What exactly is an over-cut or undercut picture mount? What is it that creates these effects? How do you stop them from occurring? What level of under/over cut is acceptable?
An over-cut picture mount
An over-cut refers to a picture mount that has blade cut over-runs this can be in either one or two directions at any corner. this effect is shown in the picture on the right. In this example the over cut to the right side of the image has happened due of the start point on the mount cutter blade being set too early, the one to the bottom of the picture is due to the end stop on the mount cutter being set too late.
In the case of an over cut mount, whether it is perceived acceptable or not will depend on how bad the over-cuts look to the naked eye. They can be burnished down with a burnishing bone which can smooth them down to unnoticeable levels. However, badly over-cut mount corners (>1.5mm or so) will not be acceptable and will render the mount scrap.
An undercut picture mount
An undercut picture mount refers to a mount that has blade undercuts and as a result will leave a incompletely cut mount centre that won’t drop out. This undercutting can occur in both directions depending on the blade entry and exit point settings. In the image shown, this particular undercut is caused by the blade entry point having been set too late.
Undercuts (provided they are not too large) can usually be remedied by carefully taking a spare mount cutter blade by hand and holding it at approximately 45o and completing the cuts manually. What you should not be tempted to do is just pull the undercut paper ‘tag’ away, as this can lead to facing paper tears which will likely make the picture mount corner visually unappealing and you may have to start with a new fresh mount board and cut the mount again.
So what defines a perfectly cut picture mount? The ultimate goal is to cut a picture mount in which the centre falls away with no visible over or under cuts. ie the entry points of the blade will exactly coincide with the exit point as seen from the front of the mount.
How achievable is this?
On a mount cutter there are usually several criteria that can affect whether you produce the perfectly mount or not. They include –
- Blade depth
- Security of blade (how well is it held in place ie. if its not tightly secured, it may get pulled out further when you start to cut. This is especially true with thicker mount boards!)
- Thickness variations of the mount board (changes from board to board, or supplier to supplier)
- Accurate offset calculations for start and stop points for a particular mount board.
- Squareness of the mount board edges and corners
- Uneven hand pressure when pulling the mount cutter head to do the cut.
Changes in any one of the above criteria can lead to over-cut or undercut picture mounts.
A mount cutter can be fine tuned, but will likely need to be adjusted for every different type of mount board cut even though they are all for example labelled as say 1.4mm. I have seen +/-15% thickness differences between mount boards of a certain specified thickness. The thickness also varies with the texture of their facing papers too, heavily patterned boards can add an extra 0.2-0.4mm thickness, where as flat finishes can be more spot on at 1.4mm. Ideally you really need to create a test mount to fine tune the mount cutter based on the board that you want to make the final mount from, this can be done on a smaller piece of mount board to avoid wastage. How many times you choose to do a fine tune of the mount cutter will boil down to how much mount board you want to waste (in test mounts) and how much of a perfectionist you are.
What is an acceptable level of over-cuts and undercuts? The following table summarises what I generally go by –
|Size of over/under cut
|acceptable but will likely need manual intervention with a blade to finish it
|May be retrievable with a blade, but keeping the blade in a straight line at 45o by hand may be difficult
|acceptable mount, burnish any ragged over-cut edges away
|unacceptable mount visually