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Old April 20, 2001, 06:43 AM   #1
Brett Bellmore
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I've got Bob Stewart's plans for the .50 cal rifle, and figured, what the heck, as long as I'm doing this, do it right. I've found a decently priced source for TI stock, http://catalog.ti-resource.com/browseGroup.cfm and I just have to decide which alloy to buy.

I'm dithering between 6-4, and 8-1-1. Anybody have experience machining either of these alloys?

Went ahead and bought the 8-1-1. Price was just too good to pass up, and if I don't use it in the gun, I can always work it into my battlebot.

[Edited by Brett Bellmore on 04-20-2001 at 08:29 AM]
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Old April 22, 2001, 10:02 AM   #2
Desert Dog
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Titanium is a strange bird to machine...

VERY slow feed rates, very slow turning speeds (two digit), shallow cuts, and good coolant flow. You have to use high-impact carbide bits. Very slow going, but can be done if you have the time and th patience.

I bet you spent a pretty penny for that material...

I personally would prefer 4340 cold rolled for a .50 cal rifle... A lot of recoil happening there.

JMO...

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Old April 22, 2001, 06:26 PM   #3
Brett Bellmore
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Thanks for the machining advice.

No, I didn't pay a pretty penny; That link I provided is to a company that sells TI and other alloy scrap, odd sized pieces of this and that, and the prices are decent. About $7 a pound for the better TI alloys.

Been looking over the relative properties, and I must admit that 4340 you recommend has some impressive properties. The titanium isn't quite as strong, though it's got a better strength to weight ratio. Have to think about this. Msybe I can find a use for it in something else.
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Old April 23, 2001, 12:48 AM   #4
C.R.Sam
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Ti strength to weight much better than steel....BUT...check carefully the tensile and yeild strengths for your material. Don't have info at hand but think you would need considerably greater cross section of material to get same strength as 4340.

The stuff is quite notch sensitive so one wants to make sure all tool marks and scratches are blended. And all inside angles are radiused.

Like D Dog said...figure bout machine time of medium carbon steel times six or seven.

Neat soundin project.

Sam
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Old April 23, 2001, 10:06 AM   #5
Brett Bellmore
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4340 properties, according to http://www.macsteel.com/mdb/general_...loy_steels.htm

Normalized, tensile strength 185,500 psi, yield strength 125,000 psi, elongation 12.2%, Brinell hardness 363

8-1-1 properties, according to
http://www.matweb.com/SpecificMateri...&group=General

Anealed and aged. Tensile strength 147,900 psi, yield strength 130,500 psi, elongation 13%, hardness ?

Heat treated, tensile strength 171,100 psi, yield strength 155,200 psi, elongation 17%

Think I might just use this chunk, but get it heat treated.
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Old April 23, 2001, 01:29 PM   #6
C.R.Sam
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My Jorgensen handbook agrees with your findings re 4340 at 363 Brinell (soft)

It also shows hardened 4340 (555 Brinell/55 rockwell C) to have 280,000+ Tensile, 220,000+ Yield and 10% elongation.

I have made some parts for the M-19, 40mm repeater and If I recall correctly, the hardness specs were in the 53 to 56 Rc range.

One of the nice things about medium carbon 4340 is that it is hardenable all the way through in fairly large sections. And....it can be hardened while remaining quite tough.

Not tryin to throw a damp diaper at the project but since the .50 is not only a relatively high pressure round, it applies that pressure against a large area. If after thorough research, you do it and it works......that would be way cool.

Sam

Sam
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Old April 23, 2001, 04:38 PM   #7
Brett Bellmore
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No, no, I appreciate the information. Since I'm a CAD operator and FEA analyst, I'm going to take these material properties and Stewart's design, and hunt up some info on .50 time/pressure profiles, (Ought to be out there somewhere!) and do some computer modeling. I'm going to be VERY satisfied this thing is safe, before the chips start flying.

Of course, the first few shots will triggered with a cord from behind a pile of sandbags. Part of learning about FEA is learning how little to trust it...
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