[knighty]

hi

I don't have loads of experience with lathes, but I do have a Myford ML7 (long bed, norton box) which I've done a few small one-off pieces on

I've just bought a bigger lathe, and want to convert it to cnc... here's the lathe:









I'd like to be able to turn a mincer worm... a bit like this...


but for the big mincers at work, so about 3 foot long and 6inches wide... when I turn it, the length has to be exact, but the actual worm but doesn't have to perfectly match the original (which will make things much easier)

to buy a worm, they're £2500 each ($4000) and made of cast... we make dog food so put huge chunks of meat through the mincers and the worms are easy to break... they also wear around the outer edge, so they don;t fit tight into the barrel after about a year and need replacing... the barrels they fit into need replaying about every 2 years (about £2500 again)
if I mad my own worms, I could probably make a wider worm to compensate for the worn barrel and make the barrels last twice as long

I'm still searching on here and the internet for more information - I've looked before and know what I'm doing / the rough plan... but not which motors I should buy etc..

(I'm good with computers so that side isn't a problem... and my brother is an engineer and spends most of his time doing cad work)

also, I'm not worried about the speed of the machine... as long as the tool can move fast enough to keep up with the work spinning that will be fine by me... (I'll only be doing big one off work) so I figured I could gear down the motors a lot for higher accuracy ?

so.. does anyone have any input ? I'd like to search more and post back here before I buy anything...

thanks

Alan

[MaxMekker]

Fist off all you need to change to balls crews, and you need a indexer on the spindle.
You also need to control spindle speed, I use VFD on my lathe.
I use 6,5Nm motors on the screws but my lathe is small, only 2,2Kw motor and Ø300x750mm.
I also use a quick tool changer(manual).
My half updated webpage
If I can help with information just ask, good luck with the project :-)

[awerby]

That part seems more suitable to a 4-axis mill to me. It might be possible with a CNC lathe that's indexed to the spindle, but the CAM software that would program a lathe to do that would be quite expensive, if it's available at all.

Andrew Werby
www.computersculpture.com

[knighty]

Max: I read through your page and you've done a really nice job there

this might be a daft question... but why do I need to change the screws on the lathe, can't I use the original ones ?

Andrew: I thought about a 4 axis mill.. and I think you're right, that would be much better to make a worm with... but I don't have one of those... and a mill big enough to accommodate the worm it is a pretty big mill....

I also have other uses for the lathe later on, but not for a mill


I'm not sure about the cam software... I assumed I could design what I wanted in some sort of CAD program, and then transfer that over and have it automatically translated into a set of instructions for the program running the motors on the lathe ?

[awerby]

Knighty wrote:

Andrew: I thought about a 4 axis mill.. and I think you're right, that would be much better to make a worm with... but I don't have one of those... and a mill big enough to accommodate the worm it is a pretty big mill....

[How big are those things?]

I also have other uses for the lathe later on, but not for a mill

[You might be surprised...]


I'm not sure about the cam software... I assumed I could design what I wanted in some sort of CAD program, and then transfer that over and have it automatically translated into a set of instructions for the program running the motors on the lathe ?[/QUOTE]

[Most (all) of the lathe CAM software I've ever seen is intended for making radially symmetrical objects. I think it's theoretically possible to make things like your worm, since ornamental lathes (like the Holzappfel Rose Engine lathe) were used to produce irregular parts with eccentric chucks and special geared follower mechanisms, but I doubt you'll find software off the shelf that will do something like this for your CNC lathe. If you're good a coding, writing that program might make an interesting project for you, though...]

Andrew Werby
www.computersculpture.com

[asuratman]

There is somebody retrofitting the same lathe as yours in here http://www.cnczone.com/forums/vertic...e_di_palo.html.
You can learn alot from him.

[will gilmore]

Stepper or servo control of the spindle and some kind of live tooling and you have something like a 4 axis mill.

What is the material you want to use?

[wizard]

hi

I don't have loads of experience with lathes, but I do have a Myford ML7 (long bed, norton box) which I've done a few small one-off pieces on

I've just bought a bigger lathe, and want to convert it to cnc... here's the lathe:

Big lathe!

I'd like to be able to turn a mincer worm... a bit like this...

Interesting, looks like they are trying to achieve compression of the material. You might want to consider how this is done on plastics extrusion screws, where compression is achieved by varying the depth of the screw groove. That would likely make machining much easier.

but for the big mincers at work, so about 3 foot long and 6inches wide... when I turn it, the length has to be exact, but the actual worm but doesn't have to perfectly match the original (which will make things much easier)

With a CNC lathe you can experiment to your hearts desire. Do realize though that you will be removing a lot of material copying the current screw.

to buy a worm, they're £2500 each ($4000) and made of cast... we make dog food so put huge chunks of meat through the mincers and the worms are easy to break... they also wear around the outer edge, so they don;t fit tight into the barrel after about a year and need replacing...

Sounds like problems the plastic extrusion industry has though they get better life than that with many resins. Even so in a plastics machine the barrel often has a replaceable sleeve. They often make use of nitriding or other surface modifications to make the screws last longer. These approaches might not work for your industry.

the barrels they fit into need replaying about every 2 years (about £2500 again)
if I mad my own worms, I could probably make a wider worm to compensate for the worn barrel and make the barrels last twice as long

That is one approach but realize that the barrel most likely don't wear evenly. You may need to reborn the barrel, lap it or do other things to it to allow an oversized screw to work right.

I'm still searching on here and the internet for more information - I've looked before and know what I'm doing / the rough plan... but not which motors I should buy etc..

Your first problem is the high feed per rev will likely mean that your axis motors will have to be sized larger than normal.

(I'm good with computers so that side isn't a problem... and my brother is an engineer and spends most of his time doing cad work)

That is good, maybe he has some experience sizing motors

also, I'm not worried about the speed of the machine... as long as the tool can move fast enough to keep up with the work spinning that will be fine by me... (I'll only be doing big one off work) so I figured I could gear down the motors a lot for higher accuracy ?

This will get complex quickly as you will actually need a motor that can drive the lead screw relatively fast to maintain the helix. Further the drive will have to be synced to the spindle fairly accurately. That might mean a spindle servo or it could mean a good encoder synced to the spindle. In effect electronic gearing.

so.. does anyone have any input ? I'd like to search more and post back here before I buy anything...

Yep! Slow down a bit. You will need to consider how you will sync the lathe arises to the spindle. Most likely you will also need a mill or possible live tooling and the ability to index the spindle. Frankly I think you will need a mill anyways. Mills are extremely handy for maintenance type work.

You might need ball screws though you might get away with a conventional screw. In the end it depends upon how much accuracy you need and how much lead screw losses you can put up with. Your feed rate will have to be extremely heavy so the axis ends up cutting as much as the rotary motion of the spindle. You will also end up needing special tooling or odd mounting arrangements.

More importantly software will be an issue. You really need to figure out how you will solve this issue before you go to far. All the mechanics and electronics in the world won't help if you can't get the software to do what you want. I can see it being far easier to taper the depth but other experienced lathe programmers may be able to offer up better advice.

By the way I've never seen a plastics extrusion screw being made but the appearance of many of these screws lead to believe that they are either done on a grinder or a lathe with a live tooling axis and a specialized cutter. Probably a cutter like would be seen on a horizontal mill. I get this from the appearance of the flutes at either end of the screw. That is basically guess work and to be honest screws vary a lot depending upon manufacture and usage. Setting up live tooling for such a usage would be a non trivial exercise. Here again if somebody pipes in with how this is done in the plastics industry it might be very helpful in generating ideas.

thanks

Alan

Best of luck.