Need Help!- Grizzly 10x54 CNC conversion

[BTD]

I've been thinking about converting my Grizzly 4029 mill to CNC. I haven't done anything yet except read the posts here and some preliminary engineering and Bill of Material prep. So far the cost (parts and materials only) is just north of $3K....not including software or computer.
So far, the plan is:
Keling 1200 oz-in NEMA 34 servos...X, Y and Z...all w/ 1000 cpr encoders.
Powered knee w/ air assist.
X and Y ball screws/nuts (ground) plus dual ang contact brgs.
Retain Z-axis ACME screw for the time being.
3:1 ratio XL pulleys/belts all axis.
Keling 72 VDC power supply.
Viper servo drive.

I'm going to take this a step at a time to spread out the cost. First step is to add the air assist and servo to the knee. I need a powered knee anyway.

I'm thinking of installing two pneumatic cylinders, bolted to the outside of the knee on top and the base on bottom, and using spherical rod ends to minimize misalignment forces.

To size the cylinders, I "weighed" the knee, saddle, and table using an offset beam and a bathroom scale. The attached photos show 1) the Grizzly file photo of the entire machine 2) the offset beam with the scale on the "long" side and 3) the reading on the scale. I had to offset the beam as the bathroom scale has a 330 lb limit. I raised and lowered the knee until I found the point where the effort required to turn the Z crank was minimum. I figured that was about the "balance" point. The scale read about 250 lbs. Did the math (force and moment balance) and I get 664 lbs. Does this sound about right?.

Assuming it's in the ballpark, I think I'll need two 2.5" bore x 16" stroke cylinders. 68 psig should balance the load...but I'll probably set the pressure at about 60 to keep weight on the ACME screw and reduce backlash. I'm also not sure how precise the control from the air pressure regulator will be. As the load is lowered, the air pressure will rise until the regulator relieves. If that setting is too high, the knee/saddle/table could float and produce backlash.

Oh...a little about me. I started out rebuilding motorcycles and cars, worked as a VW mechanic for awhile, got a Mech Engr degree and have spent the last 30+ years designing gas turbine generator sets, combined cycle power plants and offshore oil and gas production platforms. Growing up, my dad had a B'port series 1 and a 14" South Bend tool room lathe and I used to enjoy cutting up his brass and aluminum stock to make .22 cal zip guns for me and my pals.

I have absolutely no experience with this type of project so any suggestions you guys have would be appreciated.

[Regnar]

Im planning a simular setup but I will watch your thread with interest. You might find this link a little helpful for ideals. http://www.cnczone.com/forums/showth...t=25895&page=2

[BTD]

Thanks for the reply Regnar. I had previously read Chich2's posts and was very impressed with his project. I like his single air assist cylinder but I don't want to dismantle the mill and cut up the base and knee. I thought that mounting two cylinders outboard of the knee, one on each side, would work. (I know I've read other posts where this has been done but can't find them just now.)

After playing around with my Solidworks model it looks like I might not have enough space for my "2 outboard cylinder" scheme. Here's the deal...I have 15" of knee travel. I picked a 2.5" x 16" stroke cylinder (an extra inch so I don't bottom out the piston.). The mfg's data shows that a standard 16" stroke cyl is about 26" long fully compressed. That's for a plain threaded rod end and a trunion base. When my knee is in the lowest position, I don't have 26" unless I build some really big outrigger-type brackets. I need to study this some more. Maybe I'll have to pick a shorter stroke cylinder and sacrifice some knee travel to get it to work. Will report back later.

[Regnar]

I believe most go with a 12" stroke. If you add on your spindle travel you can get the full 16 inches of travel. I was going to ask you how you where going to get the 16" of travel. Another option is to mount the cylinders at angle but you will not have a constant lift through the whole travel. It might just be easier to sacrifice the last few inches at the bottom and if push come to shove remove them for a project.

[BTD]

Regnar,

You're right. Trying to get the full 15" of factory knee travel is going to require some pretty wild brackets. When the knee is at it's lowest point there's just not a lot of room to mount the cylinder and still clear the saddle.

Even the angle mount concept starts to get weird as the cylinder has to be far outside of the machine base so as not to hit the base at the lowest point. You bring up a good point that an angle mount will result in varying force (for a constant air pressure) as the knee raises and lowers. Not sure if that would be a big problem but a straight lift avoids the problem altogether.

I need to think about this some more. Will I ever need 15" or 14" or even 12" of Z travel IN CNC MODE! I can't remember ever using that much travel once the work piece was mounted. I would like to have the travel for tool changes but then I'd still need to re-zero the Z. Adding air assist is a lot of effort to go thru just to avoid cranking a handle to change tools. OTOH, I have read where a powered knee is more stable than a powered quill.

BTW, I measured the backlash on the knee...dial indicator mounted on quill indicating top of table...raise knee...set indicator to zero...lower knee one complete turn...read indicator...repeat opposite direction of travel. In all cases the backlash was right at .001". I would have expected more given the bevel gear arrangement. Backlash on the X and Y are about .003"

Oh well...back to the drawing board for a while.

[Regnar]

The knee is not bad at all that can be easily fixed with software same with the other axis. I remember reading this guy had .oo4 and .oo7 on his x and y axis and made mach fix it for him. He is still using the acme screws to. http://mysite.verizon.net/fahringer/index.html

I have been thinking long about converting over the knee. On my mill I have the ability to remove the bevel gears. The shafts are half inch with a key way. I was thinking about removing the bevel gears and mount a timing pulley on the end. Then I would mount the servo motor on the inside of the knee maybe on one of the sides. The initial install would be some what of a pain but it would look very clean after that. With out a servo in hand I dont know if it would work but just by looks I thinkI would be work.

[BTD]

In the attached photo, the view is under the knee looking up. The Y lead screw is running verticle w/ the front of the mill at the bottom and the knee bevel gear shaft is at an angle. The plate w/ the three screws appears to be a bearing retainer. (BTW...I know it doesn't really matter but couldn't the Taiwanese have spent just one hour ($4?) and trimmed the ragged crap from the casting. It really ugly.)

Even thou you can't see it in the pic, there isn't much room between the top of the bevel gear and the XY yoke/Y leadscrew. I think I'm going to stay away from that side and mount the Z pulley below the bearing retainer w/ the servo motor bracket held by the three screws in the bearing retainer. That way I should have enough room for the servo/pulley/bracket and I keep the manual crank. (I'm saying all this without having actually taken the Z screw/bearing/pedestal apart. Don't be surprised if I completely change my mind once I get a little more knowledge.) Given that, the pulley and bracket will reduce my current 15" of travel by say 2" leaving me w/ 13". We're now getting closer to the 12" travel you mentioned before.

Other news...I contacted a mfg of TELESCOPING PNEUMATIC CYLINDERS today. Very cool product. I can get 16" of stroke in a fully compressed 9.22" high x 5.25" dia package that will support 800 lbs with 86 psig air pressure. Problem is they want about $1,700 per copy as these are basically custom built items...OUCH! (The ridiculously large brackets and dual, off-the-shelf outrigger cylinders are looking better all the time.) Does anyone know how Bridgeport designed their air assist Z pedestal for the Series 2? The pictures I've seen hint that the screw pedestal and air assist are all housed in one casting.

Also, I made a cardboard mock-up of the Keling 1200 oz-in servo. It looks like there would be plenty of room to fit this up underneath the knee in front of the lead screw. I'll post some SolidWorks model views as soon as I get a chance to update the design.

[Regnar]

At least yours is clean I spent 5 hours today cleaning my knee out and removing the mystery goo or should I say glue. I am hoping now that the metal chip are out of the gears and bearing that the z axis will move up and down easier. Not that it was really hard but I dont like getting a work out in when machining.

After looking at the your photos it looks like you would also be able to attach the pulley to the horizontal bar. This wouldnt add anymore accuracy but it would look cleaner when you where done. If I understand you right you want to add the pulley to the threaded section of the acme screw?

[chevdrgtrk]

hey guys, if you are going to drive the knee.....go ahead and replace the two set screws on the pinion, you can pick them up from ACE hardware for next to nothing, and it will save you a headache later. I tightened and retightened them 4-5 times just to do 1 part, and they were loose again. replaced them, not a problem since.