Techniques/sharpening

From rLab Wiki

Knife Sharpening - An aide Memoire

Starting Point: Shape of knife already done and rough bevel created, double-bevel shape already created.

This page is not finished and are just rough notes based on the bladesmithing workshop

General Notes

  • Blades can be sharpened with an included angle anywhere from 7 degrees (razor blade) to 45 degrees (axe head). Finer edges are sharper but weaker so they dull faster, wider edges are less sharp but much more robust and will last much longer before resharpening.
  • Desired sharpness depends on what you're cutting, not much point going above 1000/1500 (and strop) with wood
    • 400-1000 grit stones are for grinding of the blade, rough shaping to get the edge to the right angle and depth
    • 1000-3000 grits are for sharpening, actually putting the edge on the blade
    • 3000+ grits are for polishing, smoothing out scratches in the edge so it won't catch
    • 10000+ grits are final mirror polishing to things slide off the blade
  • Note on grit grades, not all 1000-grit stones are equal. Stones made from a hard material (e.g. diamond) will behave as if they're coarser than the grit number would suggest, stones made from a soft material (e.g. F3 stone) will behave as if they're finer than the grit number would suggest
  • All Stones used wet- to aid lubrication and cleaning off swarf, use spray bottle to regularly re-wet while using.
    • Diamond stones are stored dry but should be frequently wetted while in use
    • Water stones:
      • Kept permanently submerged in water.
      • Only use water. Certain stones can be used with oil but then can never be used with water again.
      • Water stones may not be completely flat from previous use (dished). They can flattened by rubbing against the 400 Diamond Stone with copious water and very frequent washing but as this wears the stone only do it if the dishing is bad enough to make holding angle difficult
    • English stone can be kept dry but needs to be soaked in water for half an hour before use.
    • F3 stones don't need soaking and should be stored dry but still need to be wetted regularly while in use.
  • Both Water and English stones need even more care on push stroke to prevent nicking stones.
  • Use the full length of the stones but take care not to run off the ends, longer strokes both work faster and help prevent uneven wear on the stones.

General Process

  • Choose an angle and hold it as stable as you can. Can be helpful to move entire upper body rather than just arms. Do not "Roll" the knife over the stone. Angle might get slightly steeper as you go up the grits if you're aiming for a prismatic grind but is usually constant.
  • Firm Pull stroke on stones, very light push stroke (to prevent nicking stones)
  • Should only have to do 400 Grit when first shaping edge, unless you've badly nicked the blade
  • Clean Knife between grits - otherwise previous swarf can scratch blade - water and wipe
  • Clean stone thoroughly under running water after use. For diamond stones dry them carefully, others can be put away damp.
  • Alternate both sides until there's a burr one side all along blade that can be pushed to the other side. Can feel burr with finger
  • Then do 10 strokes on one side, then the other. Then 9 strokes on each side... repeating down to 1 stroke.
  • Repeat for each grit

400 Grit Diamond Stone

  • Coarse shaping of the cutting edge, blade is un-usable at this stage
  • Result under microscope: Dull with obvious scratches
  • Can feel burr with finger

1000 Grit Diamond Stone

  • Finer shaping of cutting edge, edge is coarse and serrated and will rapidly dull if used
  • Result under microscope: Silky matt consistent across bevel with less obvious scratches
  • Repeat above process, can still just feel burr with finger

1500 Water Stone

  • Starting to sharpen the edge, edge is getting smoother and sharper, burr will still cause problems in softer materials
  • Result under microscope: Bright silky matt, little scratching
  • Repeat process - likely trouble to feel burn with finger

3000 English Stone

  • Fine sharpening of the edge, edge is now very sharp and should be reasonably stable.
  • Result under microscope: Shiny, no scratches
  • Burr probably too small to be felt

4000 F3 (triple fine) Stone

  • Polishing the cutting edge, razor sharp at this point and usable even in quite soft materials
  • Use only light pressure on both strokes
  • Result under microscope: Shiny but reflective, matt mirror
  • Burr too small to feel, have to judge when to stop based on experience and microscope
  • Often stop here, apart from stropping

8000 English stone

  • Not normally needed, will put a fine polish on the edge
  • Result under microscope: Visible shine to cutting edge across whole blade
  • Burr too small to feel, have to judge when to stop based on experience and microscope

Stropping 30000 grit

  • Puts a mirror polish on the edge to help, to help item being cut slide across blade.
  • Result under microscope: Definite shine, like mirror
  • Only pull strokes on leather strop, 30/40 on each side with stropping compound
  • Hard-backed strops make it easier to hold proper angle, flexible strops will round-over the edge if not used very carefully

Time Taken

  • 30 mins for 400 grit, 10 mins for each after that
  • High-end tool steel - double numbers above
  • Very high grade (e.g. D2, CPM140) - 6 hours total
  • Super-steels and ceramic blades are not sharpenable by this process and require diamond lapping compounds and suitable pads.

Resharpening Blade

  • Minor touch-up's can be done with just the strop which will remove burrs but leave nicks and chips
  • Otherwise start at 1500/ 3000
  • Back to 400 grit only if there's large, clearly visible damage to cutting edge