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Idler Gear Updates - Part I

By
Jerry Turner
on September 10, 2008 2:55 PM | | TrackBacks (0)


Based on my first on set experience with the idler gear (detailed in the original post), I have made some of the listed changes, as well as one or two that weren't on my original list of things to change. This post covers changes made to the armature. A future post will cover the changes made to the rest of the unit.

As you can see in the picture above, the most notable change is the addition of a 10-32 adjustable handle. In the first version of the armature, I had used stainless steel hardware. This proved almost impossible to work with at any size above 8-32, and even that was a challenge. Because of this, I purchased the adjustable handles with ordinary steel studs. The handle was purchased with a 1-1/4" stud. This I cut down to the precise size needed, so the stud length fits the width of the block exactly. The handle has added a tremendous advantage to on set usage. It greatly speeds up the process of making adjustments to the assembly.

The arm itself has also been permanently cemented to the block. I had begun to notice a significant amount of play in the connection between the two, which was leading to some of the gear slippage I had experienced. Permanently attaching the arm to the block was the best solution. The ability to use arms of different lengths is lost, but the trade off is worth it. I settled on an arm length that has worked well for everything so far. The arm now extends 5" above the top of the block. To fasten the arm in place, I used JB Weld (KWIK). It is holding very solidly, and has eliminated much play from the system.

Building a Universal Idler Gear

By
Jerry Turner
on August 22, 2008 10:30 PM | | TrackBacks (0)



The Concept:

It's quite a shock to start using a new lens, and discover that it focuses in the direction opposite to that which you have been accustomed. When I first began pulling focus professionally, most of my work was with Nikon mount lenses. Imagine my surprise then, stepping on to a low budget RED shoot without any prep time, to discover that cine glass focuses in the opposite direction. When we wrapped for the day, I promptly went home and did some research. Turns out that the direction most Nikon mount lenses focus is the exception, not the rule. All the reflexes I had built up over many shoots destroyed by the simple change of a lens.

The ironic thing is that the Nikon focusing direction actually makes more sense. When the distance between film plane and subject narrows, the follow focus knob is turned toward the back of the camera (using the top of the knob as reference). The opposite for increasing distance between film plane and subject. With cine glass, it's the opposite. Despite the apparent strangeness of cine focusing direction, I needed to adapt. I also needed a mechanism for focusing in the apparently "correct" direction with Nikon glass.

Inspired by my own experience, as well as Ryan Walters' thread over at RedUser, I decided to set about building myself an idler gear assembly to change the focus direction of Nikon mount lenses. The basic idea is that by inserting an extra gear between the follow focus drive gear, and the gearing on a given lens, the focus direction will be reversed.

I had several parameters for this contraption. First off, it had to work with any follow focus out there. Several vendors have models that fit their own lines of follow focus units, but as I work with a variety of different gear, I needed something that would work with every follow focus I encountered.

Additional Information: As far as manufactured idler gears go, there are several options. Redrock Micro will soon be releasing an assembly that fits their excellent, affordable follow focus. Arri and Chrosziel already have units that fit several of thier respective follow focus models. I believe I've seen another (albeit, expensive) universal model lurking around somewhere, but I don't recall who makes it.

It also had to be extremely versatile, working with any lens at any height and diameter. Finally, it had to be easy to use and adjust. With these things in mind, I began the design process.

Below are some early concept drawings:


A view of the whole assembly as if from the back of the camera. Click for larger.



Block and gear assembly from top. Click for larger.

The above should give a basic idea of what I set out to achieve. the final design changed quite a bit, inspired by issues of practicality, usability, compactness, etc.

With the basics in mind, let's move on to the actual build for v1 of the contraption. As we go step by step, I'll attempt to explain some of the reasons I designed it the way I did.


The Build:

Tools:

Saw w/metal blade (Jigsaw preferred. A hacksaw can be used, but will make your life miserable)

Tape measure

Drill (drill press will make life a lot easier)

Bits (high speed steel or better)

#21

#29

3/8"

15mm (1/2" shank for smaller chucks. Available at mcmaster.com)

1/4"

Tap handle

Taps (bottoming taps preferred)

8-32

10-32

4mm .7


Pencil (or other marking device)

Allen keys (set)

Small metal file(s)

Small t-square (optional)

Center punch

Vice


Materials:

Aluminum stock (1/2" thick)

3/8" round aluminum tube

1x 8-32 Hex cap head bolt ~3/8" (I bought a 5/8" and cut it down to size)

1x 10-32 Hex cap head bolt 3/4"

4mm .7 threaded rod

4mm .7 Lock nut

2x 4mm lock washer

4mm .7 nut

.8mod/32 pitch spur gear (Same size as follow focus drive gear. I got mine from Redrock)



The idler gear assembly has three main components:

    1. Armature. Connects to the 15mm rails, and runs an arm up for attaching the rest of the assembly.

    2. Connecting Block. Connects the gear assembly to the armature.

    3. Gear Assembly. The actual idler gear and related hardware.


We'll start with the armature. Here's a diagram of what we'll be building:



Note how the design has changed since the concept drawings. The mechanism for tightening the armature base to the rails has been moved. This allows the design to be more compact, and eliminates the extra parts that would otherwise be needed to attach the armature itself to the base thereof. By moving the tightening mechanism, we can also use it to keep the arm in place.

The first step is to cut an appropriately sized piece of aluminum stock. For 15mm rails, the size is 1" x 3/4" x 1/2". I haven't yet calculated the appropriate size for 19mm rails.


Having cut the bit we'll be working with, the next step is to start drilling holes in it. Start with the hole for the rails. The 15mm hole should be drilled on the 3/4" side of the block. Spacing should be equidistant on the sides. Center of hole is .375" from each side. Eyeball the top/bottom spacing. Ideally, you'll have slightly less than .375" on the bottom. Use the center punch to mark the center of the place to be drilled. This is a large hole, so you'll want to drill pilot holes. I did a #21 and 3/8" before moving up to the 15mm.


Next, drill the 3/8" hole for the arm, as per the diagram. Drill all the way through, until you break into the side of the 15mm hole. Use a circular metal file to clean out both this hole and 15mm one. I found it helpful to do a #21 pilot hole for this one.



Drill the hole for the tightening screw. This should go all the way through the block, as per the diagram. It should be spaced half way between the top of the 15mm hole and the top of the block. Use a #21 bit for this hole. Once the hole is drilled, insert the 3/8" tube in its hole.

Note: Before inserting the tube, you should cut an appropriate length of it. I found six inches to be about right. You'll know better what length to use after actually using the finished product. I keep several lengths on hand when on set.

Also, you may have to again file out debris from the 3/8" hole, which was made during the drilling of the screw hole before the tube will fit.

Again using the #21, use the hole you just drilled as a guide to drill straight through the tube. Leave the tube in the hole, and use a 10-32 tap to cut threads all the way through the block and tube.



Now we have to make it possible to clamp our block on to the 15mm rails. As per the diagram, cut a slit perpendicular to, and through the screw hole, until it intersects with the 15mm hole. At this point, verify that you can still get your screw/bolt to thread the hole properly. It will probably be a little difficult the first time, as the screw will have to push some debris from the cut out of the way. Remove the screw, and load up a 1/4" drill bit.

While you now have a block that looks like it should tighten on the rails when the screw is tightened, you'll notice that it doesn't. The solution is to drill out the section of the screw hole marked in green below, so that the screw doesn't actually hold on anything there, thus enabling it to flex as pressure from the screw head is applied.


The 1/4" bit worked well for me here.

Having drilled this part of the hole out, insert the tube in the appropriate place, and screw in the bolt. Stick the thing on your rails, and give it a try. It should hold firmly. If all works well, use a file to soften all the sharp edges and corners. Wouldn't do to be bleeding all over an expensive lens.


The finished base without hardware.


The finished armature.

Now let's move on to the connector block. This is the bit that interfaces between the armature and the gear assembly. Here's a diagram:


Again, note how the design has changed from the concept drawing. The changes here allow for an easier build, although using a modification of the concept found in the initial drawing might lead to a slightly more rigid build, and easier access to the tightening screw; however, we would lose some flexibility in the way the block is able to be positioned.

The connecting block requires another chunk of the stock aluminum. This piece should be sized 3/4" x 11/16" x 1/2".


Once the block is cut, the first hole to be drilled is the 3/8" hole for the tube (through the 3/4" x 11/16" side). Again, a #21 pilot hole was useful. Once the 3/8" is drilled, do the hole for the tightening screw and gear attachment. Similar to the tightening hole on the armature base, this hole should go all the way through the block. Use a #29 bit for this hole. On the tightening screw end of the hole, use an 8-32 tap to cut threads just over half way down the hole. On the other end of the hole use a 4mm .7 tap to cut threads for the rest of the hole.


Cut the slit which allows tightening from the edge of the block to the 3/8" hole, again bisecting the tightening screw hole. Do exactly what you did on the armature base, and drill out the first segment of the tightening screw hole (not sure what size bit I used for this, but calipers can easily provide you with an appropriate measurement). By now, your connecting block should look like this:



Connector block without hardware.

Slide the block on to the 3/8" tube. Again, you may have to clean out the hole with a circular file. Screw in the 8-32 screw, and make sure it works. Don't forget to file down the sharp corners and edges.

Cut a length of threaded rod. The length will depend on your follow focus and lenses. I'm currently using about a 3.5 inch length, but carry several different ones on set, just in case. This piece of threaded rod will attach to the connector block, and hold the gear itself in place. I have seen one design that simply has the gear right at the top of the armature. I don't like this for several reasons. For one thing, it won't work with every follow focus out there. It will be a little more compact, but with the caveat that you'll eventually run into something it won't work with. Also, it's ultimately not as flexible as this design. The design we're building here allows far greater possibilities for interfacing between lens and follow focus.

Screw one end of the threaded rod into the metric threaded part of the hole on the connector block.


Finished connector block with hardware.

Now we're ready for the most important part, the gear itself. Put a nut on the threaded rod, followed by a lock washer. Then put the gear on. It should spin freely on the rod. I chose 4mm threaded rod for this project, because the bore of my gear is 4mm, and I didn't trust myself to drill the gear out to accomodate a larger rod size. After the gear, put on another lock washer (a regular washer will work just fine here, however). Follow it up with a lock nut on the end of the rod. It should look something like this:


Finished gear assembly.


The connector block and gear assembly should now be one part:


Connector block and gear assembly.

That's it! The idler gear is finished. At this point, you now have something that will actually allow you to reverse focus direction on your lenses.


Yes, the lens gear is attached in the wrong place.


Results:

I soon as I finished the assembly, I took it on set for a two day short film shoot. It got the job done. However, using it in a real world environment showed me some problems with the design, that I plan to address in the next version.

Rigidity: There is more flex in the system than there should be. While the assembly works fine for the most part, the threaded rod has just enough flex in it at the length I'm using so that the gear will skip if I turn the follow focus too fast, or hit the end of the focus throw with too much force. Obviously, this is not acceptable. The problem was especially pronounced when using the Zeiss Planar 85mm f/1.4 which has a fairly stiff focus throw.

To fix this, I'll be replacing the threaded rod with drill rod, which is a lot stiffer. I'll use a die to cut my own threads on it as necessary.

Ease of Adjustment: When on a tight schedule, it's a real pain (and slows down production) to have to break out the allen keys every time you need to make an adjustment to the assembly. It just takes too long, and is something of a hassle.

To fix this, the tightening screws will be replaced with adjustable handles from McMaster Carr. Ideally, I'd use something really small like what RED does. However, small adjustable handles don't seem to be available anywhere. I'll be using handles that are about 1.75" long. The smallest I can get them is 10-32, so I'll have to re-drill part of the connector block. Hopefully the hole will still be small enough that I don't have to fabricate another one.

Also under ease of adjustment, I ended up needing one adjustment that I hadn't planned for. In some cases, I found myself needing to adjust the position of the gear on its shaft (no time to swap out shafts in between lens changes). While moving the gear was no problem, I had nothing to keep it in place where it needed to be. I'm going to add another nut inbetween the lock nut and lock washer, so I'll be able to fully secure the gear should I need to move it closer to the connector block without changing shafts.

With the above modifications, I fully expect this device to become a regular part of my AC kit. I'll post updates as they are made.
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