Constructing a film slitter

A great deal has been written about film slitters and I don't want to add too much extra discussion. Although Minox offer several very fine films for use with their cameras, there could be several reasons why one might want to have a source of one's own.

In my own case, all of these factors have come into play at some time or another. There are downsides from slitting one's own film, but I think on balance, they are outweighed by the advantages.

Slitters seem to consist basically of one or other of two types:

i) Those with rotary cutters arranged in pairs - the film is pulled between them and a clean cut strip emerges the other side.

ii) Variations on some kind of flat bed where the film is slit using cutting blades supported in some sort of matrix and drawn along the length of the film - either the film or the blades could be the moving element.

Both types are available commercially, but usually at a price. For the home constructor, the second variety is usually preferred as it is generally a good deal easier to construct bearing in mind the precision needed and the sort of materials likely to be to hand. Most commercial slitters tend to use 35mm film a source, but that's not essential - mine uses 120 size roll film.

My first attempt at a slitter used something like the early cutting jig I made many years ago for my home made pocket camera. It comprised a spring loaded spacer which bore down on a glass plate. Film was placed under the spacer, butting hard on the back stop and was sliced off using a scalpel. As the whole operation must be carried out in the dark, there were plenty of steps to go wrong here, and film often ended up of uneven width with plentiful scratches. I later settled on a different design which drew inspiration from one published by Don Krehbiel. Anybody contemplating construction of a slitter should undoubtedly first consult his extremely informative web pages.

My slitter works with 120 size roll film and produces 6 strips of appropriate width (~9.3mm) Roll film is about 82cm long and I've found it convenient to divide a length into two unequal parts of about 50 and 30 cm each. These give respectively about 30 and 15 exposures each with a little margin at the ends.

Complete Slitter

The slitter can be seen to consist of a shallow trough just exactly wide enough to contain the film. The base is softwood but the guides are ramin hardwood. The base is lined with soft cork as used for flooring to avoid scratching the film which is inserted emulsion side down. At one end is attached a narrow plate which can be screwed tightly to the end of the slitter using a pair of wing nuts. This is used to hold the film securely - the end is placed under the plate and the nuts done up tightly after which the film is pressed into the cutting channel.

Cutter assembly

The cutter comprises seven cutting blades (disposable double edged razor blades) spaced apart by perspex spacers, 3 layers each of nominal 1/8" thickness. The whole assembly is sandwiched between two aluminium plates of similar size to the spacers except for the fact that they project downward below the cutting face by about 1.0mm. This is to ensure that when the cutter is drawn along the film, the facing of the spacers cannot touch the film and cause scratches. It also usefully ensures that about 3 mm of film at each edge is discarded - this usually carries film identification data which otherwise would intrude into the frame.

For anyone wishing to construct a slitter along these lines, below are some constructional details. There are two parts: the cutting bed and the slitter blade assembly.

Taking the cutting bed first, this is shown in cross section in Fig. 1.

Base

Fig.1


The base (Section B) is a piece of softwood 1 metre long and 100mm wide. This should first be carefully sanded to remove any sharp edges and splints which could conceivably cause scratching. Two sections (A) of suitable hardwood (ramin in my example) should be cut to the same length as the base and screwed firmly on to it as shown. Again, they should first be carefully sanded to remove any roughness. Use countersunk screws so as to avoid interference with the cutter when film is being slit. The only critical dimension is the spacing between the inner edges of the these guides which must be 62mm - the width of 120 size roll film. It is a good idea to sacrifice one roll of film to ensure that the dimensions are correct and that the film will fit comfortably into the cutting bed.

The bottom of the cutting bed needs to be lined with a suitable soft material which will not scratch the film surface. I found that soft cork about 1mm thick (C) used for floor covering is very suitable, but other materials may well suggest themselves to you - soft plastic sheet or ragboard (from an artwork shop) could be alternatives. In use, the cutter will produce grooves in this material so it needs to be thick enough to prevent the blades penetrating through into the wood base. When assembled, it is a good idea to run the cutter through without any film a few times to cut these grooves so that subsequent slitting will be easier and won't unnecessarily blunt the cutting blades.

A means of holding the film in the cutting bed is also required. I did this by inserting two threaded studs into the end of the base; a metal plate with two matching holes was then put in place and retained with two wing nuts. To hold the film, one end is placed under the plate, ensuring that the film is aligned with the trough, and the nuts tightened. The other end can be held down temporarily with a piece of adhesive tape if necessary. See the illustration below for detail.

The cutter is a little more difficult and needs to be made carefully if film of the right size is to be obtained.

For materials, you will need:

In addition you will need a hand saw, a drill, coarse & fine files, some polystyrene glue, some fine emery paper and a roll of sticky tape.

Begin by cutting the perspex spacers. This is the most tedious part but there is no shortcut. I suggest cutting them a little larger than needed, then place all the pieces together and bind them into a unit with adhesive tape. Hold in a vice and true up one exposed face until there is a common flat face. Turn over and do the same and continue filing until one of the required dimensions 28 or 50 mm is reached. Now remove the binding tape and rebind around the perpendicular dimension. Again file until two parallel faces are obtained and you have a stack of spacers all 28 x 50mm. This will take some time and effort...

Without undoing the binding tape, mark and drill three 5mm holes right through the stack as shown in Fig 2. [Not to scale, by the way !] It is best to do this with the stack clamped in a vertical drill stand to ensure the holes run true and parallel to the faces. Be sure to drill very slowly without undue pressure, clearing the drill bit frequently. Perspex melts easily and the drill will seize if the bit gets too hot.

Spacer

Fig. 2

 

When all the holes have been drilled, remove the binding tape and smooth all faces of each spacer, countersinking each hole with a larger drill bit. It is helpful to number the spacers before separating them as subsequent alignment will be easier. Now, take the spacers, three at a time and glue them together to make six larger blocks, using the polystyrene glue. Each block should be firmly clamped and left for 24 hours for the glue to set. Finally again clean and polish all exposed faces with the emery paper. The worst is now past !

The two end plates should be cut out of the aluminium or brass sheet as in Fig 3. Smooth and polish all exposed edges, particularly those which will end up on the bottom of the cutter in contact with the film.

End plates

Fig. 3


You are now ready to try assembling the cutter. Pass one of the 5mm screws through the middle hole of one end plate and add a blade through its centre hole aligning the other holes with the openings in the blade. Add a spacer, then another blade, continuing until you have seven blades in place. Add the second end plate and the wingnut to the screw, tightened a very little. Feed another screw through from the other end right through the assembly finishing with the third screw. Ensure that everything is correctly aligned and then tighten all the screws. Offer up the cutter to the cutting bed, and if all has been done correctly, it should be an comfortable fit.

If it's too tight, then your perspex was thicker than nominal, if loose, then it was a bit thinner. In the former case, you will need to dismantle the cutter and grind the spacers down a little. This is most easily done by putting some household abrasive on a glass plate, add a drop of water and grind until the thickness has been reduced. If you have access to some sort of micrometer, this is very useful to check that they are all of the same thickness.

If the cutter is loose, this is easiest remedied by adding sufficient packing on either side next to the end plates - sheets of paper, thin card or whatever comes to hand will do. This is how mine turned out in fact.

At this point, it is worth running the cutter through the cutter bed a few times to generate the grooves in the cork already mentioned. A strip of paper 62mm wide and as long as possible should be used with the cutter to get the feel of actual cutting, and also to check the width of the final cut film. This should be around 9.1 - 9.3mm wide. If any wider, you run the risk of film jamming in the cartridge, and subsequent damage to the camera if too much force is used to move it. The contra tensioning springs which provide the one-way film motion in the EC and other sub-minis are quite easily dislodged and damaged. Whilst they can be straightened and replaced, this is a tricky job, and very much best avoided in the first place !

Finally, you are ready to cut some film. It's worth sacrificing at least one roll of film in daylight, to get some practice. Remove the backing from the film and insert one end a little under the locating plate at one end of the base; ensure the film is aligned with the trough and do up the nuts. Unroll the film, emulsion side down, on to the base and if the curl is excessive, hold the end down with some sticky tape. Introduce the cutter at the clamped end and pressing down firmly, draw it right through the film in one steady movement, right to the other end till all the film is cut. Cut the clamped end off with scissors or a sharp blade and separate the individual strips of film. Inspect for irregularities - you will soon learn to do this easily in the dark by touch. If all is well, you are now ready to go and slit film as necessary...

Below are some illustrations of how the individual parts of my slitter turned out. Click on the small picture for a better view.

ElementsAll the individual cutter elements together.

Spacer One of the six spacers, composed of three parts.

End plate One of the end plates.

Alignment Alignment of the blade relative to the spacers.

Assembled The assembled cutter.

Clamp The film clamp on the cutter bed.

For those who are uncomfortable with metres and millimetres and prefer to work in old imperial units, you might like to note that 1" ~ 25.4 mm

This slitter has been in use for a few years now and seldom gives any bother !



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Revised February 2003