cnc Centering
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cnc Centering - 11-10-2016, 10:15 AM

Quick question, Anyone have a method to quickly center your cnc cutter over the cue. Seems like I generally eyeball it. I cut a piece of wood of fit between the table and the outside of the cnc frame that seems like it puts if back about the same position every time but im still not sure Im dead nuts. Thanks for any advise. Mark
  
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11-10-2016, 10:23 AM

CNC Centering
To find dead center on your CNC machine. Put broken bit leaving only the 1/8 inch solid shank sticking out. Bring that down and touch the outside edge of the cue or better yet the outside edge of a ground dowel pin. Lets say the pin is 1/2 inch and the center is half the diameter. So the center is .250" and the bit is .125" and half of that is .0625. Add .0625 and .250 together for a total of .3125". That is how far you are from dead center. Raise your Z axis and jog over .3125" and zero it on dead center.

Last edited by cueman; 11-10-2016 at 10:25 AM.
  
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11-10-2016, 03:27 PM


Using an 1/8" or wider bit, mill a small flat on top of a freshly-turned cylinder. Without rotating the indexer any, switch to an engraving bit and scribe a short line that is equidistant from each edge of the flat. When you get your scribe line perfectly centered you will also be perfectly centered on the cylinder.

TW
  
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11-10-2016, 03:42 PM

Quote:
Originally Posted by cueman View Post
CNC Centering
To find dead center on your CNC machine. Put broken bit leaving only the 1/8 inch solid shank sticking out. Bring that down and touch the outside edge of the cue or better yet the outside edge of a ground dowel pin. Lets say the pin is 1/2 inch and the center is half the diameter. So the center is .250" and the bit is .125" and half of that is .0625. Add .0625 and .250 together for a total of .3125". That is how far you are from dead center. Raise your Z axis and jog over .3125" and zero it on dead center.
If you're afraid of crashing into the pin, you can use a piece of paper/aluminum foil/shim/whatever as a feeler instead of touching the actual pin.

It's too bad you're not setup to do a touch off. If you can, setting up a touch off would be high on my list of things to do. Then you can automatically zero your tool height, and if you don't trust your home position you can do a touch off and get that every session too and it'll always be perfect. If you do it right, you can even do tool height compensation, i.e. have one tool for hogging out cavities, switch to a smaller tool for details, touch off and finish, and it'll be perfect.
  
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11-10-2016, 04:36 PM

Quote:
Originally Posted by cueman View Post
CNC Centering
To find dead center on your CNC machine. Put broken bit leaving only the 1/8 inch solid shank sticking out. Bring that down and touch the outside edge of the cue or better yet the outside edge of a ground dowel pin. Lets say the pin is 1/2 inch and the center is half the diameter. So the center is .250" and the bit is .125" and half of that is .0625. Add .0625 and .250 together for a total of .3125". That is how far you are from dead center. Raise your Z axis and jog over .3125" and zero it on dead center.
just to add to that....... assuming you are using Mach 3... touch one side of the cue and then zero that axis............ touch the other side ...... go the DRO in Mach3 for that axis and type /2..... hit enter and the distance with be divided by 2................. press........."go to zero"................... the machine will go to the center of the cue


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cnc Centering - 11-11-2016, 01:59 AM

Can you not use an edge finder and the side of your headstock? Tommy D.


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11-11-2016, 03:48 AM

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Originally Posted by Tommy-D View Post
Can you not use an edge finder and the side of your headstock? Tommy D.
You can certainly use an edge finder if you can slow your spindle way down, but you would never do it off the headstock. It's not nearly accurate enough for that. Even the sides of the chuck wouldn't be accurate.
  
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11-11-2016, 06:58 AM

Building mine to run off a centering camera...displays to either a tablet or the main pc, once you get the offsets calibrated from the camera center to the cutter center it's just inputting the offsets into Mach then it will automatically calibrate zero... just my .02

  
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11-11-2016, 09:46 AM

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Originally Posted by Tommy-D View Post
Can you not use an edge finder and the side of your headstock? Tommy D.
We use an edge finder for our metal milling machine. But I do not have an edge finder that will go into an 1/8" collet for our smaller CNC machine.

Last edited by cueman; 11-11-2016 at 09:54 AM.
  
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11-11-2016, 09:52 AM

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Originally Posted by Thomas Wayne View Post

Using an 1/8" or wider bit, mill a small flat on top of a freshly-turned cylinder. Without rotating the indexer any, switch to an engraving bit and scribe a short line that is equidistant from each edge of the flat. When you get your scribe line perfectly centered you will also be perfectly centered on the cylinder.

TW
This is basically how I do it on my manual pantomill inlay machine.
  
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11-11-2016, 10:27 AM

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Originally Posted by john coloccia View Post
You can certainly use an edge finder if you can slow your spindle way down, but you would never do it off the headstock. It's not nearly accurate enough for that. Even the sides of the chuck wouldn't be accurate.
Actually, you can. Pick a vertical sidewall on your headstock (or bolt on a small metal block) and then set your spindle one time to true top-dead-center - using one of the methods explained in previous posts. Move over to a location that allows you to mill a flat vertical surface on your headstock, and note your Y-axis location readout after your final cleanup passes (this assumes "Y" is your side-to-side axis)

Now, using a piece of ground drill rod (or carbide blank, or shank of broken bit, or...) that exactly matches the bit you used to mill your reference surface (above), you can very accurately touch-off your spindle to the sidewall you milled for reference surface, and then use that DRO number to relocate to Top-Dead-Ccenter.

If you are using a stepper motor on your Y-axis, and if that stepper motor has a double-ended shaft, you can permanently mount a reference disk to the outboard shaft, and a reference block immediately adjacent to that disk. Once you have positioned the spindle so it is TDC, power down your machine. Now power it back up. This necessary off-on sequence will force the stepper motor on your Y-axis to seek a full step position that is as close to your desired TDC location as physically possible.

Now you are here: you have your spindle located at Top-Dead-Center and your Y-axis spindle "locked on" at a full-step position. Using a straight edge and a sharp scribe, strike a witness line from the center of your outboard shaft, across your reference disk and reference block. This will give you a mark you can very accurately use to reset TDC whenever you power up you machine. Assuming a Y-axis drive screw of at least 5 rpi resolution (.200"), the next full step on either side of TDC will put your witness mark NOTICEABLY misaligned, making your centering operation extremely easy and repeatable.

I believe Jake (and some others here) got a chance to see this method used on a machine I built for Richard Black several years ago. That worked flawlessly until I later replaced the steppers with servo motors, and he now uses a different method for locating TDC.

TW

  
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11-11-2016, 10:50 AM

Thanks Guys for all your help. Seems like there is a few different ways to determine the center of a work piece. My machine has a fairly long point on the forearm end of the cue. About 1 1/2 inches long x .4 diameter. If Im understanding this right. I can use a 1/8 inch dowel pin in my collet and move to oneside of the point and zero that. Then raise it and move it .625 for 1/2 the pin diameter and .2 for the point diameter and that should be center. I have mastercam version 8. A much older version. Is there some way I can home this position so that I can always come to this position? Thanks again for all your help. Mark
  
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11-11-2016, 03:59 PM

I would set it up so you know where the spindle of the set up is. I like using dial indicators and have an indicator set up in a solid collet with a nut for the ER20 that my router has. You can scan one side , then lift up and scan the other side, 1/2 the difference you will be in the middle. My router does not home Zero all that well, it is inside about 0.1mm or 4 thou in inches mode. In Mach 3 I have made a 4mm ball that sticks out from the spindle 20mm, it is a solid piece with it's dedicated nut as well. There is also a 7/16 ball bearing on a mount as well that sticks out 1 inch from the collet. I can use the touch probe setting and it will touch down onto the circuit board and will be the 1.50 mm thickness of the board away from the part for either height setting or for side positioning in a vice for example.
Once you have the centre of the part or the centre line of the axis of the setup, a pin pointer can be used to get the radial position correct on the part or any of the other mentioned ways of setting up. The cameras work well too if they are correctly set for that machine.
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11-14-2016, 09:19 AM

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Originally Posted by whammo57 View Post
just to add to that....... assuming you are using Mach 3... touch one side of the cue and then zero that axis............ touch the other side ...... go the DRO in Mach3 for that axis and type /2..... hit enter and the distance with be divided by 2................. press........."go to zero"................... the machine will go to the center of the cue


Kim.
nice, simple solution.
  
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11-15-2016, 02:06 AM

Once you find dead center, turn off your machine, and run the Y axis to a hard stop. Fire it back up, and then note the exact measurement to return to TDC. Do it a couple of times, and if it's reliable, you can find TDC really fast by simply going to the hard stop, then jogging over the proper amount.


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