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.

[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

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.