Rebarreling and Home-Shop Machining

            I have used my Dremel tool Post Grinder many times now. It is slightly underpowered and lacks enough rigidity to push a 1 inch diameter stone well, though. It works great on short inside bores, like the job I did in renecking a full length sizing die. I have a Dewalt Cut Out Tool (Zip tool) that is normally used to drill and cut drywall and paneling with a zip bit that is a combination drill bit and endmill. The bit is plunged and is then dragged sideways to cut. These are fairly powerful tools at 5 amps and have good size bearings and shafts required by the need to drag the bit sideways. The Dewalt model has 1/8 inch and 1/4 inch collets. This means it can take all the Dremel accessory stones.

            I decided to make a tool post grinder out of the Dewalt tool that would fill the gap between my Dremmel tool post grinder and a standard toolpost grinder that uses a large stone. The Dewalt grinder could be used to grind an inside bore with a stone just larger than the collet nut for 3 inches due to the long shaft. The long shaft in the original tool is unsupported and mainly provides room for a spindle lock and collet nut wrench. I removed these parts by unscrewing four screws on the endbell exposing an aluminum bearing plate. This metal bearing plate made a good stable surface to mount to.

Materials

            I used: a 3/16 x 2.5 inch steel plate; this thickness was a good match to the length of the factory screws so the original tool can be reassembled quickly, a 1/2 inch square shank off an old brazed on carbide tipped cutting tool, two 5/16 x 1 x 18 TPI socket head capscrews, which I cut and filed to size.

Construction

            I started by centering the tools front cover on the 3/16 x 2.5 inch wide plate. I clamped the cover to the plate and used a transfer punch to punch the four screw holes. It would have been better toremove the bearing plate and use its flat underside to transfer these hole locations. I used a ruler by eye to make an X pattern to mark the center. I cut out a 1 3/8 inch disk out of the center with a bimetallic hole saw and the drill press. I used a die grinder to fit the hole to the tools casting and bearing retaining screws, keeping as much metal as possible to add stiffness to the mount. I put lipstick (marking compound) on the two locating posts. I installed the four screws and lowered the plate onto the post. This left a round mark on the plate whichI center punched and drilled to fit the locating post, one at a time. This worked well.

            Once the plate was made, I mounted the grinders shaft in the three jaw chuck with a brass strip to protect the shaft. This put the grinder on lathe center height. See photo 2. I had died the surface to be marked with a green sharpie. I supported the end of the plate with the 1/2 shank which happened to be the right length. I used a scriber to scribe a line on the plate inside the tool post slot where the slots bottom surface touched the plate. This was the location to attach the 1/2 inch shank.

            I clamped the plate to the grinder then I used two small machinist clamps to clamp the ½ inch shaft touching the scribed line. Look at the green area of metal in photo 3. I through drilled both parts with a 17/64 drill bit which is suitable for tapping 5/16 x 18 threads per inch. I enlarged the holes in the plate to 5/16 inch in a second operation. I tapped the holes in the 1/2 inch shank and installed the plate to the shank with 5/16 socket head capscrews.

            Photo1, 4 - 7 show the finished project less paint. I’ll do some test later to see what kind of surface finish I can get with the 1 inch stone.

Test

            I tested the grinder on a short piece of Drill Rod. I used a 1 inch diameter by ½ inch wide die grinder stone with a 1/4 inch shaft. I first dressed the stone round. The enlarged photo 8 shows the finished part. The left side is turned with an indexable carbide tipped bit. The right side has been ground. The finish is not perfect, there is some ripple in the surface that I suspect is caused by a rumble in the lathe caused by the lathe motor or lathe bearings. At twice the lathe speed the ripple is more widely spaced. It seems to line up along the axis of the part is why I suspect it is the lathe rather than the grinder. The ripple is not very deep. See photo 9. About .0003 inch or 3 ten thousandths. This polishes out quickly with 400 sand paper and then final polish with 1000 grit. So the procedure is to grind the part about 5 - 8 ten thousandths oversize then polish to final size.

            I did a second test at my lowest lathe speed of 32 RPM. See the enlarged photo 9. I got the best finish at .25 inches per minute feed rate. The sides of the cylinder are more parallel if you can pass the stone off both sides of the work area. In this case the cylinder was parallel to 2 ten thousandths. This can be corrected by polishing. The cylinder was out of round about 4 ten thousandths measured by a dial indicator. The surface roughness indicated by the dial indicator needle vibration was only about 1 ten thousandth in this case.

            At the higher speed (640 RPM, Photo 8) the cylinder was round to about 1 ten thousandth but had a surface roughness of about .003 ten thousandths.

            I tightened the spindle preload and purchased a 1 x 3/8 inch finer stone. I got good results now. Finish had less than 1 ten thousandth inch of roughness. Taper of the cylinder was about 4 ten thousandth as measured with a micrometer. 4 ten thousandths bigger diameter on the chuck side. Out of round with the preload adjustment was now less than 1 ten thousandth. A slight surface defect occurred near the left side ofthe cylinder as I did not pass the 3/8 inch wide stone all the way off the left side of the part. I used a 32 RPM spindle speed with .25 inch per minute feed rate with a spring pass as the final pass. A higher lathe speed might work OK now with the adjustments that I made.

Success

            I made another attempt. I was trying to remove the slight taper by improving my technique; the chuck side of the part had a 4 ten thousandths bigger diameter than the other end. This taper, I suspected was caused by wear in the stone with the slow feed as I had setup the lathe to a better tolerance than this. The stone wore slightly smaller as it went farther into the cut. I rough turned the part .010 inch oversize. Then, I setup the grinder shaft parallel to the bed with a dial indicator. I resurfaced the stone for full contact. I increased the feed to 1 inch per minute, still at 32 RPM. I ground the part to .5 inch for use as an expanding arbor in my ½ inch end mill holder. The part now had < 1 ten thousandth taper. I was shooting for 2 ten thousandths under size for clearance. I ended up 3 ten thousandths under. The part was a nice light drag fit to the endmill holder. I measured an endmill and it had the same 3 ten thousandths clearance (3 ten thousandths under ½ inch). The finish was only slightly worse than at the slower feed. No polishing was needed for this application.

Suitable Projects

            A grinder is helpful when you need to cut a hardened part. A recent job that I did on the lathe was on an antique Alfa Romeo oilpump shaft. The shaft rides in a 1 inch long bronze oilite bushing. The metric shaft was very scored and the flanged bushing was worn oversize. I ground the hardened shaft down to about 3/8 inch with the Dremmel grinder about .0008 oversize and then polished it to .3748. It came out nice. I cut a flanged bushing from an 1 x 5/8 x 3/8 inch solid bushing to mate with the shaft and the hole in the engine block.

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