3D finish creating flat spots using offset.

[whitefishwoodworker]

I am in the process of making some curved molding for a client. I used the two rail sweep in Aspire to create the files. Because of the curves, I want to run the finishing toolpath in offset so it follows the direction of the curve. I started running some sample of the molding in a straight piece just to determine feed rates, speeds, etc and I have some strange things happening.

I have a Buddy standard with the Porter Cable router, z axis is perfectly plumb to the table, table is perfectly flat, I used Andrew's mod with the roller blade wheels to hold the table flat in the cutting plane, brand new bits, perfectly milled S4S wood.

Finishing toolpath:
1/8" tapered ballnose bit (starter set)
running at 4 ips
router speed 13000 rpm
roughing tool path done with .03 machining allowance

I am 100% sure that the roughing toolpath is not cutting deeper than the finishing.

At every high point in the curves of the molding, it is leaving a flat spot. It also left a nasty ridge in the top at the exact spot it starting the offset cutting. If you watch the control box, it clearly changes depth on every pass, maybe 1-3 thousandths but it is not running multiple lines at the same depth. Looking at the toolpath file in editor, every line shows a different z depth, indicating that is making a curve, not a flat spot. When the CNC is carving, I can see that the finishing bit is actually cutting lower than the roughing in every spot.

If I take the exact same file and change it to raster, everything comes out perfect.

Any suggestions on what might be happening here?

Phil

[gc3]

molding profile like that forget offset toolpath, raster is the way to go running the length of the blank. I never rough simple profiles like that, finish bit only

[bob_s]

It's not you or your machine This is a problem with Aspire's computation of the cutting I have pointed it out and they have constantly ignored my question. Use a raster path and avoid the problem aspire is a great program and a good value, but this is definitely a flaw in their 3d to cut file computation

[whitefishwoodworker]

Gene, the moldings are all arches ranging from 24 inch to 60 inch diameters. I want the offset to follow the curve of the wood. If I run raster, some will be against the grain, some with it, and the rest somewhere in betweeen.

[dmidkiff]

I think you want to set your start point below the surface of your wood. That should eliminate the flat spot. You should see it when setting up the tool path.

[whitefishwoodworker]

This is a 3D finishing toolpath, there are no options for setting start points like you would with a 2D toolpath. The 3D rough has a machining allowance which I have set to .03 which leaves that much wood on top of the profile to ensure that the rough does not cut into the finish. I wish it were something that simple but the two moldings in the photo are the EXACT same file, the only difference is offset vs. raster.

[gc3]

I think you want to set your start point below the surface of your wood. That should eliminate the flat spot. You should see it when setting up the tool path.

material setup>model position in material>gap above model/gap below model

[whitefishwoodworker]

I have the model gap set to .03, using .75" thick wood with a model thickness of .6875, which leaves .0325 at the bottom. Remember any error in settings would show the same for both photos as it is the exact same file. I have no doubt that this is an offset vs. raster error in the software. I am just wondering if anyone else has had this problem and can offer some advise from experience.

I have carefully watched the milling and the flat spots are being carved that way, they are not the top of the original wood. Each curve gets lower in the model plane so if it were not carving deep enough only the highest one would have a flat spot.

[whitefishwoodworker]

I went over the entire machine today and everything seems to be in place and correct. I went into my VR settings and changed the 3D ramping setting from 100 to 50, changed the 3D finishing toolpath from 2ips to 1ips on the plunge, left the x/y speed at 3ips. The two largest curves came out better, the smallest curve came out exactly the same with the flat spot. It also left the same horrible ridge where it started the offset carving.

Just to see what would happen when a raster toolpath cuts across the grain when I mill the arches, I ran this straight piece with a raster 3D finish toolpath at 90 degrees so it cut the entire piece across the grain. This would be the worst way to cut this molding, I know, but I wanted to see what would happen. It took forever but had the smoothest curves out of any test I have done so far, even rastering with the grain.

It just seems to me that offset toolpaths simply don't work, making 90 degree turns in the work area, whatever, I don't know. It makes no sense to me and will probably triple my work time for this project. I don't know if it's Vectric, Shopbot, a combination of the two, or just the way the moon is lined up with the earth right now but for $2K software and a $13K machine, one should be able to use the functions of the software, run a file in the most effecient manner, and have it work. I'm pretty dissappointed right now with my purchases and feeling a bit robbed.

[bob_s]

I have done a lot of very precise custom moldings. It is Vectric software. This problem doesn't always show up, but it can be terribly frustrating when it does. I just raster cut and try not to get frustrated. CNC software isn't simple, and it is difficult when problems occur in some "black box" situation like this. Try and get an answer from tech support at Vectric, maybe if enough of us complain about this they will fix it.