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  #1  
Old 03-26-2021, 05:39 PM
William William is offline
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Default Cutting guidelines for the new

Hi All,

I'm getting ready to make my first cuts and, like most newbies, am a bit tense about feeds, speeds, and cutting strategies. Some of the starting guidelines I've found in other threads are:

max Pass Depth = bit diameter
Feed Rate in IPM = (bit diameter / .125)*60
For best finish, climb cut for roughing passes then conventional cut with a full depth finish pass.

I'm hoping you all can help me with a few follow up questions.

First, I have a Cobra with a 5HP spindle, I'll be starting with 1/2" bits, and cutting 3/4" plywood. Does that effect the above guidelines at all? Should I be doing full depth cuts instead of multiple passes?

Second, what guidelines do you all use for lead in and lead out? If I'm taking a 1/2" or even full depth cut, I'm guessing I shouldn't be plunging straight in to those depths.

Lastly, I'm much more concerned with edge quality than cutting speed and I'll ultimately be cutting smallish parts. Combined it seams like the onion skin strategy with the a finish pass is a good one for me. So, two questions, when you're leaving material for a finish pass I assume you're leaving it both in the depth (onion skin) and radially along the edge. Is that correct? Any guidelines on the appropriate amount of material to leave for the final pass?

Thanks,
William
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Old 04-01-2021, 02:04 AM
William William is offline
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Default Could use help fine tuning

I don't know if you old hands remember what it's like to cut your first CNC project but it's pretty exciting.

The first picture below is the model I'm using as my first project. I've now cut it three times, tweaking each time trying to dial it in. On my first go, I got a lot of dust and was obviously burning things. I have since slowed my rpm and increased my feed rate to increase my chip load.

The second picture is a close up of the plywood edges. You can see that I'm still getting burning on selective layers of the plywood. There's also, to my surprise, some very small ridges or chatter marks in some areas.

The outside cut is done in two cuts:
  • 1/2" down cut bit, 2 passes, climb cut, ramped, leaving .04 offset and an onion skin. 12,500 rpm, 400 rpm
  • Finish passe: 1/2" compression bit, single pass, conventional. 12,000 rpm, 400 rpm
The difference in the rpm's wasn't intentional. Also the sound of the roughing cut, sounded like it was straining a bit (if I had to guess). Not enough rpm?

Any suggestions on which direction which to tweak?

Thanks,
William
Attached Images
File Type: jpg 135 degree shape.jpg (42.9 KB, 26 views)
File Type: jpg Plywood cut edges.jpg (90.8 KB, 50 views)
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Old 04-01-2021, 07:29 AM
Ken Rychlik Ken Rychlik is offline
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I prefer straight bits and don't do climb first. I have never had great results with spiral up down like some seem to do. IF your bit screams it is hungry. I do twice bit diam. for cut depth and for speed bit diam and feed speeds go together. .25 goes 250ipm .5 goes 500 I mostly stay in the 12-15k spindle range

For ridges, to me, it's a sign of movement. Either material moving, machine needing adjusting, spindle bearings, ect... Something moving and not tight.

I do ramp into the cutouts. That leaves a tapered area for then the part is being cut out all the way, it is only cutting the thing ramp off.
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Old 04-01-2021, 08:04 AM
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james mcgrew james mcgrew is offline
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Old 04-01-2021, 08:59 AM
TimPa TimPa is offline
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i don't think you have burning on that plywood edge view, pretty sure it is just different woods in the layers which are different colors.

bit burn would be more consistent across the (layered) surface.
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Old 04-01-2021, 01:37 PM
William William is offline
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Quote:
Originally Posted by Ken Rychlik View Post
I prefer straight bits and don't do climb first. I have never had great results with spiral up down like some seem to do.
I don't have any straight flute bits right now but it's on my to do list to try. Are you using single flute or double flute?

Quote:
Originally Posted by Ken Rychlik View Post
I do twice bit diam. for cut depth and for speed bit diam and feed speeds go together. .25 goes 250ipm .5 goes 500 I mostly stay in the 12-15k spindle range
Those are great guidelines, thanks. Between the sound and the diameter = feed rate guideline, it sounds like my feed rate is still to low. I'll try moving it up to what you're suggesting.

Quote:
Originally Posted by Ken Rychlik View Post
For ridges, to me, it's a sign of movement. Either material moving, machine needing adjusting, spindle bearings, ect... Something moving and not tight.
That's kind of what I figured. The changes from the previous try were a different piece of plywood, I increased the finish amount to take of from .02 to .04, and I added the holes in the middle. That could all add up to more vibration/wobble in the part. I'll keep an eye on it.
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Old 04-01-2021, 01:43 PM
Ken Rychlik Ken Rychlik is offline
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My normal is a 2 flute.
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Old 04-01-2021, 01:50 PM
William William is offline
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Quote:
Originally Posted by TimPa View Post
i don't think you have burning on that plywood edge view, pretty sure it is just different woods in the layers which are different colors.

bit burn would be more consistent across the (layered) surface.
Thanks for that feedback. I'm definitely unsure whether it's actual burning or not for exactly the reasons you stated. The reason I thought it might be is because the layers which are getting discolored aren't nearly that dark on the outside edges. Also the layers are obviously different materials so I wondered if one material could be more sensitive to heat than another. I do notice this same material will give little micro tare outs where the other layers won't.
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Old 04-05-2021, 04:36 PM
William William is offline
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Default Ramping guidelines

Can anyone share guidelines they use for ramp settings? In Fusion 360 the default is a 2 degree ramp and that seems conservative. For the piece I'm playing with it's over a foot of linear travel on the straight cuts and many, many circles for the helix ramp.
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