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Author Topic: [5] Inconsistent plunge feedrate in pockets  (Read 13124 times)
Jeff_Birt
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« Reply #15 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?
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lloydsp
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« Reply #16 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.
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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

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Jeff_Birt
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« Reply #17 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.
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blowlamp
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« Reply #18 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.
« Last Edit: November 13, 2013, 18:39:34 pm by blowlamp » Logged
lloydsp
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« Reply #19 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
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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.
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Jeff_Birt
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« Reply #20 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. Smiley ) 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.

* Spiral leadin.cb (16.63 KB - downloaded 151 times.)
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lloydsp
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« Reply #21 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
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pixelmaker
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« Reply #22 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
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lloydsp
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« Reply #23 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
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blowlamp
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« Reply #24 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.
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lloydsp
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« Reply #25 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
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Jeff_Birt
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« Reply #26 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.

* 80-20 Cam bolt hole.cb (3.93 KB - downloaded 142 times.)
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pstemari
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« Reply #27 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.
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