Limit / Home Switches
Last updated on Wednesday, November 21, 2012 05:26:26 AM Eastern US Time Zone
E-stop Switch, Limit & Home Switches, Cables
E-stop switch disassembled. The switch is a NC momentary & the button contains the latching mechanism.
It mounts through a ⅞" mounting-plate hole where the button bayonets into the bracket to hold the switch.
The two screws then pull the button's base against the mounting plate. System Schematic
Left-side E-stop shown. Also, note the two leveling shims under the mill's feet.
Right-side E-stop shown. The two, bench-mounted normally-closed (NC) E-stop switches are connected in series to
LPT1 pin 13 input of the breakout board. A pressed E-stop causes the breakout board to pull all LPT1 outputs to ground.
The breakout board uses a resistor to pull the pin high (and low). If no breakout board, use a pull-up resistor on the input pin.
Mach3 E-stop input setup. LPT1 INPUTS are 10, 11, 12, 13, 15 & LPT1 OUTPUTS are 1-9, 14, 16, 17.
Also see an E-stop for the Jet BD-920N lathe spindle.
Limit & Home Switches
One-piece, mirror image, ±Y-axis limit switch L-brackets. The slots & pivoting mechanism allow position adjustments.
Two, 4-40 screws hold each switch. The plate is threaded for the pivot screw. Switch holes are #5 counter bored.
Used the small rotary table to mill the short arched slot for the pivoting-switch adjustment.
±Y-axis limit switch brackets shown temporarily mounted on the milled & tapped side of the Taig carriage.
SS, 6-32 cap-head screws & washers hold the brackets. The +X switch will be located in the middle area.
Front, +Y-axis limit switch shown tripped. The extended hex nut
(a stand-off) acts as a knob when making
The notch by the roller eliminates any possible bracket interference & allows small swarf to drop through.
The switches ride along the smooth, extruded lower area surface of the Taig's carriage & trip open at the front or back edges.
The bracket protects the switches that are tucked-up underneath the table to reduce the effects of swarf.
I considered a knurled thumb nut but decided it was best to be able to lightly (3/16") wrench tighten to hold the settings better.
The X-axis switch rollers are released for normal operation & depressed at the ± limits.
The Y-axis switch rollers are depressed for normal operation & released at the ± limits.
The Z-axis switch roller is released for normal operation & depressed at the upper limit.
All switches, wired in series, are closed during normal operation & opened at the limits.
Rear, minus Y-axis limit switch shown tripped.
Milling the +X-axis limit switch adjustment plate.
switch adjustable bracket. The switch roller-lever is characteristically loose & can
move from side-to-side.
The notch by the roller eliminates any possible interference.
The switch plate is held by two, 4-40 countersink flat-head screws on the bottom of the bracket.
I changed sides to locate the switch more central to the table (lower swarf effects).
The table lead screw bearing plate trips (depresses) the +X limit switch.
Minus X-axis limit-switch bracket bolted to the bottom of the
carriage gib retaining plate.
The steel retaining plate was drilled & tapped for two, 10-32 screws.
The switch adjustment plate is, as all others, held by two, 4-40 screws.
Minus X-axis switch adjustment plate.
Centered underneath the table to protect from metal swarf.
A fabricated (left) table end-plate trips the limit switch.
Protruding Z-axis gib adjustment screw head reduces +Z travel range.
Used a 7/16" end mill to relieve an area of the bearing block to allow room for the gib adjustment-screw head.
The relieved area allows an increase of +Z by 0.3" (but it can vary according to gib adjustment).
Milling the short, curved slot for the +Z limit switch plate.
Vertical adjustment slot sized for 10-32 screws.
The counter bore not really needed, here.
Drilled & tapped 50%, 10-32 threads to mount the +Z switch bracket & cable strain relief clamp.
+Z limit switch trips off the top edge of the box-slide assembly. -Z limit is controlled via software.
The electrically shielded limit-switch cable starts here, at the +Z. Note cable clamp for strain relief.
The white Teflon-coated cable from the tachometer also runs inside the same protective sheath.
From the +Z switch, the cable connects to -X limit switch & then it loops under the carriage.
The cable is tied through a hole to the bracket for strain relief.
Note the insulation on all of the switch lugs to protect against electrical short-outs due to swarf.
From under the carriage, the cable then connects to the +X & ±Y limit switches & continues to the breakout box
Molex connector. Used shielded, Teflon-sheathed wiring, protected with cable wrap. The cable is tied through
a hole to the bracket for strain relief. Spade connectors proved too bulky so I hard-wired all connections. The
switches are easily removed via their screws. All used & unused switch lugs are shrink-wrapped to prevent metal
chips from shorting out the circuit. The cable terminates with a 4-pin Molex that plugs into the breakout box.
All shielding is connected to the stepper-side power supply ground.
Mach3 home/limits setup: LPT1 & pin#10 is the input. I wired the shielding to the stepper-side ground of the breakout board.
Under Settings Tab (Alt-6), enable (green on) the Auto LimitOverRide so you can reset the E-stop & then jog off the limit switch.
In the menu under Configuration & Motor Home/SoftLimits, there are additional settings. See Mach3 Mill Manual, page 4-8.
305oz-in steppers are relatively strong & easily jam the lead screw quite tightly at the mechanical end-of-travel.
The installation of the limit switches eliminated this problem plus it added a three axes homing capability.
The wide range of switch adjustability afforded by the bracket design made the limit-trip setup quick & precise.
E-stop Switch, Limit & Home Switches, Cables