Machining Titanium

Years ago, I was an apprentice machining huge Ti forgings for the engine beams of VC10 aircraft on Droop and Rein hydro copy mills. The forgings were about 8 feet long by 4 feet deep. The cutters were all nitrided HSS and cut using plenty of soluble oil. Machine setters set up the feed and speed of the cutters and as apprentices, we 'rode' the mill, and read our books / playboy mags and LISTENED to the cut as it proceeded. When the sound changed, we stopped the cut and called the machine setter for a new cutter. If we left it, the cutting sound changed to a 'squeak', then either the 2inch diameter cutter snapped, or the forging got pushed away from its mounting jig. In either case, it was hell to pay. The 'damaged' area had to be ground out (removing the work hardened area) before the cut could be resumed. This may have taken a day or so to recover the cut. My experience was that you spent as much time sharpening cutters and setting up as machining.

Cobalt drills cut like butter on Titanium......HSS taps can be used, but carefully....

I tried to make some light-weight piston heads for a match target air rifle I was tuning, turning the outside and screw-cutting no problem, last job was to drill and tap an 8mm hole in the centre. No chance, the drill just heated up and that was that. Work hardened. Is it possible to anneal a titanium part that has work hardened like this?

The only way to salvage the part is to cut through the hardened section with a larger endmill or such, but it will ruin your cutter. Very low revs, hand feed so you can "feel" the cut and lots of oil to lubricate. The hardening is only local so when you are through the hard section it will be ok. If it was the tapping drill that burnt out then tapping will be impossible. If there is material on the diameter then grind a small boring bar up and take small licks until you get a diameter of 7 to 7.1mm for M8

Use a spiral tap with lots of oil and take your time, heat build up at the cutting tip will cause the metal to swell and grab.....

Tips for drilling;

Never pilot drill

grind the tip of the drill offset and it will cut oversize - won't drag on the bore already cut and overheat..

Lots and lots and lots of lube....retract the drill every 2-3seconds.

If you cannot drill finished size, use the biggest drill you can and bore the rest.

Trevor

Edited By Trevor Wright on 24/04/2013 12:58:17

Thanks Trevor. I chipped a carbide endmill trying to cut out the hardened area so I gave up on it. I note for future reference your comment about 'never pilot drill' because indeed that is what I did.

Hi guys, is there no heat treatment that will soften the titanium?

Hi, I have very few particulars on the 20mm thick Titanium 6"x4" blocks I worked on once and once only for Aero Space in Bristol. They gave us a few samples to experiment on and the long and short of it all was I burnt an ID profile in them using oxy propane. It cut like butter and later Aero space gave us about 100 parts to profile.

There were no issues with fire or anything...from a profiling point of view it was easy peasy.

As I say I've no idea what the spec was but the customer sure was happy with the result.

Jesse

I turned some commercially pure Titanium rod on my 8 x 14 lathe with HSS tooling. Worked fine until I let it rub as the tool dulled and the swarf caught fire! Best not to look at burning Titanium I understand due to UV given off. The part was saved though and looked nice after sand blasting to match the other parts. I have a couple of billets of 6Al 4V titanium and it's a bit tougher to cut. Sawing it by hand is hard work and takes ages.

I've hand-sawn grade 5 and I'd rather not do that again. It can be hammer formed cold (0.7mm sheet), but it takes some effort to get it to take a permanent set and the stuff hits you back. Grade 1 is far more forgiving.

Hot forging is much easier. Some US smiths have rather snazzy forging tongs made of Ti, the lower heat conduction is supposed to be a boon (although in reality I suspect it's for bragging rights).

A laser welder will join Ti happily (under argon), but a) it's only good for thin pieces, say up to 2mm, and b) the weld can still end up brittle - I need to see if I can anneal it successfully. Can't do vacuum annealing, but I can use tool wrap and some charcoal to scavenge O2.

I also have a few small blocks of aero origin, no idea what alloy though.

I hope not. Ti forging tongs would be very dangerous. Pure Ti is highly inflamable at temperatures over about 600 C. I suspect they are made from one of the high temperature alloys.

Russell.

Solid section bar is not going to spontaneously combust at 600C given the MP is 1668C. Turnings will go up with less encouragement due to their greater surface area. I can get steel to ignite at room temperature, never mind any of the pyrophoric metals.

As for precisely which alloy of Ti they're using, no idea. Probably whatever could be scavenged.

For more information on hand forging titanium, may I suggest this as a possible starting point - http://www.iforgeiron.com/topic/22934-help-with-forging-titanium/

On combustion (specific to aero engines) - http://www.fire.tc.faa.gov/pdf/fsr-0457.pdf

There are Ti specific soluble cutting fluid. Like others have said, sharp tools, low speeds and relatively high feeds.The centre cut point drills work a lot better than regular drills. Sharp positive rake carbide turning and boring tools also work very well. HSS works well , but does dull a lot quicker than HSS. As soon as heat starts to build, resharpen the tool or go to a slower speed. When drilling through plate, I put a piece of steel underneath for the drill to break into. On the lathe I try to avoid breaking through where possible and prefer to part off or face off the back end. Ti also expands quite a lot, and is a slow conductor of heat. When drilling the common problem is the material getting hot, and then contracting as the drill is either retracted or when more coolant is poured down the hole.

Neil

Ignition temperature has nothing to do with melting point or surface area. Increased surface area increases the rate of combustion. Many materials will burn well below their melting point.

Yes, there are many different alloys of titanium but Ti is the chemical symbol for the element titanium and refers to the pure element rather than any alloy. I was referring, somewhat tongue in cheek, to the misuse of the symbol - should have used a smiley.

Russell

PS. How do you get steel to burn at room temperature?

Steel wool, drop it in pure O2

*Technically* constitutes burning it at room temp, but obviously it's the rate of reaction on oxidation that bumps the heat up locally.

The ultimate coolant! And here.

Edited By Muzzer on 25/01/2015 17:47:44