Pockets and Islands -

[2Bits]

So has anybody else experienced this? I have a desktop hobby CNC so I generally see results within .010 of the designed measurement. I can cut circles, curves and boxes all day and get within .010 except when pocketing/area clearance that contain islands. Actually the depth of the clearance and size of the clearance is within tolerance, but the islands get clobbered to where they're .025 and greater out of tolerance.

For example, using a .1181 bit, I created a file to cut:

3 holes, 2 with islands

1st hole - .175 diameter – measures .181 difference of .006

2nd hole - .5 with a .175 island: .5 hole measures .494 difference of .006, island of .175 measures .151 difference of .024 ????

3rd hole – 1.0 with a .5 island: 1.0 measures 1.010 difference of .010, island of .5 measures .464 difference of .036

The holes are within tolerance, but anything defined as a island gets whacked. The bit isn't too big, is there something I've got setup wrong or doing incorrectly to cause this? Wrecked a couple of parts, because my 'Islands' which were to be pins of a specific size are undersized.


2bits

[metalworkz]

Hello,
Are you sure the bit is .1181"? Are you using climb or conventional milling?
Try drilling a hole with the .1181" dia end mill and measure the diameter. The hole size may give you and idea what to expect when milling with that tool?
With the holes being larger than drawn and the islands being smaller it seems that maybe the bit is larger than you thought or maybe there is some run-out at the end of the bit(bent tool?) Try figuring the average of the hole sizes and islands to see what size tool would produce those dimensions and then try using that size to drill a hole, if it works then use that size for the tool dia.

[2Bits]

You bring up a good point about the size of the bit etc. I've never measured it, just relied on where I bought it, it states it's .1181 in size. It's not runout as I even bought a new better router to replace the old one (about 5 hours on it) that had some runout. I verified that this one did not and that the bit was straight. But even if the bit was bent or runout, it doesn't explain why the outer dimension is within .006 and yet the island is off > .025. I would expect both to be off roughly the same amount if runout or a bent bit.

I will measure the bit tonight to verify it is what it says it is and drill a hole with it to verify what I'm getting there. This just seems odd to only have that much variance on just the islands. To me it almost seems like the calculations are based on an incorrect offset, such as the inside or none of the island rather than the outside. Yet I don't believe this software could have a bug like this and I'm the first to notice it. It's just odd. I even changed the milling procedure to mill the .175 first, then do the pocket and when doing the pocket, it goes all the way to the island and shaves off more resulting in the error.

[John S.]

Is the tool climb cutting on the pocket but not the island or visa versa ?

To get a good comparison the tool direction has to be the same on both entities.


John s.

[metalworkz]

Yes, as John stated you will want to use the same cutting direction for both features, and cutting the pocket and the islands with different methods may explain the difference in the finished sizes(.006" and .025") ??

[2Bits]

Measured the bit and confirmed it's .1181

Verified when selecting machine a pocket, you can't specify separate cuts for the pocket (outside perimeter) and the islands. You can only select Climb or Conventional for the pocketing. In this case it's Climb.

Drilled a hole with the bit and measured exactly the diameter of the bit
This third test produced the same results so the machine is consistent. Where's the problem? I can create a .250 circle or whatever size in CAD Create my toolpaths, goto Milling AND do contour cut and get .256, but if it's created with the pocket function, this will be .225 or less.

Am I wrong in my thinking that if 3 tests give the same results, it can't be due to machine slop etc? Outside parameters are close to specs and only islands are off? I'm real new to this, but a few things aren't adding up, but I don't know where to look. I have a rather complex part I cut out and the only cuts with issues are islands.

Using Mach3, I'm no expert in that yet either, but I haven't specified any backlash offsets or anything, running pretty much defaults.

[metalworkz]

Until the problem can be pinpointed if there is an necessity to run the parts it would seem like your tests would provide the information to set up the toolpaths to cut correctly if you compensate for the difference either in the tool definition or by offsetting the toolpaths the needed amount? I know this is more of a workaround than a fix but sometimes when time is a problem it may get you through the current project and then you can try to find and fix the problem. If there are more and different toolpaths to do then it may not be feasible as you may need to test each tool used and compensate for them.
Do you think there can be lose stepper motor connectors?
There is a test for the axis to see if they are set up accurately on the Settings Tab, just above the Reset button there is a 'Steps per unit' button and when pressed it will basically walk you thru moving the desired axis a specified distance and then you input how far it actually moved etc. to work towards checking and setting up each axis. You may need to use a dial indicator etc. to get accurate measurements.
Regards,

[2Bits]

Thanks for the info Wes, time is not a big issue here, but I miss being able to cut. Kinda surprised at how much I enjoy this, but willing to wait a bit rather than figure out how much to redo toolpaths. Will it be consistent with different sizes? How many samples of sizes to I cut to compare? I haven't tried dropping a square as an island or some other irregular shape to see if it does the same thing.

I have from a couple of friends gotten some pattern tests that verify movement is good including a large sectioned off square with a spiraling circle if that means anything. I've done the Steps per unit and it comes out ok. Stepper motor connectors are good, besides, it wouldn't be bad only when doing islands. As a application developer, I can't find a mechanical reason for it only happening on islands, but I find it hard to be a bug within the software and I'm the first one to find it. I figure this is a pretty common task.

I am surprised that I haven't heard anything from Dolphin support yet. I contacted them last Friday on this and normally they reply right away, although it was Easter weekend if that has an impact. Or maybe I did find something and they're duplicating or attempting to duplicate it which is why I haven't heard anything yet. I sent them my files so they would have what I do and hopefully see the same thing. That's why I posted this here, to see if anyone else has run into this. Maybe I'm leaving out some key piece of information and I have something configured wrong. I have only been at this since the beginning of March, so I don't rule that out either. At the same time, Dolphin CAD and CAM is pretty straight forward on setup, for the machine, tools etc.

I just got an email now from Dolphin, said they're working on it, but as I stated, the Easter holiday is a factor. I'll wait and see what they come up with before I start revamping for now. Thanks again for your input or any future ideas to the problem.

Thanks!

Mark

[metalworkz]

Mark,
You are welcome, but I wish we could have resolved the problem. Rodney will take good care of you as a customer and they should be able to help you resolve this. I am a reseller for Dolphin USA myself as well as a customer and along with the other Dolphin users I like to help when possible. It is a very good CAD/CAM software at reasonable prices, and I think more people are taking a closer look at Dolphin.
I hope you resolve this issue soon and get to the fun of making chips!
Regards,

[2Bits]

Thanks, I like the software and can't find anything I can't do with it. I like the integration of a decent CAD package to to click once and I'm in the CAM section. I've cut alot already with no issues for a beginner. This is just another learning curve and I'm sure we'll get to the root issue eventually. I have to admit that I was looking at Cut2D, but to me this is easier to use. I do like there tabs feature though.

Anyway, here's a pic of what I've been recently working on. It's a throttle lever (simplified) from a Learjet 45 for a cockpit simulator. The small rectangular piece is the cap with the pins (island) that didn't come out right. But once we solve this, I'll recut the cap and it's done and ready for assembly adding microswitches etc. All the pieces designed and created with Dolphin CAD/CAM by a novice.

Exploded:


Assembled:

[metalworkz]

Mark,
The parts look great!
Speaking about tabs, did you know there is a feature in Dolphin they call bridges that does tabs with a lot of options? Cut2D is a great program as are all the Vectric software but under the GoAround machining in the CAM side there is an option in the Options tab for Partial machining where you can designate the start and stop position and create tabs by keying in the length of the tab at the begining and end of the profile. It may not be as easy to implement as the interactive tabs in Cut2D but I think it has many more options.
They used to have a video showing in depth use of the bridge/tab option.
Regards,

[2Bits]

Thanks!

Thanks also for the heads up on the 'bridges' option. I'll have to check that out. I've had a couple of small parts pop out on the final cut and get whacked, this would prevent that.

Mark