Cutting threads with die

[GrottoParticipant @grotto]

Hi Again

I'm wondering if there is a chart for determining the correct diameter required for cutting a thread using a die.

I've found plenty of info. On the correct hole to drill for tapping, but none of the charts I have seem to tell you the correct diameter a rod should be to cut an outside thread.

Currently if I wish to cut a 6mm thread on a piece of rod, I find a 6mm screw and measure the external diameter of it, and the turn the rod to that diameter.

Works OK, but I figured the must be a chart (similar to the Drill & Tap Charts) or else some rule.

Also, I often find (particularly on larger threads) that I can't get the die to bite, and just end up reducing the diameter of the first mm or so of rod, rather than cutting a thread. I've been getting around this by tapering the first 3 mm of rod before threading, and then facing off the taper after thread is cut. Is this an OK solution or is there a better method? I've been using a die holder in the tail stock.

Thanks

[GrottoParticipant @grotto]

[VicParticipant @vic]

For metric coarse it's easy, M4 = 4mm etc.

[Anonymous]

I just use the nominal OD of the thread. So for M6 I'll use 6mm, for 1/4" BSF I'll use 0.25". Allowing for tolerances I aim for a thou or two under rather than over. I normally put a small 45º chamfer on the end of the part to be threaded. As well as easing use of the die it makes the end neater, rather than having a sharp edge where the thread starts. I never start a die and diestock with the part in the bench vice. It can be a PITA to get it going and the thread will most likely end up imitating a newt. I normally trap the diestock between the work and the tailstock barrel in the lathe. I can then turn the chuck by hand and use the tailstock to apply pressure and keep the thread square. Once I've got a couple of turns done I transfer to the bench vice, if required.

For anything precise I screwcut in the lathe, and if I have a lot of threads to cut I use a Coventry diehead.

Andrew

[Roy MParticipant @roym]

My preferred method for a good thread using a split button die is as follows although it requires an extra few mm of material. Leave a couple of thou on the outside dia. Then turn a spigot to about 10 thou above the core dia, (depending on thread size). Adjust the die to cut oversized thread and then run the die along the the bar. The spigot will ensure that the die starts along the bar correctly aligned, and adjusting the die means that you can take a couple of finishing cuts. This results in an accurately sized thread, and a perfect thread with good finish and no tear marks. A similar approach can be used if your thread is less than perfect after screw cutting. Roy K.

[Roderick JenkinsParticipant @roderickjenkins93242]

Good advice there from Andrew. It may be worth looking at the die as well – good dies have a countersink on the start side. Make sure the die is the correct way round, some of my dies actually have "start from this side" engraved on one side.

Rod

[IanTParticipant @iant]

I also take the material slightly under nominal diameter – as I feel it is the equivalent to using a slightly larger tapping drill when tapping, thus giving a lower percentage thread engagement (65-70%) but making life much easier and in the case of a tap, reducing the likely hood of tap breakage. A reduced thread engagement will not generally impact the strength of the threaded union in practice. (Read Tubal Cain on the subject)

With larger dies, a 3 jaw chuck will sometimes struggle to grip the material firmly enough, as the torque required is increased. This is where a collet chuck is a much better holding device to use. A collet will grip the material more firmly, helping to prevent any rotation.

Mention has also been made of transferring the work to the vice but again (where possible) it is much better to use a collet block (itself held in the vice) as this will again provide a greater grip (and help prevent any damage) to the work piece.

Regards,

IanT

PS – I recently enquired here about 'cutting pastes' and received a number of recommendations. I finally decided to stick with what I'd used in the past (at night class) and purchased a 500ml can of Trefolex. A much smaller jar of it now sits permanently on the bench and is used for all tapping and die threading operations.

[GrottoParticipant @grotto]

Thanks, lots of useful advice which is really helpful.

I think I may need to invest in some better quality metric dies. I have an old set of non-split questionable quality ones, which I seem to have most problems with. I have a few good quality split ones which don't appear to cause the same issues (same with my imperial ones).

Interestingly, external threads I cut with my low quality dies work fine with shop bought nuts, but not with pieces I've tapped (using decent quality sharp taps).

[Alan RawlinsParticipant @alanrawlins60482]

Something I have just learned recently is that if the bar you are threading is oversize by about .005" to .010" Then the resulting thread will be a tight fit when a screw is fitted that you have drilled and tapped.

When I am threading bar I turn a slight taper on the end to be threaded, then let the bar protrude from the lathe chuck

about 50mm. With the die in its holder placed the correct way round for threading bring the tail stock, with out any centre in it upto the opposite side of the holder, and with the tail stock tightened up on the bed, use the hand wheel of the tail stock to put pressure on the die holder while turning the chuck using the chuck to get leverage. the end of the holder will probably catch the corner of the lathe bed when you start as you would need three hands if it didn't.

This may sound complicated but it isn't , it is dead easy.

[JasonBModerator @jasonb]

As BA and the smaller UNF and UNC don't have a nominal diameter in their description this chart of mine may be useful. A set of Zeuz tables is a handy thing to have in the workshop too.

Pipe threads also need the dia of teh bar a lot larger than the nominal pipe size so the Zeuz will have those if needed

[Howard LewisParticipant @howardlewis46836]

Hear hear! to advocates of Zeus Charts. They are invaluable.

I am still using the ones bought back in 1958, (They contain info that is lacking from some of the later versions that I have). ANY issue is good for details of various threads, (Dias, Core Dias, pitches, thread depths etc) and Tapping drills.

Info is available in seconds, saving ages searching or enquiring.

Unfortunately, they don't deal with M.E. threads, but Data Sheets that give those details are, or certainly were, available.

If not this is a good place to ask for someone to provide the details of an unusual thread. Someone will have come across it, no doubt and will tell you what you need to know.

FWIW, Beware, (if not already known), BSP and NPT threads are not interchangeable; different pitches and thread forms. Similarly, British Standard Brass is Whit form, Cycle is 60 degree, although both are 26 tpi.

Graunch at your peril!

Howard

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