RF45 Clone CNC Z axis question

[Ellis76]

I'm in the process of gathering information to CNC my RF45 clone mill.

I have a question about the Z axis, once it has been changed to ballscrews and servo drive, etc. When there is no power to the Z axis servo motor will the head fall? If it doesn't does this put stain of the ballscrews? Thanks

[LeeWay]

I'm pretty sure it would fall without power. The head is pretty heavy. The nuts on some ball screws will spin down sometimes under their own weight. Consider the extra few hundred pounds added. I have a similar mill in conversion now, but round column. I was considering ordering the square column for it. This particular issue was part of the equation on that decision.

There are several ways to counter the issue though. Several counterweight systems have been documented on the Zone.

[jalessi]

Ellis76,

They make a servo with a brake option, when you power up the machine it applies 24 volts to the brake and unlocks it.

When the power is off the brake is locked so the axis wont freewheel.

http://www.isa.org/Template.cfm?Sect...ContentID=9489

Jeff...

[Ellis76]

Thanks guys. I'm guessing most of the machining will be done in the X and Y direction and the Z axis will be held in position by the power of the servo. Is it better to have a brake applied when the Z axis is not moving to save current draw through the servo z axis motor? Would this be quite high(the current) just holding the head there?
Or am I just making this more complicated than it needs to be and people typically park the head @ the lowest position when the machine is off and let the Z axis servo just take care of it during production? And some would add a brake just in case of a power failure? Thanks

[jalessi]

Ellis76,

Conserving power may be good reason to have a brake however it is usually not implemented for that reason.

If you where working on a part and decided to pause the operation and turn off the machine a brake is a good idea because you would not loose your Z zero or Z home position.

The same scenario would apply during a eStop.

Jeff...

[devincox]

I just had this same question as I am putting precision ground ballscrews on my IH mill. I was advised it wouldnt need a brake. I tested it last night and it definately wont need a brake unless your gib is all worn out (and only maybe then). Even disconnected from the ballscrew the head barely moves and thats with me forcing it and my gib oiled up and completely loose.

Devin

[jalessi]

Devin,

On a RF45 style machine the Z axis dropping may never be a issue because of the design of the ways.

On a boxed way or linear rail machine it would be a big issue.

Jeff...

[pete from TN]

MY Lathemaster cnc mill has a Z axis ballscrew setup and it definitely does fall when power is not applied. I have also lightened the head considerably with a belt drive conversion. It can fall but it is kinda slow and if you let it, will pick up speed and could damage the head. I am right now running my gibs pretty tight on the Z and I just gotta remember when I shut off the machine to tighten the gib locks a touch before I throw the switch. There are a lot of people who use an electric brake which is energized to disengage when the power is on so the head can move but when the power is off the brake engages locking the head in whatever position it is in. I am running a 3-1 ratio on the Z and I understand the IH uses a 4-1 ratio so this may be why it does not fall as easy? not really sure. If your gibs are smooth and in good condition and adjusted properly they should really not hold it up too well so I am not sure of that comment. Even if it did hold it I would be wary of trusting the simple sticktion of the gibs and ways to hold it up for overnight. It might actually slowly fall... I intend to install an electronic brake as others have done eventually but I am still working on the enclosure and other items right now. This is the best option I think wether or not your head stays put.... JMHO....peace

[LeeWay]

I agree that if the dovetails are in proper form, they should not allow the head to stay in a raised position. I suppose if the ball screw is higher pitch count and geared up pretty well, then I can see it.

Regardless though, something should be used to prevent accidental drop. The brake sounds good. I'm sure that's how high end machines do it. There is however an added benefit to using a counter weight. That is that you won't need near the gearing or motor size to gain respectable movements. The motor would be assisted in raising the head. Lowering already uses gravity.

Just another thing to consider.

[pete from TN]

But now that I have my Z working without a counterweight I do not currently feel it is necessary. I can get the Z moving at near 150IPM and it changes direction very quickly. Of course I am using some VERY stout 1125 oz inch motors at the max the Geckos will run them.... A counterweigt system is nice and will definitely help but you also have to consider that even tho the weight is now even or close to it you now have to deal with the inertia of that whole mass changing direction... Just a thought....

In reality, barring some serious use of 3d profiling at very high speeds the head never really has to move that quickly. Nor does the table for that matter, I can get rapids of ludicrous speed and I have backed it way down because I am prone to fat fingering stuff on the computer so I gotta be careful, however yesterday night I ran a little engraving program which can and did run at a nice feedrate and even that was nowhere near what it is capable of.... When I was designing this system I was very concerned with max rapid speed and acceleration but in real use at least for me right now and I am sure well into the future I have my max rapids set at 100ipm and even that is probably overkill.... Let's face it the table only travels like 30 inches either way so what do you need stupid fast rapids for? The Z axis is even less important to me, right now I have my toolchange position set 5" above the work zero and it gets there in a hurry.... Maybe if this was a large router table I would worry about it but on this relatively small machine it is PLENTY fast....

The Z axis head is Real heavy especially with the gears and original motor still on there and as one poster said a ballnut will just roll off a vertical ballscrew. If your gibs are so tight that the head does not fall without the screw attatched to the millhead then your gibs are indeed TIGHT!!! If the ballscrew was not there with it's associated pulleys and belt drive atop I am quite sure my millhead would come crashing down onto the table with some serious force.... The gibs would have to be quite near a lock tightness for it to hold up there.... I never weighed the millhead but I am no small fellow and without the cast iron framed motor on there and not including the Z axis slide I carried the millhead across the shop one time and let me tell ya I could not wait to put it down!!!! What is that saying if you plan to lift an elephant you had better have a place to put it down ready, well that goes double for an RF45 millhead!! peace

[devincox]

Originally Posted by jalessi Devin, On a RF45 style machine the Z axis dropping may never be a issue because of the design of the ways. On a boxed way or linear rail machine it would be a big issue. Jeff...

I only refer to an rf45 in my post. Not sure why you mention the other styles, I guess I missed something in the post, but agree with you that a different style mill would definately need a brake.

[jalessi]

Devin,

The reason why I mentioned the other types of mills is because 99% of the larger mills would have big problems without a brake on the Z axis. Either that or the gibb needs to be tightly locked before shutting the machine off like Pete mentioned.

As Pete already stated if the operator forgot to lock the gibb the head may end up dropping and causing some serious damage.

Not everyone reading this post has your mill.

Sorry to be so confrontational, ask Lee he will tell you its my nature.

Jeff...