Centroid Community CNC Support Forum

Hi. I'm thinking about doing an Acorn CNC conversion on a 8x30 milling machine. I want something more rigid than a bench top machine and these 8x30 mills appear to be the biggest machines I can still move with my 2-ton engine hoist. I'd appreciate any insights you guys are able to provide.

I want to use the knee as the Z-axis (not the quill). I'm thinking I can install ball screws for X and Y and continue to use the factory lead screw for Z axis. Any thoughts?

Thanks,
-Franco

http://www.grizzly.com/products/8-x-30- ... Mill/G0678

I think if you only cut Aluminum, the Z backlash would not be a problem.{You always have the weight of the table bearing down} Anything else would have to be fine cuts. Would be easier to replace the lead screw on Z, well maybe it would.

I think if you only cut Aluminum, the Z backlash would not be a problem.{You always have the weight of the table bearing down} Anything else would have to be fine cuts. Would be easier to replace the lead screw on Z, well maybe it would.

I have an G1004, which is very similar, and have put a gas spring in to aid in moving the knee. Would have to look up the specs - I think it is about 80 lb of force. Have not checked for backlash since I have assumed the knee weight would rule.

Andy Olney

What about an RF-45 or one of the variants?

I like the square column design.

Just a random pic from the net.

Reedfirst wrote: ↑Sat Nov 11, 2017 7:43 pm What about an RF-45 or one of the variants?

I like the square column design.

Just a random pic from the net.

Rf-45.JPG

My preference would be for the bed mill over the knee mill IF I were to tackle a complete conversion. I do like looking for purpose built CNC machines and converting them over converting a manual machine.

Just my .02

Marty

Thanks Guys - I appreciate your input. Let me tell you how I got on this knee mill kick.

I've done a few conversions on bench top style bed mills and I've determined they all suffer from the same weakness: the slot in between the Z-axis dovetails.
The slot allows the ball screw to lift the head, but it also weakens the Z-axis column. When pushing in the X-axis, you get a lot of "flex" in the column. I've put indicators all over the machines and It's my belief most of the movement is happening at the Z-axis dovetails.

The 8x30 mill has a lot of the features I think I want:
1. VFD spindle with Angular Contact bearings and single step belt drive (easier to increase RPM by changing motor pulley
2. Bigger table with more travel
3. Larger castings that will (hopefully) make it easier to install ball screws
4. Provisions for coolant sump/pump
5. Small enough to move with my engine crane
6. More rigidity (I hope)

My biggest concern is using the Knee as Z-axis. I'd like to use the factory screw as that would be easy. I can swap out for a ball screw but that will take a bit more engineering to shoehorn in there.

I'd love to buy a purpose built CNC but most of them are physically tool big/heavy for me. I've looked at the small bench top CNC's but they either seem to be junk or are very expensive.

I just don't want to pull the trigger until I get more perspective on this and I figured this is the best place on the internet to get input.

Thanks,
-Franco

Hi Franco,
VMC's use a column and are bed mills.
I don't think cutting forces are going to flex it as much as you might think. Buy the biggest you can afford and can get in your shop. You can always fit with a standard 3 phase Inverter Duty motor. I think most of the import motors are all anemic and their controls aren't so good anyway.

Knees have been used as a Z axis, but there is a lot of weight there and friction on the dovetail working against you. Index used to make the knee their Z on their full size knee mills. They needed a way to "counter balance" it. I believe they used some sort of air spring. Maybe you could do the same thing. I think you could get away with using the exisiting screw, if you could drive it from the screw rather than the knee cranks. I think the bevel gears may have some inherent slop?

It kind of looks to me you are out growing your smaller machines...or have the disease like me and just enjoy building machines. I'm ready to shift gears and want to finish what I have move them on to new homes and learn how to use them including CAD/CAM.

Whatever you decide, thanks for sharing your builds and info. Just have fun.

Marty

Franco, did you see this thread about the PM 727?
viewtopic.php?f=60&t=1105&sid=469200d86 ... 178d12c6b0

Franco,

I'm thinking about doing a G0678 Grizzly Mill as well! Like you, I've looked at all sorts of mill types and sizes and I keep coming back to this mill. I like it for all the same reasons you listed and some you didn't list like the fact that it has a one shot oil system and a hardened table while many of the others have no oil system at all and the tables are soft cast iron. Some have gear heads and low top speeds and some use tapered roller bearings in the quill instead of the angular contact bearings that I am looking for.

I agree with you that making the pulley changes for a better top speed looks pretty easy and I think I've got the spindle encoder mounting all worked out.

I'm planing on making the Z move the quill instead of the knee, just my preference I suppose but the knee lead screw and gear is pretty much not all that accurate, maybe?

I'll do it with an Acorn setup for sure if I do one. I've done a complete lathe conversion on a Grizzly G4003G lathe using Mach 3 and it works very well. If anyone would like to see the videos on you tube just search for DrDavoCHP and you should find 4 videos on the lathe conversion.

Dave C