2. Film Gate

Design challenges

  • Consistent film image plane
  • Consistent and reliable lateral film frame registration
  • Even illumination of film frame
  • Film handling
    • No wear or abrasion of film
    • Free passage of film splices
    • Easy threading
    • Simple film release

Photograph 7 shows the relative positions of the film gate and motor sledge as the film is transported downward through the film gate.

 

Figure 5 Shows a simplified section through the film gate and where the film is positioned.

The film gate holds the side edges of the film in compression in grooves cut into the sidewalls of the film gate. The base of the groove is parallel to the film plane while its top is inclined at 45° like this ∠. This groove shape keeps the film against the base of the groove to keep a consistent film image plane to focus on, and it handles the passage of film splices well.

The fixed part of the film gate supports the sprocket hole reference edge of the film and the spring loaded floating part supports the edge created when the original 16 mm film was split. Running the film reference edge in the film groove of the fixed part of the film gate gives consistent and reliable lateral film frame registration. Only light spring pressure is required to provide the holding force of the film in the gate to exceed that exerted by the slope of the bottom of the claw on the film as it is moved backwards. Excessive spring pressure could cause the film to bow, so the gap between fixed and floating parts of the film gate is made very small to avoid this and any damage to the film, yet still meet the holding force and image plane requirements. Photograph 8 shows the gate parts.

The material used for the body of the film gate is Acetal, which has a low friction coefficient and is easily machined to a good finish. The low friction of the gate material body enabled the film handing challenges to be met and allowed the floating part of the gate to slide freely on the tubular locating dowels with only light spring pressure. Consistent penetration of claw depth in the sprocket hole is normally controlled by the sloped bottom of the claw to aid vertical film frame registration. In badly damaged sprocket holes claw penetration is limited by the claw tip running on the surface at the bottom of the claw relief slot. Photograph 8 shows and names the film gate parts.

To ensure even illumination of the larger Super 8 mm film frame size the film gate aperture was made oversize at 6 x 7 mm as it is the video camera that determines the image size captured.

Photograph 8 Film gate assembly parts

1. Film groove in floating part of film gate.
2. Tubular locating dowels.
3. Film track relief.
4. Film gate aperture.
5. Sprocket hole support lands.
6. Claw relief slot extent.
7. Pressure springs, screws and washers.

 

Photograph 9 Film gate assembled

1. Floating part of film gate.
2. Small gap between fixed and floating parts.
3. Fixed part of film gate.

Film Gate with Super 8MM Leader


1. Film entry end. See Photograph 11
2. Sprocket holes clear of sidewall.
3. Film exit end. See Photograph 12.
4. Leader shape for easy threading.

 

Photograph 11 shows the entry film track detail of; the film grooves, film track relief and sprocket hole support land.

Photograph 12 shows the exit and the claw relief slot cut into the sprocket hole support land.

Standard 8 mm film was used in both photographs to show the wider sprocket hole of the format.

 


 

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