Making Conrods, Made Easy.

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Last update: October 25, 2003


Don't ask me why, but making conrods is one of my least favourite activities. It's not hard, it's not even tedious, it's just... I dunno. Guess the thing that makes it such an un-fun thing is achieving a neat end on them. I've used all sorts of techniques and while most are satisfactory, they all leave me not wanting to do it again anytime soon. Well, all that may have changed as a restoration job I did on a Serias 66 Taipan 1.5 diesel has given me new insight and a new way of profiling those dreaded ends!

Usually (and following pictures seen in Model Engineer for locomotive link rods), I profile against the side of an end mill as seen here. I've tried using a rotary table: forget it—the setup time is off the planet and you need a sacrificial plate under the rod to protect the table. By using a stub of rod to match the big/little end ID, the rod itself can be swung perfectly around it's own bearing axes. This works fine, requires minimal jigging, but has a number rather big disadvantages:

  1. Negative: The rod needs to be large enough to hold onto, so really small guys are out, plus the smaller the rod the more painful it is on the fingers and the closer said fingers are to a very efficient flesh remover!
  2. Negative: Depending on the rod design, you may need to use a rather small diameter cutter (say 1/8") to get into the corner of the end/shank junction, but when in the corner, the rod itself is hiding the work, so it's hard to judge just how far to swing the rod. Go too far and the cutter will cut into the rod shank—not good!
  3. Negative: You need to make all the cuts "climb mill" cuts—ie the cutter rotation is trying to push the work away from you. But then you have to swing the rod back the other way to advance the table for the next pass. During this operation, cutter is now going to try very hard to snatch the rod out of your fingers, spin it around and start diligintly chewing away the shank! Forget turning the mill off for the return stroke, just not going to happen—would take forever—too inconvienient, etc, etc.
  4. Negative: The spiral flutes of the cutter will try to drag the rod up the vertical pivot shaft, so we also have to make collars with grub screws (visible in the photo) to hold the rod in position. These get in the way of the cutter, so they need a flat milled on them which can be used as a "OD Reached" indicator (minor positive).
  5. Negative: And finally, the finish is never *perfect*. Milling against the flutes will leave feint vertical striations and there will be little pips at the end/shank junction. All this can be cleaned up with Swiss files and a ScotchBrite linishing belt, but how much nicer it would be to have the rod *perfect* as machined?
All this adds up to making rods equals a tense time, hence not fun, hence we hate making rods.

But it appears that there's another way! The clue came from looking at the end of a badly bent, but otherwise good Taipan Series 66 rod. I could see that the parallel shank had been turned using something like a right-left 45 degree chamfering tool, and a little swurl on the side of the big end looked like a slot drill with a diameter about equal to the rod width had been applied to the ends vertically (the light bulb starts to shimmer and a voice in the background intones "You interest me strangely Watson, continue...") Looking closer, we can deduce that a slot drill must have been lowered to the side of the rod end producing the flat and swurl pattern. Then the rod was swung around until the slot drill occupied the same position on the other side of the rod. Conforming this theory, we even see that as the rotating face of the slot drill had passed around the end, the slight "V" ground into the tool has imparted a correspondingly slight raised "double cone" shape to the end.

So let's try it out making a replacement rod for the Series 66 Taipan.

The first step is to mill up a blank from 1/4" 2024-T3 plate. This is easily done by rough cutting on the bandsaw, then milling the sawn faces with the blank resting on a parallel in the mill vice. The sides need to be parallel because we are going to grip it on the sides in the vice to drill and ream the holes. The blank needs to be oversize lengthwise to allow for workholding on one end, and a disposable center on the other. After milling the blank, check the vice face is parallel to the mill table X axis with a DTI. There's probably a "proper" way to do this, but I'm an untrained amateur, so I position the DTI over one of the vice jaw attachment screws, zero the DTI and wind the table to the other screw—a distance of around 4"—and note the DTI deviation. Next, rotate the table to reduce the deviation by half, re-zero, and wind back to the starting position. Repeat until the deviation is less than 0.001".

Next, zero the Y table dial over the rear face of the vice and set the blank horizontally on a parallel and clamp it up. Advance the Y axis by one half the blank width and LOCK the Y axis. Position the X axis to about the middle of the blank, then wind left or right by approximately one half the distance between rod centers and take up the backlash in the reverse direction (if you are not DRO equiped). Zero the X axis, then drill and ream the first hole. Advance the X axis by precisely the between centers distance to drill and ream the other hole. Now, loosen the vice and keeping the rod sides against the same jaw faces, rotate the blank 90 degrees. This does not have to be *perfect*; I use the ends of the vice jaws to line up the blank. Find the center of the end (X axis wise) and center drill for a tailstock center. Because we have not moved the mill Y axis, the center will be in line with the reamed holes. If you want to turn the rod between centers, spin the blank 180 degrees and repeat. I'm going to grip in a 4 jaw chuck, so I'll skip this step.

To form the rod shank, we need a tool with a generous nose radius that will cut right and left imparting 45 degree chamfers. This means no top rake, but for 2024-T3, this will not matter. The blank is centered in the 4 jaw, supported by a live center in the tailstock, and the shank turned down to finish diameter. It would have helped to have the OD of the ends scribed on the blank so we'd know how far to chamfer, but this would mean marking out and you may have noticed that so far, we've managed to avoid all marking out! I just eyeballed the position, as seen here.

No photo for this step. The rod in question is thinner at the big end, than the wrist pin end, so the blank goes back in the mill vice (on a parallel) to mill away a little material from either side at the big end. Out of the vice, the excess material used for work holding while turning is roughly sawn away.

Now the magic moment. Mount a smooth pin to act as the pivot in a scrap steel block and set this close to horizintal in the mill vice. Mount a slot drill of diameter equal to the blank width (or next size up if necessary). Wind the Y axis to center the cutter over the pin, and the X axis so the cutter is centered over the rod end with one side bearing against the steel block. Slip the rod over the pin and engage the fine downfeed of the mill. Now, running the cutter at the highest speed available, perform a two handed dance: lower the quill with the left and swing the rod with the right, taking successive fine cuts. The cutter will tend to push the rod onto the pin in one direction, but off the pin in the other. This force is relatively mild and I found I could hold my rod (which was just over 1" long) with finger pressure alone. A retaining collar may have been nicer though. Stop at some point, remove the rod and measure the OD to determine exactly how much more to lower the cutter for the final cuts. I swung through approximately 180 degrees until I'd reached the desired OD, then carefully went a little more in either direction to blend the end with the chamfered cut. If I was making a lot of rods, I'd arrange some stops at this position, but for a one-off, it's not worth it.

Here's the replacement rod with one end formed and the other still only roughed out. The rods in the background are the bent one, and a worn out one. The end is "as cut" and does not require any attention from the Swiss file, so we've achieved one aim. The process was also less dramatic than milling against an end mill face, so we've achieved another. In summary:

  1. Positive: Your delicate flesh is mostly below the volume of space occupied by the scarey, spinning bits during the operation, so the method certainly feels safer than having your hand totally within the space occupied by the cutter, as it is when using the other method.
  2. Positive: Cutter and work were perfectly visible at the end/shank junctions, so there was no guesswork and no great danger of milling into the rod shank. This is a terrific improvement over the other method.
  3. Positive: The finish is pritty good. A little buffing on a ScotchBrite belt makes it perfect. Another good improvement over the end mill method.
  4. Positive: The jigging was simple and while a retaining collar would have been nice, it was not a neccessity as it is when milling against the flutes of an end mill. So jigging can be simplified (but see my thoughts on improvements later...)
  5. Negative: Juggling the downfeed and locking the new position while hanging onto the rod required three hands! The end mill method keeps the downfeed locked, putting on cut with one of the table axis handwheels. This is easier to manage.
  6. Negative: The rod still needs to be large enough to hold onto, but I think one could form smaller rods when using this method in comparison to the other.

I'm hooked; I like this method. Making rod ends will still not be my favoirite past-time, but I'm not going to dread it like I used to. The jig used could obviously stand some refinement. A "universal, general purpose" jig should:

  1. Be reusable
  2. Accommodate all (or nearly all) rods you can imagine ever making
  3. Be simple and quick to set-up accurately
  4. Last forever
  5. Protect the faces of the ends being profiled
  6. Facilitate repeat work to acceptable accuracy with minimal measuring

After a little thinking—and the need to make yet more AHC rods, a jig meeting all (ok, most anyway) the above requirements was knocked up in less than an hour. To satisfy requirements one through three, I'm using a block of mild steel from the scrap box that has had the ends milled orthogonal with sides. I didn't bother to true these on the shaper as it's not super critical. The block ends are drilled for a variety of big and little end diameters with the block trued up using a square in the mill vice. In use the block will be set in the mill vice on parallels giving all the accuracy required. To accommodate all the diameters required, pins of unhardened drill rod are used. Each has a brass end cap glued to the end with Locktite. The other end has a flat filed on it so the locking screw will not raise a burr that makes removal from the hole difficult.

The little brass rings serve three purposes. First, they move the rod away from the end face of the block allowing cutters wider than the rod thickness to "get in" to the working area. Second, they provide some kind of bearing surface that helps protect the face of the rod during rotation. And finally, they are replacable and sacrificial. So if you are making a number of the same rod type, they can be machined to the required outside diameter of the finished rod end—in other words, when the cutter is just kissing the ring, you're done—no further measurement neccessary! You can also use the rings to lightly mark the finished profile on the rod blanks as an assist when turning down the shank using the bi-directional, radiused, 45 degree chamfer tool seen earlier.

This shot shows the jig in use. The mill vice has been been swung through 90 degrees so that the rod is swinging from right to left. This affords the operator optimum view of the cutter location in relation to the finished shank at extremes of swing. With the mill running, it's one hand on the fine down-feed, and one hand on the rod. The pin needs to have been pushed in tight to remove all end-play as the cutter will be trying to move the rod longtitudinally on the pin in either direction each side of vertical. This will leave a worm-track around the center of the finish—as will a cutter with an appreciable gap between the two cutting faces. I've observed that larger commercial cutters have wider gaps between the cutting edges than smaller ones. But a wider cutter will "blend" better into the 45 degree chamfer left from profiling. So select a cutter with a diameter that is equal, or greater than the rod thickness to give an acceptable compromise on the blending and worm-tracking. The one here is a cheapie, "disposable" type and is leaving a track that is just this side of acceptable. From limited observation, the screw-in variety are ground with closer attention to minimising the gap between the cutting edges.




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