cutting threads on hex shank rod


Old 05-22-03, 07:49 PM
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cutting threads on hex shank rod

I need to thread 3 inches of hardened hex shank rod that measures 9/32 using a drill gage. Looking in the tap book this is around .281 diameter. Im thinking of using 1/4 -20 threads. Is this too large a diameter for a 1/4-20 die to cut? I know hardened things arent easy to thread, but I need this threaded. Should I use 100% threads since its hex shank and the nut wont contact all the surface area?

Is 1/4-20 going to work or will I need a dif size die to cut this? I cant find any die cutting charts in the handbook.

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Old 05-23-03, 05:44 AM
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It's my experience that hex stock materials are commonly sized 3/16, 1/4, 5/16, 3/8 and so on, and that 9/32 is almost exclusively for tool use - hex keys, hand tool shafts, powered shafts, and etc and that 9/32 is therefore produced from very hard materials .

Common tap and die sets offer - and LIMIT your choices (in the range of your stock) to: 1/4-20, 1/4-28, 5/16-18, and 5/16-24. If I was limited to using one of these four sizes - if I was forced to use just one of those four - I would IMHO choose a 1/4 die - if it were the split type or adjustable type.

Here's why: Generally speaking 1/4 dies will produce external threads on stock that has a minimum diameter of .237 and a maximum of .260. Also generally speaking, 5/16 dies will produce external threads on stock that has a minimum diameter of .298 and a maximum of .324. Your 9/32 hex stock has a .281 diameter which is inbetween the 1/4 and 5/16. The 5/16 die will produce external threads that will not allow good engagement with the nut - it will be too loose. The 1/4 die will produce better engagement, and if it is the split or the adjustable type it will thread slightly larger diameters without too much diffculty.

That said, I'm very concerned with the engagement of the nut. A 1/4-28 fine die will have 8 more threads per inch than the 1/4-20 coarse die and the eight added threads might improve engagement on the hex shaft; but both sizes might be insufficent due to a lack of thread continuity and also due to dynamic frictions and stresses. I might consider using a left hand die instead of the traditional right hand die if dynamic friction and other forces created by a spinning or rotating shaft was a concern. Locking nuts with nylon inserts and/or thread applications might restrict or prevent disengagement of the nut.

If all you have is the 1/4-20 die, use it. If you are limited to choosing between 1/4 and 5/16 dies, use the 1/4 - BUT there are other options available.

Other options might include choosing a metric die closer to the diameter of your shaft - an 8X1.0 for example, or choosing a 9/32 die. 9/32 dies are available in 32 and 40 TPI or threads per inch, and they will produce external threads on stock that has a minimum diameter of .267 and a maximum of .292, but 9/32-32 and 9/32-40 high grade nuts are sometimes rough - but not impossible - to find.
Old 05-24-03, 04:55 AM
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Josh1, are you saying that this hex rod is 9/32" across the flats or across the points of the rod? I'd also be concerned about the "hardness" of this steel. That in itself may present quite a problem in threading it.
Old 05-24-03, 08:46 AM
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Excellent point about actual measurements of hex shaped materials and products - for example, a 1/4 hex key or Allen wrench will not fit through the 1/4 hole of a drill guage.

I believe josh1's item is actually sized at 1/4 because he states that it passed through the 9/32 of his drill guage. As I stated above, my experience indicates common hex sizes are 3/16, 1/4, 5/16 and so on, but in my opinion the item in question should be viewed as 9/32 or .281 when selecting a die because .281 should fall within the minimum and maximum diameter range of the die - or be very close. In this case case the stock is probably sized at 1/4 or .250 flat to flat, but the wider overall diameter of 9/32 or .281 point to point has to be taken into consideration for threading, and josh1 was wise to check that with his guage as he did.

Hardness is always a concern for drilling and threading but with todays fine tools and all the cooling/cutting products available I really forsee no problems in threading. My worry is in the retenion of the nut.

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