Inventories (Details) (USD $)	12 Months Ended
	Apr. 30, 2013	Apr. 30, 2012	Apr. 30, 2011
Inventories [Abstract]
Raw materials	$ 11,823,000	$ 9,412,000
Work-in-process	17,170,000	14,543,000
Finished goods	11,318,000	8,734,000
Total FIFO inventories	40,311,000	32,689,000
Reserve to adjust inventories to LIFO value	(10,973,000)	(10,349,000)
Total LIFO inventories	29,338,000	22,340,000
Liquidation of LIFO based inventories	$ 0	$ 125,000	$ 34,000