How Should I Machine This?

adjusting gibs, stepping up the rpm a tad (48 to 50 hrtz on vfd) and reducing lead in speed seems to have helped (and adding the finishing pass), the features are slightly trapezoidal so I have backlash over compensating or leadscrew wear to address but I think we're okay at that point.

I was under the impression you don't want to climb cut in a machine with leadscrews unless taking very small cuts (or I could adjust the gibs /really/ tight(?) so what is the story there?
 
I climb mill on my RF-30 all the time but with a bit of caution. I adjust the depth and width of cut to minimize the chance the cutter will grab and pull the work piece into the cutter. My gibs are not adjusted tight but not loose either.

I use climb milling almost exclusively on my CNC mill leaving about .010" for the finish pass.

Tom S.
 
Metal said:
I was under the impression you don't want to climb cut in a machine with leadscrews unless taking very small cuts (or I could adjust the gibs /really/ tight(?) so what is the story there?
Click to expand...

It is related to backlash, when climb cutting the work is being pulled toward the cutter. The heavier the cut the harder it is being pulled. The machine has to be rigid enough to overcome this force. If there is backlash the table can move uncontrolled for the distance of the backlash on a transition, this can cause the tool to grab. Tightening the gibs will help by adding friction to the system, but you can only go so far with this. The way to really fix the problem is to eliminate the backlash.

Does your machine have acme lead screws or ball screws?
 
It has lead screws which is why I was sticking to conventional
 
In that case, maybe the best bet is to do the roughing conventional, then climb cut the finishing pass(es) Or just take smaller bites so the mass of the machine helps to overcome the tendency of the tool to grab. Lighten up the DOC and stepover rather than reducing the feed. The cutter needs to cut rather than rub, sharp cutters will help.

For any really serious work the only solution is to install ball screws, preferably double ball nut so they can be preloaded. Unfortunately it's impossible to have a zero lash acme screw / nut. If there was zero clearance it wouldn't turn.

I wish I had a better answer for you.
 
I've had a backlash laden mini mill cnc'd for about a year, it hasn't been a huge issue for me, the bridgeport cnc is just proving much harder to tune up to where I want it.
 
One totally crazy idea comes to mind, you could preload your lead screws with air springs. Connect air cylinders to the X & Y axis on the table and it would eliminate the backlash. I'm doing this with my Z axis, but I never thought of trying it on the table. Probably not really practical.:confused:
 
Well that certainly is one way of increasing resistance!

I could push the machine more softly, but truth be told I just don't want to run the bridgeport at full speed (over 3k rpm) until I get it tuned in, and have a shield of some sort in place since this is quite a bit scarier than my little hobby mill. But I've picked up a handful cheap of 3 flute hss 12mm endmills that I'm not afraid of breaking that I'm going to try tonight probably with some new code to be a little more gentle once I properly setup backlash comp again.

I also need to rebuild the head as low gear is noisy and I'm getting grey oil at the base of the spindle probably because of it but that is for another day.
 
For projects that require a lot of material to be removed I've become a big fan of using roughing end mills for removing most of the material and only using standard end mills for finish cuts. Smaller chips, deeper cuts and faster feed rates.
 
Metal said:
this is quite a bit scarier than my little hobby mill.
Click to expand...

I had the same problem when I first got my CNC. Was scared to death of all that new fangled stuff. I used it for about a year as a manual machine with a fancy DRO before I had the guts to actually run it as a CNC. It just takes some time to work up to that. :) It's a learning curve. Now after about 3 or so years I have confidence in me and the machine;)
 
You must log in or register to reply here.
Back
Top