Jerky motion on PRTalpha

[dougw]

I am cutting 3&1/2" octagons out of alder. I am getting significant chatter on the 45deg leg between 4 & 5 O'clock. I am using a 1/4" solid carbid upcut spiral and I have had to slow me speed down to 3. I also have significant vibration in the Y car cutting 3" dia. circles with a 1/8th" cutter. I had to slow speed down to 2 IPS.

[elcruisr]

You may need to tune your ramping values, also are you running the latest software? I had some similar issues that I couldn't tune out that where solved with the upgrade.

Also you might want to try the cut with a larger diameter cutter as a test to rule out tool vibration. If it still happens with a 3/8 or 1/2 cutter it's a software or hardware issue.

I'm assuming you've checked all your bearings and hold downs for tightness.

I've found that with certain parts geometries I've had to adjust ramping values and sometimes feed speeds for optimum results.

Eric

[Brady Watson]

Doug,
It seems to me that you are just cutting too fast. Sometimes the easiest way to get top quality cuts is to just slow things down. The time savings is not that significant by reducing the cutting speed to let's say 1.75 IPS compared to 3 IPS. The say this because if software looks ahead and sees that there is going to be an abrupt change in direction (such as the corner) then it will go into 'ramping mode' and not run faster than a certain speed depending on a number of factors.

Yes the Alpha can move very quickly...but other factors come into play when you start cutting at elevated speeds. 1st thing that comes to mind is if the parts are well secured to the spoilboard. I mean really well...If you are using an upcut spiral, then the material is going to want to lift up. The faster you go...the more this happens....particularly in thin materials.

Try slowing things down to 1/2 the speeds you listed above. I think you will notice a BIG difference in cut quality and the issues you described will go away.

-Brady

[propellers]

Doug, We cut Plywood and Maple and found similar chatter results. The above suggestions all work. I would look at multiple passes for the cut, try different router speeds, play with direction of cut, cutting in a offset then doing a clean up pass, if the chatter continues you will fatigue the bit and you will break bits, or the harmonics will cause the bit to move in the collet.

Are you also getting the chatter in the 10 and eleven o'clock position? If so it may be the effect of the grain.

[dougw]

Thanks guys for all your suggestions, it is a bit disapointing to have upgraded from a PR to a PRTAlpha and have to cut at the same speeds!
How do you "tune your ramping values"?

Thanks again
Doug

[elcruisr]

Look under your ramp values and you might try making small adjustments to the move ramp speed and ramp distance. Do one at a time and make adjustments in single increments. Keep your speed low at first and work things up slowly. Try it as air cuts first.

For some parts I've also had to work with corner speed and ramp threshold.

I've found that small, non square, parts are the toughest to dial in followed by parts with lots of arcs (which we do in huge quantities). Rectangular parts are fine with the defaults.

Eric