Confused about G18, G19 Arcs

[Jim Dawson]

I ran into something I don't understand.
The following code snippet is the lead in for a cut. The lead in is a G18 (XZ plane), with a G3 arc move. A G3 is a counterclockwise arc, but when standing at the front of the machine the toolpath scribes a clockwise arc. CamBam agrees with Fusion on the direction so I assume that the G code is correct and follows standard conventions. When standing in front of the machine the tool path is a ''J'' with the tool starting at the top and following the ''J'' down which looks like a clockwise arc from the operator perspective.

In the XY plane arc direction is as expected when looking down on the part.

N4665 G17 (XY PLANE)
N4670 Z-0.175
N4675 G18 G3 X3.1825 Z-0.2 I-0.025 K0 (CHANGE TO XZ PLANE, AND SCRIBE A COUNTERCLOCKWISE ARC)
N4680 G1 X3.1575 F24
N4685 G17 G3 X3.1325 Y-1.0065 I0 J-0.025 (XY PLANE)

Here is an image of the tool path, Z is towards the top of the screen, X+ is to the right. The little yellow ''< '' is the start point.



Help me get my head around this. I guess the real question is: From the operator perspective is the direction backwards? Should I be looking at this from the machine perspective, in other words standing at the machine column?

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[mactec54]

I ran into something I don't understand.
The following code snippet is the lead in for a cut. The lead in is a G18 (XZ plane), with a G3 arc move. A G3 is a counterclockwise arc, but when standing at the front of the machine the toolpath scribes a clockwise arc. CamBam agrees with Fusion on the direction so I assume that the G code is correct and follows standard conventions. When standing in front of the machine the tool path is a reversed ''J'' with the tool starting at the top and following the ''J'' down which looks like a clockwise arc from the operator perspective.

In the XY plane arc direction is as expected when looking down on the part.

N4665 G17 (XY PLANE)
N4670 Z-0.175
N4675 G18 G3 X3.1825 Z-0.2 I-0.025 K0 (CHANGE TO XZ PLANE, AND SCRIBE A COUNTERCLOCKWISE ARC)
N4680 G1 X3.1575 F24
N4685 G17 G3 X3.1325 Y-1.0065 I0 J-0.025 (XY PLANE)

Here is an image of the tool path, Z is towards the top of the screen, X+ is to the right. The little yellow ''< '' is the start point.



Help me get my head around this. I guess the real question is: From the operator perspective is the direction backwards? Should I be looking at this from the machine perspective, in other words standing at the machine column?

Don't under stand what you are trying to do, with using G18, if you are just cutting an arc G17 X Y plane then you would not use a G18, if your work plane is using a G17 then your arc on arc off you would be using a G17 may be these snips will help you figure out what you want to do, you can not arc on arc off with a G18 or a G19 if your work plane is X and Y G17

[ger21]

Make a fist, with your thumb pointing up. Point your thumb in the Y+ direction. Your fingers will be pointing in the direction of positive rotation.

[Jim Dawson]

Don't under stand what you are trying to do, with using G18, if you are just cutting an arc G17 X Y plane then you would not use a G18, if your work plane is using a G17 then your arc on arc off you would be using a G17 may be these snips will help you figure out what you want to do, you can not arc on arc off with a G18 or a G19 if your work plane is X and Y G17

It is not what I am trying to do, it is what Fusion 360 generated. Your images reflect what I would expect when looking at the machine from the operator perspective. But the actual tool path when viewed in Fusion or CamBam is as shown in my image above which is a G18 move that would look to me like a G2 move, not a G3 as generated. Thus my confusion.

[mactec54]

It is not what I am trying to do, it is what Fusion 360 generated. Your images reflect what I would expect when looking at the machine from the operator perspective. But the actual tool path when viewed in Fusion or CamBam is as shown in my image above which is a G18 move that would look to me like a G2 move, not a G3 as generated. Thus my confusion.

What kind of shape are you trying to make, the cam is doing what you are asking it to do

[Jim Dawson]

That move is the lead in to a straight cut. What the CAM was asked to do is lead in with adaptive clearing. This is the way Fusion 360 chose to do it. Having said that, in this case it really doesn't need that type of lead in. I'm just trying to get my head around exactly what it did and how that relates to the machine coordinate system. The machine follows the tool path correctly, but I'm just trying to personally understand it all because to me it looks like a G2 move rather than a G3.

[mactec54]

That move is the lead in to a straight cut. What the CAM was asked to do is lead in with adaptive clearing. This is the way Fusion 360 chose to do it. Having said that, in this case it really doesn't need that type of lead in. I'm just trying to get my head around exactly what it did and how that relates to the machine coordinate system. The machine follows the tool path correctly, but I'm just trying to personally understand it all because to me it looks like a G2 move rather than a G3.

Just as I said in the first post, there is no confusion, you just can't do a straight lead in/out with a G18 this could never work, it can only be done with the plane you are working on, which is G17 X Y plane

[Jim Dawson]

Just as I said in the first post, there is no confusion, you just can't do a straight lead in/out with a G18 this could never work, it can only be done with the plane you are working on, which is G17 X Y plane

Picture a radiused ramp lead in. A normal lead in could be a ramp where you might have both X and Y moving as well as Z moving down at the same time, so all 3 axes are in motion simultaneously. I do this all the time, even when scribing an arc in the XY plane, or call it a spiral lead in. But some of the newer CAM software will generate a ramp in, in the XZ or YZ plane.

[ger21]

Jim, did you see my post above?

[mactec54]

Picture a radiused ramp lead in. A normal lead in could be a ramp where you might have both X and Y moving as well as Z moving down at the same time, so all 3 axes are in motion simultaneously. I do this all the time, even when scribing an arc in the XY plane, or call it a spiral lead in. But some of the newer CAM software will generate a ramp in, in the XZ or YZ plane.

Something wrong with your cam software then, a ramp is normal, that most do all the time, you are using software that would not make the top 20 as far as cam packages go, both are new and most feed back is from hobby users, that's why you see this kind of thing, it also could be how you are trying to use it

[Jim Dawson]

Jim, did you see my post above?

I did Gerry, thank you.

Something wrong with your cam software then, a ramp is normal, that most do all the time, you are using software that would not make the top 20 as far as cam packages go, both are new and most feed back is from hobby users, that's why you see this kind of thing, it also could be how you are trying to use it

Well Fusion 360 is pretty popular, but it does have some weird quirks, especially in the CAM side. The CAM engine is actually HSMWorks with AutoDesk tweeks.