Poor cut at high speed

[bob a job]

Hi All
Having a problem here with one axis. Cutting at 1500mm P/M it’s perfect. Cutting at 3000mm P/M I get a ripple effect cut on one axis.
I have just upgraded my machine with open bearing and supported shafts on the XY axis. On the X axis I have 2 toothed open belts one either side of the table with 2 stepper motors. The belts are T5/16mm wide with a 14T pulley. This axis is perfect. On the Y axis is where I am getting the problem. It’s the same set up as the X axis but just a single belt taking the router across the bed. This really flies. On the single belt I seem to be getting a poor cut at over 1500mm P/M. It’s like a ripple effect. Now, I am cutting 18mm mdf with a 5mm down cut at 6mm per cut. I am told that the depth of cut is to deep for the cutter but I have been cutting like this for over a year now. I am told by the cutter suppliers that if you can hand touch the cutter with out burning your fingers and its clean at the end of the cut your set up is ok. Well it is so I have counted that out. My set up before was skate bearings and bright steel runners with one on each axis T5/10mm belts. Been cutting at 1000mmP/M for over a year. I upgraded because of the clogging of the bearings.

Any ideas?? I’ll try to take some photos today.

Roger

[mlabruyere]

Sounds like chatter.....I was getting this as well at high speeds. I had to go through the machine and re-align all the axis and got the machine totally rigid. This took care of my issue.

I have never used linear rails and blocks but I would assume these would be very precise and allow no slop. With the machine powered (router off of course) pull on the gantry back and forth. See if there is any play at all. Then push on the Z-Axis for the same. If you get any movement you can detect with your hands then you have too much slop.

[bob a job]

I can’t feel any movement. I even re adjusted my rails. It’s much more rigid than my other setup. I'l have a go at getting some pic up and a vid.

[mlabruyere]

Have you changed the bit and or the collet since you did the upgrade?

The belts sound pretty stout so you shouldn't be getting any rebounding.

We are starting to get outside my already limited knowledge on the subject so I'll leave it to the "big dogs" of CNCZONE to help more....

[bob a job]

I changed to a new cutter but still the same so changed back. Then while the machine was running I heard that vibrating sound. Held a bar between the spindle mount plate and the Z axis rails. And just lightly levered them apart. Couldn’t see any movement but the sound stopped. So I made a new spindle mount bracket on the quick. Now this seams to have stopped. Haven’t tried it at high speed yet so will have a go tomorrow.


I have just added some pics.

[BobWarfield]

Originally Posted by mlabruyere Sounds like chatter.....I was getting this as well at high speeds. I had to go through the machine and re-align all the axis and got the machine totally rigid. This took care of my issue.

Absolutely sounds like chatter, and that seems to have been confirmed.

Chatter is a resonance phenomenon. Changing your speeds and feeds may allow you to move outside the resonance. For that reason, always try going faster as well as slower. You just need to get out of the resonant zone.

Changing rigidity of the machine or setup will move the point of resonance too.

You can't really get rid of the resonance. It's only a question of whether it coincides with the speeds and feeds you need to use, at which point it is a problem.

Cheers,

BW

[mlabruyere]

Yeah....my whole machine vibrates when I am homing. I have the homing set to 10% so it runs at around 50IPM when homing....man I need to adjust that....that can't be good for the bearings.....

[bob a job]

Well I stopped one part of the problem by making a new spindle bracket but still got some ripple effect on cutting. After stripping down the machine twice I have found out that it’s the open bearings. When I slide the beam up and down with just the bearings connected I can feel the balls rolling. If I unscrew the bearings and slid them up and down by hand they are very smooth. But if I twist them by hand again I can feel the bearings. So now I have put it down to the bearings not in line. Its not a lot but I am wondering if I should change them to self aligning bearings. After re aligning and squaring up I can now get 2000mm per min.

I have put up some pics.