Edge quality on HPDE ellipse

[john@mindsightdesign.com]

I cut out some ellipses from 1/2 King Starboard. It was jigged on a vacuum table on a PRT 120 with a 3.5 hp (I think) Columbo. I tried various cutters, feed speeds, spindle rpms, and z axis step levels, but none gave edge quality as good as hand routing. At best, you could see little tiny serrations on the edge of the ellipse.

I put my hand on the y carriage to see if I could feel some jumping and I did. Not visible with the eye, but you could feel the jumping, I guess that translates to the part. Is there a way to get a smooth cut on this machine? The vibrating more apparent at the smaller ends of the ellipse.

If you cut the ellipse out of wood, I don't know if it would be as visible, but I imagine plastics will show it more.

john@mindsightdesign.com (mailto:john@mindsightdesign.com)

[rgbrown@itexas.net]

"I tried various cutters, feed speeds, spindle rpms, and z axis step levels, but none gave edge quality as good as hand routing."

How about "direction" and what is your gasket material for your vacuum table, "IS the part held well enough and solid enough?"

Check you "hold-downs" and the tightness of your carriages also.

Ron

[G]

Your software may have interpretted the ellipse as a bunch of straight lines. Is the quality okay when you cut circles?

[elcruisr]

Actually all curved moves on CNC machines are a series of very small straight lines. There is a time and place for CNC and a time and place for hand work. Just down the road from me is a CNC shop running 10 machines, 2 shifts a day, cutting starboard for boatbuilders. They accept the fact that they have use some sanding on the edges as part of production. Plastics will usually show what you are describing to some degree on most CNC machines I've seen. Even the edges of laser cut acrylic show the small step marks. Cnc does have it's limits!

Eric

[John S.]

You know Eric, this is the conclusion that I am coming to, I have noticed that my lazer parts have the stepping too. But the fact that they were acrylic parts and the lazer puts a nice shiny edge made me overlook the steps.

The ellipses come out smooth, no straight lines, just very jagged. The vacuum is adequate, if it was one piece, I might worry, but there are 17 pcs that come out of a sheet and I have 17 individual vacuum jigs holding down each part of the sheet. The sheet doesn't move. After the all 17 pcs are cut and the surrounding material is weeded, the individaul parts still cannot be moved.

I was thinking that maybe a bigger bit would help, but I doubt it will eliminate the steps. Just before I bought the Bot, a MultiCam guy was hot for my business and was telling me what perfect edge quality he was getting with acrylic. I believe that a good portion of the choppiness comes from the flexing in the Bot frame. It telagraphs any vibration to the part.

At least for me, the MultiCam still isn't worth the extra 15-20k that I would have to have paid. But something in the back of my mind still says - but what if?

[beacon14]

One possible reason for the rough edges could be the vibration of the bit due to the strain of cutting the entire width of the bit in one pass. On critical cuts, I will sometimes do a 1st pass about 1/16" oversized, then come back with a clean-up cut taking off only the last 1/16".
I know it increases cut time, but the clean-up cut can usually be done at fairly high speed depending on the material, and if it saves the hand work, that is what CNC is really all about.

[John S.]

David-
Are you cutting plastic? I have done what you suggested but I still get the jaggies. This is what I did for the job. Ran a program that cut my blind counter bored through holes, go home, cut the outer perimeter of the 1/2 material at 4 passes per part (.126 per pass), go home, then had re-run the counter bore portion to clean up the holes, then run a clean-up pass on top of all than. Pretty insane, huh? And to top it all off, still got the stepped sides.

I am looking for smooth... maybe that the material is black doesn't help either. It just amplifies the highlights of the steps.

John

[elcruisr]

Black is the worst! Dark colors always magnify the smallest defects. I used to own a boatyard and people couldn't understand why we charged double for dark paint jobs. There was double the work making it look good was why.

Know what you mean about the flex in the frame. That hurts smoothness to. Sometimes I watch mine and debate ways to try and stiffen it up. We considered a multicam as well but for many happy owners I know a few who curse the day they got one so the jury is still out on that one. If we build up enough work this year maybe we'll look for a used Northwood or Komo and keep the 'bot for overflow and backup. If something would just kick this economy into higher gear!!!

Eric

[papadaveinwy]

John if you have a program like Rino you can change the segmented lines to arcs this will help greatly David in Wyoming

[G]

Dave, if only John would answer my question 'Is the quality okay when you cut circles?' then we will all discover if he has a hardware or software problem. If his cicles are okay, then Rhino or similar will help. If his circles are also not to his satisfaction, then the discussion on frame stiffness etc. becomes applicable.

[John S.]

Well, I cut some 3" diameter circles, not through though. This would not be a good example of circles. I did notice that the cut quality was good though. Remember, this is black material, so it will show any and all flaws.

G- I think you are asking if the stepping is from my file. Well, they are not steps or facets like you would get in a bad DXF import, rather tiny serrations like the part was cut on a band say, but each serration is very smooth and shiny. Looks more like chatter, not stepping. But you are right, I should run some large circles to see what's up.

Eric- The stiffness in the frame plays a pretty significant role in edge quality. My PRT 120 table is pretty flimsy. If I stand on the middle of it and walk around on it, you can feel the vibration. Imagine if I lightly jumped on it with a program running - it would show up in my cut. My thoughts are that when the bot is transitioning from one bit of code to the next, it is starting and stopping the steppers, this is causing the frame to vibrate and thus, giving a poor cut.

I really don't know enough at all, to comment on different types of controllers and frame stiffness , but I would imagine if the frame is stiffer and the contollers and what ever the drive mechanisms are could be driven at tiny resolutions, this would not be a probelm.

[John S.]

For anyone that's interested; I called SB and was told that from within ArtCam, you can save your sbp paths as arcs. There is a pull down menu at the lower right corner of the save paths window.

I haven't cut from a file yet, but I saved an ellipse path the way I did in the past with the file size being 60k. The file saved as arcs was 82k. I suppose that it just adds more line segments to the file, but I will see when I try it.

[gerald_d]

John, you say you are getting "stepped" sides. Approximately how many steps per inch are you seeing?

Would you mind mailing me some of your files?
gerald@scapenotes.com (mailto:gerald@scapenotes.com)

[John S]

Okay,I was cutting up to 2.75 in/sec I got down to .5 with a 1/4 bit - decent cut quality on a clean up pass. I haven't really timed it yet, but I think it around an hour to cut the 17 ellipses with all the going home and stuff. My thoughts are that with a PRT 96, and a 1/2 cutter, could probably go a little faster, but I am thinking of going even to .25 in/sec on the PRT 120. My guess is that this would also be a good acrylic cutting speed.

Gerald, I think you know about my ellipses. I could n't go with a 1/2 bit because it was too close of a call on the sheet.

[gerald_d]

John S, I only realise today that you are the same person who started the threads called DXF Stepping (http://www.talkshopbot.com/forum/messages/31/1462.html), Vacuum Hold Down (http://www.talkshopbot.com/forum/messages/29/1451.html) and Cutting, through and blind sequencing (http://www.talkshopbot.com/forum/messages/31/1467.html). Between these posts it becomes clear that you are not trusting your table and setup enough to cut nested parts that are 0.6" apart with a 0.5" cutter. If you have such a large room for error, then we will be wasting our time by looking for "bad edge quality" that you cannot bring yourself to quantify.

[John S.]

Gerald, I don't really understand what you are saying about quantifying bad edge quality, but if you mean counting steps per inch, they are arbitrary and could go from 5-20+ steps per inch. And I only have maybe .4 between parts. Since the parts are not size critical, I probably could have used a 1/2 cutter. When I get time, I will eventually try the cutter.

John S.

[gerald_d]

...are your files telling your ShopBot to make these "steps"?

(You said that your circles were good - there is no reason why an ellipse should cut worse than a circle from the SB mechanical/construction point of view.)