I have a book "Machining Fundamentals" by John R. Walker that says for high speed steel lathe bits:

low carbon steel:
8 to 12 degrees back rake
14 to 18 degrees side rake

high carbon steel:
4 to 6 degrees back rake
8 to 10 degrees side rake

I want to cut 60 degree and 55 degree V threads on 4140 steel barrels.

What back and side rake angle should I use?

Clark---

That's like inch pounds of torque on tang screws....I've never measured it. Grind a tool and try it.

Top rake is usually less than 5 degrees positive and side rake is enough to not drag on the threads. 12TPI means it has to have more than normal rake.......I'm sure there's a clearance angle depending on pitch, but it's easier to chuck up a piece of scrap and try it. When you get it right, measure it if you like. I never have.

Like Jack I just eyeball it. The most important part is to measure the angles with a pitch gauge to make sure they're 60 or 55 and to make sure they're square to the surface you're cutting.

Well I hesitated to answer this because I too eyeball it. Thought maybe somebody actually did do it by degrees. It sure helps when grinding the V angle to have the top of the tool as flat as possible, and I've cut a lot of threads with a near flat on top tool.

The angles depend on how the steel cuts, that is what type of chips are formed .Typical barrel steel is a free machining grade of 4140 ,that cuts nicely. That would require rakes more like the high carbon steel mentioned .But there are other variables ,speed ,lube etc. Make sure you hone the tool and pick the angles that work best for you.

I checked in Machinery's Handbook, and there is a very complex formula for calculating the rake angles based on the helix angle and the major diameter. It's my opinion that the helix angle of the bit is more important than the relief angles. I have a chart of helix angles that is based on threads per inch in relation to major diameter of the threads. This angle can run from less than half a degree to almost 7 degrees. If you let me know the diameter and pitch of the threads, I'll look up the helix angle for you.

How are you grinding the bits? Do you have access to a surface grinder and a fixture to hold the bit? If you're grinding by hand, just grind one and try it on some scrap material. If it works, go for it! The precise angles are used mostly for production work, where tool life is increased greatly by precision grinding.

I have access to a surface grinder, but still grind most of mine by hand.

Nashcat

Clark,

I know you want to know angles and such, but do yourself a favor, get with your local tool supplier and have him/her show you something in Carbide.

For cutting threads, turning steel, parting, boring, milling, profiling, etc., there is nothing like carbide. For threads I use a Kennametal tool holder and KC850 inserts. If I am cutting threads for an 03, there is an insert available for these as well. Talk about precision threading...

Malm

I went to carbide along time ago. MSC ususally has sales on carbide tool holders and inserts in 10,12, 14 and 16 TPI for external threading. This way you only need to be sure your toolpost is square with the barrel shank with the compound set at 29 degrees.. The inserts are available in full profile or partial profile configurations. The threads are cut by advancing the compound.
The resultant threads are absolutely beautiful with Dark thread cutting oil.-Rob

I just got an import carbide grinder with side tables from J&L, and made some fixtures so I can grind the facets of the bit.

I have alot of tool bits and blanks of Cobalt tool steel from Boeing Surplus.

JBelk was right.
I ground some bits with the new grinder and the angle did not matter that much.

clark... at school they teach us to grind our own lathe bits... and you can aquire tables to help, however there is not only a learning curve but an art to it as you are doing by hand... who can hold 1' deg by hand on a 3/8" stick? I go over some of mine with a bench stone when I am done... but then again I dont know any better anyways. I buy my shapes for threads and most work now, but believe everyone should learn how to grind your own bits... its usefull and you can get alot of life out of some cheap cobalt from india

My brother went to community college and got a 2 degree in machining and welding when he was 20 years old. I have seen him grind lathe bits by hand and make barrel threads like jewelry, but lately [he is now 43] he can't do it anymore because of eyesight. I am 51, and I got a copy of "Machining Fundamentals" by Walker, the text he used in college, and I even have the Clausing 5914 lathe used in the illistrations, but there is no way I can free hand grind anything.

I built some jigs to slide on the tables on the side of the new ginder, and I can make any bit I want in about 15 minutes.

I just theaded my first barrel, and the threads look ok, as long as I don't use a microscope.

Buy Carbide external threading inserts-full profile. Your threads will look like jewlery!The only Hss stuff I use anymore is for flycutters for aluminum!-Rob