How to obtain the smoothest 3D surface finish?

D

Ddgman2001

Plastic

Joined

Feb 8, 2014

Location

Canada

• Jan 1, 2015

• #1

I have some parts that require a convex surface on the top. They're being machined from 6061 with a 1/4" ball mill using a parallel finishing path. If I machine in both directions, there is a distinct difference between the height of each direction of cut. Leaving a surface finish that is not much smoother than if I'd done half as many passes in a single direction. Is this normal or is my machine just really loose?

 

D

DesertCoyote

Plastic

Joined

Nov 2, 2014

Location

NM, USA

• Jan 1, 2015

• #2

One problem you may have is by zigzagging is you are swapping from climb to conventional cutting this causes the tool to deflect in both directions alternating each pass. The more stock being removed, skinny long tools pretty much everything can effect this. I like to rough with a square EM then blow through with a ball for a second rough then finish with a ball. A larger ball mill, if possible, will also help.

 

A

alexhawker

Aluminum

Joined

May 6, 2013

Location

Philadelphia, PA United States

• Jan 1, 2015

• #3

What's your stepover? Google for a cusp height calculator and figure out what stepover you need to get cusps below 0.0005" or whatever you deem smooth enough.

 

J

jrmach

Titanium

Joined

Jun 9, 2012

Location

Boise,Idaho USA

• Jan 1, 2015

• #4

How to Choose a Stepover for 3D Machining

 

zero_divide

Stainless

Joined

Feb 2, 2012

Location

Toronto

• Jan 1, 2015

• #5

I wrote this when most of my work was surfacing 6061.
Everything from stepover to material thinckneses to leave after each operation is explained:
http://zero-divide.net/index.php?page=FrontPage&shell_id=151&article_id=4509

 

V

Vancbiker

Diamond

Joined

Jan 5, 2014

Location

Vancouver, WA. USA

• Jan 1, 2015

• #6

Could be machine related as well as the difference in deflection from a zigzag path.

The quality of machine, the quality and age of the control, and how well the servo tuning is set up could all be factors in this case.

 

macds

Hot Rolled

Joined

Jul 24, 2008

Location

Milverton, Ontario, Canada

• Jan 1, 2015

• #7

Really depends on the part, but you gotta know your machine as well. On older sloppier machines, a collapsing or expanding finishing toolpath as opposed to parallel finishing has always worked best for me to maintain consistent deflection. Other thing with parallel finishing on "flat faces" is you tend to get the end of the tool path deeper than the beginning if yer spindle chiller isn't up to snuff. I know that's not the case here, just saying. YMMV.
Stu

 

D

Ddgman2001

Plastic

Joined

Feb 8, 2014

Location

Canada

• Jan 16, 2015

• #8

Thanks for the input guys. This has been very helpful.

 

C

cncdumm

Cast Iron

Joined

May 2, 2012

Location

Canada

• Jan 16, 2015

• #9

Waterline toolpaths, instead of plain old zig zag on X and Y also creates better looking finish on complex 3D surface geometries.

But if you're looking for mirror finishes or super low surface roughness finishes, thats a job for grinding/polishing.

Sounds like you have a hobby machine? As to what you're describing specifically, maybe its your machine also. Someone mentioned climb vs conventional mill, but also likely your machine is not precise leading to the effect you're describing. Could be backlash, servos/steppers skipping, etc.

 

K

keith.clausen

Plastic

Joined

Jun 14, 2011

Location

Canada

• Jan 16, 2015

• #10

Have you also considered a different strategy like possibly a scallop toolpath that will maintain a constant scallop/cusp height? Available programming features and parameters will always depend on your CAM software but for the most part Scallop should do the trick to achieve the best/constant surface finish.