Cutting a Imperial Thread

[bwal74]

Hi,

I was attempting to cut a 14 TPI 7/14"(I think - about 11 mm) thread today. Do you turn the compound around to 30 degrees or thereabouts? What did cut did look okay but what oversize so will cut it again when i get the chance.

Ben.

[pipeclay]

It's imperial so 55 degrees,half of that would be 27.5.

If you offset your compound just under 27.5 or 55 it will be fine.

The angle you turn it to will be the same it will only look different on you degree graduations.

At the end you want to tool to be set at around half of the thread angle,you should visually be able to tell if you move the compound to 27 and it doesn't look right then move it to just under 55.

[clive hugh]

Technically because they use the Imperial system of inches UNC and UNF come under this grouping. If you are cutting either of those the thread angle is 60o so you can set your compound 29.5o but even if you use true half it will come out OK.

[.RC.]

Of course it could be an acme thread so that would be 14.5 degrees. :D:D

[bwal74]

Thanks.

I'll give it a go.

Ben

[Pete F]

Set your compound to any angle you like, just so long as it's less than half the included angle. As Pipeclay says, that will be 27.5 degrees for a Whitworth. In reality however this 0.5 degrees business is a nonsense, and no lathe compound I've ever seen can be set reliably to fractions of a degree. Thanks to the internet that figure has become a bit of an urban myth.

The reason it's come about as on smaller hobby lathes they typically perform better when cutting on one flank only. Assuming you set exactly 27.5/30 degrees that's what would happen. However if you went over half the thread angle it would cut a funky thread form. To be safe therefore set a bit less than half the angle. It will cut a little on the trailing flank, but not as much as the leading flank. I typically leave my compound on around 25 degrees as I'm normally cutting metric threads, but don't lose any sleep over what the figure is. Some people, especially with large lathes and finer threads just plunge (90 degrees), so clearly anywhere between 1/2 thread angle and 90 degrees will be fine. I have some weird Swiss buttress threads to cut tomorrow and even with those you can run down just shy of the buttress thread angle, the same principles apply no matter what thread you're cutting.

[bwal74]

Hi,

Job completed. I needed to make a new longer bolt for my Hercus fixed steady.

Due to poor job planning and setup the threading on the lathe didn't work to well. I was only holding the bolt by the head, about 10mm long. The stress of cutting the thread pushed it out on about the 3rd or 4th pass. So had to take it out and finish it with a die.

I cut the four sided bolt head by mark I eyeball. Not quiet square but works.

Ben.

[OldRustyToolie]

Hi,

Job completed. I needed to make a new longer bolt for my Hercus fixed steady.

Due to poor job planning and setup the threading on the lathe didn't work to well. I was only holding the bolt by the head, about 10mm long. The stress of cutting the thread pushed it out on about the 3rd or 4th pass. So had to take it out and finish it with a die.

I cut the four sided bolt head by mark I eyeball. Not quiet square but works.

Ben.

Good to see it worked out ok. To prevent the problem you had with the job moving due to holding by the head only can be eliminated by supporting with a centre. Saves all that heartache and the thread can be completed without the use of a die.

[bwal74]

Good to see it worked out ok. To prevent the problem you had with the job moving due to holding by the head only can be eliminated by supporting with a centre. Saves all that heartache and the thread can be completed without the use of a die.

I couldn't fit the tailstock and have the compound at 27 degree's. I could've plunge cut but was already set up. A half centre would be good.

Ben.

[clive hugh]

On my lathe cutting a thread like that I have the same problem with the live centre getting in the way, I leave an extension, turned down to less than thread minimum dia. This allows the centre to be used but not interfere with threading. After I can leave it or cut it off.

[Pete F]

... or you could leave it attached to the bar, turn and thread the piece, then part it off an machine the head.

[philbur]

In reality however this 0.5 degrees business is a nonsense, and no lathe compound I've ever seen can be set reliably to fractions of a degree. Thanks to the internet that figure has become a bit of an urban myth.

easy peasy.

Phil:)

[OldRustyToolie]

I couldn't fit the tailstock and have the compound at 27 degree's. I could've plunge cut but was already set up. A half centre would be good.

Ben.

Not sure why you couldn't get clearance with the tailstock supporting the workpiece. With the compound slide set at 27 (1/2) degrees from the right angle to the axis there shouldn't be a problem. The interference is normally between the tool and full centre, dead or revolving. This problem of course only comes up when threading small diameter work using a carbide insert tip threading tool which doesn't have enough trail angle. A ground HSS tool doesn't give this headache. Also, we don't need a revolving centre when cutting threads unless we have a lathe with the necessary equipment to handle the higher cutting speeds required with carbide threading tools. Get a spare hard centre and grind it as required to make a half centre or buy one off fleabay as they are available there.

With the angular approach method (compound slide at < incl angle/2) it is most likely the better method for "the casual screwcutter", when cutting long chipping materials such as steel and soft aluminium to prevent tearing caused by both sides of the tool cutting where ultimately both chips meet in the middle resulting in a torn thread. With short chipping free cutting material it isn't a problem. The direct approach method (compound slide at zero) requires a bit more skill but is generally accepted to give a better thread and is quicker ...... a problem which shouldn't bother the hobby turner!!

[Pete F]

easy peasy.

Phil:)

Ah nope, it looks to me that compound is set slightly under 29.5 degrees Phil, so I'll mark that photo as a fail. Not so easy maybe :wink:

That's the point, so you split the divisions and think you've set 29.5 degrees. How can you be sure it's not 29.4 or 29.6? Splitting divisions on a compound is fraught with danger because firstly they're quite small and there's no vernier scale, at least not on any I've seen. The other thing is the witness mark is often stamped in without regard to the final alignment of the machine. So there's a good chance the divisions won't be exactly right, although some more expensive lathes will be better than others, unremarkably. I guess you could set up a sine plate against a bar and confirm it, good luck with that!

The whole point is that it doesn't matter, just set the compound a few degrees under half the thread angle and go at it. Anyone who insists that a compound must be set to exactly half a degree under half the thread angle is frankly kidding themselves in my honest opinion, and clearly doesn't understand what they're trying to achieve when they do so.

[Machtool]

The whole point is that it doesn't matter, just set the compound a few degrees under half the thread angle and go at it. Anyone who insists that a compound must be set to exactly half a degree under half the thread angle is frankly kidding themselves in my honest opinion, and clearly doesn't understand what they're trying to achieve when they do so.
This bloke had clearance issues, with the compound / top slide and a centre / half center issue. It's bugged me all day.


I couldn't fit the tailstock and have the compound at 27 degree's.
I am at a lose, what that means. Turning the top / compound slide normally means turning the slide away from the tailstock.

Ben. You do know this 27 deg thing. If you twist the top / compound around to 90 deg so that both the handles cross and top / compound are facing each other. Its 27 degrees back from there.

Its not 27 degrees from where it normally sits. That would be 63 degrees. If you don't have each handle remotely looking at you, your angles are wrong.

Turn the top slide square. Handle pointing at your guts. Turn it back 27 deg from square.

[philbur]

Ah nope, it looks to me that compound is set slightly under 29.5 degrees Phil, so I'll mark that photo as a fail. Not so easy maybe.

That set-up took all of five seconds, I guess if I had taken 10 seconds I could have achieve a standard that even you could not fault. So maye still easy enough for most people. It is of course more than accurate enough for the threading procedure as described, splitting of hairs is not a requirement in this particular case.

Phil:)

[pipeclay]

Just taking a guess here as to why he couldn't use the centre he had.

1) Was using a live centre that's body was too large for him to get down to thread size when using a 4 way tool post.

2) Was using a threading tool that maybe was a little too short to stick out of the tool post further.

3) Didn't consider plunge cutting the thread.

[bwal74]

This bloke had clearance issues, with the compound / top slide and a centre / half center issue. It's bugged me all day.


I am at a lose, what that means. Turning the top / compound slide normally means turning the slide away from the tailstock.

Ben. You do know this 27 deg thing. If you twist the top / compound around to 90 deg so that both the handles cross and top / compound are facing each other. Its 27 degrees back from there.

Its not 27 degrees from where it normally sits. That would be 63 degrees. If you don't have each handle remotely looking at you, your angles are wrong.

Turn the top slide square. Handle pointing at your guts. Turn it back 27 deg from square.

Now I'm confused. I'll take a photo tomorrow. Maybe I'm doing it wrong.

Ben

[Pete F]

That set-up took all of five seconds, I guess if I had taken 10 seconds I could have achieve a standard that even you could not fault. So maye still easy enough for most people. It is of course more than accurate enough for the threading procedure as described, splitting of hairs is not a requirement in this particular case.

Phil:)

Phil you seem to be missing the point that I'm trying to make here. Some people state that the compound NEEDS to be precisely 27.5 or 29.5 degrees in order to cut a thread in this way. I can say, with absolute 100% confidence, that is complete rubbish, and is a figure that is an urban myth perpetuated by the internet. If you choose not to believe me then let me ask you, why do you set 29.5, why not 29.6? Why not 29.55? Where do you stop with splitting things? For that matter, why not 30.000 degrees? The latter would in fact give the most precise result if the aim of the exercise is to cut on one flank only (all above assuming 60 degree threads). Have you verified the accuracy of the divisions on your top slide? How can you be sure, having wasted "10 seconds" precisely setting "half degrees" on your top slide that it's actually at that point? See where I'm going with this?

The whole point is that there are no hairs to split here. A threading tool is ground or made as a form tool, thus can cut on one flank, or both flanks. That translates to mean that the tool can be fed in to the work either with an offset of 30 degrees (cutting on the leading edge only) or plunged straight down (cutting on both edges equally). Clearly if it's possible to cut at both those extremes, then it's possible to cut anywhere in between too!

Hopefully without confusing those who are singing along at home, by offsetting the top slide and feeding in that way what we're doing is cutting a taper that is the thread angle. So at 30 degrees the taper is exactly equal to the thread angle and the tool would cut only on the leading edge. Happy days and what we want right? Yes, BUT .... for the reasons I outlined above we can't set EXACTLY 30 degrees on our compounds. Some may like to kid themselves that they can, and if that's the case they should set 30 degrees and not 29.5, as the former figure is what's desired. What's the problem with 30 degrees then? Because if you accidentally set more than 30 degrees the "taper" the tool is cutting will be incorrect and the form of the thread affected. Many people have done this at some stage by accidentally setting 60 degrees on their compound and then they say "What's wrong with my thread, it looks funny". So setting 30 degrees isn't a good ideas as you want a bit of wriggle room. So we normally set "just a tad" under 30 degrees in which case the leading edge of the work is still doing the majority of the cutting, while the trailing edge will just be skimming a bit of material off to match its form. Unfortunately in this digital age that "just a tad" has morphed into 0.5 degrees, a figure that's often spoke of as if handed down to Moses himself on a stone tablet! The whole point is IT DOESN'T MATTER what you set, just make sure it's less than half the thread angle.

Soooo why offset at all then, why not plunge straight in? Well the good news is you can. However there is less cutting force on the tool if it's cutting on one edge only. For those with big lathes or cutting only relatively fine threads it's no big deal, and just plunge it. However for those of us running glorified noodles, once the thread pitch starts getting larger our lathes will struggle so we need to give them all the help they can get. Personally I also find the whole process of resetting the cross slide to zero with each pass much easier to manage, but maybe that's just me and what I'm used to. I personally think it makes it much faster process with little chance of screw ups, if you pardon the pun, but to each their own.

The reason for going through all this is firstly I don't like to see rubbish perpetuated via the internet. Just because it's written down doesn't make it true, and I'd urge anyone who doesn't believe what I've said to try it for themselves. The other reason is that a good knowledge of what you're trying to achieve may have helped the OP get out of trouble. If there is a tailstock in the way with support, that's fine, then swing your compound around to "see more of your gut" and square up your tool to the work. You may not have had room for the top slide set at "29.5" degrees, that's fine, then swing it around until it fits and you can feed in. I'm with Phil, as I'm not sure why it wouldn't fit at around 30 degrees, but I wasn't there and sometimes these things happen. It may mean you can't take as large cuts, but that thread doesn't look too arduous, so a sharp tool would knock it off fine or it may mean less DOC and an extra pass. No biggie and no dies need be harmed in the process of making the thread :p

Hopefully that helps explain things a little better and will provide ammunition in order to get around difficulties when they pop up.

[morrisman]

For thread cutting info, the Workshop Practice book #3 'Screwcutting in the Lathe' by Martin Cleeve , is hard to beat . He covers just about every situation with easy to understand practical information . Mike

[DSEL74]

Just on a slight tangent… What was used to cut the thread, Ground HSS, a Carbide tip, or one of those holders with a spiral kind of thread cutting wheel?

I haven't advanced to thread cutting yet so can't really comment. But wouldn't the size of the lathe and type of tool post/holder have an effect on how close one can get to the tailstock?
I.e. If he had a big ass Dickson quick change with a short piece of HSS, and a fat centre combined all these things work against him?

Not debating anyones expertise, just trying to learn.

[pipeclay]

In answer to your question about size then yes it can/will play a part.

You can get extended nose live centres that will allow a tool to get much closer to the centre of the job.

You may find that the size of the tool post may play a part if it is excessively big, although people say you can do small work on a big lathe it is all relative to the size of the lathe.

The tool being used also can play a part, it could be to bulky/big to get in, you always have the choice to weld a piece of HSS to a bar and extend that out to give you more room.

I think in the case in question the use of an extended nose centre with a small bearing body may of worked, it may also of needed the HSS extension or even the use of a lantern tool holder if available.

You can even get some cemented carbide tipped tools that are quite thin for this purpose as well as tools that are off set.

[bwal74]

Yep, I had the compound around the wrong way. I don't know why I continually do that.

I was also using a carbide threading insert.

Ben.