When moving to titanium, it is a whole separate ball of wax, full of missteps and broken tools if you are not keenly aware of the changes necessary. As I'm just finishing up some titanium hot rod parts, I thought I'd share a few of the lessons that are worth knowing.
Know your material:
When speaking of Titanium, we might as well talk about the grand daddy of toughness, 6/4 ELI in a non-annealed state. Machining titanium is much more difficult than steel, as the material does not like to dissipate heat, instead keeping it fairly localized. This means that you need to be aware of your tool speeds, feed rates, and coolant application to prevent tool failure and work hardening. If you push the material too hard, you will find your tools quickly dulling or becoming burnished at the cutting edge, the material pushing and forming instead of cutting cleanly, and an inordinate amount of smoke coming from your work space...not good in any combination.
Ti also has an inherent springiness, a by product of it's elasticity. This makes it want to close in on your tool, so having a proper lubricant that reduces the cutting edge friction is paramount.
Know your process:
Tapping titanium is not difficult, per se, it just requires a different approach.
Preparing the hole for tapping will require a stepped approach, using more incremental sizes and a higher peck rate to insure that the heat build up is reduced. In the case of the spider interface below, I begin with a center drill to create a focal point, then use two drill bits to step up the through hole size, finishing with a third drill bit designed to work specifically with the tap I'll be using. Due to the hardness of the material, the final bit is one size larger than you will typically find on the suggested drill charts, as the 6/4 Ti does not need near the depth of thread steel or aluminum would require to for the threads to have sufficient pull retention properties.
The extra steps and measured pace in creating the through hole is worth the time as it increases the longevity of the tap.
Know your tool:
I mentioned that titanium does not dissipate heat well, keeping it localized during the machining process. This manifests in a binding action on the cutting edges of the tap, creating a circumstance where the force required to continue to advance will quickly yield to the torsional forces, breaking the tool off in your work. We need to select a tap profile that will limit that potential.
The best choice for machine tapping titanium through holes is to use a two flute spiral point tap with a relieved back edge and tooth chamfer. This allows maximum space behind the cutting edge for chips to be pushed out the bottom of the hole while reducing friction and heat build up on the cutting surface. Several manufacture's make taps for exotic/hard metals, but I like OSG (EXO TI) and Emuge (Rekord C Ti) the best as they are material specific for titanium, have a nice tooth rake, and are designed for rigid tapping. Below is an example of some new bits and taps from OSG...I always start with fresh tools when I begin a run of Ti parts so that it is a pleasant experience.
These taps are designed to be machine fed in one push...no back and forth hand work here, don't even try. The parts MUST be rigidly fixtured as vibration is one of the forces of evil to tap longevity. Make sure that you consult the manufacturers guidelines for speeds as they will differ. I run these taps at between 10-13 sfpm, That equates to an rpm around 240...seems pretty damn fast for how hard a material 6/4 is but that is what is required for proper thread shape and chip evacuation.
You'll see that I am still power tapping with the mill, I don't use a tapping head. The key to doing this successfully is to only tighten the chuck enough that the tap will self feed but if it binds, will slip in the chuck before breaking. It takes a bit to get the right feel, but it is a process that allows you to start with light pressure and work your way up.
Know your friends:
One of the best friends you have in tapping titanium is a little bottle of Moly Dee CF Tapping Fluid. Heavier and more clingy than your typical "Tap Magic" or equivalent, this lubricant will make life a LOT easier for you. Good friends do come with a price...expect to pay around 50 bucks for a bottle.
Using these processes on my manual mill, I am able to get about 30-40 holes out of a tap before it is no longer sharp enough to cut efficiently, that's 10 crank arms. At a cost of $50 per tap, you can begin to see why working with ti can be such an expensive proposition.
Know what to do when things go wrong:
Unfortunately, if you are going to be tapping a lot of holes in titanium, you will break a tap off in the work piece. When that happens, those buggers are STUCK. No amount of trying to turn it out using the brittle shards left above the work piece or breaking the tap with a punch will get them out. So what do you do?
Drop the piece in a small container of Ferric Acid and walk away for a few days. This can commonly be found at Radio Shack labeled as a PCB Board etcher.
The acid will eat away at the cutting edges and thinner portions of the tap, leaving the titanium alone. After a few days, rinse it good and give it a tiny little tap with the punch, out it comes with little more effort than the patience to wait.
If you are going to make custom Ti bikes, you need to know how to work with all the variables of the material. I hope this gives a little insight and saves you a bit of time and money in the process.