[Jay Kyle]

Just bought a Square column mill/drill that is almost converted to CNC, however I have a couple of questions about design. Here's what the mill looks like new.


1. Should the z-axis control the entire vertical head movement, or just the quill. Seems like it would be nice to have the complete range of the head under NC control. This means tossing out the head raise/lower motor and put in a large stepper (due to the weight involved), then just use the quill for any sort of quick drilling.

2. Is it better to direct drive the stepper motors or use a step-down belt system for the axis (assuming ballscrews?)

The goal is to turn this into a working 4 axis machine!

JK

[Nono]

I just bought one very similar to that from industrial Hobbies.. Very Cool I am planning on a 1200 oz for the z axis for the full head and leave the quill for manual tool offsets and quick drilling. I am planning on cutting lots of stainless and titanium with this so I will step down the x y with pulleys using 800oz steppers and ball screws from http://www.homeshopcnc.com..

[Jay Kyle]

Hey, thanks for pointing out industrialhobbies.com, there's lots of good material on that site for what we're doing.

Sounds like some pretty hefty steppers, do you really need them that strong? What does that give you?

[ChrisJ]

Don't forget the tighter you make the gibs the more power/torque you will need to move the axis's, and you may need fairly tight gibs for accurate cutting.

I bought an IH mill and their 3-axis conversion kit and I am in the process of building it back up. Very nice kit and a lot of thought went into the design. I know he is not advertising for sale individual components anymore but I would see if he would sell you at least his optical limits. You can't beat the design. Accurate and coolant proof.

If you are looking for speed you may want to consider servo motors instead of steppers.

Chris

[Nono]

I want a lot of power but that is just me.. If it was set up with the right pully system/Ratio you could put 200 or 300 oz motors but I look at it as why make your machine slow in the rapid traverse.. the table isn't that big and I will mostly be using hss/cobolt cutters anyway. I have decided on using steppers instead of servos because the encoders are built into them. Finding motors is easy,, Ebaythe word stepper and then servo and you will see what I mean.

[Jay Kyle]

Ok now I've got the bug to go to larger steppers and direct drive them. I see that cncmasters uses 1200 oz direct drive steppers for all axis. I wonder how it's working for them.

When I start looking at the torque curves for the NEMA 34 steppers, it seems that when comparing a ~600oz stepped down 2:1 to a 1200 oz direct drive, there's barely any difference in torque at 1000 rpm. Those higher power motors really loose their torque fast as RPM increases compared to the lower power motors in the same frame design.

JK

[Jay Kyle]

Hello all,
I'm still working away at it!, getting techno-isel 25mm ballscrews coming in for it. The ballnuts fit into a nice square mount and the nuts are also pre-load adjustable without having to purchase a 2nd nut.

I'm getting a gas spring to counterbalance the head, 1300 oz steppers for x,y,z axis and am designing direct drive mounts for them.

I've run into a design conundrum however, I'll be using 2 angular contact bearings on the fixed (motor) end of each ballscrew. According to popular design the inner and outer races of the bearings are tightened together separately. This makes for rather large mounts... I'm wondering if I can skip this and use just the ballscrew bearing nut to draw the 2 bearings together sandwiching the bearing housing.

.. any thoughts?

Jay

[Jay Kyle]

Ok question solved from the last post, I'm going with the traditional bearing mount style. For those who are interested I'm attaching the drawings I created to build the X-Axis direct drive mount for the new ballscrew and stepper. I created these drawings in Alibre, a nice cad package but I used about 1.4 GB of virtual memory on a machine that has only 512 mb of ram, so I was in the swap file a lot. Also Alibre really needs a good video card as I couldn't get it to draw a complete helical ball threads without visual distortion of the threads (using a NVidia MX 400 video card).

Jay

[ngr1]

Hi Jay... Any updates?

[Jay Kyle]

There is, but I'v been on vacation and out of town on work so not as much as I'd like. However all parts have finally arrived as of earlier in the week - that includes the new steppers, ballscrews and nuts, bearings, seals, etc, and gas spring for counterbalance.

The ballscrews are at my friends machine shop for him to turn the ends. When I get home this weekend I'll stop by to see how he's doing.

Jay

[MikeAber]

This bearing would simplify your design and it is not necessary to put the bearing at the motor end, it could be at the other end of the ball screw.

http://VXB.com stocks these.

http://www.vxb.com/page/bearings/PRO...ONTACT/Kit6933

1 Ball Bearing 5204 ZZ 20x47x20.6 Chrome Steel - Metal Shields - Double Row Angular ContactCode:Kit6933
Price: $9.95

[DieGuy]

Another economical way to counter balnace the Z is to use a pulley and cable system to a counterweight on the back of the machine. Then the loading is adjustable by just adding weight to the counterbalance. I suspect that a much smaller motor could be used to drive the head in that case as you have gravity to help you.

As a quick second though a couple of recycled hydralic cylinders could be used with a counter weight as well.

[Jay Kyle]

I thought about the double row bearing but the bearing house talked me out of it, something to do with the angle and application. I also looked at weight counterbalance but wanted to keep it contained in the tower and I calculated that if I used lead it would still be too big to fit nicely. I looked at containt tension springs but couldn't find anything that would be strong enough and still fit in the tower. The hydraulic idea is a good one. I ended up getting a gas spring with an 8% tension change between closed and fully extended.. as low as I could find. If I had to do it again I'd set up some sort of hydraulic assist.

Jay

[ChrisJ]

Hi Jay,

Quick question I have been meaning to ask. If you mounted the ball screw, bearings, spacers, nut, etc. to your motor mount first, how do you mount the motor shaft to the ball screw unless you have an opening in your motor mount to make this connection?
Sorry just not seeing it in your posted drawings.

Chris

[Jay Kyle]

Chris,
Good question, I'll be using Oldham couplings that clamp to each shaft, thus I can clamp the ballscrew side of the coupling tight to the ballscrew, and assemble the mount. Then loosely clamp the other side of the coupling to the end of the motor shaft, fit the motor onto the mount to slide the coupling back to the correct location on the motor shaft (blind), then remove the motor from the mount, give it another .010 clearance and tighten the coupling clamp.

The drawing doesn't correctly depict the oldham couplings, rather it shows a solid as the coupler, my bad.

Jay