I need to machine a test specimen for a tensile test machine. The dimensions are given in the attached file. What would be the best machining method? One has to use a round insert because left-hand, right-hand as well as neutral tool with diamond-shaped insert will all cause interference at the shoulder which has an arc at 90 degree. Do I have to machine the left shoulder (say, from half the length to the left shoulder) using right-hand tool, right shoulder using left-hand tool, leaving some allowance, to be followed by neutral tool with round insert, for finishing? Or can everything be done by a neutral tool with a round insert? If yes, what can be maximum depth of cut with the round insert of diameter 6 mm, if the workpiece is mild steel, and the spindle is 5kW motor? I have an intuitive feeling that a round insert is not meant for heavy cuts (say, 1-2 mm), specially when it plunges into the workpiece, before straight turning starts. Am I correct?
I regularly make similar tensile specimens out of ¾” steel and aluminum bar stock. The gage section of the specimens is turned down to ½”. I make them on a Warner & Swasey turret lathe. The tooling used is a Kennametal tool holder with ½” round insert. The round tool is used to make all cuts in the gage section of the part. The profile cut is made in 3 passes two .050” ruffling pass and one .025 finish pass. Each pass starts at the right hand side of the part and then plunges in, then cuts towards the head of the machine. The machine Is old, so the spindle speeds are low in my case (750 rpm) and the surface finish would probably be better if cut at a faster speed. But in all of my trials with these parts I have found that cutting the profile in one motion is best, because if there is any step or mark from using two different tools to cut the radius and the gage section it will result in a stress point that can throw off the tensile testing.
machining tensile test specimen
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