GH1440 Lathe Bed Conversion

[Valfar]

Hello,

New guy here and glad to be part of this forum.

I'm in possession of a GH1440 Lathe Bed + Headstock + Tailstock (a quite rigid base I would say) and I'm willing to convert it into a CNC lathe + 4th axis Router/Mill. Will be used for Al alloy, steel and perhaps some plastic, for hobby and prototyping (max 50mm O/D parts, but occasionally bigger O/D parts as well). Ideally, I'm willing to achieve 0.01mm tolerances.

I've got a lot of unanswered questions as you can imagine, but at the moment I cannot decide how to proceed with the lathe spindle, which will be the 4th axis as well. To make it simpler, I was thinking to design it to be able to mechanically switch between the lathe and the 4th axis mill. So the first question is what motor should I use for the 4th axis / lathe spindle? I was thinking to go for a high torque NEMA34 hybrid stepper (servo stepper) but I calculated it wouldn't give me much RPM for the lathe spindle (about 300RPM). How much RPM would I need anyway, given the parameters? Would I need 2 different motors instead, one dedicated for the 4th axis and another one dedicated for the lathe spindle?

Anybody who finds a free minute is more than welcome to suggest/advise anything.

Many thanks!

[Valfar]

Hi, I know nobody answered, sometimes I ask stupid questions.

But do you find the speeds for the lathe fairly accurate in the *.pdf bellow?

http://gradstudentshop.usc.edu/assets/001/64641.pdf

LE. I was thinking to use this servo stepper as a lathe spindle motor. Do you think it would suit the application?
http://www.ebay.co.uk/itm/EU-STOCK-N...wAAOSwKOJYIDqI

[m_c]

For that size lathe, I wouldn't personally got for anything less than 2HP for the spindle for turning.

For a 4th axis, that's where things get tricky. Even with a large servo, by the time you gear it to get 2-3000rpm, you end up with very little holding torque for doing accurate positioning work.

And not only that large servos are expensive. This is something I've often though about, and for ideal use, you need some way of changing servo:spindle ratios, so you can have one high speed low torque ratio, and one low speed high torque ratio. The problem is how to achieve that while minimising backlash.

[Valfar]

Thank you for the reply m_c. Good points.

My thoughts were to use 2 separate motors, and couple them both to the spindle: one geared up and the other geared down. But only one would be powered on and running at a given time. At the same given time, the other one would still be rotating, but driven by the spindle (as it won't be powered) - the spindle would rotate due to the other one.

I would create a lathe profile (in Mach3 for instance) and run the machinery in lathe mode, with the high RPM / low torque servo power on (and the other servo off).

When milling is needed, I would shut down everything, create another profile (mill profile) and run the machinery in milling mode only, but this time with the low RPM / high torque servo powered on (and the lathe spindle servo off, which would still be rotating due to the other motor), configured for the 4th axis role.

The lathe and mill would share the X and Y axis steppers.

I hope it makes sense.

Do you think I'm going on the right track, or talking bananas?

[komatias]

Hi Valfar

Bananas for sure.

Get a mill for milling and a lathe for turning. Even a small benchtop mill is better than what you are trying to do.

Also, for a lathe , there is no need for a servo spindle, just install a indexing pulse to tie every thing together. Then with a nice 3phase motor on a VFD you can do all you need to do.

Regards

[Valfar]

Hi Komatias, thank you for the input.

I'm sure there are lots of things better than what I'm trying to achieve, but in my view it all goes down to what one's able to compromise, like every single bit in this life. I think everybody agree a lathe/mill combo is not a new concept. That being said, I don's see a problem with the design idea. The challenge would be how it's implemented. I now need to understand what's wrong with my approach.

I understand a VFD + 3 phase motor would work similar say to a 3kW spindle found on the ebay, but also add an indexing pulse into the equation. Is that correct? Is there any VFD/motor you would recommend for a DIY application, or does any reasonable priced ones bought over the ebay would do the job?

Cheers!