Need Help cutting 1/2" HDPE

[jay]

I have to cut a couple hundered of these 1/2" HDPE parts.


7109
I am having trouble getting rid of the tool marks (actually chatter) ark on the ark ends of the part. I have tried a 1/4" single edge down-cut spiral o flute (onsurd 62-775)@ 1.7 inches per sec, a 1/2" down-cut spiral double edge (onsrud 57-923)@ 3"per sec, and a regular 1/4" straight cut bit @ 1.7 inches per second. All the above leave chatter on the ark ends of the work. OH, Oh yes I have tried both climb and conventional cuts with very little difference in the appearance of the chatter.

I telephoned Onsrud and was told that my chip load, speed, stepdown were exactly right.

Anybody have any other suggestions. These parts have to come out clean, I do not have time to sit in front of a sander to clean all of these up.

All suggestions welcome.
1/2" HDPE Part

7108 (15.2 k)

[artisan]

Hi Jay. It's pretty obvious that it's not your bit. HDPE cuts like butter anyway. Try either increasing your resolution....or manually adding points to your arc. For instance, in Corel, you can create this piece, and then go back and insert extra points along your arc and then use either the "smooth" command, or the "to curve" command (right click on the added points and these options appear) to make sure the points follow your arc smoothly. You know the Bot cuts curves as a series of straight lines or facets, so the more facets you have, the smoother and less noticeable the flat spaces are. This is also known as higher resolution and can be handled with software....depending on what you are using....D

[stevem]

Jay, the grain in extruded plastics runs in the direction of extrusion (this is important if you are moulding sheet plastic), but I suspect Darrell has the problem nailed.

Will the Bot control software still interpret an ark as straight lines if it converts G code I and J commands?

[jay]

Darrell, I use ArtCam 6.08 and have not had this problem with wood. (maybe one just does not see it) I was thinking I would use Corel or ArtCam and take out all the short little lines and smooth only one node on each arc. But there is something nagging at me about PartsWizard and ArtCam having a problem with arcs, in the download section there is a fix for inches and for metric which I have downloaded and attempted to use. No change in chatter.

Steve, good question re G code, I will have to look into it.

[Mayo]

I cut HDPE in 1/2" thickness and I have never been able to eliminate the chatter marks. I have been able to produce less of it though, when I slow down my cutting speed to about 1" per second.

I have not noticed much if any difference between down spiral or up spiral or using a bit specifically for soft plastics - I think it's a 5933 double flute.

You may also want to try adjusting your ramps or your circle segment resolution (the 5th parameter of the VU command) this is detailed in the command reference section of the shopbot manual under VU.

[gerald_d]

"the Bot cuts curves as a series of straight lines or facets"

This is only slightly true. The mechanical Bot makes very tiny steps due to the nature of the stepper motors. However, your software that is generating your cut file may be using coarse straight lines to approximate a curve. I understand that this is an issue with PW.

[johnny_s]

Jay - I have had this issue as well. I have actually tried everything that Mayo has with the same result. The best I could do is follow up with a cleanup cut - say .010 or so. This helps quite a bit. Not the fastest way, but results are passable.

I was cutting black, and it shows every little thing. Aside from the cleanup cut, a 1/2 cutter at the 1" per per second might be my second best choice.

Also, if you ARE getting facets or steping, I import any non-geometric curves from adobe illustrator. I just make sure that the curve in partwizard has "handles" on them and not points going along them. Handles look like points, but they have white "handles" on each side of it, that you can pull on to deform the curve.

[jay]

Well guys thank you for all your suggestions. Here is what I have done to make the product more accetpable.

Exported my drawing from ArtCam as a .dxf. Opened ArtCam for a new layout and imported my .dxf file back. Clicked on the Node tool and made sure there were as many nodes as possible in the arc ends. Went back to the Onsrud 1/4"bit @ 18,000RPM, climb cut, .7 inches/sec., with a .27 stepdown. (I was using the age old rule before of half the bit diameter as the stepdown, hence 1/8")

The arc ends are now smooth to the touch although you can still see very small tool marks but you can't feel them. If I'm happy the customer will be happy. Thanks again for all your help.

Yes sir, the forum sure helps make the ShopBot what it is, a GREAT TOOL!

[amy_brown]

I want to make sure I'm understanding you correctly. Did you say you added nodes to make it smoother on the arcs? Our MultiCam had stepper motors and I always found that if I didn't get rid of most all nodes in Corel before exporting the arcs were bumpy.

Did you mean you took out all the nodes you could?

Thanks.

[jay]

Amy, yes I had to add nodes. It did help to smooth out the arcs. In other materials I have always smoothed all the nodes and removed as many nodes as possible to get a nice smooth arc, but for some reason with the HDPE it did not make smooth arcs unless I added nodes.