Guide Bushing Fit Part II

By Dan Murphy, The Swiss Wizard:

Last month in Part 1 I discussed the problems caused by running the guide bushing too tight or too loose, along with the issues created by the bushing’s bore not matching the diameter of the bar even when the bushing is adjusted properly.

In most cases a driven guide bushing needs only to have a snug fit to the bar. With the guide bushing locked or restrained from rotating you should be able to rotate the bar by hand and feel some light to moderate drag on the bar. Often when you perform this check you will find that the bar feels lobed. In other words, as you rotate the bar it feels tight, then loose then tight again. This is caused by a combination of an out of round condition on the bar and the bore of the guide bushing being out of round. If you think about it, the roundness of the bar stock alone couldn’t cause this lobed effect. If the bushing were perfectly round and snug on the bar the high spots on the out of round bar would remain snug in the bushing as you rotated the bar. In order for the bar to be tight in some spots and loose in others, the bore of the bushing must also be out of round.

In addition to an out of round condition in the fit between the bushing and bar, there may also be a very small contact area between the two due to difference in size as shown in Fig A. This poor fit will cause chatter during turning and milling operations. In addition to rigidity problems, a poor fit between the bushing and bar can scratch or mark the bar. Scratches can also be caused by sharp edges on the carbide pads inside the guide bushing. The best way to remedy the problem is to break all the edges on the carbide pads then lap the guide bushing so that it matches the bar stock perfectly. Start by using a die grinder or Dremel type motorized tool along with a diamond coated bit to grind a chamfer on the front and back edges of the carbide pads on the inside and on the face of the bushing. While you are at it take a cylindrical shaped bit and lay it into the slots between the carbide pads to break those edges as well.

The next step is to lap the bushing using diamond lapping compound. Diamond lapping compound is sold in different grits. The compound is color coded with the color relating to the size of the diamond dust in microns or mesh size. I recommend that you keep three grades on hand: red which is 22-36 micron or 600 mesh, blue which is 12-22 micron or 1,200 mesh, and orange which is 4-9 micron or 3,000 mesh. Use the red compound to repair a worn or damaged bushing or to quickly remove carbide to bring an oversized or undersized bushing into shape to match the bar diameter. Follow lapping with the red with blue and then orange to finish.

You can use a chamfered bar end (remnant) for lapping or you can buy barrel laps similar to this one. Begin lapping by adjusting the bushing to the remnant or lap so that it is snug but can be rotated by hand without a lot of effort. Chuck the barrel lap or bar end in the sub spindle and get it to run true. Put a small pea size drop of compound on the end of the remnant and smear it around with your finger. Mix a few drop of cutting oil to the diamond compound. The oil is critical to provide lubrication which will prevent the remnant from seizing up in the bushing.

Begin honing the bushing by counter-rotating the main and sub spindles at around 100 rpm each for a differential speed of 200 rpm. You’ll find you can run faster with small diameter guide bushings and may have to slow down for a large diameter one. Use the handle (MPG) to jog the bar end in and out of the guide bushing rapidly. Don’t let the bar dwell, keep it moving quickly in and out of the bushing. Watch this video on Youtube to get an idea of how a honing machine works. You want to duplicate that rapid feeding motion.

If the bar squeaks or squeals, or if you see a wisp of smoke, stop and add more oil to the lap or bar end and/or loosen the bushing. Work the bar end in and out of the bushing until it fits loosely. The compound on the lap should turn black from the carbide if all is working properly. Stop both spindles, adjust the bushing ever so slightly tighter and repeat the honing process. Repeat the entire process until you see that the carbide pads have a uniform finish over at least 80% 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.

If you choose to use a barrel lap you have to adjust the lap until it is the right size. Then adjust the bushing tighter until the lap is contacting 80% or more of the guide bushing bore. You will be able to see the difference in surface finish to determine coverage. On Tsugami models that use a cartridge type guide bushing adapter (SS, BS20/26/32, MU, BU 26/38, and BH38) you can use the cartridge outside of the machine to hone the guide bushing. You can use an engine lathe, knee mill, or drill press to hone the bushing. As always use safe shop practices and exercise caution. I believe that by honing the bushing in place on the machine you are less likely to induce unwanted run out between the bushings bore and the centerline of the spindle.

You can use these same techniques to breathe new life into an old worn out guide bushing or to repair a damaged one. Buy some diamond compound and give this a try on the next job you set up. You’ll see a difference.