Advice on machining titanium

Hi,
The question is as title : I have heard that titanium is difficult to machine and I wanted to know what advice anyone here can give me on machining 6AL-4V
(Grade 5) Titanium rod on my Boxford ? I mean things like cutting speeds / coolant / feed rates and general aob.
Many thanks,
--
Boo

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I had occasion recently to research this, though I have not (yet) had to put it into practice myself. FWIW, here are the basics I learned - please accept it for what it is, paper research not years of hands-on experience, and worth exactly what you paid for it, but maybe a good indication of areas to research. There are several suppliers and institutes that will be able to give more specific detail. However, FWIW:
Ti has a poor thermal conductivity compared with steel, which means that higher temperatures are created at the cutting interface. It is also prone to work hardening, which can create a tough skin. To make matters worse, although the bulk metal is passivated by an impervious layer of TiO2, the freshly exposed metal is very reactive, and tends to react with the tool tip.
All this means that slow feeds but fairly heavy cuts, with high coolant flow, are desirable, the feeds and coolant to lower temperatures, the deep-ish cut to ensure penetration of the hardened surface layer. Use a *very* sharp tool as any trace of bluntness will make all these things much worse.
If drilling, using a gun drill with coolant supplied down the drill shaft is recommended. Also, if the bore is large enough, consider finishing it by reaming and honing to remove surface cracks.
Water jet cutting is a very effective way of producing shapes from sheet, and some simpler 3-D shapes too.
Bear in mind also that the elastic modulus is about half that of steel, so you need to guard against workpiece deflection - which of course conflicts with the above need for deep cuts! Good support is obviously the answer. However, if your component is likely to be fatigue-critical, then you should also try to avoid work-hardening the surface, as this is likely to increase the risk of surface cracks.
6-4 titanium is significantly tougher to machine than CP (commercially pure) titanium, as many of the above problems are exacerbated.
Again, if the application is likely to be fatigue-critical, look carefully at the heat treatment of the alloy; most high-tech uses require multi-stage treatments to get the desirable crystal structure. Shot peening is also recommended in such cases, as this can give very substantial improvements in fatigue performance.
Obviously this is not a high volume aerospace need, or you would not be asking us bunch of reprobates - there are very detailed protocols for such work, involving continuous monitoring of tool life, such as by torque monitoring, etc. etc. If you can give us some idea what the important needs are we may be able to add further thoughts.
David
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David Littlewood

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On Mon, 05 Mar 2007 22:17:06 +0000, Boo

Can't help you with that particular grade, but I have tinkered a little with some offcuts of anonymous grade bar that I picked up which seemed to cut reasonably freely.
However, I know that this is material that has to be treated with a certain degree of caution, as it is possible to ignite the chips if you cut fast/hard enough without coolant; starting a Titanium fire in your lathe swarf tray won't be a pleasant experience.
Regards, Tony
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Boo wrote:

No fear! Never let the tool skate or it work hardens the surface and is a bugger to get going again. Dead sharp HSS tooling at a reasonable speed and it turns nicely.
If drilling, again, he who hesitates is lost. If the drill squeeks, yer buggered! Low(ish) speeds, positive feeds, sharp tools. Esp. the sharp tools!
Cheers Trevor Jones
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wrote:

Boo
As I'm sure you will know the grade you have chosen is the "dreaded" aircraft grade that caused a lot of problems when first introduced. In reality as others have said it is workable with proper care and attention. We used to machine a lot of this where I worked and in general for turning and face milling we would use standard Tungsten Carbide (C-2) and for drilling tapping and end milling we would use HSS or a High Cobalt Steel. Tools MUST be sharp and the instant they start to dull they must be replaced; one of the "tricks" the Ti will play on tools is to turn them from sharp to scrap in a "squeak". Sorry had to include that as it was a saying that my apprentice instructor liked to use constantly.
A couple of other points the material has a relatively low modulus of elasticity and rod will "push away" from the tool much more than steel so good support is a necessity. I agree with the advice you have been given relatively slow speed and high feed rates, do not stop the feed when the tool is in contact with the work as it will instantly render the tool useless. Plenty of coolant and here your intended use is important, if you are going to use it in a fatique critical area then the type of coolant is critical. Soluable oil (at 15-1) works OK but for critical aerospace components avoid chlorinated oils.
For engine components we would typically use something like, C2 tools, 150 sfpm speed, 0.200" depth of cut and a feed of 0.010" per rev for roughing. For finishing it would be C2 tool, 200 sfpm speed, and a depth of cut never less than 0.010" with a feed never less than 0.006" per rev. With small diameter bar this is obviously not possible but you get the general gist. Again tool sharpness is the most critical factor. Dry machining is possible but as Tony says you might not like the consequences of getting it wrong.
We would always cold work holes etc as machining will produce microscopic surface cracking of this grade of Ti so if you are machining anything that your life depends on you will need to research the specific alloy as heat treatment might well be required.
You may have found this information but if not it gives a good basic starting point for our type of equipment, advice for the modern CNC high volume production areas will differ and be specific for a particular alloy.
http://www.supraalloys.com/Machining_titanium.htm
Best regards
Keith
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jontom snipped-for-privacy@hotmail.com writes

Keith - thanks for the detailed information - and for the website, which I had not happened across. Always better to hear from someone who has actually done it. I will file it for reference, as I also may need to machine some Ti at some point - not 6-4, but another very tough alloy.
David
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David Littlewood

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Hi,
I found this while searching a while back....might be of help/interest.
http://www.mhims.co.uk/titanium-bible.pdf
Michael
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Thanks to all for the informative answers.
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Boo

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