now I do my drilling on the lathe. As far as a lubricant, I use my air compressor to cool the blank and bit to the touch when clearing the bit. It takes a little longer to drill, but for me time is not a factor.
 

leehljp

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Feb 6, 2005
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Tunica, MS,
The material is trustone. Does that matter?
It does matter, but only to a degree - depending on the individual material. Every material has its own characteristics including different kinds of wood. Most people don't know that and most people are just lucky most of the time. You (anyone) can speed drill a 3/8" hole through a 2x4 without a problem, but drill a 3/8" hold in a 3/4 square by 3 inch long hole in the same wood . . . and without the proper support, you will get a blowout like you had. A pen blank cannot take the same pressures that the same wood in a 2x4 can!

There is a considerable loss of structural integrity once a certain small size is reached. And As said above, there could have been a number of factors involved simultaneously:
• small size (normal for pen blanks)
• drill RPM speed
• drill travel speed
• clamp pressure
• heat
• no lubricant
• specific material
 

randyrls

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Feb 2, 2006
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Harrisburg, PA 17112
Hi ndep; All the suggestions above are good ones. One thing to add to the thread; The blank likely shattered when the drill bit broke through the lower end of the blank. So don't allow the drill bit to break through the end of the blank. Cut long, drill short and trim to length: Cut the blank at least 1/4" longer than needed. Mark the length of the brass tube on the drill bit. Drill until the mark enters the hole. Stop and slip the tube into the hole to make sure it is deep enough. Now go and trim the UN-DRILLED end to the proper length. This adds an extra step but you wind up with a good blank.

PS; We are friendly bunch here and normally post our names.
 
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