[cbass]

Hi everyone,

I'm close to purchasing a knee type mill retrofitted with a 3 axis controller (see pic).

I like the rigidity that knee mill will give me over a gantry style router. However, I don't like the fact that the z axis travel is limited to quil movement. I may want to run a higher speed spindle, a 3rd party tangetial knife, or just experiment with other types of roatry tools. Having a z stage that moves up a down would facilitate this.

What I'm think of doing is removing the mill head where it bolts to the ram. Then making a z stage to bolt in its place. This will give me options for more that the standard 5-6" quill travel provided the stage is rigid enough. Has anyone seen this done? Do you have any cautions against this?

TIA,
Carlo

[cbass]

Bump

[HuFlungDung]

While you could do that, you would then not be making a lot of use of the knee slide, nor the quill, which both happen to be there already, and you paid for it.

A VMC bedmill is pretty much what you have described, as these have no knee.

So if you are determined to go ahead, I think you might be better off to make a nice counterbalance system for the weight of the knee, and use a servo drive to power it up and down.

[cbass]

So if you are determined to go ahead, I think you might be better off to make a nice counterbalance system for the weight of the knee, and use a servo drive to power it up and down.

Thanks Hu,

I had thought about a bed mill. However, cnc equipped bed mills seem much less common on ebay than knee mills. Affordable ones at any rate. I had considered powering the up/down travel of the bedmill's head as my z axis. But I question how reliable its movement is. It would probably require ballscrew upgrades, and possibly linear slides. Quite a retrofit in itself.

While you could do that, you would then not be making a lot of use of the knee slide... a VMC bedmill is pretty much what you have described, as these have no knee...

The only alteration I am thinking of would involve replacing the head with a seperate z stage. I wouldn't be altering the knee. It would still be there if I needed it.

So if you are determined to go ahead, I think you might be better off to make a nice counterbalance system for the weight of the knee, and use a servo drive to power it up and down.

I hadn't fully considered powering the knee. Would its movement be reliable/precise enough to be used as my Z travel?

[cbass]

Thanks Hu,

I had thought about a bed mill. I had considered powering the up/down travel of a bedmill's head as my z axis. That would give me a stage on which to mount a spindle. But I question how reliable its movement would be. It would probably require ballscrew upgrades, and possibly linear slides. Quite a retrofit in itself. Even if I could land a bedmill with a decent controller, cnc equipped bed mills seem much less common on ebay than knee mills. Affordable ones at any rate.

While you could do that, you would then not be making a lot of use of the knee slide... a VMC bedmill is pretty much what you have described, as these have no knee...

The only alteration I am thinking of would involve replacing the head with a seperate z stage. I wouldn't be altering the knee. It would still be there if I needed it.

So if you are determined to go ahead, I think you might be better off to make a nice counterbalance system for the weight of the knee, and use a servo drive to power it up and down.

I hadn't fully considered powering the knee. Would its movement be reliable/precise enough to be used as my Z travel?

[loves80z]

I think HuFlungDung has a great idea. I think the acme screw is accurate an I bet there would be no backlash because of the weight of the knee. My guess is the biggest advantage to the ball screw would be in less friction for the heavy load.
check out http://www.nookindustries.com/acme/AcmeCatalog.cfm
I'm trying to decide what to do myself, but I don't think I need the travel U are after.

Kevin

Kevin

[HuFlungDung]

For what it is worth, before I got a Haas VF3, I did quite a bit of machining with a knee mill, and quill travel of 5 inches. However, 5 inches is tight, and 6 inches is a whole lot better.

You have to plan your tools, start point and initial height so that you do not have to alter the knee height during the program, or to do tool changes. This may involve getting some special extended holders to make the short tools project farther from the spindle, in order to make the whole tool set for a given program, within two or three inches length of one another.

Obviously, the type of work I was doing did not have a huge Z range to try to accomodate.

Even with a powered knee, I still think I might reserve using it for a W axis (parallel to Z). If the program required a whole lot of Z range, then program in a W move to change the knee height once in a great while, but let the Z axis quill do most of the motion, such as for drilling cycles, etc.

Obviously, you would have to run the knee with the gibs unclamped, if that were your sole Z axis. I do not know in the long run, how much nor how quickly you would wear out the knee slide. By comparison, the round quill has a lot more bearing area, with a fairly light load, than the milling machine knee slide has. An unclamped knee, unless perfectly balanced on the screw, will tend to wear most heavily at either end of the slide, because the necessary clearance on the dovetail allows it.

[cbass]

Thanks for your input,

Someone else started a thread about powering the knee. There's even a few companies out there that sell new mills with this feature (Elrod comes to mind). However, I think I'm going to keep it as simple as possible and go for a z stage. Heck, I might even get away with bolting one to the side of the head instead of replacing the head altogether. This would be cool because it would allow me to continue using the quill for steel milling at slower speeds.

I just found out that Miltronics attaches their head/quill to a powered z stage. The stage gives just over 5" of travel. The quill itself is manual. Maybe this isn't such a far fetched idea

One question I still have is this: if I do go with a z stage, its going to be steel and consequently will have some weight to it. Assuming I use ballscrews to feed this stage, what prevents the stage from slowly dropping when power to the machine is off? Ballscrews are very efficient, and with a hundred pounds of weight on the stage, I can imagine it wanting to drift downwards. I know some router manufacturers use servos with brakes built in so they can't turn when power is cut off. Would you go this route? Is this even going to be a problem?


Carlo

[loves80z]

I think U get a break on the motor that does the Z so it doesn't drop in a lost power situation.

[cbass]

I think U get a break on the motor that does the Z so it doesn't drop in a lost power situation.

This brake that you are refering to... is it an option that can be added to an existing motor, or is it built in? Also, how is the brake actuated? Is there something in the g-code, or is it mechanically initiated?

[loves80z]

The breaks I've seen have been sold with the motor and are sprung so when they are powered they release the break.12v, 24v, AC, DC. Any power loss to the break and the springs cause the breaking action.

As far as the setup controls wise....not sure.

I think U can retrofit a break but I'm not sure. Most likely will depend on the motor. Someone with more experience can help U with that better than me on that.

[cbass]

Thanks for the info loves80z,

I think I'll try calling centroid/ajax or some other retrofit company that sells servos. I imagine this sort of application isn't that uncommon.

I'll post back when I learn more.

Carlo