screwcutting problems

[hedgehog]

i have recently encountered a problem when trying to manufacture male thread gauges on our cnc lathe.when i checked the thread diameter with thread parallels the o/d.was .008 below required size,this occurred using nr/er 14-20 un inserts.Anyone know what could be causing this?.

[HuFlungDung]

Thread parallels (wires or triangles) are used to determine the pitch diameter of the thread, not the outside diameter. A calculation is involved to determine from the micrometer measurement of the parallels, exactly what the pitch diameter is.

To me, it sounds like a simple case of overcutting the depth. So, back off the tool offset.

[hedgehog]

thanks for the reply huflungdung,just to clarify turned diameter is 1.75 inches,after screwcutting this diameter is reduced to 1.742 inches .Am i calculating the reading correctly,i.e.turned o/d.minus thread depth plus thickness of parallels(.250).i could not get the required reading with the inserts i mentioned,i have since tried using ag60 inserts which seem to work ok.usually we manufacture the female component first then screwcut the male component to suit,however this is a new order and we have no gauges to suit the five different unf threads.this is the first time i have had to make the gauges and i have only used thread parallels once or twice many years ago.

[gripper]

Most threads will read under size when cut on cnc with inserts, as these will round the tops off. What i do is to turn the o/d to the right size and then blue the o/d. Start threading with the wear in the x about .2 up and then rethreading taking small cuts until the right diameter is reached and the blue just off. If you then use your thread parallels do not use your formual, has you should look for the effective diameter + your parallels in a Zusu book or the machinist bible. You can also use 2 small same size drill blanks to measure this. hope this helps, we do this alot as we are a small sub contract company and have to make a lot of threads for scientific research and development companys and some times we only have to make the male thread and this is the only to get it right if you don't have the guages.