My Solution

My own design is a modified version of Transtrom’s (1952) custom increment borer that seeks to alleviate these issues by widening the cutting tip and placing the spreader bars between two sets of threads: one for cutting into the tree and another for engaging the wood when backing out the bit. The spreader bars modeled from Transtrom differ from conventional designs in that they are shaped like rounded ramps instead of semicircles with flattened tops. This allows for a more aggressive approach to expanding the size of the hole around the main shaft by shearing rather than compressing the wood. The dimensions of the spreader bars were mathematically derived such that in one revolution, the entire inner surface of the bored hole would be covered by the top face of the spreader bars in its entirety exactly once. Originally my design for these spreader bars was to be similar to those used by Haglöf, so the derivation relates the width and height of the spreader bars in order to fit the above design requirement. However, after revising this design, only the width of the spreader bars was needed since its top edge is the only portion that comes into contact with the inner surface of the hole. Two sets of spreader bars grouped in three were spaced evenly around the shaft and set apart such that after the first group covered the surface of the hole for one revolution, the second group would re-cover that same area to shear or compress any residual wood. It is expected that this design will greatly reduce the friction caused by wood rubbing against the shaft and thus lower the amount of torque required to turn the bit when it is deep in a tree. Furthermore, the second set of threads behind the spreader bars should greatly decrease the likelihood of a borer becoming stuck because there is nothing to impede it from catching the wood behind it. A double-start thread configuration was chosen based on anecdotal evidence suggesting that double-start borers are less likely to become stuck compared to triple-start borers. Shown under Dimensioning the Spreader Bars in Data and Derivations is an illustration of my new design along with the mathematical derivations for dimensioning the spreader bars.

Material selection was a crucial step to this process as it determined the maximum amount of torque the borer could withstand, as well as its ability to stay sharp and resist corrosion. Because I do not have access to a gun drill, materials were limited to those sold in the same pipe size as the borer shaft. I was therefore limited to 4130 steel for the shaft and a compatible material for welding on the threads and handle insert. 4340 steel was used for the threads and handle insert due to its weldability, compatibility with 4130, higher yield strength compared to 4130 and 4140 (after QT), and higher degree of hardenability, edge retention, and corrosion resistance compared to 4140 steel. Given that for a hollow shaft, the maximum torque it can withstand is equal to (τ ∙ J) / D, where J = (π/16) ∙ (D₄ – d₄), it was determined that 4130 would be sufficient for most trees so long as it is oil quenched between 1570°F and 1625°F and tempered twice at 900°F. The heat treatment will be carried out after welding through Micro Arc Welding, Inc. to avoid cracking. Therefore, both the 4130 and 4340 will need to be simultaneously quenched at 1570°F and then tempered at 900°F. Calculations and comparisons for available materials, as well as their heat treatment procedures are given under Material Selection in Data and Derivations.

After modeling my borer design in SolidWorks and programming it with Autodesk HSMWorks, a 3+2-axis simulation was conducted using both programs to verify the output of each machining operation. An assembly file of the Haas VM-2 vertical mill was downloaded from haascnc.com. Mates and a coordinate system were then created that matched the axis directions and limits specified by Haas. Similarly, the Haas T5C 5-axis tilting rotary assembly file was downloaded and mated within the larger VM-2 assembly. A machine configuration file was specified that allowed the assembly’s parts to move according to the operations programmed in HSMWorks. Lastly, the Haas Trunnion (pre-NGC [rev 43265]) post from cam.autodesk.com/hsmposts was downloaded and modified to enable the 4^th (with rewinding) and 5^th axes using their createAxis command.

Machining the increment borer required that a blank first be profiled using the Haas SL-10 lathe. Next, the tapered portion of threads will be machined on the Haas VM-2 coupled with Haas T5C tilting rotary. Due to HSMWork’s limitations when it comes to producing high-quality 5-axis toolpaths, it was decided that the part be should manually tilted on the A-axis to match the angle of the tapered threads and offset in the Z and X axes accordingly. The Z and X toolpath offsets were calculated as shown under Calculating Toolpath Offsets in Data and Derivations.

This method of machining produced cleaner, 4-axis toolpaths that, when combined with these offsets, produced tapered threads. The rest of the borer is machined with 4-axis operations and then hollowed out with three different drill sizes to create a rough inner taper. Other than heat treatment, the completion of the borer consists in the bit and handle insert being welded to each end of the shaft by Micro Arc Welding, Inc.