Laurie. I have read your post again and I may have been giving you a fix for a different problem.
I thought you meant a gap at the
back of the join. That is, on the underside of the frame. I realise now that you probably meant a gap in the outer corner, as if the mitre 45 deg miter angles where off.
This being the case, I would do this:
Change the blades for a freshly sharpened set. Pay extra attention to making sure they are correctly seated, no gunge underneath and exactly the same hight. Then do Jeff's suggestion and check the fences are perfectly in line. Next, get some scrap pieces of scrap moulding. wide as possble. Pieces of 3" wide plain timber will do fine.
Cut some small frames, maybe 8" sq. See if these go together dry without any gaps. If there is still a gap, try slackening the left fence and swing it toward you a whisker. Tighten it again and do another test frame. The gap will either get smaller or bigger. If it gets smaller, move the fence another whisker until it goes altogether. If it gets bigger, move it away from you. It's a question of trial and error. Remember, each tweak in the angle will be multiplied by 4.
This may or may not fix the prob. There are a lot of factors to consider, so it is hard to give advice. It may be that the blades are not ground correctly. Or sometimes when you have been cutting a lot of narrowish moulding, the blades may be dull at the front and still razor-sharp at the back. So when yo come to cut a wider moulding, the sharper part of the blade bites first and the back of the moulding gets cut with the less sharp section. On wide deep-scoop mouldings the blade is going to hit the inside first, then the back and the middle last. This can make the moulding wriggle about as the blade is trying to make two different tracks.
