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FeedBack => Bug Reports => Topic started by: lloydsp on November 10, 2013, 21:32:19 pm

Title: [5] Inconsistent plunge feedrate in pockets
Post by: lloydsp on November 10, 2013, 21:32:19 pm
Doing a pocketing operation today, with a pretty small cutter in a fairly large pocket, but not modifying any default parameters concerning stepovers, etc., I had an 'oddity'.

SOMEtimes, it would cut one increment of depth, then rapid back to zero in the middle to start the next increment.

Sometimes, it would use the plunge feedrate, which from a large clearance height, used a lot of time.

I've never noticed this before.  If you cannot reproduce it, I'll give a simplified file to show it.

LLoyd
Title: Re: Inconsistent plunge feedrate in pockets
Post by: pixelmaker on November 11, 2013, 10:57:57 am
Quote
I'll give a simplified file to show it.

where is it? I never see this.

ralf
Title: Re: Inconsistent plunge feedrate in pockets
Post by: Dragonfly on November 11, 2013, 11:42:30 am
I have also noticed on several occasions that in a pocket MOP the horizontal transition (stepover) speed alternates randomly. Although in the MOP it is set to cutting speed, sometimes it moves with plunge speed. I haven't reported it because can't see any regularity.
Title: Re: Inconsistent plunge feedrate in pockets
Post by: lloydsp on November 11, 2013, 12:27:41 pm
"Regularity...", yes.  That is the problem.  The example I'll post is exquisitely sensitive to changes.  Sometimes a small change that appears meaningless to the issue will change the symptoms entirely.

But this one does it reliably on my computer.

About half the moves to the work surface consist of rapids to within 0.0625" of the surface, then a feed-rate plunge the last 0.0625".  About half are missing the rapid entirely, so they make the full excursion from clearance to worksurface at the slow Z feedrate.

For instance, look at the 'increment layer' between lines #159 and #161 of the g-code.  You'll see the rapid down to near the surface.  Then look at the one after that... only a feed-rate descent.

All the increments prior to 159-161 had a rapid.  The ones between 162 and 243 do not.  Then all the rest do, again.

This seems to 'break' with any little change, including which post-processor is used --- though I can see nothing in the post that's related to this behavior.

I included everything so that Andy may look at it.

Lloyd
Title: Re: Inconsistent plunge feedrate in pockets
Post by: kvom on November 11, 2013, 14:00:37 pm
I noticed that myself recently as well.  Cutting a profile with several depth increments.  The slow descent was on the last couple of passes.
Title: Re: Inconsistent plunge feedrate in pockets
Post by: dh42 on November 11, 2013, 15:31:26 pm
Hello,

I generate your file with Mach3 standard PP, ... and the same missing Z rapid is present, from Z-0.2 to Z-0.28

The rapid come back for Z-0.3 up to the end.

If I replace the automatic value (-1) in fast plunge height by 0.0625 I get the same problem, at the same positions

With 0.06 instead of 0.0625, the problem is also between Z-0.2 and Z-0.28

++
David
Title: Re: Inconsistent plunge feedrate in pockets
Post by: lloydsp on November 11, 2013, 15:35:50 pm
Thanks, guys.

It's likely some sort of singularity or rounding issue in the math.

Lloyd
Title: Re: Inconsistent plunge feedrate in pockets
Post by: dh42 on November 11, 2013, 16:03:32 pm
I retry the file up to -1.6 in target depth ... and the problem stay between -0.2 and -0.28, no other missing rapids up to the end ...

++
David
Title: Re: Inconsistent plunge feedrate in pockets
Post by: lloydsp on November 11, 2013, 19:58:33 pm
Ooooo!  THAT sure doesn't sound like a math problem... more like a variable going wonky on its own!

Could it be related to tool diameter and stepover, somehow?

Does anyone hear "The Twilight Zone" theme playing in the background?
 ::)

Lloyd
Title: Re: Inconsistent plunge feedrate in pockets
Post by: pixelmaker on November 11, 2013, 21:26:01 pm
Quote
Ooooo!  THAT sure doesn't sound like a math problem

No, sorry it again a "operator problem". You use again a spiral leadin without a spiral angle. This work if you use a profile mop.
If you set a spiral angle of 90° degree or above you get your G0 plunge to Z-0.2775.
Then a plunge with G1 to the start of that depth increment.
Perhaps you write a small nota with "no spiral leadin in pockets without spiral angle" and stick it at the middle of your monitor.  ;)
This is a behavior that sometimes work, sometimes not.
It shure works at closed polylines with a profile mop.
Use a profile mop with a cutwidth of 0.5 and a  spiral leadin without angle and it works ok.
But in this case you have to set the max crossover to 0.59 to get a retract to save height. With the standard setting of 0.7  you get no retract to save Z. Cb moves from the outside toolpath to the middle at the cutted level and spirals down to next level.

In the pocket mop the distance between the toolpath is 0.19754. Max crossover of 0.7 is 0.175. It retract.
In the profile mop with cutwith the distance between the toolpath is 0.15036 and there is no retracting movement.

ralf
Title: Re: Inconsistent plunge feedrate in pockets
Post by: lloydsp on November 11, 2013, 21:48:22 pm
Perhaps you write a small nota with "no spiral leadin in pockets without spiral angle" and stick it at the middle of your monitor.  Wink
-------
No... THAT is a bug if it doesn't work right every time.  If a feature like 'no angle' spiral lead-ins doesn't work, and it isn't prohibited in a particular MOp, then it's a bug, pure and simple.

Not operator error, no.  (ignorance of the bug I'll accept, but not an error in programming the part.)

Lloyd
Title: Re: Inconsistent plunge feedrate in pockets
Post by: pixelmaker on November 12, 2013, 09:13:14 am
Lloyd,
What I agree with you is that a pocket should have the same behavior then a profil mop with a cutwith.
By now I prefer the profile mop with a cutwith for pockets.
And using the spiral leadin without an angle not should result in different speeds on different  levels.
That is a bug.

What I learned from CB is that a spiral leadin basically needs a spiral angle. In the manual there is written on the page http://www.cambam.info/doc/plus/cam/LeadMoves.htm (http://www.cambam.info/doc/plus/cam/LeadMoves.htm) : If Spiral Angle is set to 0…..For closed shapes, the lead move will then replace the toolpath at each depth level,
In a pocket the lead move can´t and don´t replace the toolpath. So for me it is consequential that this behavior works only in profile mop. This is how I use this behavior, I use a profile with cutwith for this jobs.

ralf
Title: Re: Inconsistent plunge feedrate in pockets
Post by: Jeff_Birt on November 12, 2013, 12:47:36 pm
With a spiral leading set to zero what you are asking CamBam to do is plunge to your DOC over then length of the geometry selected. For a profile operation this type of behavior is very handy with a closed polyline as you can do a continuous plunge type operation on something like a bolt slot and remove the material very quickly.

For a pocketing operation what would one expect a spiral leadin set to zero to do? If you are going from the inside of the pocket towards the outside what should the leadin in the center look like? The actual distance traveled to clear the stock in the center will be small so the plunge angle would be steep. Remember a spiral leadin angle of zero is asking CamBam to derive the plunge angle from the length of the polyline but at the center of the pocket does it use the length of the toolpath or the length of the polyline that defines the pocket?

For a pocking operation using a zero angle leadin does not make sense to me as it is not immediately clear what I would be asking CamBam to do. I'm not saying the resultant code your seeing makes sense either but rather wondering aloud what type of result one would expect. When pocketing I will typically set an angle of 5 degrees or so which seems to be fairly gentle on most bits and still provide a relatively quick plunge.

So I guess what I'm asking is what is the use case for a spiral lead in angle of zero with a pocketing operation. If the expected behavior is defined then perhaps Andy will have a better idea of how to approach the problem.
Title: Re: Inconsistent plunge feedrate in pockets
Post by: Jeff_Birt on November 13, 2013, 14:09:54 pm
Seriously guys I'm curious, what is the behavior you expect when using a zero angle spiral lead-in on a pocket operation. I have been racking my brain for a day now and I can't visualize what I think such a toolpath should look like.
Title: Re: Inconsistent plunge feedrate in pockets
Post by: lloydsp on November 13, 2013, 14:55:33 pm
I expect it to do exactly what a profile with cutwidth would (because that's all a pocket IS) -- to rapid to a very small distance above the (current) stock surface, to move at feedrate that last tiny bit, to spiral down to the increment DOC in one revolution of the first cut, then to step over to the bounding profile for the rest of that increment.

Then I expect it to retract to clearance and repeat that first paragraph over and over until the target depth and cleanup pass.

Now...
What I get instead is exactly all that, except that sometimes it rapids to the current stock surface, and sometimes (in a weird, non-sequitor range of depths) it moves from clearance to current surface at feed rate.

When my clamps are 2" above the surface, and the current milled surface is an inch below the stock surface, that means moving 3" at a small feedrate, perhaps dozens of times;  it takes huge amounts of unnecessary time.  I want it to rapid on every increment's first move down.

It's not the general behavior of pocketing for which I called "bug", but for that unwarranted inconsistency in the plunge moves.  I believe that was clear to all the other folks who tested and confirmed the same behavior.

LLoyd
Title: Re: Inconsistent plunge feedrate in pockets
Post by: Jeff_Birt on November 13, 2013, 15:25:09 pm
OK, you want to do a pocket with a spiral lead in (let's ignore the inconsistent federate bug).

By using a zero angle you are telling CamBam to calculate the angle of the plunge so that it reaches the DOC by the end of the geometry. For example for profile cut of a circle of 1" in circumference with a DOC of 0.1" the depth would start at 0.00" and after moving 360 degree around the circle, i.e. 1" of travel, the depth of cut would now be 0.1". This makes sense with a profile operation and can be very handy.

For a pocketing operation each pass will have a different length so the plunge angle will be different on each pass. This type of tool path does not make sense to me. I want the plunge angle to be consistent and based on what is safe for the tool I'm using, I don't want it to change on each path.

What I'm suggesting is that a zero spiral angle on a pocketing operation does not make sense. But, that leads to the question of what should CamBam do if that is what a users selects? Should it calculate the plunge angle based on the outermost pass on the pocket? That would result in potentially undesirable operation as the plunge might a long time on each pass. Should CamBam not let the users use a zero spiral angle for a pocketing operation? That might seem surprising to many folks who are used to using a zero spiral angle on profile operations. If you select a spiral lead-in on a pocketing operation should CamBam default to something other than zero? Maybe it should always default to something other than zero? I typically use a spiral angle of 3-5 unless I specifically want a continuous plunge type toolpath, for example cutting all the way through a thin sheet on a small feature.

I'm not trying to say that there is no bug in the GCode that is being generated, I'm saying that asking for a zero angle plunge on a pocket does not seem to have a well defined or obvious type of toolpath that one would expect to see, i.e. how should CamBam calculate the plunge angle in such a case?
Title: Re: Inconsistent plunge feedrate in pockets
Post by: lloydsp on November 13, 2013, 16:46:50 pm
For a pocketing operation each pass will have a different length so the plunge angle will be different on each pass. This type of tool path does not make sense to me. I want the plunge angle to be consistent and based on what is safe for the tool I'm using, I don't want it to change on each path.

What I'm suggesting is that a zero spiral angle on a pocketing operation does not make sense.
------------

What you 'intuit' will happen is not at all what actually happens.  It spirals on the first, inner-most pass.  Then it steps out horizontally to the profile boundary.  It's exactly what you want to happen... you can plan the safe rate of descent based upon the length of the first pass, then all the others happen at your safely selected stepover rate.

Why not actually try this?  It's pretty clear you haven't.  My example posted early in this thread will show you exactly how it should (and does) work.

Lloyd

Title: Re: Inconsistent plunge feedrate in pockets
Post by: Jeff_Birt on November 13, 2013, 17:01:32 pm
Lloyd, it is not clear what it will do, what angle it will plunge at for every case. Pockets do generally start at the inside and work out but what if you have an island? How do you know what the plunge angle will be? I'm asking general questions here, not specific to your example. I'm not trying to be argumentative but rather understand what others think a zero spiral angle should do on pockets.
Title: Re: Inconsistent plunge feedrate in pockets
Post by: blowlamp on November 13, 2013, 17:21:09 pm
I haven't tested this, but I think that as no Spiral Angle has been specified, CamBam calculates the angle that is necessary for the Depth Increment to be achieved after traveling exactly one circuit around the pocket and this angle varies depending on whether we're near the centre or the edge of the pocket i.e. the length of a particular toolpath.


Martin.
Title: Re: Inconsistent plunge feedrate in pockets
Post by: lloydsp on November 13, 2013, 17:44:05 pm
Lloyd, it is not clear what it will do, what angle it will plunge at for every case
------
It is clear.  You yourself made the point that if you knew the length of the circuit, you'd know the angle.  You know the length of the first circuit, island or none.  But regardless, unless it's a very long or very short first circuit, the angle to get one increment's worth of DOC will be 'safe'.  Essentially, if it's safe to step over at the increment specified, it's safe to work down at that rate or slower. 

The only exception I'd make to that is one of taking too thin a cut on devilishly-hard materials, where a certain minimum initial plunge is necessary to prevent skating.  But in that case, you'd normally want to make a plunge or a very steep angle.  That's not what the zero-angle spiral is for.  It's specifically designed as a convenience to allow you to NOT do the homework you'd have to do on special materials, and to generally give prettier finishes than just whacking down into the work hard.

LLoyd


Lloyd


Martin's assessment is correct.
Title: Re: Inconsistent plunge feedrate in pockets
Post by: Jeff_Birt on November 13, 2013, 19:09:59 pm
@martin, yes a zero spiral angle asks CamBam to calculate the angle so that you reach your DOC by the end of the selected path. The question when using this with a pocket is 'what' path.

@Lloyd, I think I see where our different takes on this is coming from. You see the zero angle as a safe/convenient  default and I see it as a useful tool only for certain cases. Instead of talking this to death I whipped up a sample project. There are two Parts which each Part has the same three MOPS, the two Parts only differ in that one uses a zero angle and one used a 5 degree angle.

In one MOP I have two different geometries selected a rectangle and a circle. What plunge angle will be used? Will it calculate them separately or use the same one? The second MOP uses a small diameter circle, since the DOC is relatively deep compared to the diameter the plunge angle is very steep; probably not what we would want. The third example is a large circle with two small circles inside of it. Notice what a mess this one is CamBam is using different spiral angles everywhere, when going around an inner circle it is not too bad but then there are some rather spirals elsewhere.

Now look at the same three MOPS with a 5 degree spiral angle. While the angle may not be optimum for each case there are no surprises. The spiral angle remains consistent and predictable. This is the point I have been trying to make (but have not been doing a good job I guess. :) ) If your net setting a spiral angle you may be really surprised at what you get. I like to think of the 'zero' setting for spiral angle as a special case that is handy but you have to have a good idea at what the outcome will be before using it.
Title: Re: Inconsistent plunge feedrate in pockets
Post by: lloydsp on November 13, 2013, 19:34:11 pm
Jeff, I agree that if you don't have a good 'feel' for what the cutter will do, it's safer to use the spiral angle setting, but even that can introduce errors (like [say] not reaching DOC by the end of the revolution of the cut), but I find the zero setting to be a simple convenience, and not just for "special cases".

To each his own, I guess.

Lloyd
Title: Re: Inconsistent plunge feedrate in pockets
Post by: pixelmaker on November 13, 2013, 22:27:24 pm
Quote
(like [say] not reaching DOC by the end of the revolution of the cut)

What I say, set a angle with 90° or more and it works also in pockets, but set an angle.


ralf
Title: Re: Inconsistent plunge feedrate in pockets
Post by: lloydsp on November 13, 2013, 22:47:45 pm
This is all "preference" discussions now.  The SUBJECT was that there is a bug in the plunge logic.  If a feature (that is permitted to be used) causes an error sometimes, but not always, that's a bug!

If zero-angle spirals aren't useful in pockets, or they don't work in pockets, then they should be prohibited in pockets, not left there for the unwary to discover.  (This gun has an auto-fire mode.  We added that feature for people who want to use it --  but don't ever use it, or it might explode and kill you <G>  C'mon!)

If you ALWAYS must set an angle, then why is the feature even there?

Lloyd
Title: Re: Inconsistent plunge feedrate in pockets
Post by: blowlamp on November 13, 2013, 23:07:44 pm
@martin, yes a zero spiral angle asks CamBam to calculate the angle so that you reach your DOC by the end of the selected path. The question when using this with a pocket is 'what' path.


Each individual path that is offset from an island or outer profile.


Martin.
Title: Re: Inconsistent plunge feedrate in pockets
Post by: lloydsp on November 13, 2013, 23:51:42 pm
Martin, did you run my example?  It does not spiral down except in the center-most revolution of each increment.

I milled the part.  I watched it work. (Heck, I simulated it in CV first, and it showed the same.)


Where's this stuff coming from?

Lloyd
Title: Re: Inconsistent plunge feedrate in pockets
Post by: Jeff_Birt on November 14, 2013, 19:10:52 pm
Lloyd, Martin is talking about what happens if you have an island in your pocket. Take a look at the example I posted and you'll see what he is talking about. The plunge differs for different parts of the pocket.

I had a need to machine the holes in some 80/20 extrusions for the draw/cam bolts (what ever they call them, don't remember the proper name). This is a case where I like to use a zero spiral angle with a profile MOP. The effect is just that of a spiral drill MOP but it can also be used for bolt slots, etc. I attached the 80/20 project for reference.
Title: Re: Inconsistent plunge feedrate in pockets
Post by: pstemari on November 15, 2013, 04:27:41 am
What I seem to remember:  0 angle spiral leadin on a pocket ramps down on the first lap, takes another lap to flatten the bottom, and then proceeds to step outward.

On a circular pocket, that first lap is really small.

Thinking out loud, if you were to spiral down over the entire pocketing operation, you're wind up with a pyramid in the center of the pocket that you've have to go back and flatten out.

Possibly you could do a conical helix that spiraled down and out, then down and in, until it hit bottom and you flattened it out, but the depth of cut would be varying up and down in a strange way.

Jeff, I'm really surprised that you're using those anchor fasteners for 80/20. The end fasteners are (oddly) stronger and cheaper, and the plates or brackets can handle angled joints.