IT COMES down to a fight over screwdrivers and stepladders.

Members of the Carpenters Union walked off the job last week at the Pennsylvania Convention Center to protest demands made by management that they agree to new work rules.

The nub of the argument comes down to the right of exhibitors at the $1.3 billion center to set up their own booths.

Under existing rules, if exhibitors want to use a manual screwdriver to screw in a sign, they can do it on their own. If it requires a "power tool" - a battery-driven screwdriver - they must use a carpenter.

They can stand on their tiptoes to hang a sign, but if they can't quite reach and need to use a stepladder, they must use a carpenter.

In this day and age, these restrictions seem ridiculous - especially to exhibitors and convention bookers who can choose to hold their conventions in any of a dozen or more convention centers around the country without these antique rules.

And that's exactly what has been happening. When the state spent $780 million to expand the Convention Center in 2012, the promise was that it would be large enough to draw 25 to 30 major conventions a year.

Last year, it had 20. This year it has 14. Next year it has 11. In 2016, 13 are booked. The word on Philadelphia is that the labor hassles aren't worth it and it's easier to go elsewhere.

No wonder the center's management is anxious to change work rules. And no wonder the other five unions that work at the center have agreed to changes in what is called the center's customer-satisfaction agreement - an agreement first created in 2003 when labor issues at the center also threatened its future.

These unions know that fewer conventions mean fewer hours for their members. If they give an inch today, they will gain tomorrow through more bookings, which will mean more work.

Not the Carpenters Union, not under president Ed Coryell Sr., whose son, Ed Jr., heads the carpenters' unit at the center.

The Coryells refuse to budge - and have pulled carpenters off the job twice in recent months rather than make work-rule changes.

Last year, carpenters struck the center, too, leading to a postponement of the labor agreement. In other words, this current mess is a deja F-U from the carpenters to the city.

Coryell's stand hurts the Convention Center and the other unions that work there, but that's just a start. An empty center means less work for chambermaids, cabbies, concierges, busboys, waiters and the other 62,000 workers who depend on convention business, all of whom make far less than the $65-an-hour that a carpenter makes.

Coryell's refusal is threatening an entire sector of the city's economy that taxpayers have spent more than a billion dollars to build.

Today, the Convention Center board is expected to hold a special meeting to ratify the new agreement among the five other unions at the center. If the carpenters don't agree, the work should go to the unions who are willing to do their jobs without screwing the rest of us.