Thread: What Diameter for threaded rods/shafts etc

Whenever I hand thread a metal rod I slightly taper the start of the rod so that the die can get an easier start but I also notice that if the rod is the exact size ( ie rod to cut an M6 on is exactly 6 mm) then it is really hard work to cut the thread so I usually take off 0.05 to 0.1 of a mm or so all the way along the section of the rod to be threaded. I have looked up a few references to see what the recommendations are for diams of rods/shafts to be threaded but cannot find any. Is there an algorithm or rule of thumb that is used for this?

Thanks

It depends on the thread tolerance you are working with. I think most stock standard nuts are 6H tolerance, and most stock standard bolts are 6g tolerance (unless something else is specified). And I think most stock standard taps and dies (of decent quality) you buy are for 6H/6g tolerance too. If I look up my thread table for M6, the outer bolt diameter for 6g tolerance is max 5.974mm and min 5.794mm.

If you are using a split die, you may have to open it up for the first pass. If you only have closed dies, you may forget about tolerances and make your rod a bit more undersize, say 5.6 or 5.7mm. It also depends, when you say hand threading do you mean freehand, or do you mean the rod is in the lathe chuck and you keep the die flat to the rod with the tailstock barrel, or are you using a tailstock die holder?

A TAFE metalwork teacher told me that a thread that is only half its full height still has 70 per cent of it's maximum holding strength. (That assumes the mating thread is full form though - otherwise if both were truncated to half height there'd be zero contact!)
Many rolled threads can be seen to be less than full height.
I give myself an easier life by reducing the diameter slightly. No problem so far.

Jordan

Bob,
I made some 3/8" T-bolts last week. The first bolt was turned to 9.5mm and I found cutting the thread with the die really hard work. I turned the next one down to 9.4mm and cutting the thread was much easier.
Chris

To save calculations, you can just measure a manufactured bolt out of your bolt stock. You will be surprised how much under they are and is why your having trouble threading it at 6mm.


If you find a chart please post it up as I would be interested in having it.

Dave

According to the metric thread standard, its 7/8 * (major dia - minor dia). Alternatively, its 1/8th of the thread height (from the min to the maj diameters) off the stated diameter. So for an M6 x 1.0 thread, the thread height is sqrt(3) * 1.0 = 0.866mm. The removal from the top is then 1/8 * .866 = 5.891mm

The picture on wiki will help it make sense. Id make a table up if you were keen on always removing that from the stated diameter before running a die over it.