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Metric thread cutting in a lathe

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Richard brown 124/01/2019 22:36:15
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Hi

Sorry for this basic question but I have only cut 1 thread in a lathe and now need to do something fairly important.

I know how to set the tool the cross slide etc but my problem is more to do with lack of knowledge of threads.

Am I correct in saying that to cut a 12x1mm thread I have to turn the bar down to 11.8mm? I'm asking because in The Model Engineers Handbook by Tubal Cain he shows a drawing and I can just about see it says 0.1P for the amount of thread crest that is taken off.

Is it better to take this off the diameter before I start or does it not matter? I am thinking I should take it off before or I will end up not knowing if I should cut the thread deeper or take some of the diameter to get the nut to fit! Some advice would be great.

One last question, How do I work out how deep I need to cut in order to get the thread properly formed. Is it simply 0.86mm as he seems to show what looks like 0.86P. Or does it have something to do with the Minor Core diameter that is listed in the tables, which in this case is shown as 10.77mm

Sorry if this seems basic but I have always stayed well away from threading in the lathe through lack of knowledge but now I need to start doing some.

Thanks for any help you can give

Rich

Emgee24/01/2019 23:44:21
1086 forum posts
199 photos

Hi Richard, firstly welcome to the forum.

Turning the bar before threading will ensure you have a better chance of a straight thread.
I am not certain about the .2mm undersize but the author was far wiser than me so you could follow his advice before threading, although you may find the OD has grown during the threading process.

If you are threading to use a nut then best have that to hand and trial fit when approaching the depth of thread, it's easy to go to deep.

For the depth of thread use the table provided but note that if the threading tool does not have the stated nose radius you may have to adjust the depth of cut to suit.

Emgee

ega25/01/2019 00:49:42
1133 forum posts
94 photos

Some at least of the answers to your questions are to be found in Martin Cleeve's "Screwcutting in the Lathe" (no 3 in the Workshop Practice Series).

You don't say what your job is but I think most amateurs aim to get the screw to fit the nut rather than to produce a theoretically correct thread form; by "fit" I mean on the flanks of the thread - as long as the crests and roots clear each other their precise dimension and form are less important.

It would also be helpful to know something about your lathe and tooling: single point threading techniques are different from those applicable to insert tooling.

If you are a beginner then some practice on scrap before embarking on the real thing is a good idea.

Hopper25/01/2019 01:30:19
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When screwcutting in the real world (ie home workshop) it is best not to try to hold exactly to the ISO thread form drawings such as those you refer too. They are far too exacting for practical home use.

For practical work at home, yes make the outside diameter a few thou undersize so as to allow clearance on the thread tips. For a 12mm thread, I would turn the OD about .005" or 0.1mm undersize, or a little bit more. So 11.8mm is ok too. Yes, machine it down before you cut the thread.

For thread depth I always use the classic formula for a 60 degree thread form (metric, UN etc) of .6134 X Pitch.

For 55 degree thread such as BSW, the formula is .6403 x Pitch.

Run a file down the top of the thread groove to remove the burr and round off the crest a bit. The corner of a common flat file works well for this.

As said already, final sizing is by trial fitting a nut of known quality. (Cheap hardware store nuts are a notoriously loose fit so you might want to make your own test piece using a tap if needed.)

Chris Trice25/01/2019 02:31:41
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1354 forum posts
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Visual comparison with an M6 machine screw is a useful tool too. The thread form is the same but yours is just 6mm greater in diameter so if the M6 thread is measured as 5.9mm diameter, cut your 12mm master 11.9mm. You can also put the M6 in your chuck, advance the threading tool up to it so it sits fully into the existing thread, note the slide position reading and then add 3mm to the reading as your finished depth position when cutting the bigger version. Another useful tip is use a jewellers magnifying loupe to look closely at the thread to check it looks like your M6 version. Coincidentally, I've been cutting 12mm x1mm threads this week.

Richard brown 125/01/2019 07:43:10
75 forum posts
27 photos

Thanks for the replies you have all been very helpful.

With the 0.6134 x pitch I can now see that this will indeed cut down to the Minor core diameter in this case 10.77 so that's that solved also.

Thanks again

Rich

Andrew Johnston25/01/2019 08:39:05
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Posted by Richard brown 1 on 25/01/2019 07:43:10:

Thanks for the replies you have all been very helpful.

But unfortunately incorrect. sad

The major diameter for an external metric thread is nominally the diameter of the thread minus a fundamental deviation, and also a machining tolerance, both dependent upon the class of fit. You do not take off the truncation of the thread form, that is already built in. Incidentally the height of the crest truncation is H/8, where H is the theoretical sharp thread depth.

For example take a 12x1.25 thread. The nominal OD is 12mm and the fundamental deviation for a 6g fit is -0.028 giving a maximum major diameter of 11.972mm. There is then a machining tolerance of a maximum of -0.212mm. A 12x1 thread is not standard, so you'll have to work out the numbers yourself.

That's the theory. In practice I start with the work at the nominal diameter or very slightly above, in this case 12mm. I then cut to the calculated thread depth and check final fit with a mating thread. I find that one needs to cut a thou or two deeper than the theory in most cases. For standard thread forms I use full form threading inserts. That way all the crest and root shapes are taken care of.

Andrew

Richard brown 125/01/2019 08:45:34
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27 photos
Thanks for the info but i have bought 12x1 locking nuts and it is mentioned in tables so it seems standard at first glance.
JasonB25/01/2019 08:54:35
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Posted by Richard brown 1 on 24/01/2019 22:36:15:

 

Am I correct in saying that to cut a 12x1mm thread I have to turn the bar down to 11.8mm? I'm asking because in The Model Engineers Handbook by Tubal Cain he shows a drawing and I can just about see it says 0.1P for the amount of thread crest that is taken off.

Look again at the book and you will see as Andrew points out the "Major Diameter" is to the crest not the projected point that is "taken off". The 0.108P or H/8 is over and above the Major Diameter.

If using aground tool or partial profile you may need to run a file down the crests as you get close to remove burrs so that they do not contact the nut first and give a false sense of a good fit. Also depth of cut will depend on the type of cutter as a pointed one will need to go in further than one with the correct radius on the end.

Edited By JasonB on 25/01/2019 08:58:37

Hopper25/01/2019 09:18:20
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Funny. I've been using the "incorrect" method of slightly undersized OD and .6134 x Pitch at work and at home for about 45 years now without problem. Like I said, trying to duplicate the exact ISO thread form drawings is overkill in the home workshop, and in the professional machine shop most of the time. As Tubal Cain points out, you only need about 65 per cent thread engagement to have about 95 per cent of maximum thread strength. I believe it's best for a beginner to stick with the tried and true .6134 x P formula that has served generations of  machinists well.

But like most things, there's more than one way to skin a cat.

I forgot to mention too that if using a standard V tool bit, it's best to put a small radius on the tip by rubbing it on the bench oil stone etc. Not because of the thread form that results but because it helps pevent the tip of the tool from chipping off.

 

 

Edited By Hopper on 25/01/2019 09:42:53

Bazyle25/01/2019 09:44:47
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Gosh people working to 1/10000 of a mm. I feel like a real wood butcher wink

Richard brown 125/01/2019 10:04:49
75 forum posts
27 photos

I work to to tolerances of things like a Bee Space or a Nad.

I think I will go just slightly under 12mm and then go in by 0.61mm and see how it goes.

Regards

Rich

Hopper25/01/2019 10:11:41
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That;s the best way: see how it goes. Try a few test pieces first and get the feel of things before attacking a real job. And you can always use a die to finish off your thread if you feel so inclined. Usually gives a good finish with minimal time spent on final finishing.

JasonB25/01/2019 10:12:46
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I only see people mentioning calculating to 1/1,000 mm not 1/10,000 mm.

Whether they work to that is another matter.

Andrew Johnston25/01/2019 10:27:20
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A 12x1 metric thread is not standard in the sense that it isn't in the coarse or fine series. It appears in the constant pitch series, for which hardware, and taps and dies may be available, or not, as the case may be.

I normally use partial thread engagement for threading. However you only need the reduction on one half the combination. If it's on both then the thread engagement is a lot less. Since it is simple to produce a full depth external thread, but difficult for internal threads (especially when using taps) that's what I do. I cut external threads close to full depth and drill for internal threads to give the required engagement.

I don't work to micron accuracy, but I do like to understand what I'm trying to do. Understanding the theory to a reasonable degree of accuracy eliminates it as a source of error. Once you have numbers they can be adjusted to whatever tolerance is needed, or accepted. Misunderstanding the basics and making incorrect approximations just leads to sloppy work - possibly in more than one sense. smile

Andrew

Barrie Lever25/01/2019 10:34:50
177 forum posts
38 photos
A 12x1 thread is not standard, so you'll have to work out the numbers yourself.

Andrew

That thread is a metric 'super fine'.

Pretty well all metric threads that we use are 'course' then 'fine' then 'super fine'.

Example M8x1.25 is course. M8x1 is fine and M8x0.75 is super fine, M8x0.75 as a matter of interest is a specialised glow plug thread in competition model engines.

That info is all on the back of the Tracy tools price list.

Regards

Barrie

SillyOldDuffer25/01/2019 10:37:21
4122 forum posts
831 photos
Posted by Richard brown 1 on 24/01/2019 22:36:15:

Hi

Sorry for this basic question but I have only cut 1 thread in a lathe and now need to do something fairly important.

...

 

What's meant by 'fairly important' is the elephant in the room. It decides whether Andrew's answer is more appropriate than Hopper's.

In my workshop, I'm usually with Hopper. I mostly make threads where the only requirement is they have to fit together. The threads don't have to maximise strength or to minimise backlash. I don't bother reducing the rod slightly below nominal size, I use an ordinary 60 degree cutter, and I don't worry about getting the thread form perfect. I cut down towards theoretical depth and then test with a nut, only going deeper if the nut won't go on or jams. I usually run a file over the crests to flatten them and remove any excess diameter that may have squeezed up during cutting. Rough work in terms of precision and accuracy but plenty good enough for 95% of what I do.

Occasionally I need a better fit. My preferred approach is to rough out about 60% of the thread with a lathe and then finish off with a die. The die puts a more accurate finish and profile on the thread. If I don't have a die of the right size, and the thread needs to be a good fit for strength or backlash, then I use carbide index thread tools and take much more care with dimensions and finish. This is much more like Andrew's approach. As I'm not skilful and have poor concentration, about 1 in 4 of my 'best effort' carbide cut threads are rejects due to small flaws.

Where thread strength and fit are both paramount - like the bolts that hold aircraft wings on - it's unlikely that a high-end fastener would be made by a cutting process. Rolled threads are considerably stronger that cut threads. Most commercial fasteners are rolled for cheapness, but the posh ones are made more carefully from better metal, finished by grinding, and inspected.

If 'fairly important' only means it has to fit together with moderate strength (60-70% of theortetical), Hopper is plenty good enough. But if the thread must be strong and/or a tight smooth fit best proceed as Andrew, or compromise with a die. Finally, if the part is safety critical - essential to holding a car's brakes together - then I'd recommend buying the right commercial part.

If it's important because the thread has to be cut on an existing part that can't be replaced, practice on something else until you get good results!

Dave

 

 

 

 

 

Edited By SillyOldDuffer on 25/01/2019 10:42:50

JohnF25/01/2019 10:55:49
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779 forum posts
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Lots of thread information here plus a great deal more at Maryland Metrics **LINK**

Plus one for Andrews last comment because it’s always best to try and achieve a high standard, I often use a thread mic for some work where it’s critical and I don’t have a callipers gauge. Most threads I cut I do truncate and I use both hand ground tools and full form tipped tools.

Having said that for many things in the home workshop as long as it fits together and does what you need then it serves its purpose!

John

Richard brown 125/01/2019 12:39:47
75 forum posts
27 photos

Success!

Just kept trying it as I got close until I got a nice fit. When I said fairly important it was a ball screw for converting my SX3 to CNC.

Regards to all

Rich

20190125_115430.jpg

Andrew Johnston25/01/2019 12:50:14
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Posted by Barrie Lever on 25/01/2019 10:34:50:

That info is all on the back of the Tracy tools price list.

Some suppliers call it extra fine; Machinery's Handbook says constant pitch so that's what I'll use. I stopped purchasing from TT a good few years ago, for several reasons. sad

Andrew

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