Rebarreling and Home-Shop Machining

Making a Flat Surface

There are several ways to make a surface truly flat after milling: Surface Grinding, Lapping, and Marking with a surface plate and then scraping the high spots. Surface grinding requires a specialized machine that passes a grinder over the surface. Lapping requires a cast iron lapping plate. A lapping compound is applied to the plate and then the object moved over the plate to remove the high spots. The plate is occasionally lapped flat with a ring shaped cast iron lap. I chose the method marking with a surface plate and a modified scraping method as I had a surface plate on hand. Milling will generally leave a surface flat to about .001. Since this surface would be controlling the alignment of my mill to some extend I desired a slightly flatter surface. A flatter surface will also increase the rigidity of the mill as it will not allow the cross arm which was also made flat to rock on the surface.

Marking

Photo 1 shows the setup for this method. I put a layer of Prussian blue dye on the surface plate and then put the milled surface onto the plate. Photo 2 shows the first dye impression on the plate. The dye will be left on the high spots. I then used a scraper that I made from the end of a large file by grinding the end to a smooth square edge and then honing it smooth with the large honing stone in photo 1. The work actually went a little faster when I sanded the entire surface relatively smooth with a 6 inch orbital car body sander. This took out some of the ridges at the edge of the mill passes. The better your mill is trammed the less these ridges will be a problem. Aligning and Tramming the Mill is discussed in this Journal.

Scraping

The scraping pattern, that I used, was to scrape the high spots at an angle to the plate and then scrape again at a 90 degree angle. I watched the scratch marks and made an even pattern over the high spot, then covered the pattern on the second pass. Then I made a third pass on the original angle. Making three or four passes to begin with gives you more rapid progress between markings. Reduce this to two passes when you start getting closer to flat. Photo 3 shows the resulting pattern when the plate was close to flat.

The scraping went pretty well, but left a fairly rough finish on this mild steel. Scraping works much better on cast iron. The scraper tended to cause tearouts after a bit and then dragged these tearouts along the work creating a small gouge. This tendency was much reduced by lowering the handle of the scraper toward the work and using lighter pressure. The marking went better if I passed the file part of my scraper across the work lightly a couple of times with some rotation to remove any tearouts that were sticking up. These will interfere with the plate laying flat on the surface plate. I reached about the limit of this technique with photo 4 and changed to sanding with 240 grit paper and one finger using the same cross-hatch pattern. This did two things. It improved the surface finish and made further progress possible.

Sanding the High Spots

Photo 5 shows a marking pattern which I surrounded with a Sharpie Marker. The dye must be removed before sanding. This can be done all at once like in photo 6, if you can remember where to sand, or one section at a time. The Prussian dye ink pattern will tell you whether to do a light sanding with only two passes or a heavy sanding with four passes. Alternating at 90 degree angles for each pass leaves a nice frosted finish. Cover the area with the pattern. Photo 6 shows the plate almost done after sanding.

Finished

Photo 7 shows the last marking pattern. I got good broad coverage. I sanded one last time. Photo 8 shows the final surface finish. The steel is quite smooth and flat, probably to .0002 inch.

|  Add to del.icio.us |  digg this