A very important part of any knurling application is the set-up. The dies must be correct and the holder be rigid enough to withstand the rolling forces encountered.
- Since proper tracking is usually established after only one complete revolution of the part, the secret to sucess is to Ram the Die into the Blank! By forming a deeper, wider impression on the first revolution, the die teeth are more likely to step back into the initial grooves the second time around. Many tracking problems we solve are merely a matter of increasing the feed rate. For bump knurling from the cross-slide, the feed rate should be fast enough so that the part is completed in 5-20 revolutions. Other solutions to tracking problems include: altering the blank diameter slightly, stoning or grinding the die O.D. approximately .002 smaller, honing the bores .002/.003 over nominal size.
- If you are knurling on a CNC, and are having double tracking problems, you may want to stop (or gently slow down) the spindle for the initial contact. Resume normal speed as soon as the tool has pentrated approximately 20-40% into the blank.
- Use plenty of lubricant. Knurling generates some extreme pressures, and improperly lubricated dies are likely to bind up on or gall the pins. Slower spindle speeds and Carbide Pins reduce the possibility of the dies seizing as well.
- Do not over-roll with knurls. In general, try to roll the pattern only about 90% full. When rolling stainless steels (especially 303) roll up the pattern in the fewest revolutions possible to minimize work-hardening the piece. The same can be said when rolling brass and other soft materials, but here it's to prevent "Flaking" caused by rolling the part too many times after it has been completely formed.
- Control the blank diameter withing a reasonable limit. A variation of .0005" in the blank diameter will usually result in an O.D. difference of .001". One large blank can destroy a set of knurls.
- If you are knurling from the end with a straddle-type holder that has blocks that can be swiveled, we generally recommend rolling male diamond patterns by using straight knurls set at 30° to part axis. This eliminates the need for bevels, and provides a gradual build-up and lead-out for better part finishes. Diagonal dies can produce straight knurls with the same benefits. Unfortunately, this method cannot be used if the knurling has to be done close to a larger shoulder on the part.
- If you are knurling a "Wide" pattern, you should be concerned about the amount of force required to roll up a full form. There are many ways to reduce these forces and avoid unnecessary loads on your machine's spindle bearings and lead screw. For "Diamond" knurling, using a straddle-type holder instead of a single "Bump" knurl is one option that will greatly lower the forces on your machine. "Straight" knurling with a straddle holder however, sometimes results in a certain percentage of "Double-Tracked" parts due to the fact that there is no connection between the two knurls as they each start their own track on the first revolution. Other options for reducing knurling forces are:
1) Try our new "Convex" knurls if you can axial feed the knurl.
2) "Cut Type" knurls make small chips with much lower pressures.
3) Special knurls with helical grooves can be furnished. (call for details)
If none of the above tips solves your problems, please give us a call at 1-800-433-4933.