G0720 CNC Conversion

[Velocirex]

I am starting out on a conversion of the Grizzly G0720 which is Grizzly's version of the Sieg SX4. First up, I am working on a ballscrew conversion. After looking around quite a bit online, I ended up ordering the linearmotionbearings2008 screws, nuts, and bearings off of ebay. These appear to be the same product as homeshopcnc is offering for quite a bit more money, or at least the drawings appear to be identical. I would have liked to order domestic, but the costs appear to be prohibitive for similar quality.

With regard to end fixity (who invented that word anyway?), the G0720 has a fixed-free screw support setup on both Y and Z axes. As part of the conversion, it looks to me like it would not be difficult to add an end bearing on each axis to provide additional support to the ballscrew end. On both axes, the ways extend back almost all the way to where the column and base meet. The saddle and column can't usefully get there on this mill so it would appear straightforward to locate end supports there without sacrificing any travel. This obviously would provide more support to the screw and potentially take some loading off of the nut. I haven't seen much discussion of this with the other builds so am curious to hear if anyone here has contemplated this with other mill ballscrew conversions.

[Fastest1]

Hoss' conversion has support at both ends as do many I see.

[Velocirex]

I'm winding my way through the Hoss threads... haven't come to the ballscrew conversion part. There is alot of material from early on regarding spindle mods which the G0720 needs in a big way with the 1600 rpm top speed. I'm not sure I need the speeds that Hoss is getting, but 1600 is pretty slow. Fortunately this mill has a belt drive already and it looks like a simple mod to size up the motor pulley or replace them both if I can't find one that matches the belt pitch.

[hoss2006]

To clear things up, the only axis I've done that is supported on both ends is the g0704 X axis
because it comes that way from the factory.
Hoss

[Velocirex]

Ahh... thanks for clearing that up Hoss. I think most of the Asian mills are like that... the G0720 is the same. X axis has support at both ends but Y and Z are both floating on the ends.
For a direct drive installation like with steppers, I think this is probably just fine for Y and Z, but the concern is with a belt drive from a servo, you're gonna be putting a decent moment in for primarily the ball nut to react, particularly when fully retracted towards the fixed support.
On a slightly different subject Hoss, I looked at your G0704 electronics page where you have all the various stepper "packages" and also some of your "hogging" videos which are impressive. Any thoughts on the applicability of your heavier duty stepper configs to the G0720 which is a little bigger? Would you go that way (steppers) if you were gonna CNC your ZX45? Seems like the RF45 class is probably near the dividing line for steppers/servos.
Jeff

[hoss2006]

The new digital stepper drivers are very impressive and I would seriously consider them for my RF45 as well.
Servos certainly aren't necessary for driving a mill that size as most of the commercial CNC RF45 sized
mills such as Tormach and Novakon are stepper driven.
Still haven't decided which way I'll go but that's way down the road for me.
I'd definitely be going with the 8056D drivers with appropriate Nema 34 motors as an option.
Hoss

[Velocirex]

The fact that Novakon and Tormach are stepper machines is a good data point. I've polled many but must admit that I've come across few stepper fans in the RF45 size range. Is the simplicity the reason you seem to favor the steppers?

The big advantage of steppers at the moment seems to be the simplicity both in use and in installation. The ballscrew supports for instance... I can see leaving the Y and Z axes ends floating with a stepper but not with a servo belt drive.

I don't have much mid sized mill experience since I'm coming from micro mill land. I had a pile of stepper issues early on with my Taig but once I switched to a beefier driver setup (G540) my lost step issues went away. My only remaining issues were wrt very hot stepper motors which I cured with heat sinks. Those two experiences are why I was leaning towards servos.

So I thought I had made this decision but the more I reread the more comfooseder I get. There is about a hundred bucks difference between Keiling's comparable servo and stepper systems so its not a huge thing. That ignores some additional time and materials to work up the servo reduction drive pulleys though. Maybe I'm overthinking it.. pick one and go?

[Poseidon]

Have you decided on the type of motors you will be using for your G0720 CNC conversion? I have been watching Hoss' videos on the g0704 and the 8056D controllers really kick some serious @$$. Hey Hoss, do you think those controllers will perform just as well with 878 to 1600 oz/in NEMA 34 motors?

[hoss2006]

Have you decided on the type of motors you will be using for your G0720 CNC conversion? I have been watching Hoss' videos on the g0704 and the 8056D controllers really kick some serious @$$. Hey Hoss, do you think those controllers will perform just as well with 878 to 1600 oz/in NEMA 34 motors?

I noticed Keling has 8070D drivers that would be much better for most of the Nema 34 motors.
Run them with a 72V power supply for some serious @$$ kicking making sure the motors are suited for the 72V.
square root of the motor inductance X 32 = motor voltage, bipolar parallel is best.
Hoss

[Velocirex]

Have you decided on the type of motors you will be using for your G0720 CNC conversion? I have been watching Hoss' videos on the g0704 and the 8056D controllers really kick some serious @$$. Hey Hoss, do you think those controllers will perform just as well with 878 to 1600 oz/in NEMA 34 motors?

I went back and forth between stepper/servo setups and eventually chose the Keiling 8070D drivers with their 1200 oz-in motors for all 3 axes. In fact, their shipment should be arriving today .

The mechanical part of the conversion is lagging... I still have a fair bit of work to do on the end plates and ballnut mounts.

Another thing this machine really needs is a spindle speed increase. Stock is only good for 1600 rpm. I looked over the controller in there hoping for something like the simple pot "tuning" that hoss was able to do on his G0704 to get a little more rpm but came up empty or at least its not obvious.

Plan B is to swap the motor pulley out but I'm having a little trouble sourcing an HTD pulley on the order of 56 teeth for a 25 mm wide belt. They're out there but not from the usual "order online friendly" sources. The stock motor pulley is 28t so this would double the RPM range. It looks like I'll have to bore and broach (haven't done any broaching yet) anything I find since there doesn't appear to be any "drop in" replacement pulleys with the shaft size I need.

The spindle pulley may eventually have to change too, but that guys a tad more work to swap out. I guess we'll see how much these bearings can take before they have to be addressed also.
Jeff

[Bob La Londe]

I'm winding my way through the Hoss threads... haven't come to the ballscrew conversion part. There is alot of material from early on regarding spindle mods which the G0720 needs in a big way with the 1600 rpm top speed. I'm not sure I need the speeds that Hoss is getting, but 1600 is pretty slow.

What kind of speeds is Hoss getting?

[Velocirex]

What kind of speeds is Hoss getting?

Not sure where he ended up, but his site has references to 4700 rpm ... he may have more than that now. I think he approached it in steps which is how I'm proceeding.