Concentricity Tools

The updated drawing (not a whole lot is different except for the roller sizes because I'm using the size increase along with a shorter side to give clearance) is attached. I've started cranking through squaring up [most] stock. I'm only going to worry about flattening the base. I have the sides and gussets squared, but I need to slice the gusset stock once more at an angle so I have two per-piece. As soon as I have the frame made up I'll post pictures (probably a few weeks out with how often I can run the mill).


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Okay. I've run through half of a build. There are some changes, and some liberties, but here's my write up so far. Here's the resulting BOM (Bill of Materials) :
  • Stainless Steel flat bar stock (I used 2" x 3/8" x 36" and had enough for two tools)
  • Bearings (I'm using 624zz bearings, 10mm outside diameter, 3mm inside diameter, and 4mm cross depth)
  • 5/8" stainless steel round bar stock
  • Screws (plans called for #10-32, I'm using #8-32)
The tools needed :
  • Milling machine (optional, really, depending on how crude you are willing to have the tool - it's used to flatten and square up the flat bar stock)
  • Lathe (if you want to test the true concentricity, this is absolutely a must so that the bearing areas of the rollers are concentric with the outside of the rollers)
  • Drill press (you could use a mill, if you get the right speeds)
  • #27 drill bit (tapping) and #18 drill bit (close fit) for the #8-32 screws
  • 1/4", 5/16", and letter "X" drill bit (for the bearing cavities
  • Countersink bit to let the screws be sub-surface
  • Transfer screws to match the screws in your bill of materials
  • Taps to match the screws in your bill of materials
  • Center punch (I used an automatic)
First, I ordered some stainless steel bar stock. I'm using 3/8" hot-rolled (cold-rolled will be closer to specification, and probably cost me a bit more getting things "square"), and while the plans called for 1.5"-wide, I ordered 2"-wide stock. Oh, well, larger is going to be a little more solid, but still usable. Once in hand, I sliced off 8 pieces (I'm doubling the parts, right?). The first two were 7" long, the next four 1.5" long (for the sides), and two 1" long sections for the "gussets". These two gusset parts will be cut at an angle to yield 4 parts down the road.


With the cut list done (bandsaw cuts are rough), I began to mill things square. I milled the gusset blocks, and the sides square. Two of the sides were milled 1/8" shorter, as one end of the devices will allow for clearance if a larger cartridge is to be used (4" is large enough, but you never know what some expert will try).


Note, we still had those "two" square gussets - and we need four. They were marked on the height with the small side, and then I could scribe a line for them at an angle. These were run out to the bandsaw for a quick cut.


Bandsaws leave a terrible finish. The angle was a custom angle to get two gussets out of each one, so I couldn't readily calculate the exact angle without having a gusset too tall. So, we clocked the newly-cut gussets into the vise using a bit of a trick. I took 1/8" stock as "risers" and a longer flat piece to stretch over the two risers, and set them on the vise jaws. Then, I slid the gusset angles that had just been cut up against the longer flat piece.



This let me cheat and set the angle for these things so that it was close to the top of the vise, but gave enough clearance to get it milled flat. Again, the angle wasn't critical, but it needed to be short enough on the tall side to not exceed the short side.



While I was finishing the milling (you can see in the last picture that the base is still "raw", I milled the base's top surface, and then "about" 1/2" on the bottom surface so I could have a parallel surface to put rubber feet on and protect furniture if the users so choose to use this in the kitchen.



With the mill work complete, it was time to drill. I was only going to drill all of the holes that were threaded, not any of the matching through-holes. This was so that each piece could be matched for a good fit.

All I had to use was a cheap titanium nitride drill bit (a cheap Harbor Freight set). I'm not going to rant or rag on the quality of these drill bits - they helped me get my foot in the door. Also, I later realized that it wasn't my drill bit that was the problem, it was how fast I was spinning the drill bit. Still, I did a little cussing about cheap Harbor Freight specials at the time before I really understood that the problem wasn't the tool, the problem was the tool who was operating the tool. Anyway, I had it marked up :


I've already got the 10mm end mills for boring out the roller bearing bores (should match up), and I have the 8-32 transfer screws (I opted for a change since I milled enough material off). Once I got the replacement drill #27 drill bit, I quickly dulled one on the mill, and decided to just go to the drill press. That worked much better.

I tapped the holes. On one, I stupidly broke off the tap. It's one of the frame sides, so I could easily move the hole for the side closer to the edge and drill a new hole. Also, when drilling one of the gussets, I went just a little too deep, and that gusset has a bulge on it now (I'll just file that off when I'm done and expose the hole). However, those blemishes won't alter the effectiveness of this. Those are simply cosmetic, and one will be hidden because it will be sandwiched between the side and the frame bed. You can see all of the blemishes in the following picture. The two gussets had holes that were slightly off-center on the gussets. All of these blemishes are annoying to me, but most will be hidden.


At this point, mark all of your parts. With what we are doing next for a custom fitment, if you get things mixed up after this, it may not fit together well.

Once all of the drilling and tapping was done for the threaded holes, it was time to start marking the mating holes for the screws. This is where those transfer screws come in. I put the transfer screw into the vertical surface of the gusset (it's the only horizontally oriented screw). I could then mark the parts up with marking fluid (I'm cheap - Sharpie is my go-to, not Dykem layout fluid). With the two parts sitting flat on the frame, I could then use the gusset to mark the vertical point on the side. Then, a caliper to center the gusset screw gave me the point I needed to punch to drill the close-fit hole. I did one centered (the one with the frame and gusset close to the end), and the other end offset a bit to allow for clearance from the dial test indicator stand (about 0.625" from one side).


I got the sides drilled out using a #18 drill bit (close fit for an #8-32). That allowed me to screw the sides and the gussets together. Once that was done, I could then put the transfer screws in the bottom of the side-gusset assembly, put my expensive, high-contrast marking fluid down on the base, and get it lined up. Then a quick whack with a hammer gives drill points. Note, I'm going to suggest six hands here. Seriously, if you have four 1-2-3 blocks, use them. I used a tool makers' vise on the end to keep the gusset flush, a 1-2-3 block braced on the side, and another 1-2-3 block against that to keep the side perpendicular to the base.



I used a center punch to get those marks a little bigger, and then drilled them out. I did one end at a time, starting with the flush side. With one side complete, I dropped my 1-2-3 blocks in place (and kept a flat on the side to keep the side-gusset perpendicular to the frame base). I'm working with a 4"-between-sides distance, so I used the 1" side and the 3" side of two blocks against the face and set the next side in position. Another whack, and repeated the drilling.



Wash, rinse, and repeat for the other tool's frame, and I could screw them together for the first time. They'll be taken apart again for the boring operation for the rollers' bearing cavities. This will let me keep things in place, though, so I don't have to mix up parts.


That gave me two nearly-complete frame assemblies. I marked 0.500" from the the side and the end on the base to locate the hole for the dial indicator stand itself. The dial indicator stands I bought are M8x1.25 metric threads [7.5mm diameter with a thread every 1.25mm]. I grabbed a 9/32" drill bit (best 50% tap fit for stainless steel). I hand-tapped using a titanium nitride tap from Harbor Freight. It got the job done, but there was certainly some pucker power to get it to tap all the way. Once that was done, you can see how this is going to be set up.


To complete the frame assemblies, I need to bore out the bearing cavities, and also drill and tap the frame base to take the dial indicator arm. I used 1-2-3 blocks to mark a line 1" from the outside of the frame base on the sides. This will give a valid measurement, and although it's not critical to the operation (even if the rollers were not parallel to each other, if they roll and are concentric, you'd get valid measurements), I'm doing it because my OCD gets in the way. Make sure that, when you mark them, you are marking them "backwards". When you have the faces out toward you, if you mark them exactly the same and rotate them inward to each other, it won't line up (unless you centered it on that line scribed using the 1-2-3 blocks. So, from that center line, I measured to the outside 0.500". On the short side, I measured 0.500" to the right, and on the other side I measured 0.500" to the left of the scribed line. I measured up 0.1875" from the base and marked the other lines needed for the centers. Then I center punched one hole on each side. Using that as the starting point, I measured 0.620" from that center punch mark toward the other side, and scribed the cross har on the 1.1875"-from-the-bottom line I'd done earlier on each side, and then center punched those.





Now, on each face, I have two center punches that are properly spaced apart for the rollers. Thinking this was going to be a walk in the park on the milling machine (remember, it's a Harbor Freight machine, not big at all), I quickly burnt up my 10mm end mill. No, that's not an exaggeration - the cutting edges are all charred and the outside supposed-to-be-square corners are lifted. Here's me using a center on the hole to line it up on the mill before I went through all of that.


I wasn't going to have the prettiest time doing this. [sigh]. To the drill press!

I know this isn't "accurate". I know the holes aren't even "round" (inside joke - ask a machinist, and he'll tell you that a drill press is just for putting holes in things and that if you need them to be round, use a different machine). But, it's the only thing I have that is able to do it. I started with a 1/4" bit, went to 5/16", and stopped at 25/64". That left a bore about 9.95mm, or about 0.3910". I should have it around 0.3937", that's about 0.003" undersized. Drill bits always drill slightly oversized. I expected the drill bit to produce a larger hole that the drill bit, so in theory it should fit. Anyway, I drilled about 5mm, or 0.196" deep. That is right about the depth of the bearing on the outside edge of the drill bit (remember the cone on the end - we are using that as a bearing clearance end, which I'd originally called for a 1/4" or 7/32" hole that was about 0.010" deeper than the bearing). The drilling left a burr on it.



I filed off the burr, and set the bearings in place only to realize that the drill bit is slightly undersized. The 25/64" drill bit produced a hole that was 0.386" in diameter - 0.007" undersized. This means that the drill bit was quite a bit smaller than it was supposed to be. Hoping that the next drill bit up would be similar (or I might have to resort to brass shim stock to keep bearings in place), I grabbed the "X" drill bit (0.004" larger than the hole I needed, in theory), and enlarged those holes. I ended up about 0.392", so a 0.001701" undersize should work.

I used c-clamps to press the bearings in place, and to keep them on the same plane, I used the base of the second tool as the other end of the clamp. Frankly, you can use anything flat. I was too lazy to go find something, and I didn't want to use the hydraulic press. The bearings still turn, so that 0.001701" was perfectly sufficient to use, and keep the bearings in place.




The frames are complete.

Now I need to finish getting my lathe running again so I can turn the rollers. Note that the rollers can't just have the bearing areas turned - they'll need the whole thing. Knowing that as whatever distance away from center the round stock is, that will manifest itself in the cartridge measurement. I'm not going to spend an inordinate amount of time trying to dial in perfection, and I planned this into the whole thing. I'm going to take 5/8" round bar, chuck it in place, and turn the whole thing down to 0.610" in diameter (which is why I went with 0.620" between the roller axis'). Here's hoping I can get the lathe running again soon.
After finally getting my heavy 10 up and running, I could work on the rollers. I first tried it with the chuck, but the combination of that and the follower rest not being aligned properly bent my raw stainless stock. So, I switched gears and went with the hand-lever collet attachment. When I used the indicator, it was within 0.001", so I thought I could live with runout like that in the end result - a maximum of 0.002" (two rollers, could be anywhere from dead on to 0.002" depending on the stars aligning).

I did one test run with the buggered up roller, just to see. That roller was 0.006" on extreme travel (0.003" off centered). I included the numbers from that at the bottom here.

I had a .50 "dummy round" (primer divot indicates it was fired and then a new bullet put back in it) that I put on to test capacity :


This thing fits. The .50 was 0.030" in it's range (so 0.015" off center). I didn't care about that measurement. I loaded up the 308 my buddy had loaded and one factory (sequentially, obviously). and measured both by the neck as well as toward the tip (this will tell me if it is axially aligned, just off-centered, or if the axis' aren't even aligned) :


I took measurements with the Hornady tool, just for a baseline on both :


And I ended up with the following datum :

ToolFactory MeasuredFactory PotentialReload MeasuredReload Potential.50
Hornady0.007"0.007"0.002"0.002"no capacity
Off Centered - Roller0.0150"0.0150"0.0075"0.0040"-0.0110"unmeasured
SilverHawk Special0.0110"0.0110"0.0035"0.0035"0.0300"

My conclusion is that this will work. I doubt we really need to get this tight in tolerances, but if you are pushing down-range a long distance, this little tool works well and can give you more information than the other design. The other thing I found is that the Hornady tool still (sort of) works. It is "close enough" for what the vast majority of reloaders would need. Just multiply by 2, and that's a ballpark of your concentricity.

A lot of effort, good information, and maybe it was worth it to someone. Not sure it was worth it to me (snicker - making something in the shop is always worth it).
I'm having a tiny bit of heartburn with this picture. From here, it looks like there's and adjustable spring plunger with a nylon tip - at least of the kind I've seen many times. (In fairness, i'm unfamiliar with the Hornady device.)
Diametrically opposed to the plunger is the indicator tip. While the plunger isn't "solid" or certainly not robust, it's far more solid than an indicator tip.
As the shell revolves, this is not measuring concentricity. It's measuring Roundness. Not the same.
If the bullet tip is captured and centered, and the plunger is set, perhaps from the factory, with a master gauge of sorts, then yes, it could be construed into a concentricity measurement, albeit a clumsy one. If the plunger is user-set, against the case, it will never yield accurate results.

Your device, however, is a true concentricity device and will yield a true concentricity runout measure.
I was thinking that plunger would alter the numbers, too. Enough pressure on the side would cause it to either flex or push the spring back on the bullet end, minimizing the numbers. I still moved the plunger out, but kept backing it off because it did effect the numbers.
My concentricity gauge has a roller at the case head and another at the neck. A DI sits against the bullet. I have a set of Forrester Micrometer seating dies with the sliding bushing to keep the bullet and case aligned until the bullet is seated. Works great for concentricity and after checking a bunch of rounds I never used the concentricity gauge again. What I found that makes the biggest difference is the bullet jump to the lands. I seat each bullet .003 long and then screw the mic collar down until I get precisely .001 jump according to my Hornady head space gauge. On my 300 WSM doing this shrinks my 200 yd groups from 2" to 1" with Berger 210gr VLD Hunting bullets.