Zaxis repeatability

[forseral]

I'm curious about what others are doing to position the zaxis. I'm on the way to completing my CNC machine and was exploring options for placing the home switch for my z-axis.

My machine uses a dedicated bosch wood router and supports 1/2" and 1/4" collets.

I've noticed a lot of others speak about using an zeroing plate where you jog the machine downwards until you touch the top of the plate. This seems to work just fine but my curiousity had me thinking of other options that required just a little less setup time.

I figured I could always home the z-axis at the top of its range but i quickly realized this is not as repeatable because the length of my bits will vary. In talking w/ a coworker we arrived at the idea of using a stop collar on all of my router bits so I could switch and ensure they always had the same cut depth.

Has anyone tried anything like this? If not, any other ideas?


thanks!

[louieatienza]

I've come up with an easier solution. When you place the bit in the collet, hand tighten the nut until it's just snug, but you can still pull the bit out with a little tug, but not tight.

Drop the Z down till you're close to the workpiece, then jog slowly and you'll see the bit slide up the collet ever so slightly while the tip rests on the workpiece. Zero the Z in Mach3 and raise the Z and wrench-tighten the collet nut, and you'll be within a couple thousandths every time.

If you're using a v-bit, get a small piece of Mic-6 or other aluminum plate that the thickness is known, say .25". Follow above, but jog slowly to the aluminum (it won't harm your bit if you go slow enough.) Zero the Z, raise it, remove the plate, and manually move the Z to -.25 using G0Z-.25, and re-zero the Z.

[louieatienza]

BTW, I have learned here that the home switch is still beneficial, if for example, you had to stop a job, and restart your machine. You could jsut home the axes, and you maybe able to save your work...

[jsheerin]

I use a home switch on all my axes to home my machine. Then I set my work offsets for whatever I'm working on with whatever tool. To do this in z I sometimes use a touch block that lights up when I make contact with it (made by Fowler), and sometimes I just stick a piece of paper between the end of my bit and the part and jog down until it sticks between the two. Then I set my Z to 0.002".

Doing it this way lets you save your work offset and if you lose steps or turn off your machine, when you start it back up you can home it, load your work offset and then you're back to exactly the setup you had before and you can make more parts. If you want to use multiple tools, you need a repeatable way of setting tools in your spindle. This usually is about impossible with a wood router because as you tighten the collet, the bit will move with it. So I just rezero my z work offset every time I change tools.

[ger21]

Originally Posted by louieatienza Drop the Z down till you're close to the workpiece, then jog slowly and you'll see the bit slide up the collet ever so slightly while the tip rests on the workpiece. Zero the Z in Mach3 and raise the Z and wrench-tighten the collet nut, and you'll be within a couple thousandths every time.

I would think that 1/2 a turn or more of the collet nut will move much more than a few thousandths?

There's really nothing easier than an auto zero plate. Even better is to take it one step further, and have two plates, one being fixed. This allows you to change tools, and with the macro I use, the second tool (and subsequent tools) is zeroed automatically with no user intervention.

I also have the ability to zero to a specified offset from the table (usually equal to the material thickness). By doing it this way, I can always get the exact same Z position, even days later if need be.

See Big Tex's thread for more info.

http://www.cnczone.com/forums/mach_w...r_big-tex.html

[louieatienza]

Maybe the collet nut, not the collet itself. The trick is to get it just snug enough before it stops moving. I find with the precision ER collets it doesn't take much to get them locked in.

I only posed the idea as anecdotal experience, currently working without my home and limit switches set up yet. But for a down-and-dirty quick way, it works fine.

Another lazy way would be to turn off the controller, and manually drop the Z using a handwheel mounted to the shaft, and then turn the controller back on, which kind of works...

Originally Posted by ger21 I would think that 1/2 a turn or more of the collet nut will move much more than a few thousandths? There's really nothing easier than an auto zero plate. Even better is to take it one step further, and have two plates, one being fixed. This allows you to change tools, and with the macro I use, the second tool (and subsequent tools) is zeroed automatically with no user intervention. I also have the ability to zero to a specified offset from the table (usually equal to the material thickness). By doing it this way, I can always get the exact same Z position, even days later if need be. See Big Tex's thread for more info. http://www.cnczone.com/forums/mach_w...r_big-tex.html

[ger21]

Maybe the collet nut, not the collet itself.

Your right, I wasn't thinking correctly there.

[zeeway]

Here's another vote for an auto-zero plate with Big Tex macro...I use it on the top of the material. I have a home switch at the top of my z travel, but I do not believe it is consistent enough to use as a reference point.

Using a built in zero plate on the table top and offsetting for material thickness sounds like a good idea, but the material I am buying these days seems to vary a lot in thickness. I guess I could measure the thickness by moving the z axis to the material top...but that just seems like an extra step to me.

[ger21]

Originally Posted by zeeway Using a built in zero plate on the table top and offsetting for material thickness sounds like a good idea, but the material I am buying these days seems to vary a lot in thickness.

That's why I zero from the table. You can cut parts with varying thickness using the same g-code. Say I'm cutting parts from 3/4" plywood, but it's never 3/4" thick. I program to cut .745 deep, and no matter what thickness the part is, my parts always finish with a paper thin skin, which I quickly knock off with a sanding block.

This doesn't work with V carving, where knowing the top surface is mandatory, but for cutting most parts, it works great.

[forseral]

So here's what I gathered from your responses:

1. Zero on top of the work piece. This works for me but I'm using emc and suppose I could configure the it as the home switch then use the built-in homing feature.

2. Zero on the table. This seems likes better option since I could allocate special place on the machine for homing all axes. I guess I would have to build my programs to compensate.

3. Use the loose collet idea. I tried this today and found it worked well but a little cumbersome for regular use as my original intent for this post is also about automating homing.

Have any of you tried something like using a preset stop collar on your bits? Does this make sense?

[louieatienza]

For engraving, I use Amana's InGroove system, which utilizes a shank and interchangeable inserts; so once it's zeroed, you're set to go for all the inserts.

You can order bits at PreciseBits with the stop collars installed; the only thing I see with that method is trying to hold that stop collar against the collet while simultaneously tightening the collet nut! Though I guess you could just hand tighten, then push the collar up against the collet, then wrench-tighten. That seems a bit kludge as well...

I don't know how bad the runout or repeatability is on them, but Sears makes quick-change tool holders for routers. Not too big a fan of the set-screws for the bits, but if you could make a jig to install each bit the same depth, it might be a go. I bought them a while back for manual routing, and have always pondered if there was a way to make an ATC using them... they require you press down and twist to engage/remove the bit.

[ger21]

Originally Posted by forseral 2. Zero on the table. This seems likes better option since I could allocate special place on the machine for homing all axes. I guess I would have to build my programs to compensate.

Not zero on the table, but a fixed distance from the table. Zero is still the top of the part, or where the top of the part should be.