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View Full Version : Z Axis mounting opinions please



cncjunky2011
05-23-2012, 08:10 AM
Hi,

I have been browsing some of the commercial manufacturers sites for inspiration and recently noticed a picture of a Z Axis on a camaster cobra machine.

The image attached shows the ballscrew is connected to the motor and no top block support being present (like the Chinese BK). I dont have a photo of the bottom but have seen they use a pillow block bearing to support it.

Thinking about it I see no reason why this wouldnt work, and clearly it does as the camaster machines are excellent from what I have read.

My question is now, why do we use the BK blocks in the first place? Wouldnt it be cheaper and easier to just use a coupler and connect the top of the ballscrew to the motor shaft using a coupler?

Just a thought but as the linear rails keep everything stiff and aligned I see no reason why on the Z axis this wouldnt work? Any thoughts?

gio666
05-23-2012, 06:33 PM
It would not be good because motor would have to take up all the forces from plunging and all up and down cutting . Motor would burnout fast and also shafts will push back . Just like have large amounts of backlash

gio666
05-23-2012, 06:41 PM
You can make a support like this one

JerryBurks
05-23-2012, 07:17 PM
..... I dont have a photo of the bottom but have seen they use a pillow block bearing to support it.....

That is what I use on my machine. It is simple and works fine. I have an axially fixed and laterally floating lower screw bearing (pre-loaded pair of tapered rollers clamping a flat plate, no pillow block) and the stepper is attached to the top with a rigid coupler. That way the bearing is self-adjusting its position. For a longer axis you would need to allow the stepper mount a few 1/1000" axial play to compensate for screw expansion but for a short z-axis that should not be needed.

louieatienza
05-23-2012, 08:41 PM
Hi,

I have been browsing some of the commercial manufacturers sites for inspiration and recently noticed a picture of a Z Axis on a camaster cobra machine.

The image attached shows the ballscrew is connected to the motor and no top block support being present (like the Chinese BK). I dont have a photo of the bottom but have seen they use a pillow block bearing to support it.

Thinking about it I see no reason why this wouldnt work, and clearly it does as the camaster machines are excellent from what I have read.

My question is now, why do we use the BK blocks in the first place? Wouldnt it be cheaper and easier to just use a coupler and connect the top of the ballscrew to the motor shaft using a coupler?

Just a thought but as the linear rails keep everything stiff and aligned I see no reason why on the Z axis this wouldnt work? Any thoughts?

The stepper bearings are not designed to take axial loads. Also the use of a fixed end bearing allows for use of a flexible coupler to take out any misalignment. The stepper bearings would absorb the forces when plunging, and there's no gurarantee that there is no backlash in those bearings either.

ger21
05-23-2012, 09:11 PM
You'd have to assume that there's a fixed bottom bearing holding the screw in position.
If that's the case, most of the above mentioned concerns shouldn't be an issue.

Typically, when only one end of the screw is supported, it's the top end, with the bottom being free. I've seen a lot of machines like that, from a $150K router, to a machine built by XZero.

cncjunky2011
05-24-2012, 03:05 AM
You'd have to assume that there's a fixed bottom bearing holding the screw in position.
If that's the case, most of the above mentioned concerns shouldn't be an issue.

Typically, when only one end of the screw is supported, it's the top end, with the bottom being free. I've seen a lot of machines like that, from a $150K router, to a machine built by XZero.

Yes the bottom is supported with whay seems to be a pillow block bearing from what i have seen. Understandably everything needs to be well aligned even with out the bottom bearing but I see this as an easier and cheaper solution than placing a BK and BF block on the screw which now seems like overkill for a small Z axis, a longer Y and X I can of course understand why they would be needed.

JerryBurks
05-24-2012, 10:59 AM
As mentioned in an earlier post, I use the fixed coupler to the stepper and think it is a good solution, but here is a warning: this works only if the fixed coupler aligns the screw and stepper shaft very precisely. I noticed this is not trivial.

If the alignment is a bit off , the stepper will want to wobble. If the wobble is too big and the stepper mount is very rigid, the deformation has to be taken up by the screw or the stepper shaft which might lead to fatigue or bearing wear.

I had to re-do some of my couplers to get it acceptable. Actually, one of my x-steppers still wobbles visibly but the motor mount allows for that.

louieatienza
05-24-2012, 07:36 PM
To be fair, the Cobra uses Teknic servos with heavy duty oversized bearings. It's hard to tell if the bearing in the pillow block has a setscrew or clamp for the Z axis screw. The carriage is pretty sizeable so they must feel the bearings in the servo are sufficient.

I guess then it would depend on yoru application - if your drive motor has bearings sufficent to carry the weight of yoru carriage and thrust loads...

ger21
05-24-2012, 09:41 PM
The drive motor's not carrying any weight.

As I said, my preference would be to support the screw at the top, and let it hang free at the bottom. You'll have less stress on the motor shaft , as the alignment should be much better.