Meaning-2 1/2D CAM

In another inquiry folks suggested SheetCAM as a 2 1/2D CAM package. Just what does 2 1/2D mean?

Is it something that could be made on the mill by moving X, Y and Z but nothing more fancy in Z than changing the depth of cut?

Don't flame me, I'm new to CNC and trying to learn.

It sounds like you know the definition already. The only other thing I can think of for 2-1/2D is draft, or angled walls on a pocket or contour cut.

s it something that could be made on the mill by moving X, Y and Z but nothing more fancy in Z than changing the depth of cut?

Click to expand...

You got it. X&Y can do anything but Z remains constant throught the machining of a feature.

It is my understanding the 2 ½ axis means that X Y and Z-axis’s cant move simultaneously. An example of 2 ½ axis would be a drilling cycle where X and Y position and stop then Z drills. I would assume this would also be applicable to the 2 ½ D cam package.

Mark

Right, X&Y get to position before Z moves. Drilling, cutting stairsteps, etc.

OK, I guessed pretty close.
How about this: Can one move ANY two axes at once? X and Z while holding Y constant? I'm thinking that to mill a cavity one would not want to plunge the mill in (especially if it wasn't center cutting), but could do that while moving in X while increasing the depth (Z) and then once at depth move X and Y to complete the pocket. Is this feasible?
Will SheetCAM allow one to generate code to do this?

Someone may correct me, but I believe the most basic cam programs will generate code to cut a minimum of 2 axis.(milling) In your example ramping down to the Z level with a changing X is common.
Although a powerful CAM pkg is importatnt, what are the capabilities of your current machines and the PC to run it? Will they move in only 2 axis at once or will they move in 3 axis simultaneously?
If you can only move in 2 axis at once Sheetcam may be the answer for you.

Go to ONEcnc's website and see the demo. GREAT viewing.

A repeat of some, but here goes.
Simultanious movement of all three axis's does not make the machine a 'true" 3 axis unit.
The challange here is knowing the difference between linear moves and non-linear moves.
Straight Lines are linear. When X starts, it keeps the same speed till it reaches the desired spot
Arcs are non-linear, because the axis movement VARIES ie. X starts fast and then slows, while Y starts slow and finishes fast

When all 3 axis's can do (Simultaniously!)non-linear moves, as Doug pointed out, then you have a true 3D machine.

Most machines that can do 2 axis arcs with the third axis being only Linear, are known as 2 1/2 axis machines. NOTE which axis's do the arc ( like Y Z )is not relevent.

My Hurco can do Helical cutting , but only constant pitch, so it is a 2 1/2 axis machine.
I can do what some refer to as 3 D, but it really only looks like 3 D, for only 2 axis's are arcing at one time ( any 2 will work)

To determine what a machine is, watch the machine go the "tool change position"
If Z retracts first, before X and Y move. it is a 2 1/2 axis.
A true 3 axis machine will move all three axis's at the same time ( which can mean "Craash") if you have not programed around such a move

Blame the manufacturers for the bewildering array of claims

Almost forgot the question
So 2 1/2 axis CAM will design around the above facts.
3 axis CAM will not, it gives a series of moves that cannot be done on a 2 1/2 axis UNLESS it makes each move a linear move and gives microscopic positions ( ie steps of .0002). Of course this would create enormous programs when memory is measured in Megabits,

Rich,
I probably agree with your other points, but would you explain this one?

"""
To determine what a machine is, watch the machine go the "tool change position"
If Z retracts first, before X and Y move. it is a 2 1/2 axis.
"""

Somehow can't grasp the relation between true 3 axis and toolchange movement.

To determine what a machine is, watch the machine go the "tool change position"
If Z retracts first, before X and Y move. it is a 2 1/2 axis.

Click to expand...

I was confused by that also,maybe means if you execute g28X0Y0Z0 on a 2 1/2 axis machine the Z will rapid home first,then the X & Y will rapid home together.On a true 3 axis X,Y & Z will rapid all at once.

Trupo That is correct !

The machine code on 2 1/2 axis machines requires the Z to retract first. So you get one axis , then 2 axis (X & Y )movements for a canned cyle like tool change or home on some machines.
This occurs, even though you can get 3 similtanious axis movements at other times.

On true 3 axis where all moves can be non linear , the machine code (software) allows full 3 axis simutanious movement ( But Linear to home) and this can cause crashes into the part if the programmer is say at the bottom of a pocket and decides to "go home" for a reason. The software also is trying to save time by going directly to the desired position.
In short, the programmer has to be on his toes.

3 axis rapid interpolation does not, IMO, determine whether a machine is a true 3 axis machine. On some machines, you can turn this option on or off in parameters, so that certainly does not mean you have changed the capability of the machine.

Machines that move at maximum rapid speeds will drive the machine in a series of straight/45 degree angles, as the axis with the longest distance to travel will move by itself until it reaches a position where the remaining movement is the same as one or more other axis, at which time both axis will move together (the 45 degree move). 3 axis interpolated rapids do give a programmer a better concept of where the tool moves, whereas the 'max rapid' concept, with its attendant dogleg moves, can sometimes surprise you, even in 2 axis mode, as you drill bolt circles in cramped quarters, where clamps can unexpectedly get hit during a dogleg rapid move.

3 axis linear interpolation is the definition of a 3 axis machine, IMO. Way back in the beginning of cnc, some controls could not do this, but I don't think this is a major concern with most controls made within the last 10 or 15 years.

I have never heard of a machine yet that could cut arcs in any but one of the three standard planes at a time (unless it is a 5 axis machine), but the majority of 3 axis CAM software generates gcode that attempts to simulate 3d curves by means of short segment linear movement. This software commands a higher price, "just because" its not as simplistic as top down 2 axis paths.

I suspect that 2 1/2D means only what the CAD/CAM designer meant it to be. I worked for a firm that had TekSoft with 2 1/2D. If you drew a profile in X & Y, and included a Z wall definition over to the side, it would pocket out that profile according to that definition. It was not restricted to simple angular drafts...kinda a cheapo 3D for infrequent users with modest needs.

Hu

"""
I have never heard of a machine yet that could cut arcs in any but one of the three standard planes at a time
"""

I believe that is pretty much a must on modern CNC ram EDM-s. Have not seen any mach. centers do that though.

As far as the homing or tool change position, I'm pretty sure that the sequence is programmed to be in a particular order that can be changed on some machines, and can't think of any reason how that would translate as a determining factor for 2.5 or 3 axis machine. My Wire EDM is for sure simultaneous 5+1 axis, and it certainly homes out 2 axis at a time, Z first, X-Y second, U-V third and B fourth.