Shop Doc – Dysfunctional Machine

Today’s Machining World Archives September 2010 Volume 06 Issue 07

Dear Shop Doc,

We have a machine that occasionally frustrates us to no end. Sometimes a dimension goes out and we can’t get it back unless we touch all the tools off again. Then it works fine. I’ve had it, I’m almost ready to sell the machine. Can you help?

Touched-off in Dayton, Tenn.

Dear Touched-Off,

I’ve had similar experiences with CNC machines. Normally, when the wear offset value is adjusted, the dimension on the part closely follows the offset change. Life is good when this is happening. Unfortunately, I’ve had some experiences when it didn’t work quite this way because certain offset changes led to a condition where there was not enough material to make the complete part. Basically, I’m referring to a non-cleanup condition, which can be difficult to detect on parts made from a solid bar.

My most-recent experience with a similar problem was on a Takisawa lathe. It cut the OD and right end of the part from a bar, then the pickoff spindle moved over, clamped the part, fed the bar out for the next part, cut the part off the bar, and returned home to finish the part ID and left end.

After a relatively new operator had been running the machine for a few hours and making miscellaneous offset adjustments, he ended up in a situation where he couldn’t bring a specific dimension back in. The offsets, which were supposed to control the surface, no longer had any effect. He had no idea what he did to make this happen and attempts at resolving the situation made things worse.

Working with a more experienced operator, we inspected the tools in the machine to make sure none had built-up edges or other problems. They were all fine. Next, we tried to paint the end of the bar with layout dye and verify that the tool was actually cutting the part. We found that it wasn’t even touching it anymore. This was an example of the proverbial “I keep cutting it, but it’s still too short.”

The machined surface on the end of the part was the same surface that was left over from the previous part cut-off. The bar was no longer coming out far enough to allow for full clean-up of the surface.

To find out why, I looked at the program to determine how far the bar was fed after cut-off and calculated how much cleanup stock was available on the ends. The total cleanup stock was only 0.335 mm, or about .0065” per end. It was small enough to increase the chances of this problem occurring.

As it turned out, the operator had made a Z- change to the cut-off location. By making this change, he actually reduced the bar protrusion past the main spindle. The machine relied on a certain amount of bar-length from the main spindle to ensure enough clean-up stock on the right side of the part.

The solution was to simply move the cut-off tool 0.2 mm in the Z+ direction, which allowed a full cleanup of both ends of the part.

In this example, the problem was with both the roughing and finishing tools. I have seen other instances where the rougher removed too much material and the finisher didn’t have enough clean-up stock remaining.

A relatively inexperienced operator may not have had a full understanding of how the program and machine functioned.
The problem of too little clean-up stock can be fixed easily, as can creating some type of visual reference for the operators to use.

If you are having this type of problem with your dimensions and wear offsets, pull out the layout dye and make sure the tools are actually cutting the surfaces as intended. Give the operators training on how to identify and remedy this situation. Make some changes to the program to reduce the chances of this problem occurring.

Mark Bos
Robert Bosch Fuel Systems