Hi Richard,

Just in case it ever comes up, you might want to know the following. You might get to impress your friend (or whatever you call him) by helping him restore some of the value to his expensive drill.

Actually I wasn't going to bother with this response because the timbre of your posts suggest that you're not really interested in helping this guy out, even if you could. But I remember Chris Bertrand mentioning that he uses T-Tech routers to make circuit boards, so this information might be useful to him as well.

I am not familiar with the T-Tech system, but the LPKF software does let the user adjust both the revolution speeds and the lateral feed rates of every tool in the library. Moreover, the software keeps track of each tool's usage so as to warn when the specified lifetime of the tool is reached. For users like me who are cost conscious (polite for "cheap"), that warning, and the ones that follow it (the software continues to issue such warnings until you change the tool), serve as a reminder to start paying attention to the tool's performance.

When the LPKF universal cutter bit begins to wear out, it stops penetrating all the way through the copper to the substrate, but only in certain areas of the board. In other areas it still works fine. It can be quite puzzling, especially to a non-machinist, but once you understand the dynamics and effects of what's happening it all makes sense. Also, if you are going to squeeze as much life out of each tool as possible, you must understand what's happening and why.

First, you need to know that smaller diameter tools require higher revolution speeds in order to maximize their useful lifetime. Running them at slower revolution speeds will cause them to wear out sooner. It is also important to know that the smaller the diameter of the tool, the shorter it's lifetime will be.

Knowing these two things, consider the functional tip of the universal cutter. It has a V-shaped tip that cuts a V-shaped groove in the board (penetrating all the way into the substrate). The user adjusts the width of the cut by adjusting the depth of the cut. This geometry means that two things are true. The tip of the bit has a smaller diameter, and the tip rotates more slowly, than the bulk of cutting edges. Remember that both of these facts, slower revolution speed and smaller diameter, independently imply that the tip of the bit wears out faster than the bulk of the cutting edges.

Now add the fact that the copper layer is not perfectly uniform, and you have a circumstance where a worn bit will not fully penetrate the copper layer in all areas. This is exactly what LPKF users see when bits reach the end of their lifetime. The bits actually begin to map out microscopic fluctuations in the copper layer's thickness in the form of poorly milled paths.

The intuitive response is to adjust the depth of the cutter. And to some extent this will work to extend the lifetime of the bit. But remember that the deeper you set the bit, the wider your channel will be. Of course, you have to be milling pretty thin traces for that to become a problem.

No, the first problem you will experience is burring. The tip may wear out first, but the rest of the cutting edges wear out too. And when they do they begin to leave burrs along the edges of the milled channels. These burrs combined with copper left in the bottom of the groove give an appearance that could easily be described as "smearing" the copper. Initially the burrs can be easily removed with ScotchBrite, but with continued tool use they become worse until you are sacrificing production reliability for extended tool life.

The bottom line is that you can extend the life of your tools, but only so far. In my experience there have been times that I was able to adjust the depth of cut as many as three times in order to extend a tool's life. On the other hand there have been times that I had to change the bit after only one such adjustment. But, by understanding what is happening, and with a bit of experience (no pun intended), you can maximize each tool's lifetime on a case by case basis. But in the end you really have to accept the cost of tools as part of the board cost. You have to think of the bits as consumables, not as either assets or equipment.

Joe