The Problem of Straight Form Knurling in a 2 Die Straddle Holder
While common two die straddle holders can be used for knurling straight patterns, it carries with it a risk. The problem is that the 2 wheels are not synchronized with each other. That being said, if the straight knurl pattern is medium/fine to fine, with a fairly high infeed rate, the two knurls will often track and produce a good knurl pattern. Alternatively, If the knurl pattern is coarse to medium and/or the infeed rate is not fast enough, it will have a higher chance of randomly mis-tracking, resulting in a poor knurl form as well as low tool life.
Ways to minimize the mistracking:
Increase the feed rate in the X axis and/or Z axis to form a deeper, wider track on the first revolution.
If possible, make first contact coming in from the Z axis on the end of the part and then axial feed to get the required knurl length. Convex, axial feed type wheels are best suited for this (unless you must go up very close to a shoulder). This method sometimes mistracks for a short distance, then finds itself for the remainder of the axial feeding. If there is a small mistrack at the beginning, it could be chamfered or faced off. Another beneficial feature of convex axial feed knurls is that the relief on the dies becomes progressively smaller enabling adjustment of tracking diameters by contacting at different locations on the relief. This would be done by feeding in the Z or X axis with the X contact diameter of the knurl die varied within the width of the relief varied for optimal tracking. Once tracking is established the X axis can be brought to the final point required to generate the appropriate final knurled diameter. Then feed in Z to get the appropriate length of knurl required.
If the workpiece diameter is greater than the width of knurling, a single wheel bump holder may be the best option if, the knurling is done close to the collet and, the machine being used has the power to do the job.
If the knurl pattern is too wide for above:
A) use a single wheel as above at the very end of the knurled area and roll a narrow section (with less pressure) to get the knurl started, perhaps about 1/3 to 1/2 full. Then come in on the partial existing knurl with the 2 die straddle holder. It should pick up on the existing partial knurl and can now be rolled to full form. This requires an extra operation, but will normally will solve the problem.
B) Low force designs can allow single wheel bump applications to work where a full face knurl will not. Contact Accu Trak’s die design staff for further information about low force knurling dies.
There are many 2 die thread rolling a attachments with gear trains available that, with special knurling wheels with drive slots, will solve the problem. These holders are many times more expensive than simple non-synchronized knurling wheel holders, but for large, long running jobs they may be more cost effective.