Chatter on diagonial strokes of pvc letters

[roy_shimp]

I am still getting chatter on the diagonial strokes of pvc letters. I have read the posts regarding chatter. The chatter also shows up on O's, but not as much. The chatter is on the diagonial strokes like the letters K,M,M,R,V and W. I run at .5 ips, 13k rpm (3/4" pvc). Height of the letter doesn't seem to make any difference.
Some of the ramp settings are as follows:
X-Y MRS .1 / MRR .2 / MDC .3 / SCS 40.
The bit i use is an Onrud 52-282B. I realize it is an hdu bit, however i get the detail i want and the rest of the letter is smooth (just issues with the diagonial strokes and some O's). The diagonial stroke chatter is more pronounced than the chatter on the O's.
Has anyone else cut pvc letters and had this chatter issue? How did you resolve the problem?
Thank You, Roy

[jerry_stanek]

What machine are you using. I cut faster you may be at a speed that causes resonance. I know if I run to slow on diagonals my machine seems to stutter.

[roy_shimp]

PRT 96. 5Hp Colombo spindle. I got the shopbot in 2000. Since then (last year) i double checked for rails level and square and had the gantry squared and tack welded.

[Brady Watson]

Has anyone else cut pvc letters and had this chatter issue? How did you resolve the problem?

Hey Roy - glad to see you're back.

1) File the edges of your steel rails on the outer edges. Small burrs will cause micro vibrations that will telegraph into the cut. Just use a file by hand and take down that burr. Then follow up with some sandpaper and a Scotch pad to smooth them out. Use no power tools - they can get away from the steadiest of hands...When the rails are smooth, hit them with a rag and WD40, then keep them as clean as you can. Sawdust and chips laying on the rails can get embedded and act like little speed bumps. Your Scotch pad should take that off - or scrape with a razor blade.

2) Replace and grease your pinions. These are consumable and should be replaced when cut quality degrades. You can order them from ShopBot. Get 2 for the X and one for the Y. The Z is probably fine, unless you see signs of backlash in the Z (3D jobs only). Make sure you apply grease or you'll kill them pretty quick. A little dab every 6" will do ya - wipe off the excess or you'll be wearing it.

-B

[roy_shimp]

Brady - I did replace the gears late last summer, however i will remove the speed bumps. By the way, your explanations on ramp settings have been a great help. Thanks

[roy_shimp]

I got side tracked; this post is about chatter ON THE DIAGONIAL STROKES OF PVC LETTERS.

[Brady Watson]

Roy,
With the control box on, grab the body of the router and give it a good shake in all directions, noting if there is any slop present. If there is, see if you can nail it down. Spend a good 10 minutes really looking it over and wiggling to see if you can sense any deflection or slop. On PRTs, the T-rail that the router is attached to can get sloppy between the t-rail & v-rollers on the Z tower.

-B

[srwtlc]

Roy, you say that you got your machine in 2000. If it hasn' been upgraded in any way (meaning motors and electronics), and it has the direct drive motors that were used at that time, the diagonal chatter was common and quite pronounced with them. I had one of those PRT's (Nov 2000) with that kind of motor and diagonal cuts were quite bad. I ended up changing out the motors to some 3.6:1 gearbox ones that they started using a year or two later. That helped considerably but never removed it completely. I'm sure that the old controller has to take some of the blame too.