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Lathe Screwcutting Idea

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djh82uk:
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

GrahamC:
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

djh82uk:
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

GrahamC:
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

Weston Bye:
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.

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