Cody II/3A Arm

[James Sutherland]

Hi All,

I've been following Tom's rig progress closely and everybody have been stellar with pointers and advice (I'm kind of hoping for the same ). Big shout out to Cody, I bought his book and it forms the basis for my arm. Below is a few photos, please make suggestions if you guys see anything that could cause a potential problem. The arm is basically an adaptation for Cody's II arm, using the measurements and putting something together with the available resources.

I was seriously inspired by Michael Thonfors arm (which he also based on Cody's second arm). So I started off looking for a similar profile alu for the bones, but with no success. It was also a no go in finding a dye to bend a 5mm sheet like his . My father then rocked up one day with an old security boom arm. It had the perfect profile but it was really thin (2mm). So what I did was to combine the 2 designs.

Milling here is South Africa is insanely expensive so the only alternative was to "compose" the elbows from separate pieces off alu. (for me it ment handsaw and sanding from dawn till dusk...or was it the other way around )

The bearings I used for the bone are rated at 70kg each (680 newton) if I remember correctly, and the bolts are M5's.

The bones inside the bones are 8mm T6065 alu. I hope that the 2mm round profile would add strength and extra stability.

As you can see it is far from done. I was humbled by this project! It is a lot of work and working form out of a garage it is near impossible to get the accuracy that I think would be needed to build a truly professional rig. I take my hat of to everybody one here that have completed their rig! I hope this design allows for some tolerances seeing that is so difficult to get that precision only a machine shop can give. I'm seriously concerned about the arm not being able to handle the weight, bearings, bolt failing etc. The thing is I have never seen a Steadicam or anything similar in real life so I have absolute no reference other the reading about it.

[James Sutherland]

Hi, I need advice please.

Is it necessary to have bearings in the link that connects the 2 arm section. Cody's II arm uses such a method but the again Michael Thonfors did not use it in his design. Charles your arm makes use of 8 bearings in the middle section of your arm. What would be the determining factor (purely weight? if so how much weight = equals how many bearings/combined bearing load capacity?).

[Phil Kindred]

I assume you are going to use shorter bolts;otherwise you are apt to have your springs hanging up on them.  You can get by without the bearings in the arm link, but it is not the way I would go.  It's better to use at least four because of the increased performance and because you might as well do it now rather than wish later you had done it.

[James Sutherland]

Thanks for the reply Phil, yip those bolts still need to be shortened.

[Martin Amada]

James, I was also impressed with the photos of Michael Thornfor's arm and we're now making my wrists based on what I saw there. They will be similar to yours though perhaps a bit taller. It's hard to tell from the photos. The other difference is, the U-shaped inner parts inside each of my wrists (which are made from 3 pieces each in your photos), will be milled as solid pieces from billet. My outer plates will be bolted on as you have done.

I'm curious about the size of the bearings you are using. Mine have an OD of 22mm with 8mm bores. I would have preferred 6mm bores because my bolts "axles" will pass all the way through the bones and 8mm bolts are somewhat heavier than 6mm. The alternative would have been to use 19mm OD bearings with 6mm bores but they seemed a bit small to me.

Mind if I ask what size yours are?

Martin

[MikkoWilson]

James, Yes you will want bearings in the "elbow" of the arm too. It makes a huge difference to performance.

You will really want them in the "shoulder" too if you can.

Looking great so far!

- Mikko

[James Sutherland]

Martin, the bearings I'm using are 16mm OD and takes a 5mm bolt. I'll be using bigger bearings on the rest of the arm. The wrists are 9cm tall, what size are you planning on making yours?

Mikko, thanks. I'll then definitely try and incorporate the extra bearings into what I already have.

[Martin Amada]

James, my wrists are 13.5 cms tall, but my design differs a little from yours in that less of the bone widths extend beyond the tops and bottoms of the wrists. So in theh arm segment's fully rectangular configuration, the outside of the bottom bone to the outside of the top one measures 15 cms, which is exactly what my first arm was. I figure a little more height = greater boom range. The length of each segment will be 28 cm.

I am using bearings that have a 22 mm OD and 8 mm bores, though I will probably put a spacer in the inner ring of each, to reduce the size of the bolts needed to just 6 mm. I used 19 mm bearings in my first arm and somehow I felt that this time I should go as large as space would allow.

[James Sutherland]

Martin I think you doing the right thing going with the bigger bearings. I hope these smaller bearing I'm using don't come back to nip me in the bud

[Martin Amada]

James is it too late to widen your holes to 19 mm? If you have room that might not be a bad idea. That size should be all right I would think.

[James Sutherland]

I'm done with the weight reduction on the wrist and must say it makes a huge difference. I've tried to make the bearing holes for the elbow link my self but made such a bit mess I will have to take it to a professional to fix.

The pulleys weren't that hard to make, I'm using 19mm OD bearings in them.

I had to have the springs made here by local shop. I gave the specs of the century springs in Cody's plans. The springs are extremely stiff, I hope it is correct because a part from the bearings its one of the most expensive components.

Next up will be the spring plugs.

[Martin Amada]

Looking good James. What size bearings are you using to connect the elbow?

You couldn't get Century Springs to ship to you in SA?

Martin

[James Sutherland]

Martin I'm using 22mm OD 8mm ID bearings in the wrists with a 8mm bolt going into the elbow link.

It would have cost me more then double the amount (cost plus shipping) to buy for century springs. Hopefully the oaks here gave me the right stuff, I'll sure be peed off if its not the and it buggers up the rest of my rig

Does any one know how to setup a lathe to make the spring plugs?

[Martin Amada]

A spring plug is basically just a big screw as far as I can tell. In my experience some lathe operators have the skill and the tools to thread rod and pipe etc. while others cannot or don't want to. If you find one who is willing, he'll simply need the spring to measure the size of the threads he needs to make.

[Tim Le]

Does one one know how to setup a lathe to make the spring plugs?

It's been years and years since I made spring plugs (and the first time I did it was the only time!) but here's what I remembered:

- You set up the lathe like you would to cut custom threads. A guy from the University's shop that I was working with photocopied some pages from a machining book to show me how to do it. Nowadays you could probably google it. I remember it wasn't as hard as I thought it would be. It involves something like measuring the pitch of the spring (rev/in) and setting the power feed and lathe speed to match the pitch. Then you set some stops or something like that so that when go back to make another pass on the cut, the pitch lines up again. I also had to grind my own cutting tool to match the profile of the spring. It's really not that hard because the plugs don't have to be that precise like real threads.