Have you actually jogged to your limits to see what they truly are? The actual machine-coordinates limits of Y travel, in millimeters, I have set on my Shapeoko XL are:
0mm – Machine Home-430mm – Forward-most Y
I’m not at home to check whether Carbide Motion allows me to reach the full extent (I know my X limit is greater than CM will go), but -430mm is what I have written down in my notes as a rounded-down safely reachable limit. With that being my actual limit and 15.85in being just a hair under 403mm, that’s a good inch leftover to play with.
If you do end up in that shadowy edge where you’re beyond Carbide Motion’s hard-coded limits but comfortably within your machine’s actual limits, you can always use another sender. If you do, my strong suggestion is to turn on GRBL’s soft limits (see “Appendix: Setting Soft Limits”, below). Carbide 3D’s advice is to leave soft limits off so they cannot possibly cause any issues, which is logical enough since travel limits are hard-coded into Carbide Motion. If you’re going to use any other sender, you get more range, but you don’t have the crutch of built-in hard-coded limits protecting you from dumb mistakes, so turning on GRBL’s soft limits makes a lot of sense (unless you assume perfection, of course). If you enable soft limits, you’ll know they exist and that you’ve set them, so there’s no danger of mysterious behavior (and you can always turn them off if you’re debugging something).
Locating the stock on the bed would be pretty straightforward. I’d collet up a nice little pointy bit, then jog Y to just off your Y limit switch. Jog down to the wasteboard, then make several pencil marks at various points X and connect them with a straight edge. Or make it easier and more official: turn on the router, jog just a tiny bit below the surface, and jog across X to engrave a tiny little line onto the wasteboard. Use your line (however made) as a reference to be sure the back/+Y edge of your stock is within the limit. (For the front/-Y edge, just jog over it to verify.)
Note that if you’re going to be making outside profile cuts, you may want to move your limit reference line forward half the diameter of your mill. I’d likely just scribe the line at Y-5mm in machine coordinates, as that’s a nice round number that would have the machine just off the Y limit switch when cutting an outside profile with a 1/4"/6.35mm endmill and the stock butted up to the reference line.
If you’re going to probe XY, having the stock origin set to left front adds convenience (and can also show if you try to work beyond your forward/-Y limit). On the other hand, if you are right at the Y-travel limit, you may not have any surplus travel for the probe shoulder, tool width, and probing cycle – you definitely don’t want to crash during the probing cycle and skip steps. If you’re not going to probe XY (due to lack of opportunity or simple choice), I’d definitely go with stock center for this. I’d use a straight edge corner-to-corner to make a couple intersecting lines near the center. (A piece of blue tape for the pencil marks works if it’s stock that I don’t want to mark directly.) Use any bit with a nice pointy tip to manually jog to where X marks your spot, zero the axes, then change to your actual tool to pick up your Z.
That’s pretty much all I can think of. It’s not hard to work near the limits, but it certainly can be intimidating. To ease my mind on jobs I don’t want to mess up, I’ll sometimes jog the extents of the job before I run it (and I get the extents from previewing the G-code after it’s generated, not from whatever design program I’m using).
Appendix: Setting Soft Limits
If you want soft limits, usually because you’re going to be working outside CM’s built-in hard-coded limits and you cringe thinking about belts skipping while your machine tries to reach other dimensions, it’s quite easy.
- Home.
- Note your current GRBL settings:
- Soft limits, boolean:
$20=___ (1: on, 0: off)
- X Max travel (mm):
$130=___
- Y Max travel (mm):
$131=___
- Z Max travel (mm):
$132=___
- If soft limits are currently enabled, disable them so you can jog without limits:
- Collect your values:
- Jog an axis to the far end and stop before you actually crash.
- Note the value, in millimeters, in machine coordinates (e.g. -834.000mm).
- Repeat for any additional axes, as desired.
- Update any new max travel values (fill in the blank with your millimeter values):
- For X:
$130=___
- For Y:
$131=___
- For Z:
$132=___
- Turn soft limits on:
$20=1- Your “X/Y/Z Max travel” settings ($130-132) will only be observed when soft limits are on. They have no effect on anything when soft limits are disabled.
Note: If you’re doing physical changes to your machine and want to check your limit switches, you’ll likely want to unlock without homing and then carefully jog into the limit switches. You’ll need to disable soft limits temporarily to jog +X/+Y/+Z from your unlock point since it lacks a home reference. This is the type of exceptional circumstance where not knowing about soft limits could be a headache, but since you’ve set them yourself, you can just type $20=0 and think of happy belts.