need help, cuts wrong width

[tatinc2000]

I have been having issues with pocket cut being a little smaller than they should. I have test my machine with Mach3 using micrometers and such. It is within .002.
it is in when i manual jog in mach or when I cut profile cuts. When I do a pocket it is off. Here is a test I set up. 1 inch squares, inside, outside, and pocket, also a smaller 1"x.220" pocket. All using the same cutter. .125 end mill, it mics at .123 and is set up in the tool as .123. I don'y know what I am doing wrong that it is off in just the pockets. Any help would be great. Here is the test piece, screenshot of tool tab, code, and V-Carve file.

[scottp55]

tatin, What are measurements when a conventional cut is used? Thinking possible bit deflection? Have you tried 2 toolpaths? One with a pocket allowance or profile offset of say .015" and then a clean-up pass to take the skin layer off? Just thinking.

[llwood]

I am having a similar problem, and I've found that it has something to do with the cut depth. In the attached .crv file, there are two toolpaths. They both use the same tool and the same geometry. However, one pocket is bigger than the other. The only difference is that they have different cut depths.

I know this has nothing to do with the machine, because the toolpath preview shows them as different sizes. In fact, if you check both toolpaths to make them visible, and check "Show 2D previews" and show the 2D drawing view, you can see that the toolpaths shown are different. Even though it's the same geometry and the same tool!! How can this be?

If anyone knows how to fix this or to workaround, please let me know.

Thanks,
--Andy

[scottp55]

Ilwood, It's the .25R on the bit. On the .1" cut only a portion of the radius is cutting so it can get closer to your vector. In the .3" cut you are using the full tool diameter so it has to "step back" to stay within vector.

How do you like that cutter for smaller pockets? Is bottom finish good?

[llwood]

Oh, yes, you're right, Scott. Thanks.

The pocket I'm cutting is 1.5" deep. For efficiency, I am using a 40% stepover for the first 1.4", and I wanted to do the final 0.1" with a stepover of 10% to have a smooth bottom. But when I did that the last 0.1" was cut too big! I guess I need the second toolpath to be at least 0.25" in depth.

Thanks, again,
--Andy

[scottp55]

Andy, Just thinking and wondering if it couldn't be deflection? What's the shank on that bit, and is it an actual radiused end mill-or a bowl and tray style bit. For a similar project but not as deep (I was just playing with yours) I used this type of strategy for my SE Tools 1/4" shank 3/4D .25R bowl bit and it seemed to work well. Are the nodes on left arc necessary?
Wish I knew what was going on with Tatin's problem

[tatinc2000]

I havent had a chance to get to the machine since I posted. i will try the cut with a conventional next time I get out. I may not make it out there til next week sometime. Busy with the kids going back to school and such.

[tatinc2000]

I've made a file that is
climb - offset,
conv. - offset,
climb - raster,
conv. - raster
all with the same tool as before

Maybe I can squeeze 20 minutes in tomorrow to cut and measure.

[scottp55]

Tatin, Just a thought, but could 1 be run with a pass depth about .06" just to eliminate deflection. How much bit is sticking out?
Sorry if I'm being a pain but it's just bugging me.

[vgraves]

I had a similar issue some time back. You might want to read through this thread.

http://forum.vectric.com/viewtopic.php? ... 02#p133802

Van

[tatinc2000]

None of you are being a pain. Just helpful. I ran this tonight
climb - offset,
conv. - offset,
climb - raster,
conv. - raster
all with the same tool as before

then I ran it a second time to check deflection. I will do it with a 1/4 next time around. The numbers changed a little but not much.
There is a little deflection in the raster cuts, look at the corners.

[tatinc2000]

I had a similar issue some time back. You might want to read through this thread.

http://forum.vectric.com/viewtopic.php? ... 02#p133802

Van

Some good info in there. I know my machine has some deflection, it has 1/4 alum for the y legs and z plates also. I just need a machine that can get me into a better machine. I'm getting there, one sign or cribbage board at a time.

[Gc3]

forget offset on something that small...raster with profile pass first using ramp option...

as the saying goes...you can't make a silk purse from a sows ear