3D milling 8mm bit issue

[yafimski]

Hi,

I am working on my first 3D milling wood piece. The 3D model is a concave fruit bowl, with multiple concave parts to it.
I'm using a stepcraft 840 machine, with a MM-1000 spindle.

The first settings I used were:

8mm, 3-flute bit
30% stepover
Pass depth 4mm
12000 RPM
15mm/sec feed rate
10mm/sec plunge rate
machine allowance 0.5mm
ramp plunge 5mm

The program settings were a 'roughing pass' for the 8mm, and I also have a path for a smoothing with a 3mm bit.

Once the job started, at first it went well, but then I noticed it was a bit jumpy as some spots, so I thought to raise the RPM.
I raised it to 16000.

The I thought it might be good to lower the feed rate, so I lowered it in UCCNC to 60%, which makes it 9mm/sec.

But at this point, the bit was strangely waaaay too deep in the wood, kind of double the depth it should have been.
Then quite quickly the whole piece jumped, I think because the wood touched the shank of the 8mm bit, because it was too deep in the thin cut it made in the wood.

The questions are:

1) Are the feeds and speeds I set wrong?

2) After this fatal error, I would take out the 8mm bit from the spindle by hand.. so I think the collet is broken... how can I check and make sure if that's the case?

3) How is it possible that the spindle went down to a Z of more than 4mm, if the pass was set to 4mm? (the first few cm of top-surface were cut to 4mm, but then it went down further without completing a first pass).

4) There was literally a hole straight through the spot where it jumped. Only by chance it didn't go through the entire 38mm. It's not supposed to go any deeper than 20mm into the material, more or less, according to the file.


I would really appreciate any help and insight to prevent this from happening again...

Thanks!

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[ZASto]

It seemsthat you did not tighten the bit in the collet enough and it creeped out.
Anyway, by FSWizard with rpm set to 12000 your feed should be around 25 mm/sec, and at 16000 rpm around 33 mm/sec

[yafimski]

Hi,
I used the FSwizard before setting it up, but I didn't get your results. Can you explain perhaps a bit what you put in the setup there? I put the diameter and sizes of the bit, and the material as wood, with a depth of cut at 4mm.

What else should be there? It tells me that with 12000 rpm, the feedrate is 98mm/sec...

[yafimski]

Hi,

Followup issue:

I tested another time with the 8mm bit, with the following settings:

8mm bit, 3 flutes
5.1mm pass depth
10.2mm cut depth
30% stepover
12000 RPM
15mm/sec feed rate
10mm/sec plunge rate

This is a simple curve file with a Profile toolpath, so some of the settings don't matter.

It started nice, but my spindle has a manual set of the RPM value, and I forgot that it's still on 25000 RPM. About 30% into the job, the depth increased, so I stopped it to see what happened. I lowered the RPM to 12000 and ran it again, but still - same issue of suddenly going deeper into the wood..

I thought it might be again the clamping nut and collet, but it isn't, I checked and the bit is still the same and it didn't slip out.

The wood plate was secured from the 'top' on one end, but from the 'side' on the other end, meaning I placed another piece of wood and clamped it down, to block one end from going forward, while the other end was held down by screws.
So I think that one end was open to movement in the Z direction, and the power of the bit, regardless of the precise settings, 'pushed' up that end of the wood plate and that's what made the cut deeper.


What do you think?


Thanks!