thermwood model 67 question

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thermwood model 67 question

Postby machineman » Fri, Jun 23 2006, 12:29PM

our cnc machine axis-4 would not turn when a negative number is punch in. it'll go -2, if more than that, then it'll say out of bound, but if a positive number is punch in it'll go, but it'll touch the maximum stop pin and the machine would stop. i try to change the inlimit and outlimit, but it wont let me. i don't have much experience on cnc machine. anybody have any idea.

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Brad McIntosh
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Model 67 C Axis (Axis 4) Limits

Postby Brad McIntosh » Fri, Jun 23 2006, 7:16PM


The C axis (axis 4) on a Thermwood Model 67 5 Axis router has a range from -2 degrees to +362 degrees. (Some models MAY be configured -182/+182, depending on a clients special request.) These limits are intended to prevent the various electrical and electronic connections that run through the C's rotary \"shaft\" to the B axis (axis 5) from winding up on themselves. If the wiring is allowed to twist up upon itself, it may be damaged and render the machine in-operative.

** Many, many years ago the 5 axis machine did not have such hard limits on the C axis. It was the responsibility of the programmer to limit the number of \"winds\" and to ensure that the C axis was \"unwound\" at the end of the cycle/program. - Too many \"oversigts\" were probably made. I have seen older machines with wires so ensnarled that it is hard to differentiate what wire was what.

Again, the current C axis limits with the hard-stop is in place to prevent potential damage to the wiring running to the head.

There is a over-limit switch which will E-Stop the machine BEFORE it actually hits the HARD-STOP. This is to prevent the machine's C axis drive from tripping out if it were to actually be stopped by the hard-limit. This limit switch is what is throwing your machine into an E-Stop condition (and rightly so). (There is a Over-travel limit switch on the front of the cabinet that will allow you to \"short\" the limit switch while you \"teach\" it back into the working envelope. Use it with care. If you teach the machine the wrong way, the hard-stop will be hit and the axis 4 drive will kick-out.)

Note: (IMHO) Changing the In and/or Out Limits of any axis without being advised to do so by an authorized Thermwood representative could be very dangerous for you and/or your machine.

This hard-stop on the C axis is usually accounted for by the post processor of the CAD/CAM system that you are using. The -2/+362 degree range allows for an overlap that, with the use of a good programming strategy, allows a post processor to handle any transitions - in one direction or another - that are required.

If a toolpath attempts to cross over this boundary, the post processor will perform a \"retract and unwind\" routine which will:

1. Retract from the workpiece.
2. (Usually) lift up and clear the part.
3. Perform a 360 degree rotation of the C axis (the direction will be dependant on the toolpaths direction and from where the C is prior).
4. Lower the head to the previous XYZ position with the equivalent compound C/B vector angle.
5. Return to the former cutting position relative to the workpiece.
6. Continue cutting...

There are programming techniques to minimize these \"retract and unwinds\", but sometimes they cannot be avoided. A programmer will usually evaluate the machining operation and will select an appropriate C and B angle starting combination that will eliminate or at least minimize \"retract and unwind\" situations.

The above \"retract and unwind\" sequence can also be done by a programmer who is \"teaching\" the machine manually.


If you would like further information about this subject, I am sure that a Thermwood Technical representative would be happy to help you out - or you can post any further questions here and I will try to help you.

You may also want to consider contacting Thermwood about attending a training class at there facility.
Brad McIntosh
CNC Automation

Twitter: @bmcncautomation

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