ended June 30, 2023 from 9.8% of net sales in the three months ended June 30, 2022 reflecting the hiring of engineers, related product development and increased R&D projects.
Selling, general and administrative. Selling, general and administrative expense increased by $632,000 or 37.3% to $2.3 million in the three months ended June 30, 2023 from $1.7 million in the three months ended June 30, 2022. As a percentage of net sales, selling, general and administrative expenses was 30.1% in the three months ended June 30, 2023 compared to 24.4% for the prior year period. The overall increase in selling, general and administrative expense in the quarter was primarily the result of increased sales and marketing costs, legal and professional fees incurred as part of the Honeywell asset acquisition, as well as non-cash executive stock awards.
Interest income. Interest income increased by $175,000 to $186,000 in the three months ended June 30, 2023 from $10,000 in the three months ended June 30, 2022, mainly a result of increased cash balance and higher interest rates earned during the current year period compared to the same period in the prior year.
Other income. Other income is mainly composed of royalties earned and increased by $68,000 to $90,000 in the three months ended June 30, 2023 compared to the same period in the prior year.
Income tax expense. The income tax expense for the three months ended June 30, 2023 was $340,000 as compared to $359,000 for the three months ended June 30, 2022.
The effective tax rate for the three-month period ended June 30, 2023 was 19.3% and differs from the statutory tax rate primarily due to permanent items and state taxes.
Net income. The Company reported net income for the three months ended June 30, 2023 of $1.4 million and remained flat compared to net income of $1.4 for the three months ended June 30, 2022. On a diluted basis, the net income per share was $0.08 for the three months ended June 30, 2023 and remained flat compared to net income per share of $0.08 for the three months ended June 30, 2022.
Nine Months Ended June 30, 2023 Compared to the Nine Months Ended June 30, 2022
Net sales. Net sales were $21.8 million for the nine months ended June 30, 2023 compared to $20.5 million for the nine months ended June 30, 2022, an increase of 6.5%. Product sales increased $1.1 million, customer service was flat, and EDC sales doubled to $234,000 in the nine months ended June 30, 2023 compared to the same period in the prior year.This increase in product sales for the nine months ended June 30, 2023 primarily resulted from increased sales to our OEM customers, which include Pilatus, Textron and Boeing. Sales increases were also seen in our new auto-throttle installations.
Cost of sales. Cost of sales increased $347,000, or 4.2%, to $8.6 million, or 39.5% of net sales, in the nine months ended June 30, 2023, compared to $8.3 million or 40.4% of net sales, in the nine months ended June 30, 2022. The increase in cost of sales was primarily the result of an increase in product sales volume for the nine months ended June 30, 2023 compared to the nine months ended June 30, 2022. The Company’s overall gross margin was 60.5% and 59.6% for the nine months ended June 30, 2023 and 2022, respectively. The increase in gross margin percentage for the nine months ended June 30, 2023 is attributable to favorable leveraging of fixed costs resulting from the increased sales and production volume and a favorable sales mix.
Research and development. R&D expense increased $325,000, or 15.8%, to $2.4 million in the nine months ended June 30, 2023 from $2.1 million in the nine months ended June 30, 2022. As a percentage of net sales, R&D expense increased to 10.9% of net sales in the nine months ended June 30, 2023 from 10.1% of net sales in the nine months ended June 30, 2022 reflecting an increase in hiring engineers and working on product development and related programs/internal projects.
Selling, general and administrative. Selling, general and administrative expenses increased by $1.8 million to $7.0 million in the nine months ended June 30, 2023 from $5.2 million in the nine months ended June 30, 2022. As a percentage of net sales, selling, general and administrative expenses increased to 32.6% of net sales in the nine months ended June 30, 2023 from 25.5% of net sales in the nine months ended June 30, 2022. The increase in selling, general and administrative expense in the period was primarily the result of personnel additions in sales and marketing, legal and business development related to the acquisition, investor relations and investor facing activities and non-cash long-term compensations post COVID.