did 35 last night
Bill Tindall, E. TN
>From our previous posts most of you know that we made a bunch of tools from a highly abrasion resistant steel, CPM3V. We recently accumulated an assortment of chisels and plane irons for a marathon flattening session, which was the successful culmination of many failed efforts. Here is what we have learned....
1.We don't believe that it is possible to flatten steel with abrasive sheets, or anything else that has any "give" to it. The steel edge leading the sanding stroke always abrades faster than the trailing metal. All will look fine until near the end and then at the very fine grits the leading edge no longer engages the abrasive and it does not polish.
2. I expect a collection of proper, flat,stones will work just fine. The problem with stones is the number($$) that are necessary to go from initial heavy material removal to final polish. If too big jumps between grits are taken rogue scratches will likely show up along the way(and there are a lot of other sources of rouge scratches as well).
3. Silicon carbide is an exceptionally poor abrasive for tool steels. It is brittle and fractures readily under the forces of grinding. So it wears out very quickly, especially in the coarser grits. 120 grit silicon carbide charged on a cast iron plate lasted seconds with this tool steel. Alumina-zirconium is a good abrasive. Aluminium oxide is Ok and diamonds are best.
4. Diamond abrasive is the fastest and cheapest abrasive, providing you can find something cheap and appropriate to charge it onto. We did 35 tools and used up no more than 1/2 ml of paste per grit for a total cost of less than $10, including the cost of initially charging the plates. It took about 45 minutes per grit for total of the 35 tools. Much of the time was spent insuring cleanliness to avoid contamination between grits. 20 strokes per grit was sufficient.
5. Plate material- The secret to back flattening is a nondeformable, flat abrasive surface. the following was found to not work- various abrasive sheets on 1/2" glass, diamonds on copper circuit board or wood. Diamond manufacturers recommend charging diamond onto cast iron. We had some surface-ground cast iron plates given to us. Charged with but a tiny amount of diamond paste they cut aggressively and the metal remained dead flat. Hoooray!! Once the plates were initially charged it took but about 10-20 mg of paste per tool. Even down to 6 micron one could hear the diamonds rapidly abrading away this very abrasion resistant metal.
Diamond on copper circuit board was ok for bevels, but not flat enough for backs. The cut was a bit less aggressive than with cast iron(perhaps the diamonds embed more). It took a lot of diamond to charge a piece of beech wood and the cut was much less aggressive. So, 6 micron on wood was about like 1 to 3 micron on cast iron. I think diamond on wood could be good for polishing. Did not try diamond on plastic.
Our procedure- Initial grinding was with Makita horizontal water stone. It was kept flat by occasionally grinding a big-long piece of hardened CPM3V (the draw knife). Due to my inexperience some of the tools were left with deep 36 grit fabrication scratches before heat treating(big mistake). The Makita removed these mistakes fairly quickly. Alternatively, initial flattening could have been done by hand lapping with 60 micron diamond charged onto cast iron, but this was slower than the powered Makita. Then the tools were lapped on a succession of cast iron plates charged with 30, 15, 9, and 6 micron diamond paste. Final polishing was with 6 micron diamond on beech wood (but we would have used a finer grade of diamond on cast iron if we had an additional cast iron plate).
The inside of curved tools(sweeps and gouges) were done by charging diamond onto aluminium rod and turning the rod at about 100 rpm in a lathe. 60 micron diamond cut very rapidly with this arrangement such that some very coarse fabrication scratches were quickly smoothed.
If there is a source of cheap, flat cast iron out there diamonds are easily the fastest, cheapest way to abrade tool steel. We need 3 more plates if anyone knows a source.
