Surface machining questions

[BillGEGHV]

Surface Machining,

Hey all, Im programming an AR-15 (air soft lower) and have some general questions regarding surface machining as it pertains to metal parts. As you can see this is a standard Colt style AR, not a square corner on the darn thing!  As I experiment with different tool path options I like geodesic and flow. I will be machining all these surfaces with an 3 flute1/8 ball end mill.

Parts will be sand blasted with garnet media (not sure what grit yet) then Cerakote for pain finishing.

A very quick bit of info on my past experiences. Back in the day I used to be in the toy industry as a CAD designer, 3D modeler, sculptor, model maker, prototype creator, and digital sculptor.  I used CNC machining to create toy sculptures and models and I milled them in toy sculpting wax. this was WAY before anyone was doing stuff digitally back in 1999-2000 I started digital. I was the 1st vendor doing stuff digitally. EVERYTHING was organic surface machining and took hrs and hrs to machine. It was not unheard of to mill stuff that was 8-10 + + hrs.  I used to machine most sculptures with a .03-.02 ball end mills to get down into all the details of the sculpt at .003-.002 step over. Back then it was all parallel finishing. The sculptures would come off the mill 98-99% finished and only a sculptural Nathan/alcohol brush down and polish was required to finish.  When ever I would deviate from the .003-.002 step over and say used a .005 I would not get the results I needed and would require more hand clean up. The goal was always to pull finish sculpts off the mill. In the toy industry “much like in manufacturing,” perfectly smooth sculptures/surfaces is the benchmark and is always what is expected.  I only mention this because that is where I’m coming from and I need to improve on those experiences with this project. At this point the machining time are starting to really spook me, LOL!  Although I do need perfectly smooth paint ready surfaces I need to know how to approach the surface machining and improve on my past. 

Questions

• What is a perfect amount of step over with an 3 FL1/8 ball EM? I currently have .005, is this overkill or not enough?
• If you see the screenshots I’ve posted I want to know why there is such a difference in the step over when I programmed both with .005? 
• If you have a recommendation for garnet media LMK.
• ANY additional tips would be appreciated?  
• This is an off topic question but has AD implemented a way to measure random distances in space. For example, I would like to measure the actual distance between the 2 tool paths step over.  This has been my Achilles' heel in Fusion, coming from using Rhino 3D for 25+ years. You CAN measure ANYTHING in Rhino and you don’t have to snap to anything to do so.  Drives me NUTS in fusion it is ALWAYS is a limitation IMO!
• 
  Thanks 

Geodesic tool path 

Flow Tool Path

  

 

 

 

[programming2C78B]

im so glad its only for airsoft, and not just an unnamed fillet feature 😉

find your RA - 32 is PLENTY. 64 is probably fine and half the time to mill.
https://www.harrisonep.com/electropolishing-ra.html#:~:text=Ra%20is%20calculated%20as%20the,measured...

convert it into a decimal
https://www.unitconverters.net/length/microinch-to-inch.htm

then do the math
https://theoreticalmachinist.com/Surface_Finish_Calc.aspx

flow obviously looks best. Make sure its actually doing the stepover amount programmed. Some of them are off by a factor of 2. 
Scallop may save you more time. 

[BillGEGHV]

Thanks for the info.  I just want to make sure I get this and understand the numbers to use, took a bit to grasp what numbers I needed to use for the calculations so I have a couple of questions.

• If I’m going for an Ra surface finish of 32 I simply enter 32 in the (convert to microinch to inch) calculator. Right?  This value is now my scallop height, correct? 
• I then use that scallop height in the step over distance calculator on the Technical Machinist site and enter my tool diameter and that give me the step over value for that Ra finish? Correct?
• I assume the Ra values are infinite and you could just split the difference if your trying to fine tune machine times and such?
• Do these values apply to both milling and turning?  BUT I assume when turning and finish passes are a sweep across the work piece surface it would not apply to finishing operations???

• Isn’t an Ra of 32 considered a kind of standard for a typical machined part, generally speaking?

Based on the results here I guess I was not to far off back in my toy days by just trial and error.

[BillGEGHV]

Thanks for the info.  I just want to make sure I get this and understand the numbers to use, took a bit to grasp what numbers I needed to use for the calculations so I have a couple of questions.

• If I’m going for an Ra surface finish of 32 I simply enter 32 in the (convert to microinch to inch) calculator. Right?  This value is now my scallop height, correct? 
• I then use that scallop height in the step over distance calculator on the Technical Machinist site and enter my tool diameter and that give me the step over value for that Ra finish? Correct?
• I assume the Ra values are infinite and you could just split the difference if your trying to fine tune machine times and such?
• Do these values apply to both milling and turning?  BUT I assume when turning and finish passes are a sweep across the work piece surface it would not apply to finishing operations???

• Isn’t an Ra of 32 considered a kind of standard for a typical machined part, generally speaking?

[programming2C78B]

theres no need to double post as a way of bumping a thread. I was in time out for being too helpful.

1) yes 
2) yes

3) yes
4) yes. not sure what you mean about a finish sweep. A stepover is always a stepover.

For ref, all our customers prints have "RA125 unless otherwise specified" 
Aiming for a 32-64 on all surfacing 3d stuff is a safe route on the first run. change as needed on finish VS cycle time. 

[BillGEGHV]

Thank you for the clarification, much appreciated!  

The double post was a mistake.  

[programming2C78B]

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