The Steel Supply Company manufactures Anchor Bolts and Turnbuckle Rods every day… and that’s also how often we see Anchor Bolt Drawings calling for threads that appear unnecessarily long. With regards to the exposed end, extra thread length is requested so the fabricator can be sure there will be sufficient thread for the washer and nut to “bottom out” regardless of any problems with positioning.
Typical Size and Grade Bolts
This would hold especially true on typical size and grade bolts – for example, F1554-07a Grade 36 anchor bolts with a diameter of 1”. For production of this bolt, the threading machine would be set at approximately 75 rpm and cut the full thread on one pass. Standard UNC threading for 1” diameter is 8 threads per inch. So the bolt threading time is:
75 rpm ÷ 8 threads per inch = 9.375 inches of thread per minute.
This can be expressed as one inch of thread every 6.4 seconds.
The more pliable Grade A-36 steel does not cause as much wear on the cutting tools, known as “Chasers,” so very little cost is involved there as well.
Extra thread length does not change the bolt’s tensile or yield strength, so requesting extra thread in cases like this may be prudent. Viewing the above equation, the cost of adding a couple of inches of extra thread would be minimal.
Impact During Different Situations
Now consider the opposite situation. Anchor Bolts made to ASTM F1554-07a specifications calling for Grade A-105 round bar at 2” diameter. Two important changes will greatly effect threading time. Thread depth and material resistance.
- The yield strength is approximately 2-1/2 to 3 time greater than Grade A-36.
- The depth of the thread has gone from 0.15” to 0.28” (approx. when adjusting for tolerance range).
To attempt to thread this bolt with the same system as the 1” grade A-36 shown above would result in even the hardest Chasers at 65 Rockwell shattering almost immediately. The first adjustment to accommodate this situation is to step the speed down to the slowest rpm the gearing will enable. Freewheeling that would be 24 rpm. Once the load is applied, that drops down to 16 rpm.
UNC Specs for 2” diameter bolts call for 4-1/2 threads per inch. Using the same calculations as above:
16 rpm ÷ 4-1/2 threads per inch = 3.55 inches of thread per minute
This can be expressed as one inch of thread every 17 seconds.
Comparing only threading speed, the 2” Anchor Bolt requires 3 times longer making the thread… but that is only a small part of the added time, tool wear and cost.
Three Passes for Proper Cutting
Due to the hardness of the A-105 steel, the thread is not cut in one single pass. To cut this bolt properly, considering not only finished product quality but tool longevity and operator safety this thread requires 3 passes as shown below.
Each pass increases the thread penetration with the final pass bringing the thread into UNC tolerance. (Pitch diameter tolerance for 2” diameter bolt = 1.8433 min to 1.8528 max.)
When threading in multiple passes, all the bolts of a given lot are threaded on each setting. That is, if 100 anchor bolts are being made, all 100 get the first pass. The Chasers are then drawn tighter to cut the second pass and all 100 bolts are threaded again. Finally, the chasers are brought to the pitch diameter that will cut the thread to within the above-mentioned tolerances and all 100 bolts are run again.
Looking back at the original calculation for the 2 different bolts, it shows the 2” Grade A-105 requiring approximately 3 x the threading time as the 1” Grade A-36. When the manufacturing reality described above is included the 2” Grade A-105 requires 3 full passes plus all the loading and re-loading as each bolt is re-threaded.
Adding in the slower cutting speeds and all the extra steps and cautions required it could take as much 8 to 10 times longer to manufacture the 2” Grade A-105 Anchor Bolt. No one inch of thread would change the cost much, but multiplied by all the inches times all the bolts the cost is significant.
For this reason when ordering Anchor Bolts or Turnbuckle Assemblies, Clevis Rods, Hanger Rods, etc. it is worth spending a minute considering if what is being specified will actually provide benefit or if it is unnecessarily adding cost.
Ordering Anchor Bolts
If you are in request of anchor bolts, connect with The Steel Supply Company. Our anchor bolts are manufactured at our West Babylon, NY facility, and our extensive round bar stock allows for fast delivery to keep all of your projects on schedule. To get a quote, call 631-385-7273 or click the link below for more information.