The shears to which the workers objected were shearing frames. Steam engines had been installed at the mill in 1795. The Luddite's attacked the mill on 9th April 1812. [i]
Since the nap was uneven, the final step in cloth production was shearing (or cropping) to produce a smooth surface. The work had originally been done by hand with large shears. When the first cropping frames were introduced, one machines could do the work of four hand croppers. [ii]
References:
[i] Ossett - the history of a Yorkshire town http://www.ossett.net/beyond/Horbury_Bridge.html
[ii] Armley Woolen Mill website http://www.maggieblanck.com/Land/Mill.html