Machine Bed Zero

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Machine Bed Zero

Postby tshelb619 » Fri Feb 16, 2018 6:55 pm

Hello,

When doing cutouts with the cnc, is it better to reference from machine bed for zero? If so, what is the proper way to set it up on the shark after you have the cut file loaded and ready to go? Thanks!
tshelb619
 
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Re: Machine Bed Zero

Postby Rando » Fri Feb 16, 2018 8:50 pm

The "correct" location is always the one that the toolpath is referencing ;)

If you're taking Z0 right on the bed, that usually means you're not using a spoilboard, which is popular in most Shark-typical situations. (I use a machining vise, but that's a whole different thing). So, I'd say maybe consider a spoilboard and a fly-cutter to get it really flat, and then use that top surface. That's really common. I tend to use the top of the part, but that's because I mostly cut aluminum, so it's conductive and I don't have to worry about holding the plate over some uneven top surface.

Here are some additional considerations that tend to guide the choice:

1) If you're making a set of parts that need to be an exact thickness from that bottom side, and you're dealing with varying top-surface height, AND (this is important) the cutting can tolerate the spikes in cutting forces that come from the varying cut-depth, then it's good to use the bottom of the part as Z0.

2) Most people use a spoil board of some form over their metal bed, so their perimeter (profile) cuts can go all the way down the sides of the part. In that case, if 1) above is also true, then you'd use the top of the spoi lboard. Many people also buy a "fly-cutter" surfacing bit to really make that MDF of other spoil material flat as flat can be, and then you can really rely on that top of the MDF as your Z0.

3) If you're making parts where the overall thickness isn't so important, and you're not flattificating (that's a technical term ;-) ) the top of your part, and instead you're just carving something into the top surface, then by all means take your Z0 from the top of the material.

4) If you're using tabs on the bottom of your part (so it doesn't go flying across the room at the very moment it gets finished), and you have any uncertainty about the top surface, and you are supporting the stock on a spoilboard or similar, then absolutely use the bottom. Otherwise if the Z0 is set lower than expected, it's entirely possible that the tabs will actually be cut fully through. On the other hand, the opposite is also true: the top-surface variation can cause problems with the top-side carving. So, my advice is if you're using tabs: make sure your material is dimensionally accurate; consider flattificating at least the top and possibly the bottom as well, and then use Z0 from the bottom.

Now come the more-tricky ones.

5) If you're doing a 3D-style part where multiple surfaces of the stock will be machined, be very careful to take your X0, Y0 and X0 from places that continue to exist as the part gets made. The reason this is important is that if you're making a bunch of those parts, often you'll do the first cut on all the parts, then the second cut on all of them, and so on. This means that each individual part may be removed and re-mounted multiple times. In this case, again, make sure your zero locations, and the material faces that are use to hold the part down are features in the stock or part that will continue to exist throughout the creation process. If, in the series of cuts (typically when the final profile is cut) that removes that material, unless you're using a jig that holds it in place reliably, you won't be able to hold, much less locate the part if you need to re-do some of the cutting.

6) Some people buy a setup block that they get their Z0 from, and then use "machine offset" and "part offset" values to get the cut in the right location. This allows them very precise and consistent location of the bit tip in the machine envelope, but accuracy can then lost if the measurement to the actual part/stock isn't accurate or consistent.

7) Some loopy folks (like me) actually make a 3D model of the deformed stock solid, and then map the toolpath onto that. In those cases, you definitely want to use a top Z0 capture location, and then "shim" the toolpath with software to put it exactly in place. In those kinds of situations, you want that Z0 to be as nuts-on accurate as possible, so again, consistent top-side location is going to be best. I use that method for engraving 0.002" deep into the bead-blasted surface of hunks of aluminum angle bracket. Angle bracket, like 2x4s, are not perfect. The have allowed tolerances like any raw material. So instead of being constantly frustrated that the engravings weren't hitting in all areas, I measured the material's actual deformation, built a proper fixture and 3D model, and it worked like magic.

8) Always watch for movement in the material or touch-plate when the touch-off happens. If the stock is slightly warped and comes up from the table where you're taking the Z0, sometimes the pressure required to complete the circuit, or of you holding the plate in place, can push the material's actual top surface down a little. So, when you put the plate down, push down and see if you can feel any give. If there is, choosing a more-stable spot, or flattificating the whole thing can help there. But, if that area is still warped off the bed, it can still move when the cutter is working there, so beware.

I think what you'll find about Z0 over time is that when it's critically important to get it right, like when you're using multiple sizes of ball end-mills, you're going to do things like make test cuts that allow you to precisely know the depth, not just as the machine thinks it is, but rather exactly what's needed to make the carving come out. There's a recent thread here involving that very issue where the solution was indeed to make a set of test cuts in each subsequent bit to allow for very-slight adjustments in Z0. He doesn't use the probe to set the final distance, but rather uses the actual quality of the test cuts (at barely-varying depths) to set them. It's a pretty cool idea when Z0 is critical. This is the thread: viewtopic.php?f=2&t=5738

Anyway, that's what comes to mind for now; Hope that helps.

Regards,

Thom
=====================================================
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Rando
 
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Re: Machine Bed Zero

Postby tshelb619 » Fri Feb 16, 2018 9:45 pm

Flattificating! That's great...
Thanks for that information. You answered questions I didn't even know I had where issues were causing me headaches. I have found that throughout this learning process I become frustrated and having people like yourself willing to help really makes things easier and makes me feel much better about it.

One other question, I have a spoil board and when I check the box for use machine bed zero in vcarve how do I properly set it up on the machine? I have a touch plate so when i set the zero do I set the plate on the spoil board or on the material surface? Thanks again!
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Joined: Fri Sep 01, 2017 11:50 am

Re: Machine Bed Zero

Postby Rando » Tue Feb 20, 2018 12:10 pm

tshelb619 wrote:...I have a spoil board and when I check the box for use machine bed zero in vcarve how do I properly set it up on the machine? I have a touch plate so when i set the zero do I set the plate on the spoil board or on the material surface? Thanks again!


tshelb619:

You're of course quite welcome; thank you for those kind words. Knowledge is of much greater worth... not value, worth...when widely and generously shared*. I'm glad it helped.

You're right that "machine bed zero" means that the Z0 is set on whatever surface the bottom of your material will be held on to. It's not possible to measure (or the related "indicate against" or "register to") the bottom surface of the material, since that's resting on something. When you use the spoilboard, you'd normally take the measurement off to top of the spoilboard, if for no other reason that many peoples' spoilboard (Flattificated, of course!) covers the entire bed. If you're clamping the part directly to the bed (and you know you won't be cutting through!), then you'd capture the Z0 with the plate directly on the surface of the metal bed.

When using the touch-plate, the control panel software and/or the pendant expect to know (set in preferences) the **exact** thickness of that touch-plate. So, when it makes the electrical contact and decides it knows the Z0, it actually calculates it including the thickness of the plate set in the preferences. Sadly, it does not directly measure it ;-). Although the plates are 3/8" aluminum, I was never confident in trusting it was 0.03750", and so I measured it and set the value myself. The "plate" doesn't have to be that specific one, or any specific thickness; you can control that in the preferences. For example, I use an 0.020" thick piece of sheet metal, 1" square as my touch-plate, and tell the machine there is no touch-plate. (Someday ask me about the missing 0.0185" in the old controllers... :D).

Since I work in metal mostly, I use a 1" square piece of 0.020" thick metal as my touch-off plate. The smaller 1" square size means I can get more accurate readings on smaller parts, but gotta be very careful to keep those fingers out of the way during the touch-off.

So, for your specific question: at some point check that the system believes your touch-plate is indeed the exact thickness the plate really is. And, as you guessed, set the plate on top of whatever you plan to mount your material to.

But, this is where it gets a little weird: you can actually do it EITHER way. If you intend to get your Z0 using the plate directly on the metal bed, you would include the spoilboard thickness in your stock material definition inside Vectric. On the other hand, If you want to take the Z0 with the plate on top of the spoilboard, then the Vectric stock material definition would be of only the block you're cutting. That second way is more typical, but both are feasible and reasonable, and in some situations the first might be required.

Consider the case where the stock material has to be screwed to the spoilboard from underneath the spoilboard, because the cutting requires no clamps on the topside. First, be sure to recess/countersink the screwheads in the bottom ;-). In that case, it's not possible to capture the Z0 from the top of the spoilboard in an area where the stock will be present, because the stock is in the way. In that case, hopefully you carefully measured that now-attached spoilboard thickness. ;- ) You would add that spoilboard thickness to your stock definition in Vectric, and take the machine Z0 reading with the touch-plate directly on the bed. Then mount the wood. It would be a GOOD IDEA to then do a sanity check by jogging the bit to a known place on the stock, and making sure the numbers in the CNC machine correctly correspond to what's about to be carved. But, if all is correct, and it's all held down well, that will work as well.

Cheers and happy cutting :D

Regards,

Thom


* in this case, Value = what other people see in it and will pay for it; Worth = how much overall benefit it generates
=====================================================
ThomR.com Creative tools and photographic art
A proud member of the Pacific Northwest CNC Club (now on Facebook)
Rando
 
Posts: 583
Joined: Tue Jan 06, 2015 3:24 pm
Location: Hoquiam, WA


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