GH1440 Lathe Bed Conversion

[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!

[komatias]

Valfar,

Are you looking to make money from your machine or are you just doing it for a hobby? If you are planning to make money, do not be cheap.

Any lathe has been designed to take cutting forces in the direction that causes the saddle to compress against the bed ways. Even the machines that have a milling head on the top of them take forces in that direction. Using the lathe spindle to mill causes a torque which compresses the one side of the saddle while lifting the other side. Not very good for the saddle clamps and will undoubtedly make milling a pain in the rectum.

Myford users get away with it because they only take light cuts. With the 1440, it would be a waste really.

Yes, you are correct in the watercooled spindle comparison, only difference is that the lathe spindle motor can be 2.2kw and have lower RPM, thus higher torque ;)

[m_c]

Komatiad, my understanding is the spindle would become an A axis, with a separate milling head bolted on elsewhere.

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.

That wouldn't work. Say you have the low speed with a 10:1 ratio, and the high speed direct drive, that would mean at high speed the low speed motor will be getting driven with a 1:10 ratio, so at 3000rpm, it would be doing 30'000rpm.

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.

There is a Swapaxis function within Mach3 that you probably want to look at.
If you've not already seen it, you may want to have a read of this thread - http://www.machsupport.com/forum/ind...c,11422.0.html

A possible option, depending on if you don't need the 4th axis to move while machining, is to add a brake, as then you don't need lots of torque to hold the spindle. You just need enough torque to locate the spindle, then use the brake to hold the spindle.