Author Topic: Single point threading on the lathe  (Read 45027 times)

Offline Darren

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Re: Single point threading on the lathe
« Reply #25 on: August 21, 2009, 05:54:42 PM »
I was thinking about your calculator trick the other day while out shopping of all things....yes I was indeed waiting for the other half yet again and find this is a time the mind tends to wander....


I was wondering if a calc with some "calcked" number could count the revolutions on an imperial machine to give a metric reading. of course backlash would still be an issue unlike a proper DRO.
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Offline John Hill

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Re: Single point threading on the lathe
« Reply #26 on: August 21, 2009, 08:29:42 PM »
Darren, I do not think backlash would be relevant when trying to find the point at which to re-engage the half nuts, I have a couple of ideas on the subject but the sun is shining today so not 'shop playtime until chores are done!

I will do a short topic in the calculator thingio.
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Offline HENNEGANOL

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Re: Single point threading on the lathe
« Reply #27 on: August 25, 2009, 04:05:48 AM »
When screwcutting in the lathe I was taught to set the cross slide to half the included angle of the thread ie., 30 degrees for metric or unified threads and 22.5 degrees for imperial threads.  The tool is set up using a gauge as described earlier, then the  feed is put on with the cross slide only.  Which results in the left face of the tool doing all the cutting.

When I served my time all our machines were calibrated in imperial units, all the lathes were supplied with a 127 tooth gear for use when cutting metric threads!

In order to cut metric threads on my Myford lathe fitted with a quick change gearbox it was necessary to purchase an additional set of gears and quadrant.  Even then you must not disengage the nut after each cut, the saddle has to be wound back by reversing the motor.

Oops, I should have said set the top slide to half the included angle
« Last Edit: August 28, 2009, 04:17:12 PM by HENNEGANOL »

bogstandard

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Re: Single point threading on the lathe
« Reply #28 on: August 29, 2009, 05:54:02 AM »
When screwcutting in the lathe I was taught to set the cross slide to half the included angle of the thread ie., 30 degrees for metric or unified threads and 22.5 degrees for imperial threads.

A couple of mistakes there HENNEGANOL, but I understand what you mean.
 
Actually, this is a much better method of cutting threads, using the compound (topslide) set to half the included thread angle, and using that for putting the cut on when leaving the leadscrew permanently engaged, and was the way I was always taught how to cut single point threads. Only using the cross slide for the final clean up.

This is due to the fact your cross slide will always be at the same setting when retracting it and putting it back to its former position, as you should do when winding back on the leadscrew, rather than trying to remember the setting you were at on the last cut, otherwise if you don't retract, backlash will kill the threads by taking extra metal off. Just use 0 for your cross slide setting, a rather easy number to remember.

When, eventually, I get my retracting topslide made, everything will be taken care of automatically.


Bogs

Offline arnoldb

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Re: Single point threading on the lathe
« Reply #29 on: August 29, 2009, 10:03:43 AM »
HENNEGANOL, Bogs gave good advice. May I just add that the angle for the topslide for cutting a 55 degree thread using this method is 27.5 degrees and not 22.5.
Not being picky - just to make sure someone does not accidentally fall into a trap and use the wrong setting  :D

Hmm... less computer and off to cut that 12 tpi internal thread - never done that; hope it comes out OK  :scratch:....
 :beer: Arnold

Offline NickG

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Re: Single point threading on the lathe
« Reply #30 on: August 29, 2009, 11:36:19 AM »
I need to read through this thread more carefully!  :bang: :scratch:

I don't understand this not disengaging the half nuts thing? Judging by my questions you can probably tell I haven't cut many threads. I thought if I engaged the half nuts to start cutting on 1 of the numbers on the thread dial indicator, I could withdraw the cross slide at the end of the thread disengage the half nuts and wind the carriage back, put the extra cut on then re-engage the half nuts on the number I started on?

Nick
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Offline Darren

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Re: Single point threading on the lathe
« Reply #31 on: August 29, 2009, 11:43:37 AM »
I don't understand why the need to retract the tool......the old books I have read suggest this is only done to save time when cutting long threads.

It seems to me that the fact that the tool may take a little off when reversing is neither here or there as you're about to take more off with the next cut anyway...

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Offline spuddevans

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Re: Single point threading on the lathe
« Reply #32 on: August 29, 2009, 12:16:39 PM »
I don't understand why the need to retract the tool......the old books I have read suggest this is only done to save time when cutting long threads.

It seems to me that the fact that the tool may take a little off when reversing is neither here or there as you're about to take more off with the next cut anyway...

Maybe in a really good quality lathe ( I dont have one, so I'm just guessing ) there is very little backlash on the leadscrew. But on my little C2 lathe I can (with the motor turned off) engage the half-nuts and then move the carriage about 1mm in either direction. Also when the lathe is running with the halfnuts engaged and I then reverse the motor, the chuck will rotate anything between 1/4 and a whole turn (depending on what change gears are set up) before the carriage will actually move in the reverse direction.

All of that would add up to a ruined thread. It can also shorten the edge-life of your threading tool as it is rubbing the work but in reverse, this is pretty fatal for carbide tools, they really dont like to be run backwards against the work. ( go on, just ask me how I found that out  :bang: )


Tim
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Offline No1_sonuk

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Re: Single point threading on the lathe
« Reply #33 on: August 29, 2009, 12:38:26 PM »
Leaving the half nuts engaged also means you don't have to mess about working out which number on the indicator to use for which thread, or if you can use it at all.
I'm seriously considering taking the thread indicator off my lathe.  I don't use it, and it's getting in the way.

My lathe has electronic speed control.  That means I can run it forward slowly for the threading, then retract and run it in reverse at higher speed to get the carriage back to the start.

I just need practice getting the angled compound thing right.  :scratch:

Offline HENNEGANOL

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Re: Single point threading on the lathe
« Reply #34 on: August 29, 2009, 03:40:59 PM »
My thanks to Arnold and Bogstandard for pointing out my obvious mistakes, at my age I should know better.

Referring to my comments regarding cutting metric threads on my Myford and the requirement to leave the the half nut engaged at the end of the cut and reverse the saddle.  These are the instructions given by Myford when using the metric thread cutting kit on a lathe with an imperial lead screw.

Another tip that I was taught was to put a chalk mark on the chuck and another chalk mark on the lead screw and when the two marks coincided with their respective reference points to engage the half nut.  

Bearing in mind that there is a large difference between a Turner and a person who can use lathe! I must confess that I tend to rough out the thread in the lathe and then finish it either using a die, die nut or a hand chaser if they are available.  Thereby saving time and ensuring that a reasonable thread results.

Having re-read Geo Thomas'es "The Model Engineers Workshop Manual" I would reccommend that anyone who requires further information with regard to screw cutting, reads chapter 12 which contains some very useful information.
« Last Edit: August 29, 2009, 03:52:07 PM by HENNEGANOL »

bogstandard

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Re: Single point threading on the lathe
« Reply #35 on: August 29, 2009, 03:49:57 PM »
Nick,

I think you are getting confused because of the different threads that are required to be cut.

I will try to explain the reasons behind what we are on about.


Imperial Lathe.

These are almost always fitted with an imperial leadscrew, and because the imperial system uses TPI (threads per inch), then by using the drop in dial that is matched to that leadscrew, and with the correct change gears fitted, imperial threads can be cut directly by dropping in the correct numbers of the screwcutting dial.
When metric threads are being cut on an imperial machine, a transposing gear is usually fitted into the gear train to allow the leadscrew to turn at the correct rate for metric threads. Unfortunately, because metric threads use a pitch system instead of a TPI one, you cannot use the drop in dial, but instead you have to leave the half nuts engaged, and reverse the machine backwards to wind the carriage back to the beginning, to carry out the next cut.
Usually, metric threads are not true pitch when cut on an imperial machine, they are an approximation of the pitch, but usually close enough for a normally cut metric nut or bolt to fit. The problems arise when you cut a long metric thread that has to be used with a long normally cut metric hole or bolt thread. They can very easily bind up solid because of the missmatch.

Metric Lathe.

These are normally fitted with a metric pitch system of leadscrew. But unlike the imperial system, you also have to compensate on the drop in dial for the different pitches being cut. Usually this is done by having change gears on the bottom of the dial that pick up the readings from the leadscrew. There are 3 on my machine, and these little posts I did might explain what they look like.

http://madmodder.net/index.php?topic=870.0

http://madmodder.net/index.php?topic=973.0

Unfortunately, even with the compensating gears, some metric threads still have to be cut by leaving the half nuts engaged, and setting up the gear train for the correct pitch.

For cutting imperial threads on a metric machine, it is basically the same as cutting metric threads on an imperial machine. A transposing gear is fitted, the gear ratios are changed to give what TPI you require and the thread is cut with the half nuts permanently engaged. The same problem with the fit also rears it's ugly head, they are only very close approximations of the thread required.

Some very expensive lathes have all these facilities built in, just select on the gearbox. But on the sorts of lathes we normally use, we have to compromise.

I hope this has made it a little clearer.


Bogs

Offline Darren

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Re: Single point threading on the lathe
« Reply #36 on: August 29, 2009, 03:50:16 PM »
Tim,

I have a 7x12 that I cut threads with which I believe is the same as your lathe?

I can't say I have the issues you describe. So thinking about it the play can't be in the half nuts/leadscrew because they are clamped together. So the problem must lay in the leadscrew mountings at each end. They are to some degree adjustable and I believe some others have made adjustable adaptors at the tail end for this very reason.



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Offline spuddevans

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Re: Single point threading on the lathe
« Reply #37 on: August 29, 2009, 04:09:27 PM »
I can't say I have the issues you describe. So thinking about it the play can't be in the half nuts/leadscrew because they are clamped together. So the problem must lay in the leadscrew mountings at each end. They are to some degree adjustable and I believe some others have made adjustable adaptors at the tail end for this very reason.

You could be right are most likely right about adjusting the leadscrew, :thumbup:  I have done very little to the lathe since getting it and de-greasing it. After I finish the Paddleducks build I intend to start on a whole stream of mods to both the lathe and mill. Thinking about it, there is a lot of play in the leadscrew on mine. That's why I have to withdraw the tool before reversing the lathe when threading.


Tim
Measure with a micrometer, mark with chalk, cut with an axe  -  MI0TME

Offline John Hill

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Re: Single point threading on the lathe
« Reply #38 on: August 29, 2009, 05:16:08 PM »
After a long, and rather exciting, exchange on another board I have concluded (actually it was pointed out to me) that when using a metric lathe to cut imperial threads the drop in position repeats at 127 revolutions of the lead screw for all (whole number) imperial pitches.  (127 is an even mulitple of 25.4 which is the ratio of millimetres to the inch).

SO....having a turns counter on the lead screw that repeats at 127 you can always withdraw the tool, wind the carriage back and re-start without phutzing about telegraphing the engine room to run astern!

I have seen one design for a 127 tooth indicator gear but if that is to engage with a 3mm pitch lead screw it would be about 5 inches in diameter.  Other suggestions have been a sprocket on the lead screw running a loop of light chain long enough to repeat every 127 turns, the chain of course has a link or so painted to indicate the drop in point.  Neither of these is impossible but both rather cumbersom.

The  4020 (there are others) CMOS integrated circuit can drive LEDs directly and has a reset pin.  It should be easy enough to mount one of these in a little plastic box complete with a reed switch which suitably position would be triggered by a magnet on the lead screw.  You would then have a little display with a number of flashing LEDs in an easily recognised pattern and when it gets to the end of the sequence thats the time to drop in the half nuts.  I confidently expect Darren will make one of these in short order.
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Offline Darren

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Re: Single point threading on the lathe
« Reply #39 on: August 29, 2009, 05:20:23 PM »
So glad to inspire confidence John....... :lol:
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Offline NickG

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Re: Single point threading on the lathe
« Reply #40 on: August 30, 2009, 04:08:44 AM »
Thanks Bogs, yes that clears it up. That is a clever modification you carried out to your lathe.  :thumbup: And I admire anyone that can get a new machine out of a box and start drilling holes in it!   :jaw: :clap:
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