Threading on a Lathe

[Darren]

Some lathes like Monarch and Hardinge put the lever for the feed reverse on the apron and have stops that can be set to kick it into neutral at the end of a thread.

My SM has the lever on the apron as well.

 

[Susquatch]

In the hendey there’s an intermediate gearbox of sorts but the principle is the same.

The intermediate gearbox allows for isolating the spindle from any gears for best surface finish, driving the QCGB with a belt. The single tooth dog clutch is between the intermediate gearbox and the QCGB.

Some lathes like Monarch and Hardinge put the lever for the feed reverse on the apron and have stops that can be set to kick it into neutral at the end of a thread.

Good stuff. I think the key to everything is the single tooth dog clutch at the spindle. This guarantees that the spindle is always indexed at just one place in its rotation no matter what everything else does. In a way, that is far superior to a thread dial.

 

[Susquatch]

Over the years I've been fortunate to collect thread mics up to 4", all like new mitutoyos....fancy stuff and they are handy.

But I wouldn't be without thread wires. 1) they're only a few dollars until you luck out and trip over a pile of Mit thread mics, but 2), and most importantly you can use them to measure any thread. I've used them for acme, buttress, BA and Whitworth - they save the day over and over. Armed with the formulas below you can measure about any thread

Handling them can be a pain, but you can use a bit of plastercine, inner tube with a slit or grease. All work....and I just happen to have some pics!

Wow. Thanks @Mcgyver !

The putty is brilliant. But the inner tube would work for me too.

Yes, I was aware of the limitations of a threading mic for other thread types. Thank God I have not had to do any.

Maybe I could make new tips for my mic to do that........ Or just buy some putty!

Those tips are worth their weight in gold! Thank you!

 

[Susquatch]

It feeds straight in, like the Europeans do threading all the time on manual lathes as do CNC lathes. The depth varies depending on the pass, essentially constant chip load. Tool cuts on both sides, esp. suited to inserts. For the thread pitches hobby users are doing the 29dg isn't necessary. This table is from the excellent Sandvik Metal Cutting Training Manual.

View attachment 24376

I missed this post Gerrit.

The chart you provided is for that horrid mestic stuff, but I get the drift. I'll have to see if they have something in imperial for old-timers like me. Not that I never cut metric just it isn't my norm. If they don't have one, I can prolly make one.

Again something new for me. Varying the depth of cut as the cut gets wider is a good concept that makes sense. It will take some retraining for me though. I'm so used to taking off a given amount each pass for 29.5 degree threading. It will be tempting to use that instead of measuring threads...... NOT!

Keeping a chart handy and remembering what I did last will be a challenge though. Another vote for CNC I guess.

 

[Susquatch]

@gerritv - I've been thinking about this 29.5 VS plunge threading all through supper.

I remember thinking many years ago when I first tackled threading, why the 30 degrees instead of a plunge cut with the thread profile, and why the complicated 29.5 degree instead of 30?

Over the years I've gotten pretty comfy about the reasons and the method. Now, you have me upside down realizing maybe everyone was wrong and I should have simply followed my instincts way back then.

But it's hard to let go of old habits that are so engrained.

We all know that surface finish is a product of the material, the sfm, the depth of cut, the feed rate, and the tool design.

I can see how the Sandvik chart might be great when using their threading inserts, but is it really a good guide for cutting threads with HSS or a competitive carbide insert?

 

[gerritv]

@gerritv -

Over the years I've gotten pretty comfy about the reasons and the method. Now, you have me upside down realizing maybe everyone was wrong and I should have simply followed my instincts way back then.

But it's hard to let go of old habits that are so engrained.

We all know that surface finish is a product of the material, the sfm, the depth of cut, the feed rate, and the tool design.

I can see how the Sandvik chart might be great when using their threading inserts, but is it really a good guide for cutting threads with HSS or a competitive carbide insert?

Neither method is wrong, just different. For some reason North America is on the 29..5dg wagon, Rest of world is not. Joe P didn't help, I think by the time he made that video he had IMHO become a bit of a d..k and perhaps just wanted to stir the s..t or just be his arrogant self. Personally for anything finer than 18tpi/1.5mm pitch it is a waste of time and much more likely to get you to screw up. Anything coarser and you need to start considering lead angle on the cutting tool as well. The Sandvik manual does a good job on all of this in 25-30 pages.

If your HSS tool has some top rake but is otherwise flat, then it is more or less like the insert profiles, as in it can cut on both sides. Easy to grind.

My experience with the DOC charts is that they provide a constant chip load., not related to a brand of insert/HSS tool. Minimzies chatter, tool breakage, distortion of thing work pieces, etc. E.g. I cut 38.5tpi on 3.6mm brass, 25mm long with no support at 500 rpm. With 3 spring passes to settle things down, all is well under a 5x loupe.

gerrit

 

[gerritv]

I missed this post Gerrit.

The chart you provided is for that horrid mestic stuff, but I get the drift. I'll have to see if they have something in imperial for old-timers like me. Not that I never cut metric just it isn't my norm. If they don't have one, I can prolly make one.

Again something new for me. Varying the depth of cut as the cut gets wider is a good concept that makes sense. It will take some retraining for me though. I'm so used to taking off a given amount each pass for 29.5 degree threading. It will be tempting to use that instead of measuring threads...... NOT!

Keeping a chart handy and remembering what I did last will be a challenge though. Another vote for CNC I guess.

Here is the inferior inferial one

 

[gerritv]

More tables for the myriad of thread types, internal/external (imp and metric) are at page 96 of this Threading Application Guide. I have a binder where I keep charts such as this, in plastic sleeves.

Gerrit

 

[Susquatch]

If your HSS tool has some top rake but is otherwise flat, then it is more or less like the insert profiles, as in it can cut on both sides. Easy to grind.

Sounds good Gerrit. I'll look up and print that stuff for my own reference books too.

RE the above quote. Since I cut at 29.5 and not 30, I need both hss edges to be sharp. I have no worries about plunge cutting from that respect. Your comments about constant chip load are reassuring. But there is no substitute for doing a test run. Might be a while though. Crashing my taxes now that my seeds are in the ground. Never any end to crap that keeps me away from my shop.......

 

[Susquatch]

More tables for the myriad of thread types, internal/external (imp and metric) are at page 96 of this Threading Application Guide. I have a binder where I keep charts such as this, in plastic sleeves.

Gerrit

Hi Gerrit, I get an access error clicking on that link.

Edit - same problem with the Sandvik document you posted earlier

 

[SomeGuy]

I took my first shot at threading...M20x2.5. Definitely not the cleanest threads but they did actually fit. I'm going to make some shorty speaker poles (I have some PA equipment), so wanted to give it a try before I make the actual things.

 

[Susquatch]

I took my first shot at threading...M20x2.5. Definitely not the cleanest threads but they did actually fit. I'm going to make some shorty speaker poles (I have some PA equipment), so wanted to give it a try before I make the actual things.

How does that saying go.... She ain't pretty but she is mine!

I think that is a fantastic first threading go!

Especially since you didn't make it easy on yourself..... Next time, cut a thread relief up at the top where the shaft meets the thread. It will make threading easier and will not affect the strength of the threaded section or the quality of the joint.

Here is what that should look like:



It's just an image I downloaded so that is much wider than needed. But it should give you the idea. Just cut the relief the same depth as the thread root.

 

[SomeGuy]

How does that saying go.... She ain't pretty but she is mine!

I think that is a fantastic first threading go!

Especially since you didn't make it easy on yourself..... Next time, cut a thread relief up at the top where the shaft meets the thread. It will make threading easier and will not affect the strength of the threaded section or the quality of the joint.

Here is what that should look like:

View attachment 24393

It's just an image I downloaded so that is much wider than needed. But it should give you the idea. Just cut the relief the same depth as the thread root.

For sure! I wanted to see how close I could get to the shoulder, since the pole needs to thread and then the weight sits on that shoulder, so the closer I can get, the more thread engagement I'll have.

I also didn't do the 29.5 degree thing and the HSS was probably a bit dull, I've ordered new insert tooling for threading.

Pic for reference, want to make 6-12 inch versions of these poles (these are 36") so I could use the speakers on subs on a stage or while seated. For the audio nerds, that's a QSC K8.2 over top of a QSC KS112 subwoofer.

 

[Susquatch]

For sure! I wanted to see how close I could get to the shoulder, since the pole needs to thread and then the weight sits on that shoulder, so the closer I can get, the more thread engagement I'll have.

I also didn't do the 29.5 degree thing and the HSS was probably a bit dull, I've ordered new insert tooling for threading.

Cutting the relief will, actually help with that too. It allows you to cut a nice flat surface that both parts can engage on with no gaps and a clean fit. If you make the gap about 1 thread wide, it won't affect how strong your engaged threads are. I personally like about two threads wide.

The 29.5 is really about cleaning up the back side of the thread cut.

Did you do 30 or did you do a plunge cut at 90?

 

[SomeGuy]

Cutting the relief will, actually help with that too. It allows you to cut a nice flat surface that both parts can engage on with no gaps and a clean fit. If you make the gap about 1 thread wide, it won't affect how strong your engaged threads are. I personally like about two threads wide.

The 29.5 is really about cleaning up the back side of the thread cut.

Did you do 30 or did you do a plunge cut at 90?

Good info, that's the plan

Plunge at 90, just dialing in the cross slide 0.2mm or so at a time at the beginning, backing off to 0.05mm as depth of cut got greater until I got to final dimension.

 

[Susquatch]

Plunge at 90, just dialing in the cross slide 0.2mm or so at a time at the beginning, backing off to 0.05mm as depth of cut got greater until I got to final dimension.

Thanks to @gerritv , I have just recently learned that half the world does the 90 plunge thing. So it must be ok. It just isn't the way I've done it all my life.

But never let it be said that I am not open to new ways of doing things. Time will tell.

 

[SomeGuy]

Thanks to @gerritv , I have just recently learned that half the world does the 90 plunge thing. So it must be ok. It just isn't the way I've done it all my life.

But never let it be said that I am not open to new ways of doing things. Time will tell.

From what I've read, if the metal is soft and your setup is rigid and you aren't getting chatter, then 90 plunge is perfectly fine. The 29.5 is so you're cutting on just one face of the thread form. At least as far as I understand it given I've only cut literally one thread on a lathe, I only have theory, not practical experience.

 

[Susquatch]

From what I've read, if the metal is soft and your setup is rigid and you aren't getting chatter, then 90 plunge is perfectly fine. The 29.5 is so you're cutting on just one face of the thread form. At least as far as I understand it given I've only cut literally one thread on a lathe, I only have theory, not practical experience.

Just for clarification.

The idea behind the 30 degree cut is to cut on just one face of the 60 degree tool and slide along the other. Cutting on only one face reduces the chip load on the tool.

Sometimes this leaves a rough chattered surface on the sliding edge. Rarely, it can even leave a bit of a staircase effect.

The idea behind the 29.5 degree cut is to cut the normal 30 degree plus a little 1/2 degree shave off of the other surface to clean it up and eliminate the possibility of any staircasing. I have no idea how the 1/2 degree was decided. I do know that it is not critical.

Plunge cutting at 90 degrees is new to me.

 

[gerritv]

Hi Gerrit, I get an access error clicking on that link.

Edit - same problem with the Sandvik document you posted earlier

Sandvik don't make it easy to link :-( Go to this page, in the box on left select Technical Information and then Apply. Traingin Handbook, Turning Handbook and Threading Application Guide are there.
And I know you didn't want a YT link but this is as clean an example as I can find at the moment: (No compound)

Note the tool shape, no top rake at all, ground from 6 or 8mm round HSS.

 

[Former Member]

Thanks to @gerritv , I have just recently learned that half the world does the 90 plunge thing. So it must be ok. It just isn't the way I've done it all my life.

But never let it be said that I am not open to new ways of doing things. Time will tell.

I never learned the 90 thing either, I think it came into play with the wide spread use of CNC (as in the older books their is little or no mention of it). Once set up at 90 if you have enough power or time for lighter cuts who cares.