BTW, I've never actually seen a Forged Friend so have no idea how this one compares to one in terms of strength over an edge and somehow doubt that the arbitrary measurements I went with will yield similar results.

Little Cash and Few Tools? No problem! A Dirtbag Approach to Cam Building

I don’t know about the rest of you, but I was one of those kids who tended to have more fun taking their toys apart to see how they worked than actually playing with them as the manufacturer intended. So its not surprising that shortly after I took up rock climbing I got curious how camming devices worked and thought building one would not only make for an amusing project but also be the perfect thing to give my wife, who had forbidden me from building climbing equipment. Well, “forbidden” isn’t really the right word…. Let’s just say that at the time I was finishing building a set of bagpipes using nothing but PVC pipe and vinyl cloth from Home Depot, so her patience with my endless tinkering was wearing a bit thin.

Anyway, a bit of searching online turned up a couple really good explanations of the physics behind how cams work. For brevity’s sake I’ll take a pass on trying to rehash what others have already done an admirable job of explaining, so if you’re curious about it take a look at the links in the Camming Device Theory and Design section of the Lab FAQ. Of these explanations of cam design, only John Middendorf’s Cams - A Technical Review actually walks the reader through plotting the logarithmic spiral that defines the curved edge of a cam lobe. Certainly a good start, but unless you already know a bunch about materials and tooling you’re kinda up a creek as far as actually building anything is concerned.

However before I fill you in on how to take the theory and turn it into a working cam, I’m afraid I have to put forward a few disclaimers…. First and foremost, this is being written for informational use only and the reader is solely responsible for what they do with this information. The intent of this article is to provide simple, step by step instructions to build a functional camming device to be entered into RockClimbing.com’s 2009 Trango Homemade Cam World Cup. Actual use of the end product of these instructions as anything other than a paperweight is not advised by the author, RockClimbing.com, or anyone else. In fact it may not even make a very good paperweight, so be careful where you point that thing.

Ok, with that out of the way here is a picture of Ray Jardine’s first functional prototype Friend from 1974:

[image]http://www.cc.nctu.edu.tw/~mclub/meichu/project/friend/1974_friend-2_5.jpg[/image]

It’s certainly not pretty by today’s standards, but you know what? It not only worked but was fairly easy to make as well. The instructions below detail how to build a similar cam with just a few common tools, but for the most part they would still apply if you’re the roll your own sort of person and wanted to create your own design instead. Just make sure you make the necessary changes to the material list and tooling if you decide to go that route.

The cam we’ll be building here uses a 14 degree cam spiral and has a range of approximately 50-82mm, which puts it comparable to a #8 Trango Flexcam, #3 Black Diamond C4 or #3.5 Wild Country Friend. It will probably take you a good day or two to build, depending on how quickly you work and how accustomed you are with doing things like this. For what it’s worth, it took me 4 hours to go from printing out the blueprint to having the lobes and stem ready for assembly and another 2 or so to completion. I didn’t spend any time making it pretty though, so that would have been another hour or two. BTW, I cheated a bit and used a dremel and angle grinder so that I could get this article finished sooner.

Anyway, for the oddball tools and materials that are needed I’ll include part numbers from McMaster-Carr so you can get the specifics on what they are. You may be able to find them cheaper elsewhere; I just happen to use McMaster-Carr since they have a warehouse local to me, an excellent online catalog and competitive prices.

For tools, you’ll need the following:

Hand drill or drill press
Letter Size O drill bit (0.3160” diameter, McMaster-Carr 30595A46)
1/8” drill bit
5/16” drill bit
Hammer
Center punch or sharp nail
Hacksaw
Rectangular file
Small-ish circular file
Sandpaper
Pliers
Wire cutters
Small adjustable wrench
Rubber cement or double sided tape
Scissors

The materials list is as follows, and will probably come out to somewhere around $35 depending on where you get the parts:

10.500" of 0.250" x 1.750" 6061 T6 Aluminum
6.500" of 0.250" x 0.750" 6061 T6 Aluminum
2.750" of 0.250" diameter 6061 T6 Aluminum
¼ pound spool of 0.026” Spring Wire (McMaster-Carr 9666K54)
8x60mm shoulder bolt (McMaster-Carr 92981A215)
8mm x 1.0mm thick steel shims (McMaster-Carr 98055A115)
5 pcs 1/8" diameter x 3/8" long split pin (hardware store)
6x1mm nylock nut (hardware store)
24” of 2mm accessory cord
A short length of 9/16” tubular webbing or 7mm accessory cord

One word of advice about the aluminum bar stock… For small quantities like this you’re mostly paying for someone to cut the material rather than for the material itself, so you may be better off getting a much longer piece and making more than one cam since it won’t increase the cost very much. For example, the 2.75” piece of 0.250” rod for the trigger will cost ~$14 if you get it at Metal Express, but a 6 foot length of it is ~$16. Shopping around doesn’t hurt either, as that same 6 foot length would be less than $7 at McMaster-Carr.

On to the instructions…..

1. First, order/borrow whatever tools and materials you need from the lists above. If needed, there’s a list of suggested suppliers is at the end of this article.


2. Once you have everything, print out a copy of the blueprint. When you print it, be sure to change the Page Scaling setting from “Fit to Printable Area” to “None”. If you don’t, you’ll end up with a cam that’s smaller than intended and you’ll have a lot more material to file off (we’ll be using it as a template rather than actually measuring anything). If the box on the printed copy measures 2” on a side, you’re all set. Otherwise you’ll need to fiddle with your print settings.
   
   On a side note, one of the wacky things about logarithmic spirals is that reducing its size like this has no effect on the geometry, so the reduced cam would actually work just fine.


3. Next, take your blueprint and cut out the cam lobes and stem (with the lobes as one long strip with all four of them), leaving a bunch of extra material around the lines. Then glue them to the aluminum bar stock with the rubber cement, making sure the drawings are not hanging off the edge of the material. In case it isn’t obvious, the lobes go on the 10.500”x1.750” piece of aluminum and the stem goes on the 6.500”x0.7500” one. Take special care to get the stem drawing centered on the material, as they’re the same width and if you have it off center there isn’t much you can go about it after the holes are drilled. You should end up with something like this:
   
   


4. Once you have the templates attached to the material, mark the center points of the circles in preparation for drilling. Simply place the material on a firm surface, put the end of your center punch (or sharp nail) at the mark on the blueprint and tap it firmly with the hammer. Do each one in turn. No need to really smack it; all you need is a small divot to keep the drill from wandering. That said, a slightly larger whack for the large holes might be helpful. The center punches should look like this: (note- 2 of the holes have not yet been done)
   
   
   
   In case you’re wondering why there are so many on the lobes, it makes cutting the excess material off much easier since you only have to cut hole to hole. Plus they’re inset a bit, so it will leave a nice divot for the cam teeth rather than putting them in by hand with a file after the fact. Oh, and don’t forget the marks going down the center of the slot on the stem….


5. Next up: Drilling. If you’ve never done much work with metal be careful… It has a tenancy to catch when the drill breaks through the other side, so consider clamping it down before you end up like me and be able to count fractions with your fingers. There are only 2 sizes of holes marked on the blueprint, so small circle = 1/8” drill, large circle and slot = Letter Size O drill. I used a drill press so I could get the article done faster, but there's no reason you couldn't do it with a hand drill:
   
   
   
   It is very important that the large holes in the cam lobes are exactly perpendicular to the material, so take care when drilling them (using a drill press makes it much, much easier, but is not absolutely necessary). If you drill them skewed the cam lobes will not be perpendicular to the axle and, depending on how skewed the holes are, may jam on each other when the lobes are retracted. The other holes aren’t as big a deal and the effect of having them a bit skewed will mostly be cosmetic. And depending on how your large drill was made/sharpened the large holes may be easier to do if you first do a pilot hole with the small drill. Give it a try and do whichever works better for you. Oh, and if you do use a drill press, make sure to check that the table is actually perpendicular to the chuck.... They're usually adjustable and there's no guarantee that its set up correctly.


6. Once that’s done, it’s time to cut out the cam lobes. Simply clamp the material to something solid and go to town with the hacksaw. Make sure you stop short of the line marking the edge of the lobes though and don’t worry about getting all of it, as we’ll deal with that in the next step. That said, if you can get it with the saw it will save you time later.


7. Now the tedious part… Filing. I hate filing. Absolutely hate it. I spent my teens (and beyond) working in a machine shop and have done several lifetimes worth of it. If you have access to an angle grinder, bench grinder, bench sander or drill press with sanding drum now’s the time to use it. Basically what you need to do is remove all of the material up to the line. Don’t remove the line; just everything up to it. While you’re at it you might as well do the stem as well.
   
   Depending on how much material is left over from drilling out the slot a round file might make life easier (or even be necessary). If you don’t see them in the hardware store look over by the chainsaws. And remember, files only cut on the push stroke and pulling with them does nothing more than make them dull. I think the exact quote was something like “If I wanted it to work both directions I would have given you sandpaper, you idiot!”. But I digress….. Once you get two of the holes connected you may be able to pass the blade of the hacksaw through and cut the rest out that way. A dremel would be wonderful to use here as well, so if you have one you may want to go find it.


8. At this point you’re probably wondering what the small hole on the stem is for…. Well, since this is a Dirtbag Approach to Cam Building we’re doing everything we can to keep costs down and will be making our own springs rather than buying them. To do this, we'll be using the shank of the Letter O drill as a mandrel for winding the spring and the stem as a makeshift wire guide. There’s a good explanation of the process here, so I’ll just gloss over it rather than explaining it fully. Be sure to read the section on safety as well, since spring wire isn’t exactly the safest thing to be playing with.
   
   First, push one of the 1/8” split pins partway into the small hole in the stem with the split facing away from the large hole. Then take 2 washers, place them around the split pin and then tap the pin down until the end is flush with the top of the washers. You may need to set the end of the stem on top of something so the pin can pass through the other side, so use whatever is handy. You’ll end up with something like this: (kindly ignore that I didn’t use a split pin… I didn’t feel like running out to buy them so I substituted some 1/8” aluminum rod instead.)
   
   
   
   From there, simply follow the instructions linked above, making one spring with a left and one with a right hand rotation. If you don’t have a vise you can instead use the chuck of your drill and clamp the end of the wire under one of the jaws (with the shank of the drill in there too, of course). Be extremely careful if you try to wind it by turning the drill on…. Between the sharp edges of the drill and the energy stored in the spring things could go… ummm…. badly.
   
   You’ll need to end up with a spring that has 5 or 6 coils with the legs roughly parallel and on the opposite sides when the spring is sitting at rest. In other words, it will look something like this:
   
   
   
   It will probably take some trial and error until you get it right, so turn on some music and get comfy. FWIW, it took me roughly 8 rotations of the stem to get a spring that ended up with 6 coils. Here is some information on the setup I used…
   
   For the pickup block I took a piece of scrap from cutting out the lobes and sawed / filed a groove down the face of it (for the drill to sit in). I then filed a very shallow flat spot at the top to catch the wire. If you make this flat spot go too far down the block it will cause you trouble, so keep it short like mine. By which I mean the left side of it, not the right. Guess how I found out having it too long would cause problems?
   
   
   
   The setup is just like they show in the article linked above, where you clamp the mandrel (drill) in the vise with the pickup block:
   
   
   
   Getting the pin on the stem to catch the wire while also being held by the pickup block is a bit of a pain, but if you angle the wire like this it’s not too hard to do.
   
   
   
   Another tip is to catch the wire with the pin, push the stem all the way down and then pull the end of the wire behind the pickup block. There’s lots of ways to do it, so just find one that works for you. When you finish your rotations it will probably look a lot like this:
   
   [image]http://www.rockclimbing.com/cgi-bin/forum/gforum.cgi?do=post_attachment;postatt_id=3083[/image]
   
   If you want to cheat, you could simply buy the springs but they tend to be rather pricy compared to making them yourself and it tends to be difficult to find the right balance of size vs. force when perusing a catalog.


9. Next, lay out the cam lobes with two facing one way and two facing the other (paper up on one pair, paper down on the other). Using the 5/16” drill, drill out the small hole furthest from the large hole on each cam lobe, stopping when you get halfway through. Then gently touch the other side of the same hole with the drill to put a small chamfer on it. Don’t go very deep; just enough to take the sharp edge off.


10. We’re almost in the home stretch now, so take a moment to clean up the lobes and stem… Remove the paper, sand off any sharp edges or scratches, pull out the split pin in the stem, etc. If you’re going to paint, anodize or glitterize anything, now’s probably the best time to do it.


11. Next press the remaining split pins into the small hole closest to the large hole on each cam lobe, starting from the side opposite the 5/16” counterbore, leaving the pin sticking out roughly 1/8”. The easy way to do it is to the washer stacking thing again, this time with 4 washers. Again, kindly ignore my substitution of aluminum rod for the split pins….
    
    [image]http://www.rockclimbing.com/cgi-bin/forum/gforum.cgi?do=post_attachment;postatt_id=3082;[/image


12. Woo-hoo! Final Assembly! Toss a washer onto the bolt, followed by a cam lobe, a spring and another cam lobe. Which spring you use depends on which pair of cam lobes you grabbed (the legs of the spring should contact the split pins where they come out of the cam lobe), so use whichever one fits. Also the crossing point of the legs should be above the axle so that compressing the cam lobes causes the coil to get tighter.
    
    [image]http://www.rockclimbing.com/cgi-bin/forum/gforum.cgi?do=post_attachment;postatt_id=3081[/image]
    
    When you install the second cam lobe, hook the legs of the spring on the split pins and give one of the lobes one complete rotation before pushing it all the way in (and engaging the other pin/cam stop).
    
    Then add another couple washers (I used 8), the stem, yet more washers (8 more), a lobe, another spring, the last lobe (doing the same hook, rotate, push procedure you did with the first set of lobes), one last washer and then the nut. You may need an additional washer at the head of the bolt to fill the undercut at the start of the shoulder and more or less on either side of the stem, so play around with it until it feels right. There needs to be some lateral movement possible on the axle so things don’t bind (the springs get longer when the cam lobes retract), but not so much that the cam stops can slip past one another.


13. Now give compressing the lobes a try and see if the springs allow full movement. If not, try winding another set of springs with more/less coils or more/less rotation. Its a trial and error kind of thing, so sorry I can’t be more specific. Refresh your beverage as needed and grab a few more CDs. It might be a long night.


14. Once you have that all sorted out, take 2 pieces of your 2mm cord and pass them through the remaining holes in the cam lobes. Tie an overhand knot in each end and then pull a loop down between the lobes, wedging the knots into the counterbores made with the 5/16” drill. Pass the 0.250” diameter rod through the slot in the stem and tie it to the loops with clove hitches. You’ll have to play with the knots on the ends to get the length right, but once you get it where you want it a drop of glue on the knots and cloves might be a good idea.


15. Then take your piece of webbing or 7mm cord, tie it through the hole in the end of the stem with your knot of choice and you’re done! Time to go find a pile of papers that are in need of weight, kick back and admire your creation!

---

Suggested Suppliers
Note- I have no affiliation with any of these companies and they are merely ones I happen to have dealt with previously and had good experiences with.

Raw materials-

Metal Express
OnlineMetals.com
Enco
McMaster-Carr
Grainger Industrial Supply

Shoulder Bolts, Springs, Washers, etc-

McMaster-Carr
MSC Industrial Supply
SmallParts.com