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CNC 101 question

noahjlb

Cast Iron
Joined
Aug 19, 2002
Location
West Linn, Oregon
For those of you who have had hands on experience with cnc mills please advise: I have been told that a CNC knee mill is better for a low volume shop ie. one-offs, prototyping, than a CNC bed mill because of the knee mills greater flexibility (ie. ram that extends beyond table etc.)...but what about the Z axis?

When you program a CNC knee mill are you limited to the length of the quill in your machining programs or can you use the powerfeed on the knee to give you the z axis depth that you want in the program ?

Concrete example: during the course of cnc milling you want to drill a 7" deep hole thru an aluminum block. On a CNC bed mill with a 20" z axis you just program it in and end of story. With the CNC knee mill and say a 5" quill would you program the machine to drill a hole 5" in depth followed by withdrawal of the bit from the hole then a stop in the program so that the knee can be raised to drill the additional 2"...or can you program the powerfeed of the knee mill to raise automatically ?
 
no, the price is not higher. A powered knee usually adds only about $400 to the cost of a new mill. Is there anyone out there who can answer my question ? this should be an easy question for someone who has used both types of CNC mills. for that matter, it would be an easy question for someone who uses a CNC knee mill exclusively.
 
Hi,
Take what I have to say with a grain of salt because:
1) I'm no expert
2) My experience is limited to a 1980's vintage Hurco mill

I know of no older make of knee mill that has *both* a powered knee *and* a powered quill that are simultaneously under computer control (servo or stepper). That spans pretty much the Bridgeport line and its clones. From a logical perspective too such an arrangement just doesn't make sense. Having a powered knee for manual adjustments certainly does, but not under computer control (assuming the quill is already so equiped).

Assuming you are not doing the operation you are talking about often, however, it might be possible for you to put a wait in your program so that you could manually adjust the knee. I don't think this is where you want to go though. If you really have that much of a need for a huge Z travel then I think you would be best off either getting a bed mill or a CNC with a powered knee exclusively (I think I'd opt for the bed mill).

Alan
 
most 3 axis cnc knee mills have a power quill. running a block like you just described would not be possible without operator intervention. By the time you do that, a bed mill would have been done. Bed mills are more rigid and can have a toolchanger. I'd be going bed mill or enclosed machining center. The programming is more important than the machine, get intimate with your cam vendors.
 
thanks everyone for the input. i already suspected that full z axis programming would be found only with bed mills and that with knee mills you would have to put a stop in the program and manually adjust the knee for the full z axis range of depth ..just wanted to have that confirmed.
 
How often do you do 6" or greater holes?
I have both, 16x33 cnc knee mill and 20x33 bed mill, and can only say this.
If I have short run projects, I"d rather be in front of my knee mill any day.
I believe there are manufactures that produce bed type machines with programmable (z)axis including manual quill.

eddiez
 
Been there, done that !
On my Hurco with 5 inch travel,I go the 4 inches ( so I don't hit the axis limit switch !) and "position" 3inches higher, then raise the knee 3 inches, and "resume program" with another 3 inch drill cycle.
Now here is the problem....
Getting rid of chips...because you cannot fully retract out of the hole, and
Not moving to the next hole...till you drop the knee...spelled "crash" for those with multiple holes.

At work, we threw deep holes on our horizontals..piece of cake...did 48 inch gun drilled holes there as well

I would love to have a cnc bed mill at home also, but would raise it up 12 inches so I didn't get a back ache every time I ran it.
My hurco is raised 5 inches

BTW powered knees are OK for knee mills, but what is far more useful is a DRO on the knee so you can do all your drill "offsets" with ease...too hard counting turns
 
Very useful info guys!

first to Rich Carlstedt..good points.

I understand now thanks to your clear description how you do holes deeper than quill depth with a knee mill and it did not occur to me that you have to remember to lower the knee back down to go to the next hole. Boy, i shudder to think how hard a lesson that would be. Although you should be able to see this comming by looking at the drill bit and the workpiece, it would be just like me not to notice before it was too late.

The chips issue though is a real can of worms. I have been told by some machinists that in peck drilling you don't have to come all the way out of the hole to get good chip clearance just back up a little, others say, no you have to come all the way out. Don't know what the truth is on that one.

The back issue is something i hadn't even thought of but you are right. I did have to bend over to futz around with the workpiece on the bed mills i looked at but on knee mills i could raise the knee and keep my back straighter.

As for the DRO on the motorized knee, sounds like that would be a good idea..don't know where you would mount the bracket for the DRO but i'm sure someone has been there and done that.

In answer to EddieZ:

Appreciate your experience and opinion. you did not say why you think the knee mill works out better for one-offs and prototyping. My reasons for thinking a knee mill is more versatile are:
1) Some have a vertical head that rotates in the vertical plane, less rigidity, but then you don't have to build an angle plate fixture to machine a workpiece at an angle.
2) The ram on a knee mill allows you to extend the milling head well beyond the table for workpieces that just won't fit in the milling envelope of the bed mill.

And you are right i know of at least one manufacturer who sells a bed mill with a manual quill for those who want to switch to the feel of manual downfeed.
 
noahjlb,
The reasons you gave as in being able to move the ram forward and back lift and right on the base are exactly the reasons for my choice.
My machine however does have a rigid head and not movable to any angle. This is what I wanted when I ordered it. But the biggest reason of all, is that it is not the back breaking effect you get when you have to lean over a table all day long inspite of the fact that you can bring the y axis to you within the machine limits, but you still have to lean in when indicating project from a spindle thats 20-25" inside the cabinet.

eddiez
 








 
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