How many times have you been in or have you ever been in the situation where you’ve got all your tooling, cams, and raw material to start setting up an Escomatic D2, D4, or D6 on that “hot” job, only to discover that the guide bushing you’ve selected, even though it is clearly marked with the size for which it’s intended, is simply just too tight too allow the material to pass through smoothly? Well, all hope is not lost at that point! With just a little patience, access to a lathe, and some diamond lapping compound, it is possible to lap out an undersized guide bushing to accommodate your needs. We aren’t talking by a lot here though, because the bushings that need to be lapped by a thousandth or less are usually most successful.
You’ll need to use a section of straightened raw material that’s about 8” long, has good concentricity, and is free of any surface irregularities such as scoring, ripples, or twists. It’s usually not a bad idea to cut a couple of lengths so you have a spare if needed. Then, grind a slight taper on the end of the workpiece about an inch or so in length. This will help to get the lapping compound distributed in the guide bushing.
Once the material is ready, mount it in a lathe and apply a small dab of lapping compound (12 micron or finer) to the tapered end. Set the rpm to a slow speed; somewhere between 100 and 200 rpm seems to work nicely. Working from the back side of the guide bushing, gradually slide it back and forth onto the rotating material just to the point at which resistance can be felt. It is very important to work from the rear of the bushing to prevent “bell-mouthing” the front. Repeat this process very slowly, reapplying compound as needed. Depending on the amount that needs to be lapped, this process can take 30 minutes or more. Make sure you periodically stop to clean the id of the guide bushing and test-fit it on a fresh piece of material. Once completed, the successfully lapped guide bushing should slide onto a fresh section of straight material with minimal drag and no sloppiness.