Had an interesting object lesson in just how nasty Titanium is to machine...I was machining up a cover for a pendulum bob - simple bit of Ti tube with an end cap and a 6mm hole through the cap to take the pendulum rod. Drilling the hole was the first problem - the drill was cutting fine until it had almost broken through, at which point, I had to stop drilling briefly because the drill was slipping in the chuck, and when I restarted, it wouldn't cut at all and additional pressure just made the metal glow red hot. Finally managed to finish the hole by starting from the other side with a 5.5mm and finishing with the
6mm.
The second problem was when I came to finish the end cap on my belt sander. I noticed a couple of bright flashes, and couldn't figure out what was going on, so I carried on sanding. The next flash was more intense and lasted a few seconds - Ti dust had been accumulating in the hole in the end cap & had caght fire. Happily there wasn't enough to do more than give me a warning...
I have recently made some parts out of titanium, and thought I would relate my experiences.
I work on the railway and use an impliment called an 'EP Key' to power up the cab of the train and carry out fault-finding or move the train. These are normally supplied by the company as zinc plated steel items. Being bored one day, and just because I can, I decided to make some out of different materials.
So far, I have made examples out of brass, aluminium, clear acrylic and titanium -
formatting link
Turning in the lathe was no problem at all, and gave a very good finish. I used carbide insert tooling for all turning operations, and used Templers Blue Foam as a cutting lube.
The slow part was milling, using a 1/16" slitting saw to nibble away at the thin slot to accept the cross-bar. The only milling machine I have is a Sherline mini-mill, so with the slow speed required (approx 200rpm) to avoid burning-out the HSS saw, torque was quite limited and only shallow cuts could be taken. A 0.5mm depth of cut was taken on each pass, to a depth of
17mm, then further cuts at the top and bottom of the slot to widen it to
3mm. A 1/4" FC3 cutter was used to form the wide slot and keyway at the other end of the key. This was relatively easy, the material cutting like stainless steel.
The cross bar was formed from 3mm thick Ti strip, the outline cut with a hacksaw then finished on a bench sander. Holes were started with a centre drill, drilled through 3mm, then opened out to 6mm with a carbide stub drill.
Compared to aluminium or brass, and with the machinery I have available to me, I would say that machining of the titanium key took 10 times longer - the 3mm slot taking the majority of the increased time.
The biggest problem encountered was the dimension of cuts changing due to the material springing away from the cutter, exacerbated by a less rigid milling machine.
Overall though, I found that Ti wasn't as scary as some of the horror stories would have you believe - maybe I was just lucky!
I suspect that the fact that you were using a small mill, and therefore slow speeds/light cuts worked in your favour. As far as I can tell, one of the reasons Ti is a pig to work is that its thermal conductivity is poor, so heat doesn't get conducted away from the cut very quickly. So if you use heavy cuts/high cutting speeds, the work gets hot very quickly; the result is twofold - firstly, the metal work-hardens very easily when hot (this was the problem I had drilling the hole), and secondly, the chips can get heated to flash point (which was what happened on my belt sander). So the smart thing to do is to use coolant - for example, when Dick Stephen uses Ti to make clock escape wheels, he mills them to shape while the part is submerged in a water bath to make sure that there is no chance they (or the chips coming off) will ignite.
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