safe and clearance height

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freman
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safe and clearance height

Postby freman » Thu Nov 05, 2020 8:06 pm

Hi,

I always find the safe height / clearance height terminology rather confusing but what it means also seems confused.

In the attached FCStd I have a series of drill holes. The approach to the first one is done at clearance height but in between all others it skims along at safe height.

https://wiki.freecadweb.org/Template:Depths/Heights

The above link shows the space between safe and clearance as "space for clamping".

If I had clamped in that zone I would not have any clamps left !

Am I misusing this or is it as confused at it appears to be?

TIA.

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sliptonic
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Re: safe and clearance height

Postby sliptonic » Thu Nov 05, 2020 8:22 pm

freman wrote:
Thu Nov 05, 2020 8:06 pm

I always find the safe height / clearance height terminology rather confusing but what it means also seems confused.

In the attached FCStd I have a series of drill holes. The approach to the first one is done at clearance height but in between all others it skims along at safe height.

https://wiki.freecadweb.org/Template:Depths/Heights

The above link shows the space between safe and clearance as "space for clamping".

If I had clamped in that zone I would not have any clamps left !

Am I misusing this or is it as confused at it appears to be?

TIA.
There are a number of ways of thinking about this but I prefer to think of it this way; Clearance height is the height at which the tool will move between operations. Safe height is the height at which it will move between areas in the same operation.

Your drilling example is a good one. You would not want the drill to retract all the way to clearance height when moving between holes or pecking. That would make an operation with many holes very slow.

You should never position the clamps inside the working envelope of an operation but if you have to, you can always manually set the safe height high enough to clear.
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freman
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Re: safe and clearance height

Postby freman » Thu Nov 05, 2020 10:03 pm

You should never position the clamps inside the working envelope of an operation but if you have to, you can always manually set the safe height high enough to clear.
Thanks for the reply.

So the documentation on this is incorrect. The space indicated as being safe for clamping is NOT safe for clamping. That at least requires some explication and caveats.

If I need to set safe height equal to clearance height to clear clamps, then the distinction becomes null.

Another problem exposed by this example file is that the drilling operation is descending from clearance to safe at drilling speed , not rapid. Since this is supposed to "safe" space , outside of the stock, what is the point in G1-ing this movement. It should G0 down to safe height then G1 from there. Was this done to avoid a specific geometric problem?

If the "safe space" was traversed as a rapid the case, the objection of not moving to clearance between each hole disappears. A rapid up and rapid down would not be a problem.

I don't know of hand whether this notion of moving between different areas within the same operation makes sense on the other 2.5D tools but I can't see it as meaningful here.

The "you should never" argument does not appropriate. This bar is 800mm long. I require intermediate clamping. I have already had to machine the other pockets with safe=clearance, which is indeed a lot slower than it should be.

What is the reference for how this is done and what these terms mean? Is there a standard method, or was this just the way it was chosen to be done when Path WB was written?

Thanks.
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sliptonic
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Re: safe and clearance height

Postby sliptonic » Fri Nov 06, 2020 3:29 am

freman wrote:
Thu Nov 05, 2020 10:03 pm

So the documentation on this is incorrect. The space indicated as being safe for clamping is NOT safe for clamping. That at least requires some explication and caveats.

The diagram is indicating a cross-sectional depth and is accurate in that sense. The issue is about whether clamps are placed inside an xy area that the op assumes is clear.
If I need to set safe height equal to clearance height to clear clamps, then the distinction becomes null.
If you need to put clamps in an area that Path would otherwise expect to be clear, what else would you suggest? How is Path expected to know that you have a clamp in the working envelope?
Another problem exposed by this example file is that the drilling operation is descending from clearance to safe at drilling speed , not rapid. Since this is supposed to "safe" space , outside of the stock, what is the point in G1-ing this movement. It should G0 down to safe height then G1 from there. Was this done to avoid a specific geometric problem?
I'll need to study your file closer,including your post processor. Are you sure this is a Path G1 move to start-depth rather than the beginning of a canned cycle?
If the "safe space" was traversed as a rapid the case, the objection of not moving to clearance between each hole disappears. A rapid up and rapid down would not be a problem.
This too gets into the nuance of canned cycles.
I don't know of hand whether this notion of moving between different areas within the same operation makes sense on the other 2.5D tools but I can't see it as meaningful here.

The "you should never" argument does not appropriate. This bar is 800mm long. I require intermediate clamping. I have already had to machine the other pockets with safe=clearance, which is indeed a lot slower than it should be.
Again,I'm not sure what you expect us to do. There are real-world situations where intermediate clamps are needed but you haven't told me how we should know about them.
What is the reference for how this is done and what these terms mean? Is there a standard method, or was this just the way it was chosen to be done when Path WB was written?
Good question. Maybe someone with experience in other cam software can weigh in. Maybe we need to create another kind of object or dressup to indicate a no-go or clamping area but implementing it is going to be a nightmare.
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freman
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Re: safe and clearance height

Postby freman » Fri Nov 06, 2020 3:57 am

I'll need to study your file closer,including your post processor. Are you sure this is a Path G1 move to start-depth rather than the beginning of a canned cycle?
No, looking at the unexpanded Gcode before the post processor ( inspect g-code ) the canned cycle starts from clearance on the first hole, there is no move down to safe ht, first. So this is not related to any of the post proc code.

Code: Select all

(bar-fix-Drilling)
(Begin Drilling)
G0 F50.000000 Z21.000000
G90
G99
G83 F1.000000 Q3.000000 R-0.500000 X170.000000 Y-15.000000 Z-20.000000
G80
G0 Z3.000000
G83 F1.000000 Q3.000000 R-0.500000 X320.000000 Y-15.000000 Z-20.000000
So if there was a G0 from clearance to safe ht, before G83 a lot of this would disappear. Moving up to clearance between each hole would not be a problem. This is nothing to do with canned cycles since if I deselect Peck, the G81 does the same : much drilling of air. This is a defect I've noticed before but never got around to bringing it up.
If you need to put clamps in an area that Path would otherwise expect to be clear, what else would you suggest? How is Path expected to know that you have a clamp in the working envelope?
There may be a case for this ( though I remain to be convinced ) but my point here was that the doc is incorrect. If this is the thinking behind it, the concept of the "envelope" should be a caveat to the current statement this space is "safe".
Good question. Maybe someone with experience in other cam software can weigh in. Maybe we need to create another kind of object or dressup to indicate a no-go or clamping area but implementing it is going to be a nightmare.
Assuming that the user can tell what route FC will take and anticipate the envelope is not guaranteed, it not always that obvious. I already have a few lumps missing from my clamps where I got caught out by this unsafe "safe space". :( One thing I did intend to raise at some point is ordering of "base geometry" being taken into account.


Yes, if someone with trade experience could provide input on existing conventions it would be helpful.

I would have thought where there are separate volumes to be removed , as in this case, retraction to clearance would be preferable. I have not thought it through very much for other tool paths but in principal I would think the same would apply. Pocket and helix are pretty directly analogous, maybe you can come up with a geometry where this does not make sense.


I would think a dressup similar to holding tags could be feasible, if needed. I wasn't sure that there was not already a facility for clamps which I had not discovered yet. I'd seen "fixations" appearing the Gcode comments.
Last edited by freman on Fri Nov 06, 2020 8:53 am, edited 1 time in total.
chrisb
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Re: safe and clearance height

Postby chrisb » Fri Nov 06, 2020 8:19 am

sliptonic wrote:
Fri Nov 06, 2020 3:29 am
If you need to put clamps in an area that Path would otherwise expect to be clear, what else would you suggest? How is Path expected to know that you have a clamp in the working envelope?
I think too, that the documentation is wrong. If course can clamps be in the way on vertical moves, because they are misaligned, but as we currently don't make any proposals where to place clamps, this can be neglected. Indicating clamps in an image can thus affect only horizontal moves. And I would expect "Space for clamping" to be indeed "Safe space for clamping", i.e. it is safe to make horizontal moves above that height. For anything else it is safe to move above stock.
I think we had talked about this quite some time ago and we talked about the typos - Save vs Safe, and I think there was one more. Perhaps it was missing "Finish Depth".

So I think we should correct this (again?) and include "Finish Depth" as well.

Herbert, I think you are the author and you have the source document?
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Re: safe and clearance height

Postby herbk » Fri Nov 06, 2020 8:30 am

Hi,
chrisb wrote:
Fri Nov 06, 2020 8:19 am
Herbert, I think you are the author and you have the source document?
yes, - for both.

I'm out with my motorhome atm and back at Sunday and can make the changes.

I started allready with the change to "Finish Depth", but want to wait with the upload till it is changed at FC to.
Gruß Herbert
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Re: safe and clearance height

Postby chrisb » Fri Nov 06, 2020 8:52 am

There are different versions of this file in the wiki:

https://wiki.freecadweb.org/Template:Depths/Heights mentioned in OP and
https://wiki.freecadweb.org/Path_Workbench (Path main page)
https://wiki.freecadweb.org/Path_Pocket_Shape (PathPocket)

The latter two are the same.
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Re: safe and clearance height

Postby herbk » Fri Nov 06, 2020 9:27 am

The first is an old one, may be i forget to change the link... It's a label "Start Height" at it, - we have a "Start Depth" at Path WB.
Gruß Herbert
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freman
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Re: safe and clearance height

Postby freman » Fri Nov 06, 2020 9:28 am

Great to see agreement this needs clarification.
Indicating clamps in an image can thus affect only horizontal moves. And I would expect "Space for clamping" to be indeed "Safe space for clamping"
I think "Space for clamping" still implies it is protected space and that this is what the difference between these two confusing terms is about.

Until and unless current behaviour is changed, I think the doc needs to explicitly point out that operations will do rapid moves inside this volume and that careful consideration by the user is needed to avoid collision with clamps.

Honestly, I think the logic of current behaviour is flawed. Unless this is consistent from one pocket to the next I really to not see the point in defining two different heights.

As I suggested above , if drill, pocket and helix ops did rapid retraction from safe to clearance and then back down to the next pocket at safe height, then this would all make a lot more sense and still provide fast practical paths.

This may lead to clearer idea of what these two height are for and hopefully some more clarity in naming. Clearance height seems fine to me but since there is nothing "safe" about safe height there may be place of a better name. Maybe retract height, if the functionality I suggest is adopted.