Bridgeport Interact manual feed - why inhibited?

[swarf_rat]

This is a Series 1 Interact with 151B control. The manual feed via the joysticks is inhibited when the spindle is off, only rapid is available. With the spindle on, both manual feed (set by feed pot) and rapid are available. So says the Operator's Manual, and the schematic agrees: that is the way it is wired.

I am contemplating modifying this behavior, which looks to be a simple matter of jumping over the spindle-on contacts (1CR) in the x-y feed interlock chain (7CR) by connecting wires 127 to 128. That would make manual feed available when the spindle is off for fine positioning, etc., while still allowing rapid by pushing the button. Doesn't look like it would affect anything else.

Why wouldn't I want to do this? I haven't made all the mistakes a CNC operator can make (though I am catching up rapidly!), but I can see no reason why it was set up that way. Can someone tell me why this would be disabled with the spindle off? It isn't as though there is extra danger of crashing a tool, that seems much more likely in rapid - which is enabled. I know I can use the handwheel, but one has to keep pushing buttons to switch between X and Y while approaching a point, could be very simple with the joystick set at a slow feed rate.

[machintek]

Machine tool manufacturers are not worried about you trying to set up a machine. They have found out from experience not to feed a machine in a program (inhibit a feed) if the spindle is not running. Why? because typically a rapid move is in air. A feedrate is through material. And if the spindle is not turning you will break a tool or damage something. It is a safety issue.
Some TNC controls with a spindle drive will monitor spindle speed and will stop the feed if the RPM drops 20% of the commanded RPM. The DX controls with a spindle drive do the same thing.

George

[swarf_rat]

Thanks for your thoughts.

I can understand inhibiting feed within a program if the spindle isn't turning. On the Interact that is handled by the machine waiting for the M-code verify (which - on the Series 1 anyway - is just the spindle-on relay). It will not step to the next line until it sees that. Of course you can manually switch the spindle off anytime you like and cause a crash. You can also simply forget to put an M3 in your code and it will go ahead and crash without looking at the spindle. The relay chain I am talking about is disabled when the machine is in Auto (axiis under computer control), is only in use while in manual or handwheel positioning mode, and only feeds the joystick inputs.

What I don't understand is inhibiting feed during manual operation. This is no different than a manual mill, you have to be standing there, pull the safety ring on the joystick and be holding it over. As I said, they allow this in rapid movement. If you are going to stand there holding the joystick over watching the tool crash into something, it is going to happen far quicker in rapid motion. In fact having to use rapid to approach work seems to me to be manifestly more dangerous than at a slow feed.

So I don't see what safety feature they have added with this disabled for manual operation. Maybe they think the handwheel is even more foolproof, and you would be less tempted to use the joystick for approach if it only rapids. But with the handwheel set to its fastest speed and given a good spin, you could break a lot of stuff.

Curiously, the Z is allowed to move at the feed speed when the spindle is stopped (no manual -Z rapid is ever allowed). Why not X and Y?

[swarf_rat]

I went ahead and hot wired my machine, wired 127 to 128. While I was at it, I wired 163 to 129.

Now I can feed X, Y, and Z at the pot set feed rate with or without the spindle running. It seems to default to F200 (20 inches a minute) at 100% setting. While feeding, I can hit the rapid button and switch to rapid feed. But then releasing the rapid button stops all motion until the joystick is returned to neutral. That is because the feed interlock relay has unlatched when the neutral switch opens.

Z will still only feed down at feed rate, no rapid. They didn't connect that switch, probably a good idea to leave it that way.

Works great, like it shudda' to start with, in my uninformed opinion.