“how to knurl a bent steel bar”, this was the search that one of the Workshop shed readers had made against the site. Well, I’m afraid I don’t have any complete solution to that problem but here’s my independently thought up ideas.
1) Knurl the bar first then bend it, care would be need to protect the knurling when the bar is bent.
2) Build a specialist tool for the job.
What could be used for this second task would be a device similar to a tailstock knurling tool with a clamp to attach to the bar ahead of the knurling wheels. There would be a hole through the device so that it was free to slide up and down the work piece. The rear of this tool would be able to rotate with respect to the clamp and would hold the knurling wheels on arms that could be moved together with a screwthread (similar to a conventional knurling tool). The tool would be rotated and the wheels moved in, this would be repeated until the knurl on this section was acceptible. The wheels would be loosened, the clamp would be loosened and the tool moved along approx half the distance of the knurling wheels before engaging the clamp. The wheels would be rotated and slowly tightened again allowing the knurl to “take up” on the adjacent knurl. This would be repeated until the required distance was completed.
Levers could be added to allow the user apply more force to the rotation or the two halves of the device could be hexagonal so they could be turned/held with a wrench.
The limit on tightness of curvature of bar would be dependant on the distance between the clamp and the end of the wheels.