Stud threading

Hi I need to thread a 1/4" diameter steel rod with 1/4" UNF die.

What diameter can I turn the o.d of the rod to help the die cut the thread.?

Mike.

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.

How long does the rod need to be? You can buy threaded rod if the need is for large lengths.

Or if it's just an inch on each end of a piece of rod, you could turn it down about five thou under 1/4" to make threading easier. For much more than that, go with the advice in Phil's post above.

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

Bright drawn round rod is usually a thou or two undersize to start with. A UNF thread isn't coarse pitch so I wouldn't bother with a chamfer, turning down or multiple passes. Just set the die and cut the thread in one pass - job done. The time saved can be used to down a beer or two.

I've made hundreds of studs for my traction engines, all from stock material, no chamfer and cut in one pass:

I'll state that the studs have been made on a repetition lathe, largely using a Coventry diehead, but the end result is the same. Some of the studs were made using a split die, where I didn't have, or couldn't find, the requisite Coventry dies.

Andrew

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

Dormer recommend reducing diameter by 10% of the pitch. An M6 has a pitch of 1mm so turn down to 5.9mm before threading. If you measure a 6mm bolt it is around 5.9mm outside diameter. As Duncan says this will not affect the strength.

Jan B

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

I was taught that the female thread is cut to full size ( 1/4 in your case) by a tap but the tapping drill is selected to drill slightly oversize hole thus giving a truncated thread root ( sounds sexy eh ? ) the male thread is cut slightly undersize giving a truncated thread crown and this stops the thread binding up on either the crown or root , the thread will engauge on the thread flanks so adjusting the die to give a nice fit in the tapped hole will mean the two threads will run together smoothly . 75 % thread engagement is the usual but i have seen less work with no problem .

If the rod is a couple of thou under it won't hurt a thing - unless you are working for NASA building space probes !

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

There seems to be some confusion about thread forms and thread engagement.

For metric and unified threads the internal and external thread form definitions are different with regards to the crests and troughs. So even perfect threads will only mate on the flanks. However that is not true for the Whitworth form where the crests and troughs are the same and only one thread form is used for both internal and external.

In the case of most of the threads we cut, whether by taps, dies or dieheads, the thread form is predefined by the tool manufacturer. So the only thing we can muck about with is the diameter, moving the whole thread in or out without changing the form. It happens that it is easiest to change the diameter of the male thread by opening out the die or tweaking the adjustment on a diehead. But that's all one is doing, effectively changing the pitch diameter, not altering crests or troughs independent of other features of the thread.

I'd agree that cutting a thread on a rod that is slightly undersize won't make any difference. But that's not the same as wasting time making the rod a few thou smaller in the first place. I'd rather cut the thread on the rod as supplied and waste time drinking beer.

Andrew

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.

This seems to be true for just about every bolt I’ve ever measured. They always seem to be under size even with good brands like Unbrako. I think Duncan’s right, they don’t need to be “full size” to perform properly.

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 .

Purely an aside, having roughed the male thread with a Die at maximum opening in the holder, I finish by adjusting it to a good fit on an existing male thread. Hopefully, I don't end up with an undersize thread.

The lubricant is what you wish, Rocol STD, Trefolex, tallow or whatever. (I try not to use ordinary lubricating oil)

Howard

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