bit change causes misalignment in cut

[cfgenie]

I have a project cutting 4 round coasters out of a board 24"x5.5". The first board came out great, all 4 coasters are perfect.
The second board, which I ran last night, a week after the first board, was a disaster. I am using 3 g-code files, 3 bits, therefore 3 jobs. The first is a v-bit to cut a nice chamfer edge. The second is a .75" straight bit to cut out a pocket, leaving a 1/16" wall (to hold the cup/drink), the third cut uses a 1/4" straight bit to cut out the perimeter.
So, last night I ran the first cut - no problem. Then I changed the bit to the .75" straight bit and when it started the pocket cut, it was off center and cut into the wall of the coaster and left a thick wall on the opposite edge. I let it go through 2 of the coasters before I cancelled the job. I then ran several more tests where I kept the bit up higher so it didn't touch the wood while it ran, and basically, the fatter straight bit is off. The v-bit and the 1/4" straight bit are both aligned with each other.
What could be happening? It's the same file I used the other day that worked great. The tool is specified in the g-code correctly.

Any help would be much appreciated!
Thanks

[milo30]

There are some posts about machines not going back to zero after running a file. You need to check that before changing bits. Both on the software that it says 0.0 and physically to you starting point.

Are you running a dust boot and if so is it grounded? That can cause issues. Not sure what tolerances you are dealing with so have you measure the end mill as most aren't exactly 1/4"?

[cfgenie]

thanks for your reply milo30. I did install the dust boot in between the first good cuts, and then this problem cut, so I ran the ground wire (which I had planned on doing but hadn't yet). I also swapped out usb cables since I read elsewhere that can cause issues.
I then ran a test and it "seemed" to be all good again, including returning to 0,0,0 after each tool finished. It was after this test on mdf scrap that I noticed it was barely off. I took a closer look at my first cuts that seemed good and noticed if I looked closer, they were also off (unlike the preview within vcarve which shows its supposed to be an even 1/16" all around).

The problem cut where it cut into the side wall of the coaster

test cut on mdf scrap after adding ground wire and new usb cable

You can see the left side is thicker than the right.

I measured the bits and the v-bit is .506 (should be .50). The 3/4" straight bit is .753 (should be .75) and the 1/4" is exact. I then changed my project to reflect these tool sizes and resaved the toolpaths and ran the project again. This time it started to cut into the wall again!

this was the test run on a scrap of plywood. (I also edited the tool to not step over as much, trying to clean up those pieces in the middle, but it still missed several as you can see).

Does anyone have any suggestions? thanks

[tonydude]

Are you using the same post processor?

Tony

[cfgenie]

CNCShark-USB 3D Contour (inch)(*.tap)

is this the correct processor to use? it is the only one I have tried.

[drueth]

Cfgenie

The dia of a V bit is not as inportant as the angel of the V bit If you measure the angle of a V bit by cutting a square with a pocket cut and measuring the square you will probley find the actial angle of the bit is less than than the rated value. As far as why the 0.75" EM is lossing it place it looks like you are cutting oak and you have a very large step over. WAY to much for a bit that big for any Shark. I would try a smaller EM a step over of about 10% and lower the speed of your cut using the FRO setting on the Shark Control Pannel set to about 50% If everthing comes out great move the speed up a little and watch how it goes.


Good luck and let us know how you make out.

[jeb2cav]

These are great points - to follow up on Drueth's - this Vectric post is one I wrote down early on - http://www.vectric.com/forum/viewtopic.php?f=3&t=2164.

With regard to the CNC Shark post processor to use - I'd expect you to get consistent results for each toolpath using the contour post processor. For something like this you may also want to try the non-contour one. There is a difference in the G64 setting used by each post processor. To learn more about that - download the CNC Shark Post Processor pdf from the NWA downloads site. There are pros and cons to using either one. You'll find that you can also manually edit this parameter and then generate a new tap file to see the outcome.

[cfgenie]

I tried changing the bit used for the pocket to a different bit I had, it's a 23/32" flute bit (brand new, Bosch). I also moved the v-bit path out a hair more so that it would leave a thicker wall. Before it was set to about 1/16", now it's about 3/32". I saved all the toolpaths using the CNCShark-USB Arcs (inch)(*.tap) processor, and just to look at what the difference was, I also saved in the other shark usb .tap. I compared the files and the only difference I found was a line of code near the top, the contour usb tap file had G64 P.1, while the arcs tap file had G64 P.01.
I loaded the arcs gcode and ran the v-bit through a piece of scrap plywood. I didn't bother doing any other cuts because I noticed the bit had not exited the circle well and messed up the inner circle. (see the left side of the picture)

Next, I rotated the scrap to start a new circle, but this time I skipped the v-bit and started with the pocket cut. I used the new flute bit, with the new arcs tap file. I was very pleased with the smoothness of the surface, so I moved on to the cutout of the coaster using the 1/4" end mill. It looks to me to be pretty centered and the way it should be, it just doesn't have the nice v-bit chamfered edge.

I did notice that the cut using the 1/4" bit also left an exiting mark similar to the v-bit. I have my code set to ramp Smooth with a distance of .5". I was doing a ramp with the v-bit but I removed it because I thought it was causing a problem, so maybe that ramp needs to be put back?

I have my speeds between 60-75 FRO (because when I ran it slower it left burn marks).
Does anyone think my problem could be my v-bit? It's a brand new Bosch bit I just bought last week. I think the next thing I'll try is a smaller bit for the pocket cut.
I'll include my tap files if anyone wants to take a look. Thanks for all the help here, my machine is brand new. These coasters are my first project.

[cfgenie]

here are the other two tap files

[cfgenie]

Here is my latest test on a plywood scrap. I used a highlighter pen to help make it easier to see. The yellow (v-bit) is all over the place. I don't get it at all. The red on top is clearly not centered and the final outer cut is centered with the pocket. This time I used the 1/4" EM bit for both the pocket and cutout.
I checked zero after each cut and each time it was back where it should be. I used ramps and leads this time on all 3 tap files, but that didn't help with the witness marks.