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Author Topic: Re-starting on a partially cut piece of material  (Read 5236 times)

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Offline rickw

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Re-starting on a partially cut piece of material
« on: February 19, 2011, 07:09:13 AM »
Please forgive me for sounding so ignorant but it's because I am still figuring out the CNC process. I have been cutting with minor problems but learning as I go. I have been designing using AutoCad and importing into Mach via CamBam. So far, I am not a fan of CamBam but maybe I need to use it more but I am only using it to transfer my AutoCad DXF files to code. If anyone knows an easier way, please let me know but my question is not with CamBam. But I do have a question on why this happened in the fist place because the slots were fine in AutoCad but did not show as one entity when in the CamBam program. 

I imported a DXF file into CamBam in order to develop code and when importing into Mach, something didn't transfer correctly. I wanted to cut a a few slots in the wood but only one cut. I have a large piece of material mounted on the table with the piece partially cut. I will need to fix the file somehow (haven't figured out why it did this) but I want to finish cutting the fixed material since it is a large piece and I don't want to waste it. Finally, my question. How can one align the cutter to cut on the same path as a previously cut piece? I have saved the fixture 0,0 coordinates from yesterday to try and get it aligned today. I have previously clamped the piece in the correct spot and started with plenty of material to spare and have been successful but now I need to be more precise. I knew this time would come but now I'm stumped. If I import the same code (file) with the slot problem corrected, how can I get the cutter on the original cut path? The only thing I will have to correct is the slots and all of the other lines/cuts are in the same exact locations. I don't have one of those fancy centering jigs yet but I want to make one for this reason. But I still need some assistance finding the proper procedure for aligning.

Thanks!   

Offline djc

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Re: Re-starting on a partially cut piece of material
« Reply #1 on: February 19, 2011, 11:55:39 AM »
I wanted to cut a a few slots in the wood but only one cut. ...I don't have one of those fancy centering jigs yet but I want to make one for this reason.

Can you post a drawing of what you are trying to cut, with the outline of the material overlaid on it and the slot that is OK highlighted?

Some general thoughts. Have you cut the outline of the piece yet? Is it square to an axis or two axes? Is the slot square to an axis? Is the OK slot straight-sided? What collet system are you using?

Easiest is with two square edges both located at zero on their respective axes. Spindle off. Position the cutter until it is outside the outline of the part. Lower the cutter until it is say 70% of the part's depth. Jog until the cutter just touches the work. Then zero the axis (by typing into the DRO). Raise the cutter until it is clear of the work, use the MDI to move over half the cutter dia. and then zero again. How do you know if the cutter is touching? Use a cigarette paper and when the cutter just traps it, you will be 0.001" off the work. Or turn the cutter upside down in the collet, cover it with a marker pen and when the ink just transfers onto the workpiece, you are there.

You can do similarly with a slot the same width of the cutter. Leave the cutter slightly loose in the collet. Jog over until the cutter just drops into the slot and then enter the position in the DRO.

You can put a pencil in the spindle, true it up so its point is on the centre of rotation and draw some lines on your workpiece. Record the coordinates of the lines, measure how far they are from a known or wanted feature. Plot them back into your ACAD drawing as offsets from the same feature in the model and read what the coordinates should be. That should give you the translations you need.

If the slot is angled, make a male version of the slot that you have cut on a new piece of material on top of your current one. Square up the edges. Turn over and put into the good female slot. Indicate off the edges of this (remember to take account of any mirroring when turned over).

Offline RICH

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Re: Re-starting on a partially cut piece of material
« Reply #2 on: February 19, 2011, 12:21:09 PM »
rick,
I always align my cutter to an edge or the center of a hole. Thus you have a repeatable point to work from during set up.
RICH
Re: Re-starting on a partially cut piece of material
« Reply #3 on: February 19, 2011, 01:34:06 PM »
You really have a small number of choices to solve your problem. Because you did not plan for this eventuality, you must rely on 'poke and hope'  by moving the tool over the partially finished work until you feel satisfied with the alignment. You can do trial cuts on overlay stock. There are lots of tricks you can invent to get satisfactory results.  

To plan ahead for disaster recovery, you need to have either:
A) two reference edges that you can keep until your final cut (easy to do).  
B) a reference edge and a reference hole (easy to do). The edge gives you angular alignment and the hole gives you offset to 0,0.
C) two reference holes (harder to do)

I have recently begun to use the reference edge and the reference hole as the easiest practice to use, and it has saved my butt several times.

You also need a dial gauge, that can be easily mounted on the Z-Axis, for any reasonable accuracy in the repeat finding the reference to the X and Y axis. Your eye or a ruler is simply not good enough.

Mach 3 has an edge finder input that is incredibly nice, especially for finding the center of a hole. I made and use an aluminum corner finder that allows me to find the edge (0,0) on a rough or rounded corner reliably. It will allow me to find the corner either by the hole method or the edge method on both metallic or nonmetallic stock. It has two holes, one over the 0,0 of the corner, and the other at 0.5, 0.5 from the corner. If you don't use the Mach3 edge finder feature, then set it up and use it. You will love it.

I have created several macros (various 0,0 finder routines using either edges or holes)  that I can drop into my g-code programs that allow me to preset the reference every time I run the job.  Now each job has built in disaster recovery. As long as I don't move the work piece, I only need to find 0,0.

good luck,
Bill