Prescription knurling

When to roll form, when to cut

By Ronald M Klaucke
Manager, Engineering,
Accu Trak Tool Corp,
Cherry Valley, MA

Knurling by the roll forming method is a plastic deformation process characterized by the absence of metal chips. Many so-called "conventional" knurling jobs are performed, essentially, to enhance the gripping properties of parts' surfaces--as with medical instruments. Pre-forming dimensions are purposely made smaller than the required finished size to accommodate the outward displacement or material flow generated by the penetration of knurl teeth into the part.

A typical knurling cut form operation showing a pattern being machined into a tool part using a gear hob. The tooth crests of the pattern come up sharper than roll formed knurls.

In many instances, knurling by roll forming need not be pretty but rather functional as in a car wheel bolt or electric motor shaft. Once such parts are assembled, the knurled area is often hidden from view. However, in the case of a part such as a turning knob for an electrical apparatus or a micrometer handle or the handle of a medical instrument, the knurled pattern must also look aesthetically pleasing.

The gripping-type knurl application is the most common. Most progressive machine shops are familiar with this operation. In such industrial work environments, the rule of thumb is that as long as the quantity of parts is high, the cost of roll forming will be low.

All too often, the "mind set" is to roll form whenever a print calls out a knurl pattern. The fact is that there are times when it is unwise to roll form but instead to mill or hob. These cutting alternatives most frequently arise when the work piece being knurled has a tooling application or where the knurl produced is to make other knurls or has some other demanding function.

The manufacturing volume of such tools or parts is generally low and the performance of the knurl is critical. What types of tools do we have in mind? Feed rolls, rolling mill dies, deburring tools, sealing wheels, and marking dies, among others. Mistakenly, first consideration is given to the roll forming process to produce the patterns but the results can be less than desirable for a number of reasons:

1) Tools are generally made using alloy steels which are not the most malleable metals and which can be difficult to roll form.

2) For reason of unit cost, no extra work pieces may be available with which to establish effective knurling parameters (i.e., speeds and feeds, etc.).

3) Difficulty in estimating the exact dimension "build ups."

4) Although rolled steel may be stronger than cut steel, roll formed knurl tooth crests are not as sturdy as cut or milled tooth crests because of seams resulting from the very nature of the rolling process--thus producing shorter tool life.

With these considerations in mind, shop management would be well advised to turn to milling or hobbing as alternative means. Additionally, with the proliferation of CNC machining centers, some knurling jobs can be set up and run just as quickly and efficiently as form rolling. Gear hobbling machines also achieve good results when work piece quantities are low and the correct cutters (hobs) are available.

Although Accu Trak Tool manufacturers a wide assortment of knurling and spline application services where machining (cutting) processes may be chosen to replace roll forming. Depending on tooling availability (hobs or milling cutters), it might be less expensive and quicker to sub-contract knurling applications than to procure the necessary knurling tools to do the job in-house.

By replacing the roll forming process in selective machining operations as indicated, a machine shop can avoid the "art" factor or experimentation identified with roll forming. The knurling job can then be squarely fixed in the realm of machining science. By choosing milling or hobbing, the trial and error preliminaries which sometimes are required to produce the right rolling die and set up parameters are eliminated.

With all types of tools, integrity of tooth tips is critical to performance. When knurl teeth are made by roll forming, planes of metal are displaced upwards to form the teeth. In the case of a male diamond knurl, four different planes are involved. Where these flowing planes meet, a seam is formed. This development is especially noticeable at the tips or crests of the teeth. Even if the teeth are completely formed, small seams will be present making them more vulnerable to early failure as compared with cut teeth.

In the case of feed wheels, the tip of a tooth receives the most punishment. The tip must penetrate the surface of the material (often metal) and push it along into the next machining process. Should the tip break down prematurely, suitable job completion is impaired. Accu Trak has field reports showing 10 times more life for cut form tooling versus roll formed tooling.

In the case of a roll-formed die used on a difficult knurling application, the tooth crests weakened by seams are likely to fail prematurely if not immediately.

In retrospect, when to roll form or when to cut comes down to these essential differences: 

• Roll form when the job involves basic patterns for gripping purposes; 
• Roll form when high productivity of parts favors the roll forming process; 
• Roll form when economies of time and cost outweigh the benefits of cutting; 
• Mill or hob when work pieces have tooling applications and such jobs involve limited quantities of parts; 
• Mill or hob to meet high tolerance requirements and when limited development funding is available; 
• Mill or hob to produce intricate patterns; 
• Mill or hob when the part is difficult to form as in the case of tool and alloy steels. 

Originally published in the September 2000 issue 
of Tooling & Production. 
Please Note: some pictures or diagrams are only available through the printed media.