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It was me with the limit but no home switches. My limit switches are mechanical, and although they're good switches, they're not as accurate for homing as prox switches or manual homing would be. They're fine for limits, and they'd be pretty close for homing but...

I don't dispute the advantages of home switches, but since I don't always put a jig back in the same place (depending on what else I have going on on the table at the time), doing it manually is really no big deal. With handwheels on my motors, getting zeroed in doesn't take much more time than hitting the zero buttons on Mach 3 does. Where this method does fail compared to using home switches though is, as Andy pointed out earlier, if something happens that knocks 0,0 out of whack. With a workpiece in place, to re-zero the way I've been doing it would require removing the workpiece, re-zeroing, and then putting the workpiece back. Not a big deal, but not nearly as easy as automatically re-homing and moving on. But, if the jig isn't in a predictable place on the table (as is often the case in my not-so-organized world), even with home switches, you'd be in the same boat as I am now...

I guess the big thing for me is that in the shop, I'm pretty disorganized. Pretty heck, I'm really disorganized! I'm an IT professional by day and am expected to be hyper-organized there. The shop is the antidote to my daytime life....

Dave

I got my start in guitar making back in the 70s up at Earthworks with Charles Fox and George Morris. I still marvel at how much I learned from those guys...

Dave

Summer'80 GRD... George did 99% of it as Charles was into the production of GRD instruments at the time.... That place tied for my top 3 experiences in my life.

I was there at the beginning of 78 during the big blizzard. It was still about a 50/50 thing between George and Charles at the time--maybe more like 75/25 with George at the 75 level. At any rate, Charles was still prototyping and designing the GRD instruments at the time. That was a good learning experience in itself, since I got to see some of his jigs in the process of evolution and design.

I'll agree with you though as for top life experiences. It was pretty darned great, and I really learned a lot more than just guitar making technique while I was up there. Both of those guys are up at the top of my list of great teachers I've had.

Dave

We were never able to get reliable results with proximity detectors on ur Shopbot. Likely due to the sloppy, loose, flexy frame more than anything. We settled on homing on an index pin. Makes it easy to reference home on a drawing, just snap to a circle that resents the pin. For us this works best if there are two pins on the centerline although we'll use control holes and neck bolt holes if we have to. I'm sure the machine home on the Fadal will be reliable enough that we'll just use fixture offsets and not have to locate the part every setup.

Thanks again for all the info. Let me see if I'm getting all this, including the language.

Zeroing the machine can be done in several ways. One is based on the X and Y axes (potentially using homing switches). This is the purpose of homing switches and they are different from limit switches (which are shut-off switches on each axis to keep the tool from destroying itself?) But limit switches can be used for homing if they are accurate.

Another zeroing method is specific to the work using some kind of indexing point (or two points to establish an axis). With this method, I totally understand the need for hand wheels as I imagine it would be pretty hard to find a tight hole by jogging the machine.

Another method is a probe that will reference a point on the table or the work itself.

If you repeat the same operation on the same table with the same zero point and accurate locators, there is still some possibility that the machine will get off track over time and need to be re-zeroed?

I'm imagining that perfect placement of parts will be rarely needed. Seems like for most operations, one would be starting with over-sized work and part of the routing would be cutting the final shape. Is that how most of you work?

It seems like it would be easy to start each drawing in reference to the entire table with a zero in the center or a corner or wherever you want it. Is that harder than it appears? Specifically, I'm imaging drawing a headplate logo and copying it 10 times around the table top and then using the machine to cut it's own indexing for each piece on an auxiliary table top. Make sense?

Thanks.

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My answers (not necessarilly correct ones )


True

Only if there's something wrong with your machine or something changes.

False, there's a lot of things that need precise location, particularly when you're flipping a part or re-machining an assembly or other things like that

A lot of what I've been doing so far and what I will be doing is very 3D and multi sided so yes, you do start out with oversize stock but it might need to be oversized by a precise amount. The pic I included on my neck is a 4 sided setup (haven't run one yet, still working on stuff) and the first thing I need to do on that is to face two sides and drill holes for locating pins. Every setup needs to be precise for my plan to work....which is why I'm eyeing a 4th axis!



For a lot of this stuff, I think it's best to take Dave's approach and dive in. As I was getting started, I drew up a light saber and laser blaster for my boys as a couple of my first parts. It helped me get back into the swing of things with Solidworks and learn a bit about CAM.

I think that step one in the CNC journey is to learn a CAD package (if you don't already know one) and start drawing "things". That step goes even before you buy/build a machine.

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http://www.birkonium.com CNC Products for Luthiers
http://banduramaker.blogspot.com

That is absolutely spot on!!

..when I first build my first CNC machine, I basically had to put it aside for quite a while, until I got to learn how to model with a CAD package....it really starts with all that....

not saying you can not cut simple 2d parts with some of the wizards in programs like mach3, but your time will be better spent upfront learning a CAD package, before you go out and get a CNC machine.....

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http://www.cncguitar.com
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If it's a resolver machine, your home positions can wander over time. I'm not quite sure why, as I don't know a whole lot about how the indexing works on the resolver machines, but I do know that they adjust the home positions every few months on resolver machines at Taylor.

I'm running encoders without index pulses on my machine now, so I have to line things up again every time I power down if I want to use the holes in my table as indexes. Worth the sacrifice to be using Teledyne Gurley encoders, though.

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Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services

I was pleasantly surprised with the repeatability of the homing switches on my Techno as they are just simple microswitches.
I will sometimes do a homing routine (Mach3) just as a double check to make sure the software/controller/encoders haven't "wandered".
A good test of the homing accuracy is a power loss while cutting inlays.

Hi Bob,

1. How do you know if you have resolvers?
2. Have you drilled your table for indexing purposes? I'm thinking of doing that to my machine but not if things are going to drift.
3. Where do you learn all this s**t?

1- If you have AC motors, then they're encoders. So far as I know they never used encoders on the DC motors, so if your machine is pre-1997 then it has resolvers.

2- Yes. You only need to re-align it every few months, and it's only a couple thou of drift. Having the pin holes is nice for lining things up parallel to the table, too, since they function as a straightedge with gage pins in them.

3- There's all kinds of good stuff in the manuals, so read all of those. : http://fadalcnc.com/FadalTechnicalDocs.html

Besides that, I know the machine tech at Taylor and there are probably about five guys on the planet who know more about Fadals than Randy. It's one thing to run 50 of them at a shop, and a whole other thing to rebuild them in house as well.

Retrofitting a machine will teach you all kinds of stuff if you've got a couple hundred hours to spend with a multimeter, an oscilloscope, and a constant sense of dread.

_________________
Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services

Sounds to me like you're getting it. I do what has been described by some posts in this thread.

I use my home/limit switches to establish and maintain my machine zero point. As long as I keep my belts tensioned, grub screws tight, etc., I find that this is a reliable method for me and accurate enough for me, considering I've got an entry level machine and I'm not building aircraft parts or nuclear reactor doodads (technical term.)

I only cut solidbody necks and bodies, so I've dedicated half of my table to necks and half to bodies and pickguards. My CAD drawings use the lower left corner for my X0Y0 point, but that part is not really crucial. The crucial part IMO is that you A) know where your reference point is and B) can reliably, repeatedly, get there.

I don't have permanent fixtures yet. So for example, I attach a piece of MDF to my "neck area" and surface it flat. Then I pick a reasonable spot in the lower left corner of that MDF and create a G55 offset in Mach3 establishing that point as my X0Y0 for that offset. When I'm cutting necks I use G55.

The other thing I do is for each offset (G55 for necks, G56 for bodies), I will go to X0Y0 and drill a 1/8" hole in the MDF and insert a 1/8" steel pin leaving it about .3" proud of the surface.

Before I do a cut I will chuck a 1/8" bit or pin in the router, then zero the machine using the home switches, then travel to X0Y0 for whichever offset I am using. Then I jog down and eyeball the pin alignment. If all is well, it will align perfectly (by sight at least), then I will insert my tool of choice, set the tool height, and start cutting.