SEATTLE (Reuters) - Some suppliers to Boeing (>> Boeing Co) and Airbus (>> Airbus Group) are putting the breaks on expansion plans, saying they fear that planned increases in production by the big plane makers may not be sustained in the face of slowing growth and low oil prices.

Consider AccraFab Inc, a precision manufacturer in Spokane, Washington, that produces parts for Boeing's 787 and other aircraft.

It recently spent $500,000 for high-speed punching equipment that can stamp complex metal cutouts used in housings for cockpit display screens, said President Greg Konkol.

"We're dragging our feet on two more investments," worth about $1.3 million, Konkol said. "We know we have to ramp up, but we're being conservative about investing too much and getting overextended."

The situation is similar at Tech Manufacturing, a maker of milled wing and airframe pieces that supplies Boeing, Airbus, Bombardier (>> Bombardier, Inc.), Embraer (>> Embraer SA), Lockheed Martin's (>> Lockheed Martin Corporation) F-35 Joint Strike Fighter and business jet makers.

"We're taking a wait-and-see attitude," said Frank Kimball, senior vice president of sales at the company, located in Wright City, Missouri.

Tech Manufacturing bought three new machines in the last five years, he said, but now is "looking very carefully at investing." Executives have priced other new equipment they want, he said. "But it depends on some of these bids coming in."

The comments, made in interviews during a supplier conference in Seattle last week, reflect growing caution among the thousands of companies that produce parts for plane makers.

Boeing and Airbus continue to forecast strong growth, and say they are fully committed to ambitious plans to increase production sharply through the end of the decade.

Boeing Chief Executive Dennis Muilenburg dismissed investor concerns about a cyclical downturn on Wednesday, noting large backlogs of orders are enough to keep factories running for many years without selling another plane.

"This is fundamentally different than anything you have seen in history," Muilenburg said at an investor conference. "We're not seeing any cyclical behavior. This is a strong long-term growth market."

Passenger traffic is rising more than 6 percent annually, above its historic 5 percent average, Muilenburg noted, adding that Boeing plans to deliver more than 900 jetliners a year by the end of the decade, up from 762 last year.

But increasingly, suppliers are voicing concern that the high rates of production won't last. They fear that a weakening global economy, particularly in China and other emerging markets, will undercut growth.

"It causes us to play things a little tighter to the chest," said Konkol. His company now wants new equipment to pay back costs in one to two years, down from two to five years.

Not all suppliers are being so cagey. Some say the lackluster growth forecasts and fears about falling sales of new fuel-efficient planes due to low oil prices are just temporary.

"We all know oil's going to go up," said Rick Taylor, a vice president at ALTEK Manufacturing, a parts maker in Liberty Lake, Washington. "And if oil goes up, you're right back into new aircraft demand," he added. "This is just a snapshot in time."

But automation companies also report caution among some suppliers. "They're trying to plan, but they don't want to get too far ahead," said Tim Shumate, a vice president at Ascent Aerospace, an automation and tooling supplier based in Santa Ana, California. "Nobody would. You don't want to make an investment and have it sit idle."

Mark Irby, senior account manager at KUKA Systems' aerospace group, which supplies automation to Boeing, said his company is bidding for work on the 737, but it's diversified in autos and other sectors, protecting it against a downturn.

But many smaller parts makers are especially worried about production targets Airbus and Boeing have set for their A320 and 737 assembly lines, rising to 60 and 57 a month in 2019, respectively, said Brian Murphy, an aerospace-focused investment banker at Meridian Capital in Seattle.

"Very few I know are taking steps today to meet the 2018 or 2019 rates," Murphy said. "They're saying let's get to 47 and then 52 and they we'll worry about 60."

(Reporting by Alwyn Scott; Editing by Phil Berlowitz)

By Alwyn Scott