Introduction to Titanium Machining
Discover how to machine titanium, it’s easier than you’d think, especially with our free checklist.
Titanium exhaust for McLaren sports car…
Titanium and its alloys are staples in aerospace, medical, automotive, and many more applications. The metal’s benefit are properties like these:
- Resists rusts and chemicals
- It’s recyclable
- Perhaps its biggest attraction is that it is extremely strong for its weight. It is one of the lightest metallic elements in the periodic table–only slightly more than half the weight of stainless steel.
These properties are highly desirable for a number of specific purposes:
- Auto, Military and Aerospace: Any aerospace application will value high-strength + low weight, but titanium also brings corrosion and chemical resistance which may make it valuable in propulsion systems. Interestingly, it also makes good armor. It saves 15-35% wright compared to steel or aluminum armor with the same ballistic resistance. As Auto makers work harder to reduce the weight of vehicles, whether for performance or fuel economy, they’re considering more titanium. Some of titanium’s properties are ideal for things like the valves on an internal combustion engine.
- Ships: Titanium’s corrosion resistance is helpful in ocean applications.
- Medical and Dental: Implants such as hip joints, knee joints, bone plates and pacemakers are some of the many applications that benefit. Titanium is an ideal biocompatible material because it has low electrical conductivity, high corrosion resistance, and a thermodynamic state at physiological pH values. It also promotes integration with bone
- Sports, Jewelry, and More: Let’s face it: titanium one of those “magic” materials that the public will pay for regardless of whether it is actually needed.
But, there are obstacles to working with titanium, it is quite challenging to machine. Fortunately, there are also solutions for each obstacle. In this article you’ll learn how to choose the proper tools, as well as how to use those tools with proper feed and speed parameters, axial and radial engagement, and all the rest of the parameters needed to be successful with Titanium.
Why is Titanium So Difficult to Machine?
Image courtesy of Wesdel…
When you have heard the full list of what makes Titanium so hard to Machine you may wonder how we do it at all. But, take heart, we’ll have a strategy to deal with each obstacle.
- Titanium alloys have a low Young’s Modulus. The Young’s Modulus is essentially the stiffness of the material. Practically speaking, that means titanium will suffer more from springback and chatter than other materials. This can lead to surface quality and other problems in finished work.
- Titanium is sticky (much like aluminum is sticky and wants to weld to the cutter). That combination of work hardening and stickiness creates long chips that want to wrap around everything. These tangles make completely automating titanium machining almost impossible. An errant chip sticking to a cutting edge can be responsible for tool’s chipping, especially on entry or exit from a cut.
- Titanium does not conduct heat well. Titanium’s toughness generates more heat to start, and it’s harder to get rid of it. Compared to other materials, we are much more reliant on coolant and not the chips to remove that heat before it damages our cutting tools.
- Titanium is very likely to work harden. Work hardening is triggered by not controlling the heat well enough in the cut.
- Cutting pressures in titanium are very high. This means that the cutter receives a strong shock when entering or exiting a cut. Failure to control this will result in excessive chipping of your tools on entry or exit.
Titanium and Its Alloys
Like most metals, there are many Titanium alloys (nearly 40 ASTM grades as well as several additional alloys).
Grades 1 through 4 are basically commercially pure titanium graded for ultimate tensile strength. Grade 5 (Ti6Al4V or Ti 6-4) is the most common Titanium alloy. It has 6% aluminum and 4% vanadium added.
Titanium is generally divided into 4 classes:
- Untreated, commercially pure Titanium
- Alpha alloys: Add Al, O, and/or N.
- Beta alloys: Add Mb, Fe, V, Cr, and/or Mn
- Mixed alpha + beta allows: A mixture of the adds for each is present.
Tooling for Machining Titanium
First thing is choosing the right tooling for Titanium. Don’t grab the first generic end mill you find and hope for good results. Follow these 6 rules to choose tooling that will work better for Titanium.
1. Use More Flutes when Machining Titanium
The properties of Titanium keep rpms and chiploads low. A 10 flute endmill is too tight for the chiploads most materials can handle, but it can work fine with Titanium, especially given you need to reduce radial engagements. Cranking up the number of flutes is the only way you can get cycle times into the realm of productivity.
The ability to keep more teeth engaged in the cut will also tend to reduce chatter. Titanium’s lack of stiffness can make it more susceptible to chatter.
2. Stay Sharp and Avoid Interrupted Cuts for Titanium Machining
Titanium is both tough and elastic (remember the low Young’s Moduls). What that means is we need sharp tooling to get under the surface and slice of a chip efficiently and without rubbing.
Unfortunately, that sharp edge is delicate. Try to avoid Interrupted Cuts which will hammer chips into your sharp edged tools.
3. Coatings Matter when Machining Titanium
Coatings can tremendously enhance your tooling’s ability to survive the conditions that Titanium machining products. A good coating to consider is TiAlN. It is particularly well suited to the temperatures found in Titanium machining, and it adds lubricity to help fight galling and chip welding.
If you’re having problems with TiAlN, you can also try Titanium carbo-nitride (TiCN). PVD diamond-coatings work well too.
Avoid ceramic and CBN. Ceramic won’t work with coolant and CBN’s binder degrades too quickly in titanium machining applications.
4. Insert Shapes for Titanium Machining
Round inserts (toroidal or button cutters) are great for titanium, but the shape won’t work for every application. Try to go with a cutter that mounts the insert with some lead angle. A 45 degree facemill is preferably to a 90 degree. The lead angle promotes chip thinnning, as does a round insert.
5. Try High-Feed Mills when Machining Titanium
The admonition to keep engagement low both axially and radially with Titanium is music to the ears of High-Feed enthusiasts. That’s precisely what those tools are designed to do well.
6. Secondary Relief Makes Your Tools Tougher
You need a sharp edge for titanium. A Secondary Relief simply means having some beef right behind the edge to help support it.
Tips for Machining Titanium
1. Reduce Radial Engagement to Control Heat
Titanium doesn’t conduct heat, almost the opposite. Keeping radial engagement low is critical. You have to leave the time the cutting edge is exposed to heat and leave time and clearance for coolant to work.
Finishing Titanium requires a very small percentage of the tool’s radius be in contact (i.e. small radial engagement), a honed cutting edge that’s super sharp, a high surface speed (hit the upper end of the range), and a very minimal feed per tooth.
If you must cut a slot, try to use a Trochoidal toolpath rather than full width engagement. For full slots, you’ll have to dial the rpms WAY back. It’ll depend on your tool and coatings, but it can get down to tens of SFM.
2. Prevent Tool Pullout
Sandvik’s high precision hydraulic chucks can really grip a tool tightly and prevent pullout…
Huge cutting forces, shocks, and vibration are all common when machining titanium. They’re also a perfect storm when it comes to pulling the tool shank right out of the holder. Even a little slip radically increases cut depth which raises cutting forces which causes more slip. It’s a vicious cycle and one you must avoid.
Use proper procedures with collets to make sure they hold as tight as they can. Keep everything clean and torque the collets to their optimal spec.
Weldon shank holders are even more secure from pullout as are hydraulic mill chucks.
3. Make Chips Thick-to-Thin: Climb Milling
That initial thick portion of the chip will carry away more heat. Getting rid of heat in the chip is essential because the more that goes in the chip, the less that will stay in the workpiece of tool. The thin end of chip is also less likely to weld onto the cutter, so Climb Milling delivers a double bonus when machining titanium.
4. Arc Into the Cut
This is a good strategy for many tough machining materials like stainless or high temp nickel alloys. The arc in reduces the shock as the tool is entering the material for the first time. If you feed straight in, your tool will often chip from the shock and it won’t live long after.
More about this in our article about CAM toolpath considerations for cutting.
5. Pre-Mill a Chamfer at End of Cut
You can arc out of a cut like a surfacing cut atop the material. But it’s harder to arc out of something like a slot where the cutter comes out of the wall. Putting a 45 degree chamfer at the end of the pass is a good way to minimize the shock of exit. Leaving that chamfer may mean planning ahead and leaving material for the chamfer that will be machined away later.
6. Choose a Much Smaller Tool Diameter than the Pocket’s Minimum Radii
With aluminum, we’re used to using a tool only slightly smaller in radius (diameter/2) than the minimum internal radius of a pocket. This can result in the tool being very shrouded from the coolant, and it’s a no-go when machining Titanium. Instead, choose a tool whose radius (diameter/2) is no more than 70% of that minimum internal radius. That’ll ensure room remains for plenty of coolant to get in.
7. Change Axial Depth Each Pass
Titanium is nasty, and a place where the is particularly strong tool wear is right at the top of the cutting zone. That interface where the chips release. That wear is so severe that you will benefit significantly from changing the amount of axial engagement on each pass when machining titanium.
That’s right, don’t make 3 equal depth passes to get to the bottom of your pocket. Make 3 passes where each pass is a different depth.
8. Limit Axial Depth with Thin Walls and Slender Features
Titanium’s flexibility means it wants to vibrate and chatter. Thin walls and slender features just make it worse. Any time a wall is more than 8x higher than it’s thickness (or a slender feature has the same issue), you need to take special care.
The best way to take care is to leave a finish allowance large enough to include the flex, and then make your finish pass with a low axial depth of cut but a high radial depth of cut.
9. Use High Pressure Coolant
Remember, titanium conducts heat very poorly. Your oven mits probably conduct heat better, LOL!
It’s hard to get the heat out of the workzone. Left there, it will wind up destroying our cutting tools and thermal effects will make holding tolerances difficult. Therefore, we want the best flood coolant setup we can get when machining titanium.
A high pressure coolant system really helps. Depending on the application, through spindle may also be critical.
You may also find that increasing coolant concentration is helpful with titanium. It needs the extra “juice” to promote lubrication and prevent chip welding.
10. Use Proper Feeds and Speeds
I often refer to a “Sweet Spot” balance of feedrate and spindle rpm:
The Sweet Spot is not shown to scale because it varies from one material to the next. Difficult materials have a very small sweet spot. The range of values that work is small. Accordingly, you want good feeds and speeds, preferably generated by a high quality feeds and speeds calculator like our G-Wizard.
Hey, if you don’t believe good Feeds & Speeds matter, check out this G-Wizard customer who made a Titanium AR-15 Lower Receiver on a tiny little CNC mill.
11. Rigid Machine and Rigid Workholding
Here come’s Titanium’s flexibility weakness again. Because it is relatively flexible, you’ll find your setups need to be as rigid as possible to cut down on vibration and chatter. The more rigid your machine and the more rigid the workholding, the better is is for titanium machining.
12. Try Plunge Milling and HSM
Plunge milling is one of my favorite roughing toolpaths for touch situations because it puts as much of the cutting force as possible up through the axis of the tool, which is the most rigid part of the tool and (usually) the CNC machine.
The only real downside is all the scallops mean Plunge Milling is strictly a roughing strategy. You will need to come through with a finishing pass to get the walls smooth for titanium machining.
Conclusion: Plan Ahead and Be Systematic and You’ve Got This
Whoa! That’s a lot to remember!
Wait. You’ve got this. You know what you need to do, how to pick the right tooling, and how to use that tooling. You know why, what the obstacles are that titanium machining must overcome.
The fact that there are a lot of steps just means you need to plan ahead and be systematic when machining titanium. Keep a checklist for titanium jobs and make sure you’re employing every strategy. Soon enough that checklist will be second nature.
But, until it is, it will get you through the problems and pitfalls.
You’ve got this.
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