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Hi, this is my first post but I've been reading this forum for a while and find it very helpful!
Now about the problem
Until recently I was using clamps + manual V nail tool and was quite happy with the frame quality/look. But few weeks ago I decided to buy an underpinner as wanted to save time on Clamping each frame.
Now the frames are more solid, which is good, but I can't tight them as good as with clamps. I can't really say the inner corners have gaps but the difference is noticeable. Am I doing something wrong?
I think that the problem isn't with the underpinner but the mitres themselves. What are you using to cut the moulding? Guillotines often need a bit of fine tweaking to get the perfect joining angle. The usual thing is to move the left-hand fence toward you a tiny amount.
Ideally, when you come to pin the last corner the faces should be just a gnat's whisker apart.
I agree with Prospero, sounds like your blades are starting to dull, or are slightly out of alignment, you should get perfect corners without having to clamp.
Attached an example pics. This is deep rebate oak moulding but I've got a similar problem with a soft wood as well. Need to touch up at the end.
Today ended up clamping the frame and underpinning it while being clamped to make sure these don't split up.
Deep mouldings in hard wood are always a problem with underpinners. Your method of clamping and then pinning is a good one. Sometimes it's tricky to stack v-nails in hard wood. I never do it. In the case of deep but tall mouldings I would underpin all four corners with a shallow v-nail, toward the inside of the moulding. Then strap clamp it and drill'n'nail near the top. The v-nail just helps keep the joints in alignment while you clamp and nail it.
Sounds a bit of a faff and it is, but you get a good strong corner.
