Problems milling a "perfect" straight line on a VTL with live tooling
Problems milling a "perfect" straight line on a VTL with live tooling
(OP)
We have a 1991 Berthiez (french) Vertical Turning Lathe (model TVM-100). The vertical ram (Z axis) has a live tooling attachment for milling & drilling. In turning / boring mode, the table can reach 600 RPM. In milling mode, a precision, low speed gearbox servo engages the table for a max of 5 RPM. We are trying to mill a rectangle shape into a surface. There is no Y axis ... only X, Z, and B (table rotation axis). Therefore, to mill a straight line, the B and X axes must be synchronized within the FANUC series 15 parameters to yield a perfect point-to-point straight line. However, no matter what line length or feed rate we program, we are consistently getting what appears to be a slight angle at the center distance of each straight line cut. However, it is possible the "out-of-straight" line is really a very shallow curve. For example, in a 16" travel cut, we are getting about 0.005" "out-of straightness" on what should be a perfectly straight edge. As the low speed table rotates clockwise, the part initially rotates "away" from the tool, and then rotates "into" the tool after passing the midpoint of each straight mill cut. It appears the "bends" in the straight milled edges are at the center distance where the X axis starts to reverse direction to maintain the straight cut. In other words, it appears there is something not working correctly with the synchronization of the table rotation (B axis) in relation to the X axis movement. Hope I explained this correctly. Anyone have any ideas where to start, or what to look for? It looks as if this problem may be a common one for any machine that uses this type of syncro between axes to mill straight cuts ... or any other specified shapes as well. One person has raised the question about whether backlash in either axis could be the culprit. If so, how does the Fanuc program compensate for the backlash (assuming it exists)? We have measured near zero backlash in the table, but we have not measured the X axis. However, we have recently completed many precision acme threaded, mating parts using the high speed turning / boring mode of the table, where it must syncronize with the X andf Z axis ... all with no problems. So we assume there is no backlash problem. Thanks in advance for any help on this issue. BTW, we have tried to get technical help from the factory on past issues, but with very little success. Our 1st attempt using the GE Fanuc technician has not worked. GE has another tech coming soon, but thought we may get some help from this forum too. Thanks again.





RE: Problems milling a "perfect" straight line on a VTL with live tooling
You may be able to approach straight but you will never get there.
It seems from your posting that the axis syncronization is working correctly.
Check your setup and programming parameters, live tooling attachments at notorious for not being as rigid as main spindles on CNC's. Are you removing all the material in one pass? Is this too much to ask from the live tooling head in terms of rididity?
You say that
It appears that there may be backlash in one of your axis drives. When the direction reversal occurs, this is where you are having your problem.
If this is the case, try taking multiple lighter cuts and see if the problem diminishes. You may need to either baby the machine with light cuts in the future or look into having the axis rebuilt. We are not exactly talking about a new machine here, so this may be the correct way to fix the problem.
Good Luck!!
RE: Problems milling a "perfect" straight line on a VTL with live tooling
Thanks for your insights. I realize there is no such thing as perfection (unless you talk to my girlfriend). We are getting a much greater "out-of-straight" condition than what the machine is capable of achieving. I don't believe it is a tool rigidity problem due to the way the live tooling head is designed & attached to the ram. This is the first attempt at using the live milling function since the machine was installed in 2003 (it was purchased used at an auction). The original factory parameters are reported to be still in the software. One thing we did during installation was to rebuild the low speed gear box. In doing so, we reduced the backlash in the main table spindle from about 0.010 to 0.015" to less than 0.005". This was measured as indicator movement at the OD of the 1 meter diameter table while prying back & forth on the table with a 16" long prybar. So what we are wondering is whether there is a "compensation" parameter in the FANUC software that is set for the original 0.010-0.015" backlash. Maybe we are now over-compensating with the reduced backlash. In this particular operation, the cutting pressure on the X-axis (i.e. the cutting tool) is always "pushing" in the same direction, even when the X-axis reverses direction, so we don't think there is an issue with backlash in the X axis. However, the cutting pressure on the B axis (i.e. the table spindle) is in the clockwise direction for the 1st half of each striaght line cut, then reverses to counter clockwise pressure past the mid-point. So we may have an issue with backlash compensation on the turning table. We will investigate that possibility next, as soon as Fanuc tech rep arrives later this week. This run of parts is made from 3"T 316ss plate cut outs. We are taking lots of relatively light cuts. I don't believe we are over stressing the ram or live tool holder. The live spindle drive motor is only about 9-10hp, with only CAT-40 tooling. The main spindle motor is 60-70hp, so the X & Z axes are very rigid for the heavy duty turning & boring functions. In any event, I will pass your suggestions on to the operator to see if that makes any difference regarding taking lighter cuts. Thanks again.
RE: Problems milling a "perfect" straight line on a VTL with live tooling
I have never seen this exact same problem before but have you tried to compensate the "out of strighness" by modifying the program. Since you dont have a Y axis youll have to make the table spindle go back and forth a little in the are where you have the problem. The controller should have a backlash parameter but I dont think the problem you have is there.
Good luck.
Colega
http://www.corporacioneg.com
RE: Problems milling a "perfect" straight line on a VTL with live tooling
RE: Problems milling a "perfect" straight line on a VTL with live tooling
RE: Problems milling a "perfect" straight line on a VTL with live tooling
Somewhere in the background of your control there is a type of logarythm that calculates how to "synchronize" the X and rotational axis. This Log uses a constant number in the form of a parameter setting somewhere (probably well hidden, and for good reason). If you try this same type of cut at different distances from center you will most likely find a pattern to how far "out of straight" the line is milled. If you have the machine lazer calibrated by a reputable company, they can probably fix this. Unless it is a mechanical problem (backlash as suggested earlier, or a guideway or encoder problem). This is the gamble, you have to pay them even if they find a mechanical problem.
RE: Problems milling a "perfect" straight line on a VTL with live tooling
Maybe you just can't do that piece on that machine, it may be out of capability. Sad, but may be true.
Colega
http://www.corporacioneg.com
RE: Problems milling a "perfect" straight line on a VTL with live tooling