What is the difference between a bolt with rolled threads and one with cut threads and does a fastener with a reduced body and rolled threads meet ASTM specifications?
Threads of a mechanical fastener, regardless of whether it is a headed bolt, rod, or bent bolt, can be produced by either cutting or rolling. The differences, misconceptions, advantages, and disadvantages of each method are described below.
Roll threading is a process by which steel is extruded to form the threaded portion of a fastener, instead of being removed as in cut threading. In this process, a bolt is manufactured from a reduced diameter round bar. For example, a 1″ diameter bolt is manufactured from .912″ diameter round bar. This “pitch diameter” material is approximately the midpoint between the major diameter (peaks) and minor diameter (valleys) of the threads. The bolt is “rolled” through a set of threading dies which displaces the steel and forms the threads. The end result is a fastener with a full 1″ diameter threaded portion but a reduced body diameter (.912). Roll threading is an extremely efficient process and often results in significant cost savings. Therefore, Portland Bolt will roll threads whenever possible.
Roll Threading Misconceptions
- Roll threaded bolts do not meet ASTM specifications.
Technically, any specification with the exception of A325 and A490 structural bolts can be produced with a reduced body and rolled threads.
- A bolt with a reduced body will be weaker than a bolt with a full sized body.
The weakest area of any mechanical fastener is the minor diameter of the threads. The thread dimensions of a cut thread and rolled thread fastener are identical, so the weakest part/minor diameter is the same in both methods. Rolling the threads actually work hardens them, resulting in threads that are typically slightly stronger than their cut-thread counterparts Additionally, cut threading interrupts the natural grain structure of the round bar whereas roll threading reforms it. One could argue that cutting into the grain of a round bar when cut threading may produce threads which have less structural integrity than a part which has been roll threaded. That all said, both methods are acceptable (expect for structural bolts as noted above) and will result in identical thread geometry.
Advantages of Roll Threading
- Significantly shorter labor times means lower costs.
- Because a roll threaded bolt has a smaller body diameter, it weighs less than its full bodied counterpart. This weight reduction reduces the cost of the steel, galvanizing, heat-treating, plating, freight, and any other costs associated with the fastener that are based on weight.
- Cold working makes threads more resistant to damage during handling.
- Rolled threads are often smoother due to the burnishing effect of the rolling operation.
Disadvantages of Roll Threading
- Portland Bolt is limited to a maximum diameter of 1″ and a maximum thread length of 8″.
- The availability of pitch diameter round bar is limited for certain material grades.
- 150 ksi minimum tensile strength material is too strong for Portland Bolt to roll thread.
- A325 and A490 structural bolts cannot be produced with a reduced body diameter.
- In rare wood applications where shear strength is the critical requirement, bolts with a full sized body may be required.
Cut threading is a process by which steel is cut away, or physically removed, from a round bar of steel to form the threads. A 1″ diameter bolt, for example, is produced by cutting threads into a full 1″ diameter body of the bolt.
Advantages of Cut Threading
- Few limitations with regard to diameter and thread length.
- All specifications can be manufactured with cut threads.
Disadvantages of Cut Threading
- Significantly longer labor times means higher costs.
|Coarse Thread||Rolled Thread Stock Weight||Cut Thread Stock Weight||Weight Savings %|
|Size||diameter, in.||lb. per ft.||diameter, in.||lb. per ft.|
|3⁄8" - 16||0.330||0.291||0.375||0.376||22.6|
|1⁄2" - 13||0.445||0.529||0.500||0.668||20.8|
|5⁄8" - 11||0.562||0.843||0.625||1.043||19.2|
|3⁄4" - 10||0.680||1.235||0.750||1.500||17.7|
|7⁄8" - 9||0.797||1.696||0.875||2.044||17.0|
|1" - 8||0.912||2.221||1.000||2.670||16.8|