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Strange Behavior

Bill Arnold

Bill Arnold

1974
Staff member
Messages
8,633
Location
Thomasville, GA
No, not me - a carving process!

I posted this on Vectric's forum, but wanted to see what you folks had to suggest.

I just had a strange thing happen on a cut. I'm trying to carve radius rails for two door panels. They are 3/4" thick by 1.5" wide by about 6.75" long. All looked fine on my drawing and preview in VC, but when I started the carve on my CNC, I got the results below. When the carve started, it was cutting only 5/8" wide for the first four passes. The remaining nine passes were cut at about 11/16". It should have cut at 3/4" thick all the way through.

Top of carve
100_1486.jpg

Bottom of carve
100_1487.jpg

Screenshot of my settings
Screenshot 2018-08-10 13.54.40.jpg

This ain't my first rodeo, but it's the first time I've had something cut smaller than it should have.
 
I posted the same response on Vectric, you don't need to post the CRV file here


Bill,


I can think of several things.


Can you post the v-carve program .crv file?


First off the cuts look rough, maybe I am not seeing it well enough.


How are you holding it in the machine?


There may be some flexing of the material, or the machine.


Do you have a good sharp cutter?


Posting the .crv file will also help answer questions


How is you machine calibration? Have you checked to see if the machine accurately cuts a circle or a square?
 
Leo Voisine said:
I posted the same response on Vectric, you don't need to post the CRV file here

How is you machine calibration? Have you checked to see if the machine accurately cuts a circle or a square?
Click to expand...

I responded to your post on Vectric, attaching the .crv file.

The machine accuracy seems fine. I've cut other radius pieces with the same bit and they were dead on. One piece I cut is a full circle and it was perfect.

ShelfParts_a .jpg

I'll do a test cut of the piece in question today using a different bit to determine if that could be an issue.
 
Bill,

I don't see anything wrong in the CRV file. A bit fast on the feedrate maybe.

As the other poster mentioned about bit sizes, I don't see anything wrong there either. The actual bit would need to be about .06 larger diameter than .25 to cause the problem that you are seeing.

You are set to conventional cutting. Maybe Climb cutting will help. I am climb cutting almost exclusively.

You have said that the machine accuracy is good. I am assuming that you did some test cutting and actually measured to determine the accuracy.

Over all though, it sounds like the part is moving as it is being cut. It is a better dimension where the screws are holding, and does it taper smaller at the top of the cut?
 
I'll back the feed rate down and change to climb cutting to see how that affects the cut.

I did a quick check this morning by making one shallow pass in MDF. The bit measures exactly 1/4", is cutting at exactly 1/4" and the overall dimensions of the test were correct.

Accuracy: as I said above, the circle and radius pieces in the photo were dead on using the same bit.

The work piece did not move in any way, nor did the base board.

The cut didn't taper. As I said, the first four passes were at one dimension and the final nine were wider, which really had me wondering . . .
 
Bill Arnold said:
The cut didn't taper. As I said, the first four passes were at one dimension and the final nine were wider, which really had me wondering . . .
Click to expand...

The first four were at the top? The final nine were at the bottom and wider? That sounds like a taper to me. That would indicate that the material at the top was moving, flexing, and the bottom, being closer to the screws holding the material did not flex.
 
Leo Voisine said:
The first four were at the top? The final nine were at the bottom and wider? That sounds like a taper to me. ...
Click to expand...

From M-W:
Definition of taper
1 : to become progressively smaller toward one end
2 : to diminish gradually


The cut started small at top and jumped to a little larger on the fifth pass, so it was a stepped cut. Another test run held the same dimension all the way through. I'm beginning to suspect an issue with the bit and will run another test with a new bit Monday.
 
"I'm beginning to suspect an issue with the bit and will run another test with a new bit Monday."
Or maybe the bit wasn't tightened properly. I've had similar issues with my router a couple of times.
 
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