Pockets > Some »rounds «a little bit too deep

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typomaniac
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Joined: Mon Oct 29, 2012 6:11 am
Location: Hamburg, Germany

Pockets > Some »rounds «a little bit too deep

Post by typomaniac » Mon May 27, 2013 3:41 pm

Hey all,

i am sorry but i do not know the right term for this. »Round« means the incrementing path which makes the pocket.
I used a 1 mm 2 flute endmill für milling the pockets (500 mm/min, 0,7 mm per pass und one 0,1 pass for the finish pass).

Around the islands (in the attached picture) you can see that there is a visible difference between the rest of the pocket and the areas which i marked with an arrow. I did another
different graphic with the same wood and settings before and everything was fine (except the tooling marks which can be removed easily). I had this problem several
times bevore but still now idea where this comes from.

Can you help me with this issue?

thanks a lot

andre
imperfections.jpg
imperfections.jpg (301.7 KiB) Viewed 1531 times
imperfections2.jpg
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Proud owner of eShapeOko #17 :-) (Dual-X, Grbl 0.8a)

jhllt67
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Joined: Fri May 04, 2012 1:57 am

Re: Pockets > Some »rounds «a little bit too deep

Post by jhllt67 » Mon May 27, 2013 3:57 pm

My guess is you are loosing steps on your Z axis, probably when it retracts. This would lead to the next cut being deeper than it is supposed to be. Slowing down the rapid speeds should help if that is culprit.

Enraged
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Re: Pockets > Some »rounds «a little bit too deep

Post by Enraged » Mon May 27, 2013 4:25 pm

This may be an obvious thing, but also make sure your Z axis screw is lubricated. I just use an all purpose tapping/lubrication oil.

typomaniac
Posts: 64
Joined: Mon Oct 29, 2012 6:11 am
Location: Hamburg, Germany

Re: Pockets > Some »rounds «a little bit too deep

Post by typomaniac » Mon May 27, 2013 8:35 pm

@enraged: ok i will give it a try although the rod seems to be lubricated well. thank you!
@jhlit67: hmm. Maybe ... What do you mean with "rapid speeds"? Do you think 500 mm /m is too fast or did you mean the speed of the z-axis? I do not know if i can select a different speed for z in cambam ...

thanks!

andre
Proud owner of eShapeOko #17 :-) (Dual-X, Grbl 0.8a)

WillAdams
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Re: Pockets > Some »rounds «a little bit too deep

Post by WillAdams » Mon May 27, 2013 9:33 pm

Grbl uses the same acceleration for all axes (for older versions?).

Some Communication / Control programs will limit the Z axis when using G0 --- try testing w/G1 at different speeds after lubricating and see if it has problems.
Shapeoko 3XL #0006 w/ Carbide Compact Router w/0.125″ and ¼″ Carbide 3D precision collets
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jhllt67
Posts: 47
Joined: Fri May 04, 2012 1:57 am

Re: Pockets > Some »rounds «a little bit too deep

Post by jhllt67 » Tue May 28, 2013 12:43 am

I meant the speed set by "default seek" ($5) in grbl. That controls the G0 speed when your Z axis raises up after a cut (you can't directly control it from Cambam). Unfortunately the current grbl uses the same speeds and accelerations for all axes as Will said.

I think I used to have my "default seek" set at 250 mm/min before I upgraded to an acme thread, so I think 500 is a little fast if that your setting. I would try lowering it, remember to reset your arduino after making a change or it wont take effect (at least in the older versions).

typomaniac
Posts: 64
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Location: Hamburg, Germany

Re: Pockets > Some »rounds «a little bit too deep

Post by typomaniac » Tue May 28, 2013 5:44 am

Ah! Ok! I thought you were talking about the feed speed. I did not yet do anything with the $5 but i made a dump and
it told me 500 mm/min as the default seek speed which may be to much when considering your experiences. I will give it a try today!
Maybe one more thing: I use nema 17 (42BYGHM809) on all axis and set $0=87.489 steps/mm and $1=87.489. My z axis is set to $2=640 steps/mm ...
Do i need to change this or won't this have any effect on speed and maybe the depth issue?
Proud owner of eShapeOko #17 :-) (Dual-X, Grbl 0.8a)

jhllt67
Posts: 47
Joined: Fri May 04, 2012 1:57 am

Re: Pockets > Some »rounds «a little bit too deep

Post by jhllt67 » Tue May 28, 2013 12:14 pm

Changing $5 won't affect your steps/ mm, so as long they were correct before you won't need to change them ( I don't remember the settings off hand but they seem to be in the right range)

typomaniac
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Location: Hamburg, Germany

Re: Pockets > Some »rounds «a little bit too deep

Post by typomaniac » Tue May 28, 2013 12:19 pm

ok. Hmm what can i do now?I even slowed default seek to 100 which is incredbly slow on x and y but nothing has changed. Still the same »halos« ....
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WillAdams
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Re: Pockets > Some »rounds «a little bit too deep

Post by WillAdams » Tue May 28, 2013 12:38 pm

If your Z-axis moves smoothly and consistently when not milling, I suspect the problem is one of path direction and the differences in tool physics between the two different styles of milling (unless it's a bug in how your CAM program calculates the finishing path? What is your toolchain, settings for same? Could you post your G-Code?):

- climb
- conventional

You should be able to influence this by either reversing path directions, or by making your machine stiffer (try metal spacers on the Z-axis?).

There's a bit on these in the glossary: http://www.shapeoko.com/wiki/index.php/Glossary (copied from http://cncmentor.com/glossary/?tab=1 )

Climb Milling[14]
A milling cutter can cut in two directions: Conventional and Climb. Climb Milling on the other hand moves the cutting tool the same direction that it wants to travel. Each tooth engages the material at a definite point, and the width of the cut starts at the maximum and decreases to zero. The chips are disposed behind the cutter, leading to easier swarf removal. The tooth does not rub on the material, and so tool life may be longer. However, climb milling can apply larger loads to the machine, and so is not recommended for older milling machines, or machines which are not in good condition. This type of milling is used predominantly on mills with a backlash eliminator or ballscrew setup. The nature of climb milling is that it will pull the workpiece into the tool and so if there is any backlash in the table, that “slop” will be sucked into the workpiece with dangerous results. <to do copy in images, duplicate text in Climb vs. Conventional milling, make link, edit further>

Conventional Milling[17]
A milling cutter can cut in two directions: Conventional and Climb. Conventional milling moves the cutting tool opposite the direction it wants to travel. The chip thickness starts at zero thickness, and increases up to the maximum. The cut is so light at the beginning that the tool does not cut, but slides across the surface of the material, until sufficient pressure is built up and the tooth suddenly bites and begins to cut. This deforms the material, work hardening it, and dulling the tool. The sliding and biting behaviour leaves a poor finish on the material. Surface finish is also poor because chips are carried upward by teeth and dropped in front of cutter. There is therefore a lot of chip recutting. Coolant can help carry away some of those chips and increase the quality of the finish. Conventional milling creates upward forces that tend to lift the workpiece during face milling. <to do copy in images, duplicate text in Climb vs. Conventional milling, make link, edit further>

and I started making a page specifically on it: http://www.shapeoko.com/wiki/index.php/ ... al_Milling but punted when I decided that I'm too confused by the whole direction thing until I have the time to actually start milling and experiencing it myself, but there is this:

Please note that in MakerCAM Help it's advised that
“If you're cutting an inside profile, choose counter clockwise. For an outside profile, choose clockwise.”
Shapeoko 3XL #0006 w/ Carbide Compact Router w/0.125″ and ¼″ Carbide 3D precision collets
Nomad 883 Pro #596 (bamboo)

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