Again, Larry, are you suggesting that a late-spring trunk chop on red and silver maple will be OK from first hand experience? If possible, please let us know, so that we can judge whether we should take your advice seriously or instead with a
grain of salt. Though I haven't worked with either of these particular species myself, general experience with other maples leads to me recommend against doing a chop now, after the plant had thrown a great deal of energy into developing foliage above the chop site. Nor would I recommend a chop in early spring when the leaves were unfurling. Rather, I've found that trunk chops work best in late winter, just before the buds begin to swell.
There's a more critical issue, though. While as mentioned above I haven't worked with silver maple or red maple myself, I would suspect that the leaves and in particular the internodes will not reduce sufficiently to allow you to fashion a convincing mame. I'd guess that these species if used for bonsai at all will be better suited for large-sized bonsai. Perhaps others with direct experience can confirm or deny this conjecture.
Best regards,
Carl