Quill often oscilates... Bridgeport Interact Sereis 1 Mk 2

[MeTri]

Hi again,

I now can say I've machined something with the CNC. I've written a small prgram and cut a slot. Not so earth shattering, but exciting to me.
I have noticed a small issue with the quill. If I set a depth it will go to it and oscillate about a thou, maybe a thou and a half. if I put pressure on the drive pulley it seems to stop. I don't have to continue to put pressure just a bit with my thumb and it gives up oscillating. My first thought is parameters, but if anyone has seen this before I'd love to hear about it.

Also a general question. I'm using ISO programming mode because I'm familiar with G Code. Well I set the thing to Inch, but the feed rates still seem to be set in Metric?! Can anyone confirm this is true. mm/min seems to be how it's acting... it's not thousanths per sec or something silly?

Thanks again

Brian

[machintek]

Quill could be a parameter (have you checked them against the list that came with the machine). Look at backlash parameters too. Could be drive tuning. Is the belt too loose?

How many places are displayed to the right of the decimal point? If 3 then it is displaying metric. If 4 it is displaying inch. Just because you are doing G code does not automatically put you in inch mode.

George

[MeTri]

First things first. Inch vs Metric. The machine readout is in inches and yes it has 4 decimal places. When I start a new program, the first thing I'm asked is "Inch or Metric" so I choose inch and put the G70 in the first line of the file. all moves use inches, it's simple the F that's added to the G01, that seems to be metric.

The hunt and peck with Z. Parameters are exact as came with the machine. I entered them myself and double checked. No backlask in Z in the parameters. It's both the readout that shows the hunt and peck and I can see the motor/belt moving. Noting is loose. Parameter seems most logical. PO sent the servo out for repair for this issue but they sent it back saying it was perfect.
So that, to me, leaves a parameter or the servo driver... The parameter in question could be the derivative for Z or possibly the "square root characteristic"? If you don't know, it's alright, I'll tinker and see what I come up with.

George, since you seem to be reading my posts, I'll address this to you directly, Is there any oiling or anything that should/could be done the the spindle motor? Mine seems to squeak a bit when it starts up but goes away after 15 seconds or so... I can deal with the noise I just fear doing damage.

This machine is new and I have a lot of unknowns. Like I always hear a transformer buzz. I assume this is normal?

Last night I finished by saying the machine shorted out and blew a fuse. I fixed that this morning and nothing else was damaged.

Thanks again.

Brian

[machintek]

Brian, do a test. Put in a feed rate of 10.0 which should be 10 inches per minute. See if it traverses that distance in a minute.

I believe the spindle motor has sealed bearings. But below it is a cooling fan. I am not sure what bearings that has. It has been a long time since I worked on one of these. There is a filter there that you may want to clean. Also removing the fan will expose the spindle motor brushes which may squeak a bit as well.

Personally, I have seen a lot more dramatic oscillation of a axis when the TACH feedback loop was compromised. A thou oscillation I have not seen. What mode is your display in? Actual usually shows a flicker of a tenth or 2. Thus usually the display is left in nominal. Yes, I did read that you were watching the belt move.
I have to find a procedure to tune those drives.

George

[MeTri]

I did a test with the feedrate a 2, and it crawled. I did a bunch of milling at 80, which in inches per minute would be flying! I wanted about 3, and 80 seemed about right. It's just so wierd to put the feed rate that high when all my coordinates are in inches. I keep expecting the machine to "wake up and smell the units" and just fly the tool and have it break off.... stranger things have happened.

If by display mode you mean units it's in inches. The other axes are bang on, but the z moves about a thou. 2 tenths or so is no big deal, in fact I don't think you can actually reliably make the machine move less than a thou or maybe half a thou on a good day. Oscillation, in my mind should be a control issue. If it was only a single oscillation I'd bet money on the control issue, but since it's almost always present... The good news is the PO had this trouble and it's never gotten worse. For contouring it's not even a real problem (unless it chips my carbide tooling, then I'll be pissed :P ). It's only for 3d shapes and what not that it would show in the finish. Right now I'm cutting slots in steel, which is not exciting, but needed for a project.

I'll leave the problem for now as annoying but livable and check the square root characteristic with the oscilloscope when I feel inspired. I'll check the fan for the noise and look at the brushes on the spindle motor. I'm a bit paranoid with this machine because it's an expensive toy and right now I can't afford to replace it. I'm still paying off the electricians bill! I just don't want anything to happen because I didn't oil a bearing...

Thanks again for the help

Brian

[gus]

feed: 10 is a feed of 1 ipm, 100 is 10 ipm 1000 is 100

If the parameters are set right and the drive is good, the trailing error needs to be set. 151 and later have a mod function, 145 has a parameter, that will display 'LAG' set the display for this, write a program to run the z back and forth to near its limits and adjust the 'tacho' pot [if it is bosch drives] to get it as near zero as possible. start with the feedrate pot down and work it up until it is max.
If that doesn't fix it then it is something more obscure.

[MeTri]

Thanks Gus, I'll try that. Very much appreciated.

Brian

[MeTri]

I fixed the quill oscillation problem today. It took quite a while but I figured it out. The PO told me he had removed the Z servo and sent it for inspection and it came back ok. I assumed the problem was a driver board. My machine is 4th axis ready so I have 4 driver boards. I don't have a 4th axis so I figured I'd swap out the 4th axis servo driver board for the z-axis board and be done in no time. On the Bosch driver boards is a small plug in board with 2 pots on it. In the maintenance manual it says to check certain values of pots and resistors. I did that and one was out so I adjusted that, but the oscillation just slowed down and the X and Y axes didn't seem to stick to a value as well as before. Then I swaped the whole driver board and still didn't fix the problem.
Discouraged I decided to replace the z-axis drive. The PO said it wasn't needed but... so I swaped out the motor. The old servo was the problem. It turns, but in distinct steps. Maybe 20 "steps" per rotation. It felt just like a stepper motor, but it's a servo! When I turned it back on with the new motor everything was perfect!
The machine would seem to hunt if it had to stop in between steps. There was always a force trying to twist the servo to one of the step positions. It took a fair bit of effort to move past these steps moving the servo by hand. Also it would cause my quill to pulse downwards when drilling slowly. Not good for bits etc.

A very happy CNC owner

Brian