Drilling titanium pencil blanks
Description of part
For my titanium pencils, I make the body from a 112mm long piece of 8mm AF hexagonal titanium (6Al4V/Grade 5). This is drilled 6.5mm half-way through from one end, then 5.9mm through from the other (so the drills are doing about 8x-10x diameter deep holes). They look like this:
Machine setup + drill choice
Manual lathe. Lever-action tailstock so the drill can be advanced/retracted quickly. I have rigged up a coolant arrangement which flows coolant around the hexagonal part as it’s held in the lathe chuck. Running about 350rpm. Pecking 1mm at a time.
Here’s a quick video showing the drilling in action:
I initially used Dormer A002 drills (HSS, TiN coated, 118° split point, 30° helix). However, I went through a period of very poor performance with these drills and tried others. I tried fancier drills from Dormer, drills specifically for titanium from other manufacturers etc. and all suffered from the same problem (see next section). As of June 2024, I am using Europa 810504 drills (HSS, TiN coated, 118° split point, 25° helix). While these are better, they still are far from perfect.
The problem
The main problem is the lands of the drill rubbing inside the hole. This causes material to eventually stick to the drill, increasing friction/heating and generally making it progressively more difficult to drill. The reason for this is that the drills typically produce either exactly on-size or slightly undersize holes. The sole exception to this was one 6.5mm drill which happened to produce a slightly oversize hole (something like 0.07mm oversize). This drill performed beautifully and did maybe 30-40 holes before finally wearing out.
Here’s a photo comparing several drills. The top drill is the 6.5mm which drills oversize - note there is no material stuck to the lands. The others show significant material where they’re rubbing.
Things I’ve tried/thought about
I’m aware of the existence of back taper on twist drills. However, I determined that any lack of back taper did not explain the rubbing problem. For example, I could have two drills from the same series, with identical (and good) amounts of back taper, yet one drilled undersize and rubbed while the other drilled oversize and didn’t rub.
I’ve tried different kinds of coolant, cutting/tapping fluid etc. but with little effect. With that one good 6.5mm drill, all I needed was mist coolant.
My solution
Remember that one drill which did work well? It was a 6.5mm nominal size, but produced a 6.57mm hole. I reckoned that it might be possible to deliberately grind a drill with a slightly off-center point, which would hopefully cause it to drill oversize. I bought the drill grinding machine shown below. Drills are held in a collet chuck and this chuck is inserted into two different stations on the machine to grind the primary and secondary reliefs on the tip. I made up a couple of shims of slightly different thickness (1.00mm and 0.95mm). By placing these between the collet chuck and the body of the machine, it’s possible to grind each cutting lip to a slightly different length and produce an off-center point.
The results from this are extremely promising. A 5.8mm Dormer A002 drill, reground using this arrangement, can produce anywhere from a 5.85-5.95mm hole. The variation is most likely due to slight inconsistencies when mounting the drill for regrinding - e.g. not perfectly centered in the collet, pushed slightly to one side when grinding, etc. A 6.4mm A002 drill does a 6.47mm hole. When the drill does produce a suitably over-sized hole, it drills perfectly, without any trouble. The only downside is that the tip doesn’t seem to last long, but this is perhaps due to the lack of coating (removed when grinding) and the relatively coarse grind produced by the machine.
However, the general principle seems sound, so I’m now going to track down someone who can produce proper drills that are ground slightly off-center.
Random point - I don’t care about the exact size produced, since I’m reaming both ends to exact 6.5 and 6 diameters anyway. I’m more concerned about making life easy for drilling.