Where Does Diamond Make Sense In Machining Titanium?
Boeing machining experts say one application is drilling Ti-Composite stacks. PCD does well here. The group recently addressed this question submitted through our Aerospace Machining Zone.
Personnel from Boeing's Research & Technology group recently answered readers' questions. One reader asked the following. To see the list of experts taking questions from readers right now, see our Ask An Expert page.
I am curious about titanium. I know that diamond tooling has been successful in machining various grades of aluminum, but have you seen PCD at all in any applications regarding titanium? If so, can you comment on the pros and cons of diamond tooling in this application?
Response from Boeing’s Research & Technology group
For applications machining titanium 6Al-4V alone, our team has seen limited advantages to PCD coatings. However, we have seen advantages when drilling composite-titanium stacks. The PCD holds up well cutting the abrasive composite. The PCD costs more, but produces quality holes for a longer period of time.
If you choose to PCD-coat a carbide cutting tool for an application such as this, we recommend the carbide substrate cobalt content to be around 6%. If the cobalt content is higher than 6%, you risk cobalt leaching disbanding the diamond coating from the substrate.
When it comes to milling applications, we have run numerous simple tests where we have a cutting tool coated and make a single pass across a titanium block. Looking at the tool under 100x magnifications, we frequently see the coating removed and the substrate exposed after a single pass. This can’t bee seen with the naked eye, but the coating is gone from where it matters most—the cutting edge.
Another concern is that when a tool is coated, the cutting edge is dulled slightly. This is very detrimental to titanium 6A1-4V machining. We always recommend sharp tools for titanium. The sharper the cutter is, the less heat that is generated, resulting in longer tool life.
Liquid coolant offers advantages unrelated to temperature. Forced air is the fluid of choice in this shop...but even so, conventional coolant can't be eliminated entirely.
Cutting holes by interpolating a face milling cutter may be a better process choice for many rough and even finish boring operations. Software improvements and better cutter designs allow expanding use of the versatile face mill for hole making.
One of the most common methods of tapping in use today on CNC machines is 'rigid tapping' or 'synchronous feed tapping.' A rigid tapping cycle synchronizes the machine spindle rotation and feed to match a specific thread pitch. Since the feed into the hole is synchronized, in theory a solid holder without any tension-compression can be used.