Have run chippers only two years so recently had same problem.
Two friends with chippers say cannot be bothered with sharpening...go to Zenith Cutter or Wespur and new knives are $20 to 25, plus shipping.
I called grinding shops and equipment dealers in north NJ. Quotes were about $35 per blade, or $120 per set, drop them off and rep comes by once a week, picks them up and returns job next week.
Best blade grinder i found on internet is Tennessee Saw and Knife, 1695 Barcrest rd, Memphis TN 38134. Small specialty shop, low key, blade sets matched on a jig, will hold a CC number for repeat orders without confirmation hassle. Allow 10-14 days round trip.
Double sided 7inch blades are $6.50 each, a package of four ships for about $15 each way.
Re use and recycle. :msp_thumbup: