Machining a pentagonal shaped drill hole in rock
Pentagonal holes and star holes are created using ultrasonics.
To make a five-sided pentagonal hole in hard rock, we must first drill a precision star shaped hole with five equal points or lobes into the rock. Below is a 5-point metal tube along with a slab of hard igneous Dacite rock, which I used to ultrasonically create a 5-point star shaped drill hole.
The evenly shaped 5-point metal tube allows for a clean precision cut, star-shaped hole with evenly spaced internal lobes (5-points).
(Below) to create a pentagonal hole, drive the same star tube into the same star hole where now the round potion of the tube touches the star points.
The round portion of the oscillating metal tube will grind away the internal lobes of the star hole, creating a pentagonal hole. The free-abrasive slurry is the cutting action as the lobes are ground away.
By demonstration, we know pentagonal and star holes are created using ultrasonics and a 5-point shaped metal tube. One tube makes both shapes in rock.
The shaped metal tube was created by a drawplate; The drawplate forms even spaced points along the metal tube, leaving behind evenly spaced lobes inside the rock.
If the metal star tube has a twist in it from stress during fabrication, the star hole will have the same twist (rifling effect). A slight twist in the metal tube will cause the internal lobes of a star drill hole to have the same twisting configuration inside the rock.
(Above) a 5-point metal tube will ultrasonically make both a star-shaped hole and star-shaped core. Both core and hole will have a slight taper
Should you come across a pentagonal hole in hard rock, you may also come across a star shaped drill hole because they are made by the same drilling tube.
Pentagonal shaped holes are created by the round portion of the ultrasonic drilling tube, grinding down the lobes flat into a pentagon. Only ultrasonic machining can make both holes possible in rock.