Titanium machining is often discussed for milling, but there isn’t a lot of available information on turning titanium. While most titanium is usually handled by milling, it’s not uncommon to turn this exotic material.
Titanium turning is often used for making flanges or tubes that will be used in corrosive environments. It’s also used for strong, lightweight parts that need to bear load, and in turbine parts.
In this article, I’ll share some of the tips that I’ve picked up that will help you successfully handle titanium on a lathe.
Tip #1: Titanium Moves
Since one of the most typical applications for titanium relate to lightweighting, it’s really common to find very thin titanium parts.
One thing that you’ll find out immediately when machining titanium is that it does not sit still. It’s very rare that you’ll be able to take a rough, finish, rough, finish approach as you machine all sides of the part. It warps significantly as you remove stock.
You might already be familiar with some of these challenges, especially if you’ve dealt with thin aluminum or stainless steel. For titanium, though, expect even more warping.
Thin titanium parts usually need to be roughed on both sides, then unclamped, then finished. Creep up on finished dimensions to ensure that you can hold tight tolerances.
Actually, it might make sense to try a stress relieving heat treating cycle between roughing and finishing. This is especially practical if you’re removing large amounts of stock and accuracy is required.
Usually flatness is very challenging if the tolerances are tight. As the part deforms as it’s being turned, holes can also deform so that they become out of round and can only accommodate smaller pins.
To sum up this tip: Don’t remove large amounts of material after hitting final dimensions; titanium warps like crazy. Do your roughing first, and creep up on tight tolerances.
Pro Tip: Using a finishing tool with a small nose radius (like around .008″) might mean a longer finishing cycle, but the lighter cutting pressure and lower heat could mean less warping on precision finishing cuts.
Tip #2: Titanium Insulates
Heat doesn’t dissipate quickly with titanium. Actually, compared to most other metals, titanium is a thermal insulator more that a conductor.
What does this mean for turning?
The chips don’t remove the heat like they do with steel or aluminum.
In fact, if you take an aggressive chip load, your carbide will burn out quickly. Your tool is what will take on most of the heat from cutting. The key to successful titanium turning is in minimizing heat as much as possible.
Here’s how this translates to turning titanium:
- Use inserts that are designed for titanium, which are typically much sharper than what you’d use for steel. This means that they slice better and don’t generate as much heat. Negative rakes or rounded cutting edges will not perform well on titanium.
- Make use of chip thinning. If at all possible, use an insert geometry that thins the chips. Round inserts can work well, same with using the 110-degree corner of a CNMG insert instead of the 80-degree side whenever possible.
- Good coolant delivery is critical! The heat will be going into your cutter, and only coolant will keep your carbide from burning out prematurely. Not only does your coolant need to be directed powerfully into the cut, but you’ll probably also need to have a more concentrated mix to get the lubricity you need. It can be a good use of your time to have a chat with your coolant sales rep to see what they recommend.
- Don’t push the RPM. Common SFPM for titanium is usually around 150 or so, and you might get slightly higher for finishing if your insert grade allows for it. This is one that’s usually unforgiving, though. Sometimes even a 10% change in RPM can push a cutter that would last an hour into catastrophic failure. Titanium is unforgiving.
- Chip load is important, but not as important as RPM. You just can’t push off those .040″ thick 6’s and 9’s like you can with steel, but you can do better than most people realize. One study found that a change in chip load of .002″ to .020″ resulted in a temperature change of only 300 degrees Fahrenheit in the cut. If you’re trying to max out your productivity, push your feed, not your speed.
Tip #3: Titanium Eats Carbide Alive
Titanium is abrasive to cut, and carbide takes an absolute beating from it.
Common problems include chipping and notch wear at the “skin” of the cut. Insert geometry and grade can have a huge impact on tool life and process stability. For example, consider using a WNMG instead of a CNMG insert.
Tooling reps are usually always wanting to show off their latest and greatest for carbide grades for titanium. I’d highly recommend grabbing all the freebies they’re willing to hand out to test out whether they actually make a difference.
Beyond appropriate cutter selection, there are a few programming techniques that can help you get more life out of your tools.
Since notch wear is so common, try varying your depth of cut to spread it out. Bury the cutter while there’s more stock, and reduce your depth of cut as your workpiece thins out.
Tools also don’t like being buried in corners. Even for turning, programming an arc interpolation and using a tool with a smaller nose radius will likely give better tool life than burying the tool fully at a step. This is especially critical for finishing toolpaths.
Titanium Turning Studies
Lots of tooling manufacturers will showcase their own studies on how much they’ve been able to improve tool life and efficiency with the latest and greatest. To be honest, it’s kind of hard to separate what’s just marketing from what’s legit.
TechSolve did a really interesting study where they tested different coatings, feeds and speeds on turning titanium. If you’re wanting to go deep in the weeds on this subject, I’d highly recommend reading through it. You can find a copy of their tests and findings here.
If you’re wanting the Cliff’s notes instead of the deep dive, here are some of the key takeaways:
- Superfinished tool edges (like what you can get from Microtek MMP) resulted in a doubling of tool life. This is a process that gives you an extremely smooth cutting surface, which reduces friction.
- Surface speed made an enormous difference in tool life, especially when compared to chip load. A change from 20 to 150 SFM resulted in a difference of about 900 degrees Fahrenheit vs only 300 degrees Fahrenheit in changing the chip load from .002″ to .020″.
- Abundant coolant made a massive difference in the majority of cutting tools.
- Sharp cutting tools are necessary, and tool failure happens very rapidly once the tool shows signs of wear. They displayed pictures of each cutting tool after each
If you’re getting started in machining titanium and want some practical tips for milling, make sure you check out my guide on how to machine it like a pro.