Negative allowance doesn't work

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Negative allowance doesn't work

Postby jglazer » Thu Nov 01, 2018 11:32 pm

I was under the impression that I could do a negative allowance (-.001) on an outside profile for a last pass to "clean up" any roughness from prior passes. When I enter -.001 (or any negative number for that matter) the negative is ignored. It says in the documentation that a negative number will remove more material on an outside pass. What am I missing??

Thanks!
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Re: Negative allowance doesn't work

Postby Bob Reda » Fri Nov 02, 2018 12:12 am

I never put in the negative number. I just assumed that any number you put in the last pass cleanup would be negative.

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Re: Negative allowance doesn't work

Postby jglazer » Fri Nov 02, 2018 12:27 am

Well, just to try that, I put in "1" and it blew way outside my outside profile so it's definitely outside. Negative should go inside. According to their docs:

For the Outside / Right option, a positive offset value will leave more material on the outside of a closed vector or to the right side of an open vector. Likewise, a negative offset value will remove more material from a closed vector or leave less material to the right side of an open vector.
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Re: Negative allowance doesn't work

Postby mtylerfl » Fri Nov 02, 2018 1:22 am

Hi,

Seems Profile Offset is working peachy-keen, as normal (negative or positive values).

Please post a file example of yours where you have confirmed it is not working.

Now, the Last Pass topic is something I think you may be misunderstanding. Normally, you have the previous passes offset by a small (positive) amount. The last pass usually will not have ANY offset, because you want it to cut the part at exact designed size (and produce the clean edge you want).
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Re: Negative allowance doesn't work

Postby wb9tpg » Fri Nov 02, 2018 12:06 pm

Allowance is working for me on both profile and pocket cuts. I set up a document variable to contain my number and set it to -0.015. I then use that variable {my_allowance}. It allows be to adjust the fit of several mating pieces all at the same time by changing the value in a single place.
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Re: Negative allowance doesn't work

Postby LittleGreyMan » Fri Nov 02, 2018 1:09 pm

wb9tpg wrote:Allowance is working for me on both profile and pocket cuts. I set up a document variable to contain my number and set it to -0.015. I then use that variable {my_allowance}. It allows be to adjust the fit of several mating pieces all at the same time by changing the value in a single place.


The topic is the "do separate last pass" allowance which is a specific case. As the graphic in the dialog shows, a negative value doesn't make sense. You can still apply a general allowance value (positive or negative) on this toolpath and use a variable as you suggest.
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Re: Negative allowance doesn't work

Postby jglazer » Fri Nov 02, 2018 1:10 pm

Here is the file. Again, no matter what negative number I put in (i want to use -.001) it removes the minus sign.
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4x4 Coasters (CNC).crv
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Re: Negative allowance doesn't work

Postby Adrian » Fri Nov 02, 2018 1:40 pm

jglazer wrote:Here is the file. Again, no matter what negative number I put in (i want to use -.001) it removes the minus sign.

But why wouldn't it? I can't see why you would want or need a negative number for the last pass? The first passes are offset by the amount specified and the final pass is done to the actual vectors.
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Re: Negative allowance doesn't work

Postby mtylerfl » Fri Nov 02, 2018 2:00 pm

jglazer wrote:Here is the file. Again, no matter what negative number I put in (i want to use -.001) it removes the minus sign.


Ok. I see what you're doing...you are selecting the "Do Separate Last Pass Option" and attempting to put a negative number in the input box. That (as you discovered) won't work. Why? Because it doesn't make sense!

If you really, really, really want to do a Last Pass with a negative amount which will cut your parts undersize, you can:

1) Set up two separate Profile cuts - one set at no allowance, the other set at your desired negative allowance. You would NOT use the "Do Separate Last Pass Option" on either Profile toolpath

or...

2) Create an additional negative offset vector to the inside of your original vector, then create two separate Profile toolpaths - one for each vector. Neither with any allowance. Your part will be cut undersize with either of these methods.

Again, think about what you are doing for a second. It is unnecessary (and undesirable) to make the last pass Profile cut smaller than your designed part (except in the case of inlays, but that's not what we're talking about here).

Simply set your Profile cut with a small positive allowance offset (0.001" if that's what you want), then select the "Do Separate Last Pass Option" but don't set any offset. Your goal will have been achieved and your part will end up the size you designed it at.

BTW, I'm not sure that my machine would be able to precision cut a 0.001" allowance anyway. That's a pretty small amount to be very noticeable when cutting wood!
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Re: Negative allowance doesn't work

Postby LittleGreyMan » Fri Nov 02, 2018 2:52 pm

Adrian wrote:the final pass is done to the actual vectors.


Not exactly Adrian. The last pass is done at the global "Allowance offset" of the toolpath. This one can be negative, equal to zero or positive.

So jgglazer, you can have a negative allowance and use a "Do Separate Last Pass" option. But its "Allowance (A)" is relative to first passes vs last pass. Look at the little diagram and you'll see a negative value does not make sense for this parameter (not for the toolpath).
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Re: Negative allowance doesn't work

Postby LittleGreyMan » Fri Nov 02, 2018 3:00 pm

Allowance.jpg
In blue: the general allowance. Sets the final size of the cut: equal to vectors (=0), smaller (negative for outside/ right outer profiles) or greater (positive).

In red: the separate last pass allowance. Doesn't set the size of the cut, but the offset between first passes and last passes. Can't be negative as it doesn't make sense as the little illustration shows.

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Re: Negative allowance doesn't work

Postby LittleGreyMan » Fri Nov 02, 2018 3:05 pm

I didn't notice you posted a file.

Using a general -0.001 allowance and a 0,05 last pass allowance works without problem.

As Michael noticed, 0.001" allowance is absolutely not noticeable on wood and your machine won't be able to handle such precision.
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Re: Negative allowance doesn't work

Postby wb9tpg » Fri Nov 02, 2018 3:17 pm

LittleGreyMan wrote:
wb9tpg wrote:Allowance is working for me on both profile and pocket cuts. I set up a document variable to contain my number and set it to -0.015. I then use that variable {my_allowance}. It allows be to adjust the fit of several mating pieces all at the same time by changing the value in a single place.


The topic is the "do separate last pass" allowance which is a specific case. As the graphic in the dialog shows, a negative value doesn't make sense. You can still apply a general allowance value (positive or negative) on this toolpath and use a variable as you suggest.



I'm manually doing a separate profile toolpath pass (with regular allowance) with the same vector on my designs instead of last_pass and feel it gives me better control. It allows me to adjust slow down the feed rate on that pass too that way. Sorry I was not clear. It's an alternate approach but works well for me.
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Re: Negative allowance doesn't work

Postby Adrian » Fri Nov 02, 2018 3:26 pm

LittleGreyMan wrote:
Adrian wrote:the final pass is done to the actual vectors.


Not exactly Adrian. The last pass is done at the global "Allowance offset" of the toolpath. This one can be negative, equal to zero or positive.

Yes but I didn't see any point in introducing yet more confusion over allowances. There seems to be enough to go around in the thread already. :lol:

I run double toolpaths with different allowances on all my jobs as the last pass feature doesn't give me enough control over feedrates and ramps. I then use the merge toolpaths to create a single toolpath which completes each part before moving onto the next.
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Re: Negative allowance doesn't work

Postby jglazer » Sat Nov 03, 2018 5:57 pm

My goal was to do a last pass to clean up stray marks from prior profile cuts. Just a little clean up. I am cutting corrian and was trying to eliminate a bit of sanding on the edges.
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