Author Topic: Lathe Screwcutting Idea  (Read 3669 times)

Offline djh82uk

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Lathe Screwcutting Idea
« on: November 25, 2009, 06:55:45 PM »
At the moment I have a Sieg C3, I like it, but maybe looking at getting a C4 or other larger lathe.  I was then going to CNC it but had an idea:

I was thinking, If I were to remove all the gears that drive the feed screw, make a mounting and atatch a lets say 300W DC motor, now as my background is partially in MCu's etc, I was looking at getting a small screen, a propeller MCU and two methods of accuratley measuring RPM.

I input via some pushbuttons what thread i want (and is displayed on the screen), the mcu measures the spindle speed and from that works out what speed the feedscrew needs to be moving at to produce that thread.  It would do all threads, It would be reversible, and when not screwcutting it can be used to produce a very fine feed for a great surface finish.

Cost would be the motor, motor controller (preferably I2C controlled), 2 methods of measuring RPM,  a propeller develepment board, an LCD screen and something to house it.  And then i need to make a mount and coupler and get the motor parallel to the shaft (or use belts i guess)

Thoughts?

DJH

Offline GrahamC

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Re: Lathe Screwcutting Idea
« Reply #1 on: November 25, 2009, 07:46:13 PM »
Interesting idea but not new.

Do a Google search for "electronic leadscrew" 

Often abbreviated as ELS. There are a couple of threads on a couple of other online forums and there is a Yahoo group. I seem to recall that someone mentioned that even some of the commercial lathe manufacturers had similar designs, Hardinge might have been one that was mentioned.

I have some cnc parts collected and have considered doing just this to my lathe but I am working up to it slowly. I still don't really mind changing gears to thread but what I do mind is changing gears to cut a thread and then having to change back to get powered saddle feed. I have and am still considering just setting up a stepper (or perhaps a DC motor) to drive the leadscrew just to move the saddle rather than use the handwheel.

The developer of the ELS has a kit but can be a bit pricey - it is just a wee bit of hardware and software so should be easily duplicated.

cheers, Graham in Ottawa Canada
The challenge is being able to state any problem in such a manner so as to allow a solution.

Offline djh82uk

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Re: Lathe Screwcutting Idea
« Reply #2 on: November 25, 2009, 08:11:12 PM »
Yeh i kinda figured it must have been an old idea as it makes a lot of sense to me.

Making the electronics is a problem, My biggest issue is selecting a motor, I have no way to figure out how much torque I would need, Although i am presuming a 100W DC motor should be enough as the leadscrew never moves THAT fast.

I would probably use a frequency counter to work out the RPM of both the spindle and the leadscrew and I guess the easiest way to make the calculations would be to setup the mill for each feed speed and measure the rpm of both spindles to work out a ratio.

DJH

Offline GrahamC

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Re: Lathe Screwcutting Idea
« Reply #3 on: November 25, 2009, 08:32:58 PM »
Actually if you use a stepper to drive the lead screw you need only know the rpm of the spindle and move the stepper appropriately (in very simple terms). Or you could use a servo or synchronous motor to drive the lead screw. Always more than one way to skin a cat.

You will find lots of information on the Yahoo group. It is a reasonably active group with several messages a day. The group also has a file and picture area.

The ELS has been adapted to lathes from the small to the large end of the scale and even home made Gingery style lathes. All of the source code, schematics etc, etc are provided. It is built around a PIC chip (which one escapes me at the moment). Wouldn't be hard to adapt to the Propeller or AVR or what have you.  You also have the option of using and old PC running DOS and TurboCAD to do something similar (or a LINUX program called EMC).

What you may start as simply motorizing the lead screw may lead to doing even more with CNC'ing your lathe. Just like jelly beans, bet you can't just eat one. (if you have a sweet tooth that is - replace with ale if that is more to your liking).

cheers, Graham
The challenge is being able to state any problem in such a manner so as to allow a solution.

Offline Weston Bye

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Re: Lathe Screwcutting Idea
« Reply #4 on: November 25, 2009, 08:46:42 PM »
In the upcoming Winter 2009 issue of Digital Machinist I will be presenting a series on adding electronic threading to the Sherline Lathe.  The leadscrew is driven by a NEMA 23 stepper motor and Geckodrive controller.  An auxillary motor and gear reduction, equipped with an encoder, powers the spindle.  Pulses from the encoder pass through an "electronic gearbox" divider made up of TTL logic chips which in turn feed pulses to the stepper motor.  Threading rates are selected with thumbwheels and the controls mimic the typical manual machine controls as closely as possible.

The series is expected to be presented over the course of 3 issues, but elements of the system will be functional with each article.  The electronics are designed so that no special programming knowledge or tools are necessary - just a soldering iron and some patience.

Although the project is built around a Sherline lathe, the principles I will present are applicable to any lathe.
Weston Bye
Practitioner of the Electromechanical Arts.
author of The Mechatronist column
Digital Machinist magazine

Offline John Hill

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Re: Lathe Screwcutting Idea
« Reply #5 on: November 26, 2009, 01:23:11 AM »
Weston Bye, I can understand how you could turn the leadscrew at the right rate in relation to the spindle but how do you keep accurate relationship between the two?
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Offline Weston Bye

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Re: Lathe Screwcutting Idea
« Reply #6 on: November 26, 2009, 06:52:06 AM »
John,

Indeed, you have identified the fundamental issue in threading.  Being able to generate the proper ratios between spindle and leadscrew is of little value if you can't follow the same path on successive cuts.  The "trick" is to identify the starting point and return there every time a new pass is started.

The starting point includes all elements involved; obviously, the saddle, but also the spindle angle.  These are the fundamental factors.  Knowing the starting leadscrew angle also will improve accuracy.  This can be demonstrated on a mechanically geard lathe by using a dial or digital indicator to determine the starting position of the saddle, and a pointer and hash mark on the spindle.  But, the half nuts won't engage?  Indeed, the gear train between the spindle and the feed screw will only have a certain number of points where the half nuts will drop in with the spindle hash mark aligned with the pointer.  Manually rolling the spindle over until the 'nuts engage with the spindle aligned gives you a starting point.  With everything aligned and locked up, you start the spindle to make your threading pass.

What I have described is threading without benefit of the threading dial - it can be done but is cumbersome.  With electronic threading, rather than using indicators, hash marks and pointers, a few strategically placed switches are used.


Weston
« Last Edit: November 26, 2009, 06:54:21 AM by Weston Bye »
Weston Bye
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author of The Mechatronist column
Digital Machinist magazine

Offline NickG

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Re: Lathe Screwcutting Idea
« Reply #7 on: November 26, 2009, 07:14:21 AM »
I had thought of this for slow feeds but I can see it being too complex for threading (at least for me!), that relationship being the stumbling block. When you load the spindle it won't slow down at the same rate as the leadscrew so there'd have to be some sort of wizardry that would react quickly enough to maintain the ratio. Obviously not impossible but difficult. would be an interesting project to see.

Nick
Location: County Durham (North East England)

Offline djh82uk

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Re: Lathe Screwcutting Idea
« Reply #8 on: November 26, 2009, 07:33:21 AM »
Ok you got me thinking now, If you had an encoder wheel on the spindle, the MCU would always know what angle the chuck is at, aswell as where it was at the beginning, however you would little to zero control over the spindle, so the spindle would either need to be replaced with a powerful servo/stepper so that you did have control, or you would need an equivalent of an absolute dro on the feedscrew so it would work out where the carriage needs to be so that it engages the workpiece at the correct place.

But as soon as you factor in Backlash you have a problem, this then starts to become expensive and so you may aswell just CNC the damn thing.


My initial idea was just to have it so that the electronic feedscrew just replaces the change gears, other than that you do the screwcutting exactly as you did before.  The mcu would be quick enough to react to rpm & direction changes on the spindle and adjust the feed motor speed/direction to the correct proportion.  But to do this the Feed motor has to be a stepper or servo.


But now that you mention it, just adding a slow rpm motor for getting a very fine finish would be a welcome addition, it could be mounted the opposite end of the feed screw so you don;t even have to mes with the gears, just disengage the feedscew from the spindle before firing up the feed motor.  Although you would obviously need some sort of saftey device to ensure the feed motor is off if the feedscrew is engaged with the spindle

DJH

Offline Weston Bye

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Re: Lathe Screwcutting Idea
« Reply #9 on: November 26, 2009, 08:41:57 AM »

Quote
...however you would little to zero control over the spindle,...


Control over the spindle speed is not necessary - as long as the leadscrew can remain in sync with the spindle.  Any variations in spindle speeds must be proportionally matched by the motor driving the leadscrew. 

Quote
My initial idea was just to have it so that the electronic feedscrew just replaces the change gears...

This is quite feasable.  However, I suggest putting the encoder somewhere upstream of the back gear rather than directly coupled to the spindle.  This way, an encoder with a lower pulse count (less expensive) can be used but still provide enough pulses to divide down to drive the leadscrew.  But, because you don't have the fixed relationship inherent in the gear train, some sort of starting synchronization between the spindle and the leadscrew would be necessary for threading.

Weston
Weston Bye
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author of The Mechatronist column
Digital Machinist magazine

Offline John Hill

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Re: Lathe Screwcutting Idea
« Reply #10 on: November 27, 2009, 02:11:33 PM »
I suppose one could start the carriage moving at an instant synchronised with a single contact on the spindle, advance the carriage the required distance, retact the tool, reverse the carriage the same distance, advance the tool, restart the carriage again at a time triggered by a spindle contact.

Providing the pulses to move the carriage were synchronised to spindle speed the process seems almost trivial, I am probably missing something. :coffee:
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Offline Weston Bye

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Re: Lathe Screwcutting Idea
« Reply #11 on: November 27, 2009, 09:02:05 PM »
What John describe is precisely what happens with a CNC lathe.  There are a couple of them where I work that turn, face and single point threads on a part all at the same fast spindle speed.  The threading takes 6 or 7 passes of the tool and moves back and forth in the motions John describes to thread a section about 3/8" long faster than I can brush my teeth.

Powerful motors are used, as the tool holding axis is massive and supports multiple tools.
Weston Bye
Practitioner of the Electromechanical Arts.
author of The Mechatronist column
Digital Machinist magazine