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I moved your post to the relevant forum, where hopefully you will receive more replies.
Welcome to the G!
How are you chopping, and what kind of moulding is it?
With a chopper, you have to take “bites”, you can’t cut through it all at once. Some moulding might require more and smaller bites than others.
The moulding might also be the issue as a lot of gessoed moulding doesn’t chop well on a chopper.
If the blades are new, I assume that is not the issue. Blades need to be sharp of course.
What Ylva said. And I have found with softer woods, sometimes you need to be very careful with the lip of the frame and make sure it is supported with each bite.
Once you get used to each profile, you will find that you develop a memory for how to cut it. They are all slightly different. And to add a further complication, a moulding you buy this year may be on a particular wood. But next year, it may look the same, but the base wood has changed.
You can usually get a feel for how a moulding is going to cut when you make that first cut on the right-hand corner. Sometimes I may make that cut several times to get a feel for it. Until I am sure I might take a bite every 1/8 inch.
Post some pics and you will find almost every problem has been experienced and a solution found here! The G has been around a long time and there is a lot of institutional memory.
I always disliked the term "chopper" for a guillotine because "chopping" is what you do to timber with an axe. A guillotine nibbles then shaves to produce a precise, smooth mitre.
As Ylva says, small bites to get through most of the wood then, for the second last cut move the cutting head hard against the stop then back it off around 1/16" (I never used the ratchet for this). Final cut with the head against the stop should then just pare off a shaving with a soft ssshhh sound - like a librarian reminding where you are.
If your blades are sharp and you follow this method and still get poor cuts then, quite possibly, the moulding is that wretched crumbly stuff which no-one can do anything with.
ps: Sometimes new blades don't cut perfectly straight out of the box so if you have a spare pair try them first.
the guillotine chopper is one of the best machines for cutting mitres as cutting perfect mitres can be a struggle to many out there!
furniture makers often ask me how i create such clean cuts without shooting boards ( jigs used with hand planes) i'll then show them my guillotine cutter and how it works and they buzz out completely.
but these machines perform best when cutting raw wood moulding, even hardwoods which is the road i've gone down as i mill and finish picture frames from beginning to end
if you're cutting pre-finished moulding, it can come with a challenge.. but there are tricks to the trade always.
you're fence being potentially off square has nothing to do with flaking. when cutting pre-finished moulding you often end up with flaking especially with painted opaque frames because the paint is fairly thick then oppose to stained wood moulding with thinner top coats. the hardest prefinished moulding i found to cut was a painted opaque frame with side scoops on the sides because of the empty space thats along the fence and the frame. this empty space is the main issue aswell as the paint being more brittle than wood. it's the same principle as the rebate supports. if you cut moulding without the rebate supports you often crack the bottom of the rebate because there's nothing there to support it. one thing people do is tape the section where the cut is made but the edges may flake off when removing the tape.
i'd like to try out what was suggested above ^^^ if i'm to cut this type of moulding again in the future, but what i wanted to add is perhaps a bit contradicting and the opposite to what was said which is try removing a fair sized chunk off on your last cut rather than trying to graze the last cut. i found that by trying to minimally cut the last section off i was experiencing flaking still, but when i took a reasonable portion off i ended up with a cleaner cut somehow. mind you this was with one particular pre fab moulding i was experiencing this issue with and this trick worked. almost always though you need to finesse with fine sand paper and touch up...
another thing you can try when cutting box frames is to try inserting a thin strip of wood along your fence so that the last cut bites into the wood and not the empty space in between the two fences.
That is a great idea about putting a piece of wood behind the moulding for the last cut. I always seem to have lots of those thin strips around from "artist framed" canvases brought in for framing.
And I always have made my bites smaller as I got toward the back of the moulding.
When you look at the edge of the finished moulding, if you see any layers or gesso, vinyl or paint, chances are good it will be a challenge to cut on the Morso.
All mouldings are different, so it is good to do a test of each new order. These days, that is a good idea for even mouldings you are familiar with. The base wood seems to change with some frequency and that affects your cut.