What's the right way to speed up and slow down for curves?

[woodturner]

I hope this question isn't too elemental, but was wondering what is the best way to cut fast on the straight and make a significant slow down in a tight curve. I am cutting 3mm sintra with a 3/16 O-Flute (straight bit) @18,000rpm. I tried an up spiral o-flute (great chip extraction) but lifts the material as it rounds the corner. I am cutting @6"/sec. Finish is OK, but getting chatter in the corner (looks faceted). Using Part Wizard. Thought I might try entering an MS line (to go to 3"/sec) before each curve and another MS after curve (back to 6"/sec). Or do I mess around with ramping values (sounds scary to me, not very comfortable modifying programs yet). The shape is a modest sized rectangle (approx. 32" x 8") with 10mm radius and narrow slots (hence the 3/16" bit). I have to cut 600 of these by close of biz on Monday so cycle time matters. Dedicated computer (Pentium 2.4, 256 RAM) on an ALPHA. Thanks in advance.

[richards]

Ed,
The latest Command Reference Manual (download it from the Shopbot website) has an excellent section on ramping.

The chatter (facets) that you're getting may just be the way that the Shopbot cuts. Many of us have unsuccessfully tried lots of different speeds, ramps, and cutters in an effort to reduce chatter. There are photos posted that show problems with arcs, 45-degree line segments, and line segments that are part of the ramping part of a cut. You'll probably also note that the chatter can easily be sanded away without much effort. I know that sanding is an extra step and has the danger of distorting the cut, but it may be the only way to handle the chatter problem.

[gerald_d]

Mike, I think that there is consensus on the "chatter" being related to speed. So, Ed's thinking is logical.....slow down for the curves...

This has been mentioned on the Forum before: "..... a setting for Slow Corner Speed : a tool tip that pops up when mousing over that setting states this "Percentage to slow from full speed on gentle corners". The range is 20% to 99%." but have not used it myself.

[richards]

Gerald,
The photos that I posted on May 13, 2005 (Techniques for Cutting, Drilling, Machining / Quality of Cut) showed 45-degree cuts made at 1-ips and 5.5-ips. Both cuts show significant chatter.

Since those photos were posted, I've tried many different combinations of feed speeds and RPM without significantly improving the cut.

I totally agree that slowing down for the curves "should" improve the cut, but it hasn't worked for me - yet.

[gerald_d]

Mike's reference Techniques for Cutting, Drilling, Machining / Quality of Cut (http://www.talkshopbot.com/forum/messages/29/7835.html)

[woodturner]

Thanks Mike and Gerald for your thoughts. Mike I saw the same results when I tried 3,4,5,6 IPS. No change in corner cut quality. I also tried entering a slower MS before each curve and a faster MS after each curve. That didn't look so good. The head would come to a complete stop then jump forward into the curve, then stop and jump into the straight cut. I didn't even try to run a part. I figured it would look worse stopping and starting like that. I would like to figure out how to "smooth out" that curve.

[woodturner]

Oh, and I forgot to mention the latest (and even worse) issue. I am going to post it in the Troubleshooting section. Thanks again.

[gerald_d]

Ed, have you looked at your .sbp file to see if your curve is actually a "circle" command, or is it a bunch of straight lines. Apparently there is a setting in PartsWizard which can cause this.

[richards]

Ed, Gerald's right. When you save a PartWizard File, be sure that the "Machine output file is formatted for: Shopbot(arcs)(inch)*.sbp".

If you select Shopbot(inch)*.sbp - which I did - ONCE, you'll end up with a file that is LONG, perhaps thousands of lines long, depending on the circle resolution, etc.

[woodturner]

Thanks for the tip. We always save in Shopbot (arcs)(mm) since we do everything in metric. It is a CG command.