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How to Avoid Trustone Pen Blanks Cracking When Drilling ?
Strongink / 2019-07-12

TruStone is a kind of expensive and very beautiful material for pens, which is consists of more than 85% actual stone with acrylic.

Strongink has same pen blanks with same material and we call them Turquoise Stone Pen Blanks, after tests, they have almost same performance in sawing, drilling, turning, etc...



Meanwhile, they both have same brittle attribute of stone, so they are easy to cracking when drilling, turning or pressing pen parts.

I write this article to talk about the problem cracking when drilling.


Trustone cracking when drilling


Cracking is a pain and big buck loss for pen turners when drilling, please see below illustration, they are all kinds of badly crackings from drilling.


Why they are cracking when drilling and how to avoid the problem?



After many tests, i find the chief culprit of cracking, which is the overheating of pen blanks under the press.



As we know trustone made of stone powder and acrylic, the acrylic organize the stone powders which make trustone have many beautiful patterns .

but acrylic is easy to lost it's hard attribute under high temperature. so trustone is easy to cracks under press.


What factors make the trustone pen blanks get over heating ?

Of course, it is the friction between drill bit and blank.

So, we can make analysis as below factors.

Factor 1. Drill speed

Factor 2. Type of drill bit

Factor 3. The pressure onto the pen blanks.


Drill bits



Let’s talk about the first factors,


Drill speed 

This factor is easy to understand, for example, when we drill hard wood, if we drill at high speed, the hard wood will smoking, so the higher speed we apply the higher temperature pen blanks will get.


Type of Drill Bit

You must have known that Trustone can be treated as hard wood because the public statement will reach you when you buy Trustones that The trustone Can be sawed, drilled, shaped and polished with tools for wood.



So most of pen turners will use Brad Point Drill Bit, this type of drill bit is designed for wood, which can  cut and remove much wood in seconds, but if drill stone, the Brad Point Drill Bit will make the blanks get over-heating very fast and let the trustone blank easy to crack when the point getting through the bottom.

Please see again the photo for the cracking



Even though you get a not bad drilling result, they will be like below, some times we may need to add 2/5” to the length and do not drill through the bottom to avoid the cracking bottom end by sawing off the bottom.


Trutone cracking when drilling


trustone cracking when drilling


The Pressure Onto The Pen Blanks

,This is easy to understand, the greater the pressure applied, the faster the temperature rises .


So, finally, how many speed, what kind of drill bit, and how much pressure we should apply ?

After many test, we get the   CONCLUSION as below


We should use sharp split point drill bits under 500 RPM. apply pressure just less than that we drill hard wood. and in order to avoid long time over heating, we need to pull back the drill bit off  the blank per each 5 seconds and cool for around 3 seconds. you can count  by your hear , this is easy to process.


Please see below pen blanks we drilled with above solution. you can see the clear hole almost without any chips on two sides, we mark the two ends with A and B.


trustone pen blanks drilled with split point drill bit


But please make sure the drill bit is sharp enough, other wise the blank will be easy to crack again. this is important.