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Square thread cutting

Square thread cutting

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George Jervis29/03/2020 18:04:43
76 forum posts
42 photos

Hi everyone

I'm looking for a little guidance I need to cut an internal square thread to make a back new back plate to hold a small 3 inch chuck, my question is can a internal grooving tool be used to cut a internal Square thread and/or which tool type would be recommended

Many thanks


I've put a picture of thread in my gallery

David George 129/03/2020 18:38:00
1226 forum posts
415 photos

Hi George a few questions. What material is the backplate made of, have you done any internal screw cutting before and what lathe are you using to cut the thread on. As the thread is up to a shoulder I would personally cut it in reverse so the tool is traveling away from the shoulder. That assumes that it is a standard pitch thread that you can disengage the leadscrew to start and stop cutting. Here is a tool that I used to cut a nut from for a crossslide leadscrew.


The cutting edge is the same width size as the finnished thread with clearance on both sides. It is made from a piece of tool steel hand ground on a standard grinder.


Johnboy2529/03/2020 19:11:37
259 forum posts
3 photos

Hi George.

I'm not trained machist but a self taught experienced amature so I'm prepared to take a hit on this one.

With square or acme threads I would normally roughout with a 60° internal boring bar then once this is cut to close depth, pick up the thread with the square ground boring bar and finish it off to depth. It may take a bit of practice and /or fiddling about to get the thread starting in the same position as the roughing cut.

I like cutting internal threads with the headstock running in the reverse direction with the tool upside down on the backside of the job as you can see what you're doing more easily - but this isn't always possible if you have a threaded chuck as the chuck could unscrew itself.

If there's another way perhaps someone can correct this.


HOWARDT29/03/2020 19:30:07
557 forum posts
15 photos

You don’t need to create a tool with the full width of the thread form. Easier to create a narrower tool and shift it sideways to get full form and a good fit. Obviously of more use on larger thread forms.

George Jervis29/03/2020 19:30:10
76 forum posts
42 photos

Hi David G

The thread is a right hand 1.75" by 6 TPI, my lathe is a Raglan little john mk 2 (5" with a qcgb . I have cut external threads many times but never really attempted internal threads, my Lathe can be reversed with switch but the spindle nose is threaded so it might unscrew? . I was just wondering if grooving tools could be used or if there was something better? I have a P horn internal grooving tool but haven't had much success with it as the shank has been ground a away and it vibrates so I don't really use it. threading with it is impossible


Pete Rimmer29/03/2020 19:44:50
716 forum posts
49 photos

Hi George,

I was going to ask if it was a Little John. I have made a backplate for one of those and the dimensions looked familiar.

I don't recall it being particularly difficult to turn that thread. I would grind the end of a broken end mill to a square shape just deep enough to cut the thread depth and cross-drill a chunky piece of bar as a holder. Secure the bit with a grub screw and make sure it's turned slightly to match the helix angle. It should go fairly easily. Grind your tool a few thou wide rather than the exact width then you won't have any clearance problems. Turn the register exactly to the shoulder diameter first then thread it until the tool puts a shallow groove into the register so you have good clearance. A slightly loose fit on the thread is fine. Be sure to face the end on the same setup so the register, the threads and the seating face are all true to each other.

Good luck!


David George 129/03/2020 21:25:53
1226 forum posts
415 photos

Hi George

Although people say that a chuck may unscrew, at the speeds that you will be cutting at there is no chance that it will unscrew. you should practice on a spare piece of material. possibly a bit of ally, just to get the feel of engaging the feed in sequence to the thread in reverse that can be done on the outside before trying another internal trial piece before going on to a real article. As there is quite a lot of clearance I would make a toolholder by cross drilling a piece of silver steel, which is more stable than mild steel and with a grub screw from the end fit a piece of HS tool steel which you can grind to the thread width. grind a flat on the HS tool to prevent it turning in use. the same tool can be used to cut the relief at the start of the thread. My M type lathe has a screwed on chuck and it has never unscrewed even when cutting a LH thread.



Look at the first and third from the left for type of boring bar holder. 



Edited By David George 1 on 29/03/2020 21:30:13

Simon Collier29/03/2020 21:28:23
352 forum posts
56 photos

I suggest cross drilling a length of suitable steel bar, say 3 mm, and tap for a M3 grub screw to hold your tool bit, ground from the shank of an M3 drill. Grind to 80 thou wide plus suitable clearance. The bit can be rotated slightly to suit the helix angle. Take light cuts in what is hopefully cast iron. The thicker the bar you cross drill, the stiffer, obviously.

A grooving tool might not have the necessary clearance on the leading edge to cut fast threads. Presumably they are designed for grooves, not threading.

Johnboy2529/03/2020 22:18:10
259 forum posts
3 photos

David George - I would respectfully disagree with a threaded chuck revolving at slower speeds not becoming loose. I seen it happen, fortunately no injury was inflicted and I was a fair distance away from it! The reason it became loose was that there was a lot of chatter with the cutting tool which as we all experienced can happen when you you least expect it. This is my observation and I heard other people say similar.
Regards John

larry phelan 130/03/2020 09:32:12
719 forum posts
14 photos

The Craftsman lathe has a screw on chuck fitted with two simple clamps and never comes loose.

Pete Rimmer30/03/2020 14:18:22
716 forum posts
49 photos

George, I cleaned up your photo a bit. Not the best result but it's a bit easier if you ever want to print it out.

It would be nice to get a pic of the whole page if you could. If you took the pic with your smartphone there's a great app called CaMScanner that will automatically clean up photos to make them much more readable.


George Jervis30/03/2020 14:31:28
76 forum posts
42 photos

Hi Peter

Thank you for the app info I've installed it now and put the rest of the page in my album

Many thanks


Pete Rimmer30/03/2020 15:29:57
716 forum posts
49 photos

Nice one George, that has come out much better. I don;t know how they do it but they do it well!

Now, if you get really bored in your isolation and fancy doing the whole manual, send it to me and I'll make a very nice PDF for you.


old mart30/03/2020 17:57:37
1771 forum posts
138 photos

If you lock the spindle and tighten the chuck a little more firmly than usual, then threading away from the headstock in reverse is not dangerous. Small cuts will be made and you have time to think what you are doing, rather than being in a panic. I actually have a locking device which fits most of the chucks on the museum's Smart & Brown model A, but producing the 1 1/2 X 8 thread for the Atlas chuck backplate used an unmodified chuck. I still ran in reverse, and with a nice tight chuck and care with the cutting forces, all was well.

As for the grooving tool being usable, I would say yes, better if it was narrower than the thread. Tighten up the compound gibs and take up the backlash towards the head. Zero the scale and then you can move the compound to make up the width difference. If you like, you can alternate the cuts as long as you back off enough to be able to remove any backlash each time. The grooving tool must clear the helix angle of the thread.

Edited By old mart on 30/03/2020 18:05:42

Edited By old mart on 30/03/2020 18:07:39

Russ Bulley31/03/2020 19:38:18
12 forum posts
2 photos

This is a very useful text on square threads


not done it yet31/03/2020 19:59:04
4648 forum posts
16 photos

The main thing missed here is that of the thread “ being merely”. Too much credence given to a perfectly fitting thread. It is not absolutely necessary. The register dimensions are the overwhelmingly important dimensions which must be closely adhered to.

For a lathe with reversing on the feed direction and choice of cutting at the front or rear (upside-down cutter), the problem of a screwed chuck is simply non-existent.

old mart31/03/2020 20:35:46
1771 forum posts
138 photos

Since Raglan seem to be uniquely unfortunate in specifying a square thread for your lathe spindle which has no inherent self centring qualities, you need to actually measure the registers and compare the results to the drawings. The rear one is 1.750" on the spindle and the front one is 1.562". Any reduction due to wear will have to be compensated for when the backplate is made.

CHARLES lipscombe31/03/2020 23:09:59
118 forum posts
8 photos

Larry Phelan: Could you please supply details (sketch?) of the craftsman lathes clamps for preventing the chuck from unscrewing in reverse?

I have this problem on my Taiwanese lathe maybe because it sees and has seen a lot of use and I frequently change between 3 and 4-jaw chucks. The 3-jaw is due for replacement when "we get out the other side" of corona virus but even so I would like to have a positive lock.


Nigel Graham 201/04/2020 00:23:47
644 forum posts
12 photos

Looking at this thread as I may need cut a square threaded nut and screw for a particular project, I note the advice on the tool's side clearance.

Would it be feasible to use a semi-circular section tool, ground rather like a D-bit, with squared-off edge a few thou deeper than the diameter, and an axial rake? Such a cutter could also work for trepanning or face-grooving (e.g. for an O-ring seal), because the side-clearance on a cylinder is innate. I ask because I don't recall ever seeing this suggested anywhere.

Hopper01/04/2020 00:34:21
4530 forum posts
94 photos

Its a good idea and worth a try. Only problem might be a bit of weakness on the sides of the tool where the semi-circular shape does not support the edge as well as a conventionally ground square tool. But most of the cutting is on the front edge that would be well supported in the conventional manner so well worth trying out.

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