How to Calculate Thread Rolling Penetration Rate on CJWinter Pneumatic Attachments

Over the course of our 60 years of providing thread rolling solutions CJWinter has fielded a number of questions about the process — including dies, attachments, and other related tooling.

One of the most common questions we receive is: “How should I calculate thread rolling penetration rate?”

To help, we put together this quick three-step guide to determine the right penetration rate for your thread rolling application.

Step 1: Blank Diameter and Attachment Adjustments

Balancing blank diameter, pitch diameter, and major diameter is a sensitive process. With the proper pitch diameter achieved, you can begin narrowing down to the proper major diameter — important for calculated penetration rate.

Achieving the correct major diameter usually requires increases in blank diameter. Because major diameter increases proportionally to blank diameter at a rate of three times, it is important that blank diameter be increased in the smallest possible increments.

In increasing blank diameter to achieve the proper major diameter, you are also increasing thread roll penetration resistance, essentially lowering penetration rate, also known as feed rate. A skewed penetration rate leads to a skewed pitch diameter. To correct the penetration rate, control the depth of penetration by making careful adjustments of pitch diameter using the proper knob on the attachment.

Step 2: Determine Part Revolutions

The final application of your product, as well as its material, determines the ideal number of part revolutions in the thread.

In the thread rolling process, part revolutions directly relates to thread roll penetration. A method of penetration rate control, as it relates to part revolutions, is the control valve assembly — adjusting the flow control valve on the assembly’s “out” line will adjust penetration rate and part revolutions.

Because control valve assemblies have no clearly defined metrics by which to set them, penetration rate-expressing terms such as “inches per revolution” and “rolling time” are of estimation value only. To accurately and precisely determine and set a penetration rate, there is a simple five-step process:

Determine recommended part revolutions using a reference chart such as the one below
Determine number of thread starts on the thread roll using the appropriate Reference Data Chart, of counting directly on thread rolls
Divide recommended part revolutions by number of starts; the result is the recommended compensator revolutions
Observe the attachment compensator mechanism adjusting collar while machine is in operation
Adjust the flow control valve to alter penetration rate until the compensator makes the recommended number of revolutions

Thread Rolling	Brass Aluminum	Steel: Up to Rc 15 400 Series Stainless Steel	Steel: Up to Rc 15-32 300 Series Stainless Steel
8-12 TPI Acme	9-12	10-13	Consult factor
8-13 TPI UN or 3.0-2.0 MM	8-11	9-12	10-14
14-20 TPI UN or 1.75-1.5 MM	7-10	8-11	9-12
20-28 TPI UN or 1.25-1.0 MM	6-9	7-10	8-11
29-Finer TPI UN or 0.8 MM-Finer	6-7	6-8	6-9
Knurling
12-17 TPI	8-11	9-12	10-14
18-25 TPI or 64 DP	7-10	8-11	9-12
26-35 TPI or 96 DP	6-9	7-10	8-11
36-50 TPI or 128 DP	5-8	6-9	7-10
51-Finer TPI or 160 DP	5-7	5-8	6-8

As an example, a brass blank with a thread size of ¾-16 being machined with attachment 160-SA will have a recommended part revolutions of 8 and 3 thread starts on a thread roll.

8 / 3 = 2.7

The penetration rate control valve should be adjusted to the point that the adjusting collar of the compensator mechanism makes between 2.5 and 3 revolutions.

Step 3: Remember the Formula

Penetration rate can also be calculated in terms of rolling time. The rolling time calculation is particularly useful in that it can be used to determine appropriate machine spindle speeds or estimated attachment cycle times. The rolling time formula is as follows: