Lathe screwcutting

[PlasmaParticipant @plasma]

Hi all.

I'm using a Boxford X10 lathe with screw cutting gear box. It's got very little wear and tear. It as a thread cutting indicator to show when to start the cut, is the position of the saddle relative to the workpiece critical, I.e. should I be using a carriage stop? Leaving half nuts engaged shouldn't be necessary if I am accurate with the indicator.

I'm going to revisit tool grinding as suggested just to nail that but down.

The main reason for inverted/reverse running is I want to turn threads to an undercut and am wary of not stopping in time.

Setting the compound slide at an angle seems to be part of the problem, I'm not sure it's necessary but the books say don't and you tube says not.

Speed wise I am trying 65 rpm as the slowest on my gearbox but higher speeds have yielded mixed results with some good passes and some bad.

Lastly one you tuber cut all his threads on a Myford by hand cranking it and leaving the half nuts engaged, but that seems odd considering screwcutting should be done under power.

I'll keep on trying and report back.

Best regards Mick

[John RuddParticipant @johnrudd16576]

Errmmm, whether the lathe spindle is driven by motor or hand crank…..isnt that still under power?

If you are threading to an undercut and worried about not stopping in time when cutting in the conventional manner ( tool in the direction toward chuck) then why not manually crank it? Unless of course its a big diameter thread you are cutting….?

My understanding of setting the compound is so that the tool is cutting on one edge with clearance on the back….?

The thread indicator guides the user when to engage the half nuts when cutting a thread, if the nuts remain engaged all the time then that is irrelevant….

My advice fwiw, get a copy of Sparey's book on the lathe…..

Edited By John Rudd on 07/03/2019 08:35:02

[Martin ConnellyParticipant @martinconnelly55370]

You have said you are set up for a metric thread and are using a Boxford lathe which I think has an inch based leadscrew. This is the situation where the threading dial does not work and the best advice to follow is to leave the half nuts engaged to avoid multiple start threads.

Martin C

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Plasma on 07/03/2019 08:19:09:

… by hand cranking it and leaving the half nuts engaged, but that seems odd considering screwcutting should be done under power.

.

Not sure that I follow your logic, Mick

If you are concerned about 'not stopping in time' ; hand cranking seems a very sensible approach.

MichaelG.

.

My posting was delayed by a telephone call … so John beat me to it.

Edited By Michael Gilligan on 07/03/2019 08:38:40

[Martin ConnellyParticipant @martinconnelly55370]

I would suggest that if using carbide tipped tooling hand cranking may be problematic as a slight reversal of the spindle may break the tip off the carbide. Using hss this would not be a problem, we reverse hss dies to break chips when hand threading, we also do this using hand cranking at slow speeds. It is basically the same process that occurs when screw cutting in the lathe.

Martin C

[PlasmaParticipant @plasma]

The Boxford is a modern version with a 6mm pitch leadscrew.

The tool I tried using is an omega products SER 1616 H16 from a reputable Sheffield tool dealer so should be good quality?

I've checked all the gear settings and engagement chart and I'm cutting the right pitch.

I guess its possibly infeed etc. I tried flood coolant and Dormer supercut fluid.

I'll read sparey again and get my head round it all.

[Paul KempParticipant @paulkemp46892]

I think if you reverse the spindle either under power or by hand cranking without retracting the tool you will have bigger problems than just breaking the tip! Any plain engine lathe will have backlash in the half nuts even if it's new. There is absolutely no need to reverse to break the chip when screw cutting. I am surprised any commercial shop would hand crank when screw cutting, time is money and the cost of threads would be astronomical.

Paul.

[HopperParticipant @hopper]

It could be your screwed-on chuck coming loose in reverse rotation under heavy screwcutting load.

Or it could be any number of other things.

A pic or two of your set up and the results would be most helpful.

It sounds like you need to practice the basics of screwcutting before attempting this particular job that requires reverse rotation to get close to a shoulder or whatever it is. Try taking a piece of mild steel of known quality. Put a tailstock centre in it and turn the OD nice and smooth. Then practice cutting a thread of your desired pitch on it, finishing a safe inch or so from the chuck so you don't have to panic about crashing. That way you can work out the tool and technique that works for you and your machine. Then do it again and again. Once you have had some practice, then go back to the job in hand, using a tool and technique that have been proved to work..

[SillyOldDufferModerator @sillyoldduffer]

Unlikely causes of multi-start threads:

• Cutter quality
• Coolant and Lubrication
• Infeed
• Faulty lathe (unless it's damaged)

Likely causes:

• Operator Error, specially misreading the manual and making incorrect assumptions (don't ask how I know…)
• Incorrect ratio between leadscrew and spindle. This could be caused by:
  • Change wheels incorrectly set to satisfy the Norton Gearbox. (Depends on the lathe)
  • Wrong gear fitted to Dial Indicator or trusting a Dial Indicator that doesn't support the particular pitch being cut.

Have you tried cutting the thread with the nuts permanently engaged, ie winding the slide in and out with the leadscrew, and ignoring the dial-indicator entirely? The purpose of the dial-indicator is to save time and chaos ensues if you misunderstand it. Keeping the nuts engaged is slower but it eliminates this source of error.

The only reason for using the motor to cut threads is to save time. Turning by hand is better when fine control is needed, and manual turning makes it much easier to see set-up errors before any damage is done.

In cases of unexpected results it pays to go back to basics. Once the cause is understood, then you can go for speed.

Dave

[PlasmaParticipant @plasma]

Wise words hopper, I will take things slowly rather than my usual on the job learning as it were. Better to learn on spare material than the actual job.

It has a D 13 mount for the chuck or I would never consider any reverse spindle operations.

I'll take some snaps of what I'm trying to do.

Best regards Mick

[HopperParticipant @hopper]

Posted by SillyOldDuffer on 07/03/2019 10:24:25:

The only reason for using the motor to cut threads is to save time. Turning by hand is better when fine control is needed, and manual turning makes it much easier to see set-up errors before any damage is done.

Not necessarily. If cutting speed is too low, it can cause tearing of the steel instead of cutting. Plasma's 65rpm is ideal for all but the very smallest and shortest of threads like tiny BA studs etc where you might resort to hand cranking on a lathe with no clutch/brake unit for a "dead stop". Just keep one hand on the cross slide handle and the other on the stop switch. Practice makes perfect.

Edited By Hopper on 07/03/2019 10:39:31

Edited By Hopper on 07/03/2019 10:40:20

[JonParticipant @jon]

Posted by Plasma on 07/03/2019 08:19:09: Leaving half nuts engaged shouldn't be necessary if I am accurate with the indicator.

1 The main reason for inverted/reverse running is I want to turn threads to an undercut and am wary of not stopping in time.

2 Setting the compound slide at an angle seems to be part of the problem, I'm not sure it's necessary but the books say don't and you tube says not.

3 Lastly one you tuber cut all his threads on a Myford by hand cranking it and leaving the half nuts engaged, but that seems odd considering screwcutting should be done under power.

1&3 If its external thread its very easy, i can usually stop within 0.3mm then revolve chuck by hand or with a spanner around a jaw so tip is up against a face or blind hole for internal threads with no relief for strength. You physically cannot do this inverting the tool or using opposite hand tool and firing away from spindle which in its self gives problems lining up without chipping the tips.

2 As before keep it simple, certainly for now as it irons out any potential problems caused by doing that.

3 Last place of work they had a cheap asian geared head almost impossible to select any other spindle speed. Right pain revolving chuck for 1/2" UNF X 14mm long then back out every time.

Realistically you should be able to leave a superb finish from 40RPM upwards with full profile tips. Been there done it for the scary threads up to shoulders with or without relief.

Quite right Paul next lathe would have the Ainjest or similar but grieves me at the 2nd hand price they command.
Did often use an Elliott auto threading machine forget the name its been 22 years. Took longer to load the hydraulic cylinders in 10 seconds, than thread M28.6 X 1.5mm with 7 passes. Every one a mirror finish.

[Roger Williams 2Participant @rogerwilliams2]

Buy a Hardinge HLVH with the "outside" Myford changewheel set up for metric pitches if necessary, makes screwcutting an absolute joy.

Also, if you can reverse the spindle with a forward reverse clutch setup you can disingage the half nuts when cutting metric with an imperial leadscrew. Theres a good article somewhere by a chap whos name , I think, is Conrad Hoffman on the subject.

[JasonBModerator @jasonb]

Are you syre its 6mm pitch as they also did 4tpi with dual metric/imperial dials. Should have the lead screw marked on the front just under the "F&G" lever

Have you checked the chart down the side of the thread indicator to make sure you have the right drop in point for your thread

Edited By JasonB on 07/03/2019 16:51:55

[JasonBModerator @jasonb]

I'd also like to see how you can get anywhere near a shoulder with that insert holder when mounted upside down and running in reverse as it would put the cutting tip about 20mm away from the shoulder?

Unless you have it at the back of the work in which case the lathe needs to run the right way if the tool is upside down

[peak4Participant @peak4]

Plasma, a photo of your setup might help some of the contributors on this posting.

Also have a look at this video , which whilst it may not be appropriate if you are cutting a metric thread via a metric leadscrew, makes an interesting 1/4 hour for a teabreak.

It shows how to disengage the halfnuts on a metric/imperial combination, and how to re-engage without losing your indexing.

Bill

[PlasmaParticipant @plasma]

The weekend has been a thread free zone as I'm building a wind vane for the front of the house and both lathes have been turned over that job for a few days

I will set up for conventional thread cutting and send in some images.

I'm looking at using hss tooling rather than insert but will try both the see where I get. Is there a dedicated tool bit holder for threading tools in square section hss?

[John RuddParticipant @johnrudd16576]

Posted by Plasma on 10/03/2019 17:11:51:

I'm looking at using hss tooling rather than insert but will try both the see where I get. Is there a dedicated tool bit holder for threading tools in square section hss?

HSS thread cutting tools are generally ground from a blank….Use your fishtail to grind the correct profile for 55-60 degrees

Edited By John Rudd on 10/03/2019 18:51:17

[HopperParticipant @hopper]

And no dedicated holder, just held in the conventional manner like a turning tool.

[PlasmaParticipant @plasma]

Thanks Hopper, I use a multifix tool post and its tricky to put small tool bits in. Sparey shows a home made sprung threading tool bit holder which is supposed to do a sterling job due to the flex in the neck allowing for fine cuts by repeating passes with no infeed. I just wondered if anyone had seen a commercial equivalent.

[JasonBModerator @jasonb]

The sprung cutters were available commercially but don't know of anyone selling them now.

If you find it hard holding small bits then just mill a channel in some 16mm square stock and add some tapped holes for grub screws. You can then hold small square HSS blanks in that and put the 16mm block in your larger tool holder. One with a round hole will also be useful for tools ground from round material like old cutter shanks.

[HopperParticipant @hopper]

A Mulitfix should hold a length of 1/4" HSS tool bit ok. You need to put some packing pieces underneath it of course. It's best if the bit is long enough to have two clamping screws bearing on it.

[Niels AbildgaardParticipant @nielsabildgaard33719]

Posted by JasonB on 07/03/2019 16:17:13:

Edited By JasonB on 07/03/2019 16:51:55

Thats an awfull nice picture of a very well engineered slide system.

Been there myself

 

Found this from Boxford

 

From Boxford Spares

 

It does not show the   Boxford locking screw/cone very well.Does someone have a picture of underside of compound slide base with integral cone please?

Edited By Niels Abildgaard on 11/03/2019 08:20:32

Edited By Niels Abildgaard on 11/03/2019 08:21:14

Edited By Niels Abildgaard on 11/03/2019 08:22:37

[Rik ShawParticipant @rikshaw]

Plasma – I have a lot of small section HSS and made this holder for it. It gets used a lot primarily because its less time consuming to form a tool in small stuff. To give an idea of size the HSS bit in place is 3/16" square

Rik

[SillyOldDufferModerator @sillyoldduffer]

Posted by Plasma on 11/03/2019 06:46:48:

Sparey shows a home made sprung threading tool bit holder which is supposed to do a sterling job due to the flex in the neck allowing for fine cuts by repeating passes with no infeed. I just wondered if anyone had seen a commercial equivalent.

Sparey has one fault – he wrote 70 years ago! Although most of his advice remains fresh, his books have a few obsolete ideas, and – of course – he doesn't cover technology that became available after his time, notably carbide inserts.

Hand grinding a 60° or 55° pointy threading tool in HSS requires practice and not everyone is good at grinding. Give it a try, you can get started making external threads with quite a poor tool shape. The result may not be beautiful, or strong, or a tight fit, but they do work. Once you've made a few threads you can concentrate on improving them, most likely with better grinding and lathe technique. I don't think many people bother with spring holders, but they do pop up second-hand or you could make one.

Personally I find it easier to use indexed thread inserts, these cut properly shaped thread profiles, and can be changed quickly without interrupting my workflow if the point gets damaged.

Deciding between HSS and Carbide depends on your interests, goals and equipment. Some love HSS, others swear by inserts. As is the way with hobbies, I've maximised the cost by insisting on both…

Dave

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