No need to turn diameter, just put a small chamfer on the end and use a split die at widest setting, then after first cut, run it down thread again with adjusting screw loosened off slightly. Check with a nut after each pass to check the fit.

Agree with Phil, but if you're expecting trouble or want to extend the life of your die and don't mind reducing strength, you could shave a little off the rod to reduce strain on the die.

Depends on how good a fit or not the female is, but in any case I would never reduce a rod under about 5% of the starting diameter, say 0.24" in your example. A much smaller reduction is usually enough to ease a die.

If strength matters don't reduce the rod at all.

Dave

Reducing the diameter of the rod by a couple of thou won't affect the strength at all. Threads have 2 failure modes, at the root where the male spans, or along the line where the shear strength of the male equals that of the female. Neither of these are affected by the OD unless you get very enthusiastic. Similar reasoning to why tapping drills are bigger than core diameter

The nominal OD of a thread is the same as the nominal size. So for M6 it should be 6mm. However, the threads in nuts are usually slightly larger and the threads on bolts slightly smaller than nominal, allowing parts to fit together over a range of manufacturing tolerances and tool wear.

I've just measured the OD of one of the M6 studs I made recently, it's 6.00mm. Granted it's a snug fit on the home made nut tapped with a commercial spiral flute tap, but I'm not trying to achieve fits over millions of components. And I want a better quality of fit on my engines than I can get with ordinary commercial fasteners. The commercial offerings don't look right either, but that's another matter.

Andrew

topping the male thread won't affect the closeness of fit, the crest is clearance in the female. Just try running a tap down an accurately drilled /reamed 6mm hole and see the witness mark. the fit is, or should be, between the flanks of the thread. Here's a useful little calculator, whiuch suggests that Andrew's studs are in fact oversize for general engineering tolerances. They should be between 0.03 and 0.2mm below 6mm

https://www.amesweb.info/Screws/IsoMetricScrewThread.aspx

Edited By duncan webster on 26/07/2018 15:20:54

I’m in agreement with Andrew. 1/4” is not a huge thread – not like inch and larger.

A good quality die should cope easily and without too much effort. It is a fine thread, so not as hard to cut as a coarse one.

Now, if using cheap chinese tat, or a carbon steel die, the die may not last very long. OP appears only to be doing one. If the rod is of any length, a travellig steady might well be needed to remove a sliver from a thin rod.

The above is of a lot of interest to me. The clutch centres of the classic motorcycles I enjoy using have 6 6 mm studs to take the springs and adjusting nuts. They are very often all shapes ! with I assume earlier owners just jamming a screwdriver across 2 or 3 when attempting to undo the centre nut. I drill out the weld holding these studs from behind to release them. New ones are made from Allen bolts with the head cut off, then a slight taper ground to enable a die to get started. Not easy, these bolts are tough. Eventually all 6 are at the correct height, so off to a pal 3 minutes up the road to mig weld on the back to lock them securely in place. I have made a dummy mainshaft plus a guide plate to ensure each new stud is exactly vertical, last step is to skin the excess weld to the same level as the original. Slightly off topic, I am sure I read somewhere that although modern car engines use mm threads, some use unc on head hold down studs, on being threaded into the crankcase. Reason given was that there is no mm of suitable size for that particular application. Can anyone comment on that ? Someone is bound to ask, my bikes are 1960’s 70’s Spanish Bultaco’s .

The fit of nuts and bolts is defined by tolerancing just like shafts and bores. Here's an overview. Most of the bread and butter stuff we buy never mentions it but for production parts, the drawing must always include that part of the spec. The default is 6g for the bolt and 6H for the nut. However, the stuff you will get down at B&Q is likely to be a pretty sloppy fit and just what you produce from a split die is largely up to you and how you choose to adjust it. Unless you know the precise spec of the nut and bolt you are looking at (do you have the drawing?), measuring it isn't going to tell you much about "normal" dimensions.

If you buy a modern motor vehicle, all the fasteners will be metric, even US ones almost certainly. There's nothing stopping engine designers from specifying any diameter and thread pitch they fancy, so the idea that they would choose some ancient imperial spec sounds like an urban myth. Fasteners such as cylinder head bolts will be designed and manufactured with unique dimensions, so availability of "standard" parts just doesn't arise.

Murray