Best 12x36 (or similar) late for a cnc conversion.

I want a convert a lathe to cnc and I have been looking around at the different lathes available. I think the PM 1236 is the best choice from what I've found so far. I have also been comparing to the grizzly lathes. The threading and power feed will be nice up to the point I start the conversion, but after that its just waste. I know support and parts from Grizzly are good, but what about PM? If I cnc this thing will the warranty still be valid? I really like that there is a collet closer for the grizzly for under $400, I could see this being handy one day. The Preferred package PM 1236 has a foot brake, I think that would help make gear changes quicker, right?


Here is what I would like to have in a finished CNC conversion:
- cnc threading
- coolant
- torque to cut 6"+ steel parts
- speed to cut small parts from Al, plastic, etc...
- tool turret (I will probably wind up making this)
- Encloser (will build this)
- VFD, and all the benefits that come with it. ( this would probably be the last thing I do)

I also built a CNC router you can see my thread here:
http://www.cnczone.com/forums/cnc-wo...minum-cnc.html

Thanks in advance!

P.S. I uploaded a comparison spread sheet I made up

Grizzly is pretty good most of the time for replacement parts. That is when they have it in stock. I've never had to get parts from Precision Matthews, but I have gotten replacement parts from Weiss, which is who PM gets his machines from. Dealing with Weiss was very pleasant and I got my parts relatively quick considering they come from China.

http://www.weiss.com.cn/products_det...uctId=338.html

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Originally Posted by dbsharp

I think the PM 1236 is the best choice...I have also been comparing to the grizzly lathes. The threading and power feed will be nice up to the point I start the conversion, but after that its just waste.

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I am not sure I understand that statement in context, since both Grizzly and the PM have similar threading and power-feed capabilities.

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Originally Posted by dbsharp

I know support and parts from Grizzly are good, but what about PM?

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Grizzly stocks thousands of spare parts, especially if it is for the "grand daddy of home lathes (the venerable 12x36 G4003). PM custom orders parts as you need them, which can take months. People owning other brands frequently buy their spare parts from grizzly.

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Originally Posted by dbsharp

The Preferred package PM 1236 has a foot brake, I think that would help make gear changes quicker, right?

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Yes, however if I was you, I would try to eliminate having to do gear changes. Maybe look for a belt driven lathe for your conversion. Replace the spindle motor with a 3 phase, VFD driven motor, set on a fixed reduction ratio. Ideally, you want your cnc controller to control spindle speed. The VFD with a braking resistor can slow the spindle rapidly.

Why belt driven?
- You don't need all of those gears, because you don't need speeds as slow. Slow speeds are really only useful for very large diameters, or threading... and since cnc handles that for you, you don't have to run the machine slow enough so you can react in time.
- You don't want all those gears, since the gears on a cheap machine like this tend to add some vibration, affecting the smoothness of your cut.

That means (assuming you are looking for cheap lathes) looking at some of the older asian 12x37 lathes.

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Originally Posted by dbsharp

torque to cut 6"+ steel parts

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:eek:

You mean diameter?

hmm, while the belt driven lathe does allow you to get low spindle speeds, using such drastic speed reduction still means that a VFD can't provide you with all the range of speeds you want (50 - 2000 rpm), without changing belt configuration. Even the new Tormach lathe requires a belt change to get below 350 rpm... actually they require swapping motor pulley size, which isn't very convenient. :rolleyes:

Might be a good reason to go for gear reduction after-all. I am not a big fan of swapping belts around.

Using HSS, that means rpm = 70*4/6 = 47 rpm. Carbide makes that rpm=350*4/6 = 233 rpm.

I might get a vfd on down the road. There sure are lots of benefits. Yes, I want to have the ability to make large parts from steel if I need to so the low gears are a must regardless of the motor.

anyone ever take the g4003 faster than 1400 with a vfd? This is one of the reasons I wanted the 1236.

I might look into getting the machine from weiss. you've gotta save something when you cut out the middle man!

Not sure what you mean by faster than 1400, I am guessing you mean RPM's. If that is the case. Many VFD's will achieve a 300hz output. Most machines in garages in the US are running on 60hz. So theoretically it could go 5x as fast. However safety and longevity of components do come into play. Doubling the available max is easy to accomplish (120hz) with minimal if any extra stress. Plus if you install a pot (this can be done manually through the VFD too) on the VFD you can dial any RPM you want.

With the stock motor the g4003 only runs 1400 rpms. I wonder how much preload is on the bearings and how that preload is applied. Judging by the exploded view, Looks like the housing itself will have to flex a little giving the preload. The bearings the G4003 uses are rated far beyond 1400 rpm, so with a vfd, I might be able to turn the speed up a good bit and get some work done with softer materials!

You will be able to turn it up til you feel uncomfortable working in front of it.

It will be like Capt. Kirk requesting a Warp factor 9 when the machine has only been tested to Warp factor 5 ;-)

Stay tuned, I just shot a video example on my 12 x 36. 25hz of my VFD was 710 rpm in high gear. I have my VFD set to a max output of 90hz which gives me 2700 something rpm. Had to remove the chuck due to fear of it getting thrown into my M3 parked just out of view.

Here it is.
12x36 lathe with VFD - YouTube

Nice!