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David - Machinist in wood
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Discussion Starter · #1 ·
For the last 18 months or so that I've had the CNC running I've been very diligent about setting Z height. I often leave it a few thousandths high so I don't cut too far into the spoilboard, although I have no issues cutting into it 0.005" to 0.010" or so. It's easy to resurface, after all.

In the last week I've had 5 orders for Longworth chucks and also cut the Command Chief pieces so the CNC has been busy. I cut the Longworth chucks out of 1/2" Baltic Birch but it's really 0.480" and that's what I have my file set for. To set Z height and zero that setting I just use a piece of paper under the bit and slowly lower it to touch the top of the work piece to where I can no longer move the paper. That's close enough on these.

Well, today I did the same thing and proceeded to cut the first chuck plate. I cut the finger holes first and then the slots and then the outer perimeter. Because I have done so many of these and the 'sound' was correct I began working on other things while my 'employee' was busy on the chuck plates. When it got to the slots I walked over to look and noticed one of the finger holes was right above a spoilboard mounting screw. I sure am glad I chose to use plastic screws to mount the spoilboard!

I just resurfaced the board last week and promptly cut into it a bit further than I wanted with the Command Chief stripes but what I did today was a LOT deeper. While I did touch the bit to the top of the work piece I had failed to reset zero and I ended up cutting about 1/8" deeper than the bottom of the work piece.

Oh, well, I'll let it ride for a while and surface it again although I probably won't bother taking all of today's botched cut out. On a good note, the Longworth chucks came out just fine. :wink:

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David
 

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While I've never cut into a mounting screw, my spoilboard shows considerable evidence of encounters with a too-deep Z cut.

On our multicam CNC there is a setting to limit and prevent any cut from going more than .001" (or your set amount) into the spoilboard. For every cut both the top of the work and the spoilboard top are set, so the machine "knows" where the spoilboard is and can keep programming/g-code mistakes from cutting into it. I'm getting into the habit of not trusting material to be consistently thick, and setting Z on the spoilboard rather than the material top. I don't set it at zero. I set it at a negative value of what I told Aspire the board thickness was. The guarantees that all through cuts make it through, and never do more than a skim cut into my spoilboard.

Mistakes can come from boring repetition, distractions when setting up a cut, or time-travelling parallel world generating events. ;) I'm glad to see you and the bit survived this one. A lesson learned, perhaps. :)
 

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Mike
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David, definitely check the thickness of your materials. I bought 3 sheets of 1/2" BB plywood about 4 months ago and two were the same thickness that I was used to getting and one was .027 thinner than the other two sheets. I found out when I cut deeper into my spoil board than it should have. Hate the overseas quality control or is it them trying to save a penny here and there.
 
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David - Machinist in wood
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Discussion Starter · #4 ·
Thanks, Mike. I check each one and the last two sheets have both been 0.480". One I got a few months ago was 0.476" so they've been close but I check them. What I have to do is actually hit 'Zero Z' when I touch the bit to the board. For some reason it won't do that for me. LOL!

David
 

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Mike
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On my older machine I had the same problem, if I forgot to zero the Z axis after zeroing the bit to the material it never would do it on it's own for some reason. :grin:
 
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Probotix came up with a feature that no doubt was inspired by the setting of Z after changing bits. I don't have that feature on any of the Probotix CNC I oversee, but I certainly will add it if I'm ever ordering another CNC from them. It is a Automatic Tool Length sensor (ATLaS) and lets you save all your toolpaths in one file, then when a tool change occurs it prompts you to change the bit. Once changed the spindle moves over to a sensor that it touches down on so it knows exactly where the bit bottom is before continuing with the next toolpath.

They also teased a tool changer option on their facebook page, but I haven't seen any sign of them actually coming up with one.

4D
 

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Rick
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Probotix came up with a feature that no doubt was inspired by the setting of Z after changing bits. I don't have that feature on any of the Probotix CNC I oversee, but I certainly will add it if I'm ever ordering another CNC from them. It is a Automatic Tool Length sensor (ATLaS) and lets you save all your toolpaths in one file, then when a tool change occurs it prompts you to change the bit. Once changed the spindle moves over to a sensor that it touches down on so it knows exactly where the bit bottom is before continuing with the next toolpath.

They also teased a tool changer option on their facebook page, but I haven't seen any sign of them actually coming up with one.

4D
I sure like the sounds of that feature 4D .

It’s unfortunate that spindles that can do tool changes are so expensive ,as I’d go that route
 

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Generally when the tool rapids to the work surface you'll see a Z0.1. You can watch the program run if you have Mach 3 (which I don't have) But my machine has a small hesitation at Z0.1 before going into G01 mode so I have a split second to make sure I'm at the right height before continuing the cycle. Single block is also a good feature to have but when in rapid mode it moves pretty quick to catch a mistake before cutting into the part. Something else you can try is run the program with a piece of scrap foam the same thickness as your workpiece. If it looks good ya got nothing to worry about.
 

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David - Machinist in wood
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Discussion Starter · #10 ·
This is a piece I have made about 30 sets of, 2 plates per set, so I have done this many times. There were no rapids out of the ordinary involved, just the wrong Z height, which turned out to be about 0.150" below the top surface of the Baltic Birch I was cutting. And since the retraction height was at 0.200" nothing hit or seemed odd, especially since it was all behind the dust shoe and the DC was running. Everything cut just fine so the only way I knew something was amiss is that I picked up the scent of MDF being cut and it should have been strictly BB.

I could have left it to finish out but didn't want even more deep cuts into the spoilboard. I wasn't watching the depth on Mach4 because I knew I had touched the bit to the top of the wood, just didn't realize I failed to actually zero Z when I did that. Fortunately, it was only a small amount off and not an inch or something like that which would have caused crashes.

David
 

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Yep... Routine is the nemesis of perfection. I always close my garage door after backing out of the garage. My brain is imprinted with multiple memories of doing just that. Yet one day last week I got home to find my garage door was left open all day. How could that be, as I had a memory of closing it when I left? Or was that just an old memory that jumped in to fill the void when I'd obviously been distracted from my routine and hadn't actually closed it?

I set X and Y and Z so routinely that if I even have the slightest doubt about having done it, I do it again. LinuxCNC doesn't mind, fortunately. Before I run a job I typically issue a G0X0Y0 command and verify that the bit moves over to where I believe I've set X and Y zeros at. G0Z0 should bring the bit down to where I set Z=0 at. When it doesn't end up where I expect it to I know I've been distracted and simply "forgot" to actually tell LinuxCNC after touching off the bit where I wanted the home coordinates to be.

So beware of routine. I'm fortunate that I rarely cut the same thing more than once when using a CNC. At least clamping down and squaring up the stock is different each time.

4D
 

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Mike
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The worst time to forget to reset zero is after a 1 1/2 hour roughing toolpath. When you change bits for the finish toolpath and forget to reset it will make you say things you shouldn't say.
 

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David - Machinist in wood
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Discussion Starter · #14 ·
Here's some firewood that I kept around just to remind myself to set Z correctly. I cut this right after I built the CNC and long before I understood changing bits and where to set Z height, about 1 1/2 years ago. I successfully cut it the other day on the flip side but I'll show y'all that in a new post. Nothing fancy but at least I got it right this time. :wink:

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