Setting X/Y Zero

[majohnson]

I'm curious how others are setting X & Y Zeros. I was taught on manuel mill to use edge finder. After finding the edge of the work piece, to take half of the diameter of the edge finder and that will give you center of the power head.

Example edge finder is .250, you then run to -.125 should give you zero.

[steve_g]

Mark:

It's been many years since I did any work on a Bridgeport mill, but... In those days 0,0 was a point on the material being milled, hence the use of an edge finder. Today with a CNC router 0,0 is a point on the surface of the table able to be "addressed" by the router, and may not be any where near the part being routed. However, it's important that the 0,0 location is able to be determined in a repeatable fashion, the proximity sensors are good enough for most work. The design software will keep track of the center line of the bit you are using and make adjustments as necessary.

I hope I understand your question and have not muddied the waters!

Steve

[ssflyer]

Hi Mark,

I setup a simple jig - 2 pieces of ironwood (because I had some handy - you can use whatever as long as it is dimensionally stable) at 90 degrees, that I used the ShopBot to trim. This insures proper alignment. then I modified my C3 routine to use the prox switches and always set x & y zero to the vertex. You don't have to manually modify your C3 program (actually probably not a good idea, since it will be overwritten on an upgrade of the SB3 software) but I changed other things as well. The easiest way, if you always use 0,0 at the lower left corner, using a jig, is to simply use the TS command in SB3 and on the the 4th page set the offset from the x and y contact point (These should be entered as negative values). It should be accurately repeatably to within 0.002".

This is probably only applicable if you usually use sheet/square stock, but that's what I usually use

Hope this helps,
Ron

[rhfurniture]

Hi,
I have a totally different approach on my 1200 x 800 shopbot.
I rely on the fact that when the control software starts up it puts the steppers into a whole step position (on mine it runs in 1/10 step units by microstepping), and on mine this is easily readable (about 1/4mm). I therefore park my machine at 500,400 and switch off at the end of the day. I have a simple measurement routine with a 1m ruler from physical objects (eg end of rail) that I use to check if it is in the same place when I start up. If it looses position, I move the machine until it is in the same position by measurement, switch off and on at the control box, and reset the axes (VA) to 500,400. My 0,0 is at the bottom left of the table. As you can gather I do not trust proximity switches, and try to keep everything as (electrically) simple as possible.
R.

[adrianm]

I just use the C3 proximity switch routine with offsets to get to the edge of an 8x4 sheet which is always in the same place due to guides fitted to my table.

On the rare occassions I don't use a full sheet I just eyeball the XY (90% of the time within 5mm or so is more than good enough for me) and the rest of the time I use the crosshairs on my XYZ jig.

[dana_swift]

Ralph, I think your fear of prox switches is not justified. I have measured every shopbot I have visited for the past several years, employing several kinds of prox switches.

The zero certainty of a prox switch that is set correctly is about 0.002" which is approximately 0.05mm. That is double the accuracy you are getting.

I have not yet found a properly setup PRS Alpha ShopBot that did not get 0.002" (0.05mm) repeatability in home finding when I measured it.

With the copper sweat fitting zero method (or a super zero) the repeatability is 0.001" (0.025mm), which is four times what you are getting. Quite often it comes back as 0.000".

Mark- There are several threads on the forum about super-zeros. Those "should" get the 0.001" (0.025) accuracy on the corner of the stock. I am having a machine shop build me one to try (its nice to have a shopbot to trade favors eh?) but I have not gotten it yet. You can trust I will be testing it the same as the other techniques.

Hope that helps-

D

[dhunt]

As you can gather I do not trust proximity switches,
and try to keep everything as (electrically) simple as possible.

Ten years ago proximity switches were installed on our brand new PRT-96
but since then various 'local' operators have entirely trashed them!
Some of the wires remain...and one of the sensors

I view them as a cute hi-tech but largely un-necessary gizmo system.
Why?
Because in ten+ years no-one (not even the 'locals') has launched either gantry into Inner Space!
In fact just the other day I unscrewed and removed both sets of green and black contact wires at the control box end
while fiddling something else in the control box


One remnant prox wire however still provides an important service:
I use it to keep one of the X-motor wires from dragging on the rails
so it doesn't get in the way and/or cut.

Now that's essential stuff!

[pappybaynes]

Dana,
"With the copper sweat fitting zero method (or a super zero) the repeatability is 0.001" (0.025mm), which is four times what you are getting. Quite often it comes back as 0.000"." Elaborate on the Copper sweat fitting zero method please.
Thanks,
Dick

[rhfurniture]

Dana,
I have never determined how accurately it gets set, as with my method it is not necessary to accurately read the measurement. I just depend on the stepper to start up at the same whole step every time, and I can easily see when it has started into the wrong whole step. tbh, I started this way back - I ordered some proxies and gave up when I worked out how many wires they needed - I am not an electrician (well I can use a screwdriver and pliers, but don't give me a soldering iron). I am now really quick with the steel ruler, and I can SEE that it is right. It has never let me down, and is always dead right against the Y zero fence I keep in place.
PS: do proxies still work when they are buried in sawdust??? (I don't do dust extraction, it gets in the way)
R.

[dana_swift]

Ralph- sawdust should not impact the accuracy of prox switches, they are metal locators, and hopefully there is no metal in your sawdust. (I must confess sometimes there has been in mine!)

Dick- here is the long version of my answer to your question:

Background: I got started doing things this way from a suggestion by the folks at tech support. I don't remember who to credit, but they pointed out that the "Center In Hole" routine was already written and available in the SbParts directory. All I had to do was come up with a way to detect when I had touched the edges of the hole. I needed a cheap super zero.

The sweat fitting has worked out the best so far, they cost about a buck each. And if one gets dammaged they are easy to replace.

Details:

To do a high precision XY zero, first you have to prepare the spoilboard with a pocket to hold the outside of a 3/4" sweat fitting end cap. The INSIDE diameter of an end cap is specified, but the outside diameter may vary depending on the wall thickness, measure what you have and mill a pocket about 0.002 smaller than the fitting so it will fit snug. I insert it about half way so I can pull it back out with pliers and not deform it.

Prepare a jumper wire with two "battery charger" size aligator clips on each end. It doesn't need to be very long. Connect it from the outside of the end cap to the zzero plate.

Connect the aligator clip that comes with the zzero plate to the bit. (In my photos you will see the stubby collet chuck adaptor [Thanks for the tip Brady] and 1/64" bit in an ER11 chuck) In this case I hook the aligator clip to the stubby collet chuck adaptor, not directly to the bit.

Run the "center in hole" routine in your SbParts directory. The name appears to be different in the current release, but its the same thing. Find it, run it.

Run the routine TWICE. The first time it will usually be way off center when it starts which can change the accuracy. The second time is the keeper.

When it finishes use VA,PocketX,PocketY and your table is zeroed.

I modified the shopbot center in hole file a bit to make it work a little better for me, but the stock routine will work, it just asks a zillion questions each time. Ignore this step if you are not comfortable with the possible consequences.

If you run the center in hole routine many times in a row it will usually come up to the same indicated position every time, off by 0.001 sometimes. Try it!

To check the backlash in your gears, pull up to a safe z and use the K command to move around all over the table then come back and re-center in the hole. How far off is it now? That number is your gear backlash value.

Hope that helps-

D