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Thread: 2 -1/2 Axis Milling
08-10-2004, 07:22 PM #1pk_davidson Guest
What exactly is meant by 2 -1/2 Axis Milling.
How does it differ from 3 axis Milling ?
Specifically, when someone talks about 2 1/2 axis profiling, 2 1/2 facing, etc... just what are they referring to.
I thought I understood, but the Visual Mill software that I have just makes no sense at all when it creates tool paths in these modes.
08-11-2004, 04:51 AM #2
2 ½ axis normally refers to a machine were one axis is “Dumb” By that I mean it’s not under CNC control. It normally refers to Machines with an X – Y table to position the work, but the third axis, normally the Z has a different drive mechanism.
Examples are, drill / tapping machines were you have a position table X – Y (2 axis) and the third axis has a conventional feed that just cuts to a depth stop or limit switch.
Old Burgmasters and turret drills and their early retrofits used this principle.
These can be automated, but the third axis might just be an output for a coil, clutch or solenoid to advance and retract that axis.
The home / hobby market on things like Bridgeport’s, were they have CNC on the saddle and table but the quill is still manual. Great for doing lots of drilling work, bolt hole patterns etc.
It’s sometimes referred to also in the laser / water jet flat sheet type machines. Where the contour is programmed in X/Y but the Z axis in not actually programmed. Due to undulation in the sheet, they have various methods to establish the height of the head.
2 ½ axis contouring might be stretching things a little. By my understanding, you don’t actually have 3 axes’ under CNC control. 2 axis for getting to co-ordinates quickly, but they will be stationary, while the third axis works.
Then of course, I could be mistaken.
08-11-2004, 07:19 AM #3
I'll take my own stab at the question, although I think Phil already covered it pretty well...
2 axis, as Phil stated, implies there is no automated 3rd axis (usually Z). That is pretty much a no-brainer. 2.5 is a bit trickier. It means that you can't simultaneously move 3 axis at once but that all 3 are capable of moving under computer control. For instance, the machine moves to a certain X/Y coordinate, then the Z pecks down to a certain level, then the X/Y starts again. That would do a pocket, for instance.
You should be aware, however, that some machines allow you to change the two "active" axis. For instance, you might want to have X and Z active instead of the traditional X and Y. Plus, if you want to do a ton of programming, you can always "slice" your 3 dimensional parts along the Z plane and thereby get a poor man's 3 dimensional contour.
3 axis is also a no brainer... That is when all 3 axis are running at the same time. To be productive, however, I think you pretty much have to use a ball endmill.
Now, if you really want to twerk your mind, start thinking of 4 or 5 axis at the same time. Or better yet, a hexapod machine. Yikes! That is some pretty cool stuff though. Never seen one running though (seen pictures); probably make you start to think that iRobot will occur in about 10 years!
08-11-2004, 07:20 AM #4
08-11-2004, 08:44 AM #5
"What exactly is meant by 2 -1/2 Axis Milling.
How does it differ from 3 axis Milling ?"
These terms mean only what the speaker wants them to mean. They have no standardized meaning in the industry.
08-11-2004, 09:47 AM #6
The cam software that I use is advertised as 2 1/2 axis. While my machine is capable of simultainous 3 axis motion the cam system is more limited. The 2 1/2 axis software is not capable of generating toolpaths for molds or complex 3D shapes with blended surfaces. It can do simple helical pathways and some ramped contours but it is limited to working at one Z level at a time. I suspect that this is the most common meaning of the term but as Doug mentioned it can mean different things to different vendors.
08-11-2004, 10:14 AM #7
With all due respect, I beg to differ. Surely if a machine is only capable of 2 ½ axis control. That would be unanimously accepted that it is not capable of controlling 3 axes simultaneously. X, Y & Z at the same time, as in helical milling, thread milling, pocket milling etc.
Regardless of that era of machine having 3 physical axes’. It only has control of 2 of them at once. Why else would you down rate the machine classification? That’s the only reason cad programs, control & machine manufacturers, make the definition.
Same thing happened latter in the late 70’s / early 80’s when controls got to the stage that they could control 3 linear axes’ and throw in a rotary table. The early ones that had to unclamp, lift, index, clamp were considered 3 ½ axis as the rotary wasn’t under CNC control simultaneously.
08-11-2004, 10:24 AM #8
True enough. Maybe were on two different trains of thought here. P.K mentioned 2 ½ axis as in a physical milling. But then mentioned Visual Software. Now that I’ve re-read it, he could be having either problem. Software, DNC - Cad packages are spinning in a whole different orbit. They use the ½ like a marketing thing, where as on a machine it’s a limitation, but in software it’s an alleged plus. It normally means that it lacks the number crunching power to generate programs in true 3D.
If that’s the case, I stand corrected.
08-11-2004, 12:26 PM #9
We have to diffferentiate between the number of axes of the machine and the milling capability of that machine.
I have a machine, purchased new in 1990, that was advertised as having 2-1/2 axis capability, it has three axes,X,Y,Z, under CNC control. It'll move all three axes simultaneously with a linear move. It'll do circular interpolation using any two axes with the third axis not moving.
I asked the sales people why the machine was not listed as capable of 3 axis milling. Their answer was, 3 axis milling means doing circular interpolation using all three axes which that machine will not do.
Another mill of mine has the same three CNC axes, X,Y,Z. This machine will do circular interpolation using all three axes, but in a kind of round about way. The circular move has to be programmed in one of the planes, XY, XZ, YZ, etc, then an axis rotation can be done around one axis to get into position for use of all three axes in the circular move. I consider this machine to be 2-1/2 axis capable by the original definition I learned back in 1990 since it's still limited to three axis circular interpolation in limited orientations.
I consider a machine having 3 axis milling capability to be one that the circular move could be programmed in one block like, G0n Xn Yn Zn In Jn Kn.
That's just the way I learned it. Obviously from reading ads for machines and software and other opinions we all don't agree on these things though. So when someone uses these terms to describe software or machines I think it's best to ask them exactly what they mean.
08-11-2004, 11:29 PM #10
Will run 2 axis in circular interpolation and the 3rd axis in linear intepolation. Usually the axis can be designated with a G code command. I've seen tons of mold cavities, some very complex run on old Bridgeport 2 1/2 axis machines. The cad programs are really long because it's usually outputting a point to point coordinate (X,Y and Z). It works well.The newer the machine and the faster the processor in the control and the more responsive the drives are will have a big impact on how long a job takes to run through the machine. An old Boss 6 can be drip feed the program, but do you really want to wait that long? For work like that a newer machine is the way to go.
08-12-2004, 12:23 AM #11pk_davidson Guest
This forum is great.
I find that everytime I have
a confusion, it turns out that
generally the industry doesn't
exactly have a standard.
I finally found a tutorial that explained
why my CAM software was acting so strange.
Had to actually draw tool paths for 2 1/2 axis motion (and it makes huge files) while the 3 Axis stuff works off the part geometry and is generally a 1/3 the size. Go figure...
thnx to all, very helpful.
08-12-2004, 06:40 AM #12
The tutorial that you found. Was it on the Net someplace? I for one would like to take a refresher. Maybe the reason that there is no definitive definition of this is because it’s still evolving. Controls, machines, software have evolved so fast in the past 30 years.
Lots of varied opinions here in this thread, all of them valid. Mostly dependant on the era of the machine or control or software.
“I consider a machine having 3 axis milling capability to be one that the circular move could be programmed in one block like, G0n Xn Yn Zn In Jn Kn.”
Doug’s statement on what is currently considered to be true 3 axes seems to me to be about the best description of it.
In summary. 3 axes is 3 axis similtantiously. 2 ½ is 2 axis at the one time, with a third axis that is some what limited.
I’m guessing that old late 60’s / 70’s machines. Pin board and tape read, that could only move point to point in one axis at a time, could have been called “1” axis.
That’s what I like about this forum. Over some of the others I frequent. Multiple opinions, most of them valid, but from different experience, viewpoints, but it never de-generates into a shit fight. For that you should be congratulated.
08-12-2004, 10:09 AM #13
Terminology can be a pain in the butt...My Fadal is 3 axis where I can helical mill with one line of code..But I have another machine that says its 3 axis, but only in linear..its only 2 axis in helical milling...In order for me to thread mill I have to circular mill in arc segments then move z up or down, this involves a "repeat" statement..it usually chatters a bit..
08-12-2004, 05:51 PM #14pk_davidson Guest
No sorry, the tutorial was buried in an
online tutorial for the program. It was buried in a subdirectory I just happened to find. Go figure.
08-15-2004, 01:00 AM #15
The definition requires that you understand simultanious motion,linear motion, and non-linear moves.
A mill can have 5 axis's, but if only three can operate at the same time , it has a "3 axis similtanious servo drive"
A 2 axis machine, can have a manual 3 rd axis, or a powered 3rd.axis..it just will not move (x,y) when the 3 rd axis is moving.
A 2 1/2 axis Machine has any 2 axis's with non-linear control, and a third with linear only ( Some machines can swap this between axis's..like x,z with y as linear)
A true 3 axis machine can make non-linear moves in all 3 axis's at the same time.
Cutting a helical thread requires 2 axis's for the round part. If the thread is constant pitch, the 3 rd axis is linear and you really only need a 2 1/2 axis machine.
IF THE PITCH VARIES (!), it takes a full 3 axis machine to cut it. think of an Auger with course to fine thread and you get the idea.
The whole area has been confused by machinery Salesman..WOW I wonder why ?
A quick check is when sending the machine "home"...a full 3 axis machine goes in a straight line to home(x,y,z)(Crashing the clamps!)
A 2 1/2 axis will retract the spindle FIRST, then move to home
08-16-2004, 05:34 AM #16
Thanks, Rich, for the excellent description. I obviously was wrong in my initial description. I always assumed that my machine was 2.5 axis, but you are showing it as a 2 axis with 3 servos. Makes sense. I can only run 2 at a time... linear or not.
Anyhow, thanks again... You have some great posts here.
08-16-2004, 05:47 PM #17
Okay there is room for everyone's understanding of what 2.5D means...
Here we have a whiz-bang "2D" CNC on a bridgeport clone. The Mfg claims 2.5D, what that really means is the 'puter moves X&Y, and flashes a message for the operator to
It works great. If you don't need to move Z!
08-16-2004, 08:20 PM #18
Just like I said, it means whatever the Mfg says it means.........