Shop Doc – Mark S. Begone

Today’s Machining World Archive: January 2008, Vol.4, Issue 01

Dear Shop Doc,

I¹m having an issue with a job we run on a CNC Swiss. If I adjust the bushing tight enough to eliminate out of roundness and chatter on the turned diameters, the guide bushing will scratch the bar diameter. The finished part has a long section which we would prefer not to turn, but the scratches are unacceptable.

Mark S. Begone

Dear Mark,

There are two solutions to the problem; I¹ll start with the better one. A guide bushing can only be ground to one diameter; if your bar is smaller or larger the fit won¹t be as good. If you add in variations in the guide bushing adapter taper and fit, the bushing may not fit the bar well at all. Start by taking a die grinder or Dremel type motorized tool, and using a diamond coated bit, grind a chamfer on the front and back edges of the carbide pads inside the bushing. Then take a cylindrical shaped bit and lay it into the slots between the carbide pads to break those edges as well.

Next you need to hone or lap the bushing using diamond lapping compound. Diamond compound is sold in different grades. It is color coded with the color relating to the micron size or mesh size. You should keep three grades on hand: red 22-36 micron or 600 mesh, blue 12-22 micron or 1,200 mesh, and orange 4-9 micron or 3,000 mesh. Use the red compound to repair a worn or damaged bushing or to dimension an over sized or undersized bushing. Follow with blue and then orange to finish.

Use a chamfered bar end (remnant) for lapping. Begin by adjusting the bushing to the remnant so that it fits tightly but can still be removed by hand. Chuck the remnant in the sub spindle and get it to run true. Put a small, pea size dollop of compound on the end of the remnant and smear it around. Add a few drops of cutting oil to the smear and you are ready to begin honing the bushing. Counter rotate the main and sub spindles around 100 rpm each. Then, using the MPG, jog the bar end in and out of the guide bushing rapidly. If the bar squeaks or squeals, or if you see a wisp of smoke, stop and add more oil to the bar end and/or loosen the bushing.

Work the bar end in and out of the bushing until it fits loosely. The compound should turn black from the carbide if all is working properly. Next, stop the spindles. Adjust the bushing ever so slightly tighter and repeat the honing process. Repeat the entire process until you see the carbide pads have a uniform finish over at least 80 percent of the surface with the front end completely honed. Switch to a fresh bar end and repeat the process with the finer compound. Take it slowly the first couple of times you hone the bushing. Once you get the hang of the process, it only takes a couple of minutes. Clean the bushing and adapter thoroughly when finished.

The second alternative is to buy a Meehanite guide bushing which uses Meehanite (cast iron) in the place of the carbide. Meehanite has less tendency to “pick up” or have the bar material weld to it. While Meehanite is a cure for problem materials, it never completely eliminates the need for breaking the edges and lapping the bushing.

Dan Murphy
Tsugami REM Sales

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