At Winn Mfg. we have CNC machined (4) 100 piece lots of the part in this photo over the past 18 to 24 months. The material is a 1141 cold finish bar and the Acme threaded section is approximately 11-1/2 inches long. All of the turning operations are done our CNC lathe with a 10″ chuck.

The Process

The first operation of this shaft was to face and center one end, flip the part around and then turn the short straight section. A set a steel soft jaws were bored out to hold the bar stock. We built a custom stop that mounts onto the back of the rotary chuck actuator of the CNC lathe. It has a bearing built into the workstop, so it rotates with the work piece and it worked very well. This workstop allowed us to hold the tolerance of the 18-3/4″ overall length of the shaft to within +/-.002. This was much tighter than the drawing requirements, but it helped having this feature held closer for the next couple of CNC machining operations.
The part was then chucked on the small turned end with soft jaws and with the tail stock center engaged. The stock is 2-1/2 diameter and is turned to the major diameter for the 2-3/8 – 5 TPI Acme thread and turned a bearing fit and locknut thread.

CNC Machining Concerns

We had concerns when we quoted this shaft with the 2-3/8″ diameter X 5 TPI Acme thread. We expected that we were going to get chatter when cutting the threaded section, because it was so long and would be unsupported.

CNC Machining Solution:

We did a lot of research into whose cutting tools should be used. We ended up using Carmex’s laydown threading inserts. A size 22ER and grade MXC. Used the speeds and depth of cut they recommend. There was absolutely no chatter when cutting the thread.The only issue we had was when we did the first lot, determining what a good tool life would be. With the length of the thread, we could only safely get 10 parts per insert tip. This was an acceptable tool life and it gave us a high quality thread form. I believe that the thread form is an important feature of an Acme thread and there needs close attention paid to it.

The last CNC machining operation is a simple keyway for the locknut thread. This operation was done on our CNC Bridgeport retrofit. It took longer to deburr the keyway than it took to mill it.

I invite you to look through our website. We have being supplying CNC machining services to this customer since 1995 and are always looking to build another vendor/customer relationship.