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Home Workshop > Members > Forum > General Area > General > Threading inserts?

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Threading inserts?

Threading inserts?

Wonder if anyone knows of a beginner friendly resource which explains threading inserts? I know the difference between  full and partial profile, but cant find any specific info on full profile inserts, regarding things like inserts needing to be specific to thread diameter as well as pitch?

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Re: Threading inserts?

Hi Chris to the best of my knowledge inserts are capeable of cutting many sizes but come as 55 degreees for whitworh and 60deg for most other threads. The tips are very delicate and you need to keep the speed up which when you are cutting to a shoulder which is difficult. Thats why I have tried to cut left to right. I have broken many tips trying so I've  almost given up trying to cut complete threads. I am trying Coventry Die Heads that are easy to use but the chasers can be expensive. One tip that I use is to use one chaser in a holder to cut the thread like a regular lathe tool but with the screw curtting feed set to correct tpi, this works well and individual chasers are no where near as delkicate as the carbide tips of the inserts. But my best tip is to cut most of the thread with a HSS tool and finish it with a die, that is almost fool proof and you get a concentric thread.  To be honest cutting the odd thread in the lathe is is a bit of a black art and too much bother easier to use chasers or dies. A final thought probably much easier to cut threads above 12mm dia and that is my direct experience and easier the larger they get up to a point of course.
Ash.

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Re: Threading inserts?

Thanks, Ashley.  Was interested in whether full form threading inserts are diameter specific as well as pitch? A mate of mine who has been thread cutting for 40 years has shown me the basics, but as I don't currently have reverse on my imperial lead screw lathe have been experimenting with marking chuck, lead screw, and saddle to see if I can get away without keeping lead screw engaged and thus avoid crashing tools into shoulders? Mate has a threading dial with gears which can be changed, and a couple of weeks ago watched him cut a 42mm thread into a piece of CDS tube in about 5 minutes.  If you can disengage lead screw threading up to a shoulder doesn't seem that difficult.  Have an Imperial thread dial on my machine, but not much help for metric threads..................

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Re: Threading inserts?

Hi,

The thing to remember is that threads are normally 'truncated' - ie the tip and root are rounded off. "universal" inserts cut sharp threads - the specific tpi inserts have the correct truncation for the pitch. The diameter consideration is to get the helix angle correct (if you think of a thread, the thread is at an angle - which varies with diameter for a given pitch). Again, 'universal' inserts will have relief on the tip - which should cope with most helix angles (though, perhaps, not left hand threads).

Hope this all makes sense!

Simon

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Re: Threading inserts?

Thanks Simon. What I am trying to work out is if full form threading inserts are specific to material diameter as they are to pitch? For instance if I was threading some 12mm bar at 1.5mm pitch, would I be able to use the same insert that I would use to thread 40mm bar at the same pitch?

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Re: Threading inserts?

As I said, it's a question of helix angle and the clearance on your tool.
If you look at your 12mm diameter thread, you'll see that the threads aren't at 90 to the length of the screw - this is the helix angle - this angle is greater for small diameters and less for large diameters for the same pitch. If you look at the threading tool (it's most noticeable on multiple tooth cutters such as thread chasers) the profile isn't 'vertical' - it's at an angle that corresponds to the helix angle, so that the sides of the tool  don't rub against the sides of the thread. In addition, the tool should also have a little side clearance - so that the 'triangular' form of the cutter tapers slightly from the upper surface to the lower.
What this all means is that there is no simple 'yes/no' answer to your question. You should be able to use a cutter on a range of diameters (a cutter designed for 12 mm should work on 10 - 15) - but there will be a limit. Quite where that limit is will vary according to the individual manufacturer (just how much clearance he's built into the tool).
Hope this isn't too confusing!

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Re: Threading inserts?

I understand the geometry of a 12mm thread will be different to that of a 40mm. I am trying to find any sort of information which gives details of full form inserts recommended for different diameters and pitches. Got engineers "Black Book" now and it has all the info I would ever need about tungsten inserts and a lot more besides.  Thanks for all the replies,  just need to sort out reverse on lathe and give it a go!

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Re: Threading inserts?

A thread of a certain pitch is the same form for any diameter. You can use the same insert regardless. Different pitches obviously require different inserts and even if you were single-pointing you would need a bigger flat on the nose of the tool for a coarser pitch than a finer one.

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Re: Threading inserts?

As Simon has already pointed out the helix angle of the thread and the flank clearance of the insert are the key factors. The same inserts are capable of cutting virtually any reasonable diameter but..........

My external threading toolholder comes with a shim which lives between the insert and the toolholder.  This sets the angle of the insert. My toolholder came with just one shim but shims with different angles are available for some toolholders.

See pages 26 and 27 of http://www.sandvik.coromant.com/sitecol … 20-031.pdf

Hope this helps.

Phil.

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Re: Threading inserts?

"Have an Imperial thread dial on my machine, but not much help for metric threads.................."

The thread dial is designed for the lead-screw, and it is the latter that matters here.

You can cut metric threads on an Imperial lathe if you have a 127T change-wheel in the set - that is the standard way; and full change-wheel sets include one. You will need to concentrate on using the same dial mark for each pass, but that's not a great problem.

Over short distances a 63T or 42T wheel could be used instead, with appropriate re-calculations, but they will introduce a cumulative pitch-error so you'd need to determine the thread-length limit they will give without reducing the flanks; then use partial-depth screw-cutting with finishing by die or chaser.

By experimenting with a spread-sheet, I found it theoretically possible to calculate the change-wheels for such a technique with a set having 5-factor wheels only.

I had to use a somewhat similar approach when I made a special-purpose adaptor with a 19mm dia male thread of metric but non-standard pitch... on an EW 2.5" lathe with 8TPI leadscrew and change-wheels stepping in fives only - but of course did not have a die to finish the profile fully..

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Re: Threading inserts?

Thanks Nigel. I tried threading using the correct change wheels, and marking the chuck, lead screw and saddle positions, so the half nut could be disengaged, and the marks used to return to the same position.

This did work in a fashion but as it seems there is a hunting tooth, ended up having to turn the chuck a good deal of times to return to the index marks.

I cant see how a threading dial on an imperial lead screw, using metric change wheels would be able to indicate the correct position, but if it can I would be very interested?

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Re: Threading inserts?

Hi Again My Bantam manual says leave lead screw engaged for metric threads, Colchester did make an all metric machine but usually its a 127 transposing gear.  From a previous comment Bantam has a D1-3 fitting for the chuck quite safe running in reverse, but Chris doesn't have reversing motor hence needing to disengage and mark engagment point. Do you have a manual?
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Re: Threading inserts?

Hopefully motor will be reversing soon, but looks far far easier if its possible to disengage lead screw. If the marking method had worked it would have been less hassle than reverse, but it doesnt so reverse it is!

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Re: Threading inserts?

Normally, you can disengage the leadscrew when cutting imperial threads on an imperial lathe or metric threads on a metric lathe - but not a 'mix'.
Theoretically, you can disengage when cutting metric threads on an imperial lathe - but you have to wait a VERY long time to re-engage, and you'd need some way of counting rotations. For instance, cutting a 1mm pitch - the shortest distance where an exact number of eighth of an inch (assuming you have an 8tpi leadscrew) equals an exact number of millimetres is 5" (127mm). So, you can disengage and wait for 40 revolutions of the leadscrew and engage again (5" = 40 * 1/8). Alternatively, you could stop the lathe with the leadscrew engaged, then disengage the screw, move the saddle exactly 5", re-engage, then start the motor again. For other pitches, you need to find another distance where there is an exact number of 1/8" and whatever the pitch may be.

Usually it's simpler to reverse the motor!

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Re: Threading inserts?

Chris,
With reference to cutting imp and metric on the same machine. It all depends on what lathe you are using. Mine, a Chester Crusader can cut both, but depending what pitch is required for metric, I have to change the gear on the bottom of the leadscrew dial to give me the correct drop in points. unfortunately, because mine is a metric machine, imp threads have to be cut by reversing the motor. In fact, I find that reversing the motor is a much easier method, so use it for both types, disregarding the drop in dial.

Now your other question about inserts.

If you go for full form inserts it can get very expensive as they are precision ground for a certain thread type, and can only be used for that individual thread.

I use the general purpose inserts (pointed tips and valleys), in two sizes for small or large threads, and two angles, both 55 and 60.
All you need then is either internal and external, so for a full set you require 8 different tip types and four types of holders. You can double that up if you want to cut either from left to right or right to left, but I am quite happy using a standard tip for both if you take things steady.

I got all mine from China, and with ten of each tip and the four holders, it cost less than £150

John

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Re: Threading inserts?

Thanks for replies.  I have been told by all my engineering contacts to use full form inserts, but very likely not a great idea for a beginner so have got some Chinese partial form which were nice and cheap!

I am now very confused as to whether half nut can be disengaged or not, especially after watching this video by someone who seems like a very experienced professional engineer:  https://www.youtube.com/watch?v=HXt4TWa382Q

There is a link to text write up with some very useful info on threading in the comments section below the vid, and the writer does appear to have a good understanding of what he is doing?

If this method works seems it might make things a good deal easier for some jobs and I would be very interested to hear what anyone else thinks about the video?

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Re: Threading inserts?

Chris,
That video is very old hat now, and if you have reverse on your machine there is a much easier way to do both imperial and metric, left and right hand threads without ever winding back the tool off the job and never have to disengage the half nuts.

You could easily make a swing up holder that I pioneered a few years ago.

This video is very slow at showing what is happening, and the threads could be cut a lot faster. But at least you can see the tool in action.

Swing up toolholder

John

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Re: Threading inserts?

Hi I have looked at both videos the first does make stopping at a shoulder easier compared to swing up holder but in both cases you are winding the tool back with lead screw and it is that what takes the time. But Chris still has no reverse so it's going to be difficult.
I have tried stopping my Bantam before tool hits a shoulder and it's not easy so I'd go for the disengage the lead screw method.
Ashley

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Re: Threading inserts?

Looking at the video, this is, basically, keeping the leadscrew engaged - the 'disengage' is only momentary and the 're-engage' on the reverse puts the leadscrew back exactly where it was. The important thing to note here is that the leadscrew must be 're-engaged' on reverse in EXACTLY the same place (if you're using 'no 1' on the threading dial, for instance, it's not good enough to re-engage on any 'no 1' - you have to re-engage at the same 'no 1' - if you keep the motor running for three rotations of the dial after the 'dis-engage' and before the stop, then you must run the motor in reverse for three rotations of the dial). It's a good idea, though!

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Re: Threading inserts?

Ashley,
Chris was on about using that method shown in the first vid, but can't because he has no reverse. So really, it is no use pursuing that method at all, including mine.

All I would suggest to him to do is fit a VFD, rig up a bicycle disc brake (mine uses an old fashioned brake shoe) and cut off micro switch for the motor, then you can do VERY fast threading indeed.

John

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