Here's the question:
If you're boring 4140 material in a manual lathe. Minimum bore is .640 and need to bore to progressively larger diameters in steps of .010 - .015 with depths of the bore up to 4”, what tool would be best?
Our recommendation:
Aa 1/2 Kennametal Carbide Boring Bar E08-STFPR2 because it has a 0 degree lead angle and will be able to create the small 90 steps inside the bore. The insert is a TPMT21.50.5FP in a grade KCU10. The .008 nose radius will allow the user to create well defined steps and combined with the FP chipbreaker will minimize tool pressure and deflection.
Do you have any other suggestions? Maybe you prefer another brand? Share with us.
I have bored .640 dia holes in 4140 many times on a manual lathe. .640 dia is a standard dia for back bored 12 guage shotguns. I use a "Mecabore" style boring bar (availiable from MSC) with the sharp boring inserts. You did not mention if the main bore is a thru hole.
Keep in mind that a steel boring bar only has a length to diameter ratio of 4:1 and a typical carbide bar is 6:1. As you extend your overhang beyond that, chatter starts to become an issue. You can combat that a little bit by taking steps to reduce tool pressure like using a smaller nose radius, taking lighter cuts, and/or using a more positive cutting edge. There are a few carbide boring bar manufacturers that claim their bars will give a 10:1 L/D ratio if you set their tools .010" above centerline.