Tag: Improvements

Having hacked a jack in the Sherline motor driver box, this is what goes on the other end of the cable: the simplest possible tool length switch.

I’ve seen all manner of exceedingly fancy and painstakingly constructed tool length switch stations, but it seems ordinary snap-action / tactile-feedback switches are repeatable enough for most purposes. I selected a switch from my stash with these qualifications:

• physically small — fits on Sherline table
• plastic button — avoid nicking the cutters
• many more in the stash — not a special-order item
• cheap – ’nuff said

A few snippets of heat stink shrink tubing later…

After puzzling over the mounting, I snagged a chunk of aluminum U-channel from the heap, poked a 10-32 clearance hole in one end, and held the switch in place while slobbering brown hot-melt glue over it. The glue is rigid when cool, so the switch isn’t going anywhere, but it’s mounted with some air space below to allow crushing when the probe routine screws up.

The button stands slightly proud of the U-channel, so even wide tools have side clearance. If the tool doesn’t stop when the switch trips (it could happen!), the entire switch will bend downward until the Z-axis drive stalls as the tool crushes the rubble or snags on the side of the U-channel.

At which point I just cut the cable, hammer the hot-melt glue and switch rubble out of the U-channel, solder up another switch, blob it in place, and continue the mission… from scratch, though, because the stored tool height reference value will be kaput.

The U-channel can be screwed down to a T-nut, clamped directly to the table, or affixed wherever it’s needed. If the Sherline had home switches, it’d be better to mount the probe switch in a fixed location somewhere, then use a fixture offset for the part, but I’m not there yet.

The switch doesn’t have much overtravel: when the contacts activate with a tactile click, the button is pretty much bottomed out. However, unless you’re driving the tool into the switch at a dead run, it ought to stop moving fairly quickly.

Back of the envelope: I have the Z axis acceleration set to (a sluggish) 3.0 in/s/s. Approaching the switch at 12 in/min = 0.2 in/s , it’ll screech to a halt in 67 ms = (0.2 in/s)/(3.0 in/s/s). Assuming the average velocity while stopping is 0.1 in/s, the distance works out to 7 mils, which shouldn’t pose a problem.

Then drive up off the switch enough to clear the backlash and drive down at nose-pickin’ speed, so the axis stops pretty much instantly when the switch clicks.

Some not-very-exhaustive testing suggests the repeatability for a single tool is well within 0.03 mm, about 0.001 inch, which is entirely satisfactory for my purposes.

[Update: It’s pretty good, all things considered. A simple experiment is there.]

The overall procedure:

• Laser align XY to the part origin, home X&Y axes
• Execute G49 to clear any existing tool length compensation
• Insert first tool, align to Z=0 on part, home Z axis
• Eyeball align XY to the switch with the tool just above
• Jog Z comfortably high, execute G30.1 to set tool change location
• Fire up your program!

The program probes the first tool length and saves that as the reference length. Each subsequent tool change gets probed and the tool offset becomes the difference between the new length and the reference length.

The initial probing routine:

O<Probe_Init> SUB

G49					( clear tool length compensation)
G30					( to probe switch)
G91					( relative mode for probing)

G38.2 Z-90 F300		( trip switch on the way down)
G0 Z1				( back off the switch)
G38.2 Z-10 F10		( trip switch slowly)

#<_ToolRefZ> = #5063	( save trip point)

G90					( absolute mode)
G30					( return to safe level)

O<Probe_Init> ENDSUB

Note that the G30 coordinates are stored in native units, which are inches for my Sherline mill. To get to that Z height (for safety, before moving) while using metric units:

G0 Z[#5183 * 25.4]

The G38.2 coordinates are stored in whatever units the G20/G21 mode calls for, so they can be applied directly to tool length compensation. That seems odd, as EMC assumes the tool table uses native units.

There does not seem to be any way to determine which unit mode is active, although the probe speed depends on that setting. although I suppose I could set a global variable to the desired probe speed and leave it up to the G-Code program(mer) to get it right. Yeah, like that’ll work…

Anyhow, each subsequent tool gets probed thusly:

O<Probe_Tool> SUB

G49					( clear tool length compensation)
G30					( to probe switch)
G91					( relative mode for probing)

G38.2 Z-90 F300		( trip switch on the way down)
G0 Z1				( back off the switch)
G38.2 Z-10 F10		( trip switch slowly)

#<_ToolZ> = #5063				( save new tool length)

G43.1 K[#<_ToolZ> - #<_ToolRefZ>]	( set new length)

G90					( absolute mode)
G30					( return to safe level)

O<Probe_Tool> ENDSUB

With those two routines in hand, this demo code shows how it’s done…

G21					( metric units)

(msg,Verify origin at proper location, hit Resume)
M0
(msg,Verify G30.1 at tool change switch, hit Resume)
M0
(msg,Verify first tool installed, hit Resume)
M0

O<Probe_Init> CALL

G0 X0 Y0 Z0
(msg,Verify return to origin, hit Resume)
M0

M6 T2
O<Probe_Tool> CALL

G0 X0 Y0 Z0
(msg,Verify return to origin, hit Resume)
M0

M6 T3
O<Probe_Tool> CALL

G0 X0 Y0 Z0
(msg,Verify return to origin, hit Resume)
M0

M6 T4
O<Probe_Tool> CALL

G0 X0 Y0 Z0
(msg,Verify return to origin...)
M2

The M6 Tx commands make use of a

TOOL_CHANGE_AT_G30 = 1

line in Sherline.ini, which tells the Axis automagic manual tool changer routine to traverse to the G30 position and pop up a prompt to (manually) change the tool. When you click OK, the subsequent CALL command invokes the tool length probe routine and away it goes.

This whole lashup doesn’t have a lot of power-on hours, but I really like how it works!

A tip o’ the cycling helmet to the folks behind EMC2…

I’ve been mulling over adding a tool length probe for a while and finally decided that the simplest approach might be the best: a momentary-contact pushbutton switch that pulls a parallel port input pin to ground.

The motivation is that a simple switch seems to be repeatable enough for tool length probing and it’s cheap enough that I won’t form a deep emotional bond to it. When a probe crashes the switch, I can just pop another one in place without any heartache or putzing around for a day or three to build another over-elaborate probe station.

The catch is that the Sherline motor driver box doesn’t include connections for any of the parallel port input pins.

The choices seem to boil down to:

• Adding a breakout board between the parallel port and the driver box or
• Hacking the driver box to get access to the port pins

Well, I’ve already pretty well hacked up my controller, as I wrote up in Circuit Cellar magazine (Aug & Oct 2004), so I don’t have much to lose… and the box is already in the shop!

This picture shows the connection to pin 15 of the parallel port on the Sherline driver PCB. The driver doesn’t use that input pin (or any of the others, for that matter), which means the PCB doesn’t have a trace leading anywhere convenient. I ran the new wires through the connector mounting hole, rather than around the edge, and soldered them directly to the connector pins on the bottom of the board.

The jack is an ordinary 1/8″ (3.5 mm, these days) stereo (3 conductor) jack, with lah-dee-dah gratuitous gold-flavored flashed plating; anything similar will work just fine.

Connections:

• Sleeve -> driver box
• Ring -> circuit ground (pin 19 is convenient)
• Tip -> pin 15, the probe input

The cable shield connects only at the plug into the driver box, not at the switch end. That ensures there’s no current flowing through it and it can do a marginally better job of shielding the two conductors within. I’m reasonably sure that makes no difference whatsoever in this application.

The cable got chopped out of an AV-interconnect dingus with all sorts of fancy connectors on the other end. It’s a surplus find, cost maybe a buck, and has the redeeming feature of sporting molded plugs that I don’t have to solder.

The switch connections are soldered and insulated with heat stink shrink tubing. The general idea is that the driver box provides all the power, there’s no electrical contact with the mill table or spindle, and thus no reason to use fancy circuitry to solve a problem that’s not there.

I did not add a capacitor across the switch contacts, figuring that I’d solve that problem when it happened. The common practice of putting a honkin’ big cap across switch contacts is bad practice: it effectively shorts the power supply across the contacts for a brief moment every time the switch closes. Some stored energy is good (it keeps the contacts clean), too much simply burns them away. ‘Nuff said.

I marked a hole on the front panel symmetric with the LED, eased the circuit board out of the case and wrapped it in a shop rag to keep the swarf out, propped the case on the drill press table, and rammed a 1/4″ hole through the spot marked X with a step drill. Yeah, hand-held on the table, just like you’re never supposed to do.

The force is strong with me…

The (well, my) Sherline.hal file connects pin 15 to the probe sense input (maybe I defined that when I set things up; I don’t recall now), but it assumes the pin will be high when active. The parallel port pin has a built-in pullup resistor and a switch to ground makes it active when low. These two lines in my custom_postgui.hal file disconnect the high-active pin signal and connect the low-active pin signal.

unlinkp parport.0.pin-15-in
net probe-in parport.0.pin-15-in-not

You do it that way to avoid changing the Sherline.hal file, which will be overwritten if you ever run the automagic configuration program again.

If you’re doing this from scratch, just configure the whole thing using the configuration tool, it’ll set the HAL file properly and you won’t need any of that fiddling around.

Tweak the Sherline.ini file to add support for tool changing with the G30 command:

[EMCIO]
TOOL_CHANGE_AT_G30 = 1

Button everything up, then do a quick

G91 G38.2 Z-10 F10

and poke the button while the Z axis is in motion. The Z axis should stop instantly. If not, check your wiring.

Now, some Orc Engineering is in order: I need a low-budget fixture to put the switch in harm’s way.