Dale Grover wrote:
> I'm fixing up an IBC BoardMaker 2222, a 22" x 22" PCB engraver. So
> far I've replaced the Y axis with a McMaster-Carr 5/8" ball screw &
> nut, which had been a proprietary-threaded rod with self-adjusting
> anti-backlash (plastic) nut that self-destructed, and replaced the
> stepper drivers with Gecko G251's.
>
>
The glass-fiber dust may also destroy the ballnut unless you find a way to
shield it.
> The machine uses a solenoid to move the spindle down until a
> wear-button hits the PCB, thus establishing the cutter height.
> Spring return.
>
> My question is about the solenoid drive. There are two identical
> circuits--both are logic-level in (from a db-25 parallel port)
> driving a relay that supplies +45V to the solenoid (i.e., they are in
> parallel), though one has a 5W 40 ohm resistor in series. (The
> solenoid is not identified, but has 70 ohm resistance.)
>
> This sounds to me like a way of driving the solenoid hard to move the
> spindle down, then holding it with reduced current.
Yes, I'm pretty sure that is what they are doing. What a totally
primitive way
of doing it, though! Relays!
> This is often
> done with a (big) cap across the current-limiting resistor. The
> solenoid doesn't move all the way down with just the "hold" current.
>
> I don't have the original control software, so don't know what the
> original software control signals look like. (IBC seems long gone,
> but may have had connections in the past with LPKF.)
>
>
Undoubtedly, they turn both relays on at first, then turn off the direct
supply
relay after a short time. My personal preference would be to rip the
ghastly mess out, and make up a simple circuit with 2 power MOSFETs
and a 555 timer. When the single "down" command comes from the computer,
both transistors are turned on, and after a short and adjustable delay, the
direct MOSFET is shut off by the 555. Slight problem is the 555 output
will go high when the timer is triggered, so the output is opposite of
what you want. You might be able to rig the other side of a 556 as an
inverter.
> Does anyone have specific knowledge about what they intended as drive
> signals? I was tempted to replace the two circuits with just one,
> with an adjustable one-shot to drive the solenoid full, then a gated
> PWM to provide the holding current. (And replace those relays with a
> MOSFET.)
Yes, this is an even more elegant solution. Best would be to have a
current sense resistor
and a comparator, and have the PWM controlled by solenoid current. It
might need
a little RC circuit to boost the controlled current for an instant at
turn-on to pull in
the solenoid.
> But then I wondered if drilling or milling required a
> different set of timing than straight engraving? (In particular, I
> was imagining breaking a lot of 0.020" drill bits by driving full
> speed into the board.)
Umm, yeah, I think if you want to do drilling, too, then you should toss
the solenoid
and install a Z axis motor. You can plunge pretty fast with a .020"
drill at 50,000+
RPM, but you'd still a dashpot or something to control the plunge rate.
It is possible
the solenoid has a copper sleeve that acts as a single massive shorted
turn to control
the plunge rate, a magnetic dashpot.
Anyway, you use different plunge rates for different drill sizes. I
actually had
a calculation in a program that converted Excellon drill files to G-code.
Here's the line from that Pascal program (that kind of dates it, doesn't
it?)
ToolFeed := (ToolDiam/15.0) * RPM;
So, for a .020" drill and 24000 RPM, that would be
(.02 / 15) * 24000 = 32 IPM.
Jon
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