Shop Doc – Thread

Today’s Machining World Archive: April 2007, Vol.3, Issue 04

We are a Davenport shop having a problem doing a 1/4 -20 thread in 303 stainless
steel. No matter what we try we are getting torn threads. Someone suggested
changing the slip clutches to disc clutches. Before we spend the money, is there
another solution?

By a Thread

Dear Thread,

Disc clutches are a good style, but slip clutches can do this job. I will go through some troubleshooting methods for slip clutches.

Make sure your spindle speed gears are not rubbing on the threading gears, because they are running different rpm. That will cause the machine to labor and affect threading.

Check the shifting cam for the clutches to see if it has moved. The 3/8” square head collar screws are too long when they’re brand new and can bottom out inside the hole. You might think the cam is tight, but it’s not. Grind off two threads and put the screws back in.

Now inspect the clutches themselves. Take the clutches apart and inspect the parts for cracks, loose carbide blocks, undersize bearings and deep pit marks from the bearings. Pit marks are the reason we do not leave the clutches hooked up and shifting while they are not in use. Without centrifugal force to hold the bearings to the outside and pit evenly, gravity will obviously hold the bearings at the bottom, causing pitting on one side of the clutches.

Clutch cones can still be garbage if the keys are intact. If clutch cones are not gripping, you can cut them in half. You can now buy them in two pieces. Brand new clutch cones are 1.010″ or 1.011″ in length. Measure the length of the used cones (minimum length is 0.995″). If the cones are shorter than this they are garbage. Cones with bronze keys are definitely stronger but are never needed on the high side. As it is, the low side does all the work of cutting and forming the threads. The high side is just chasing itself out of a threaded hole. An easy way to tell if your clutches are slipping is to just put your hand on the clutch. It should feel warm, but if it feels hot it means it is slipping instead of driving.

A trick I use to check the timing of the shift is to put a tommy bar in the hole on the low side of the clutches, and a tommy bar in the hole of the cam lever on the threading spindle. As the machine is running empty watch the two tommy bars. The bar in the clutches should shift just before the bar on the cam lever starts to drop back. Set the depth for 4 or 5 threads to see how much pullout you have – it just needs a little pullout. This is called the “cushion.” I hope this solves your problem.

Jim Gross
Davenport Doctor
Niagara Falls, Ontario Canada


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