After a limb is fit into the riser how much play / tolerance should be considered allowable? I don’t have enough equipment experience to make comparisons, so I wondering how far off my ILF tolerances are. Even though I've got a small amount of play at the limb bolt, the other end of limb can magnify the variance quite a bit.

In my case it with low end equipment (Cartel Fantom, SF Premium Limbs) using a micrometer to measure tolerances I’m finding that there is about .015-.020” play in the dovetail diameter and the dovetail slot width, and also .015-.020” play between the limb bolt diameter and the U notch width at the end of the limb. There is also about 0.1” difference between (the dovetail slot to leading edge of Limb bolt) and (the end of the U notch and leading edge of Limb dovetail) = dovetail holds the sliding load not the limb notch.

If there were no tension on the limbs, the Dovetail and Dovetail slot has +/- .010” of play from center of U in the dovetail slot. But if the limbs are under tension the shape of the dovetail slot and the dovetail should keep the dovetail centered (unless there is enough vibration/friction to allow movement over the dovetail and the dovetail slot tolerance and not return to center). I'm not sure if this exists, but the dovetail should self-center itself.


Lots of questions maybe only needs one answer.
Question 1: Does the dovetail centering vary from shot to shot? It shouldn't but just asking- no. once set, that's it. But, when you put your limbs in- your first couple shots may be off a little while the limbs set. Once set- they don't move. You can limit this by popping the string a little (pull back a few inches, let go- no arrow)...no this won't damage anything. It does take a little experience to know how far to pull the string back.
Question 2: Is the dovetail/dovetail slot supposed to take most of the sliding load, with the Limb U cutout and Limb Bolt mostly guiding/stabilizing the alignment at one end (and the Dovetail socket making lateral adjustment at the other)? not sure- I'd think that most the load would be at the bolt due to the leverage
Question 3: If Questions 1 is NO and Question 2 is YES, then how much tolerance should be allowed between the U cutout of the limb and the Limb bolt? as much as it takes to easily slide the limb in and out but not so much that it sticks
Question 4: How much does the limb string groove aid in limb alignment/centering from shot to shot? you want the string to lay in the grove (about 80/90%)...it doesn't physically align anything but it is a reference
Question 5: What do you do to decrease tolerances? Does it change over time? you don't do anything- tolerance is what ever the manufacturer sets. If it's too tight for you, clean it up a little. If it's too loose, send it back.
Question 6: Or do you get closer tolerances by spending more money on equipment or staying within one brand? yes, higher end components have better tolerances. But that doesn't mean that inexpensive limbs wont work well.

I guess what I'm trying to figure out is... even though I've got lateral limb adjustment is it totally negated/or variable from shot to shot because I have a sloppy fit at the limb notch/limb bolt.

You're way over thinking this. Put down the calipers, get rid of the engineering mindset...it will just cause you to over complicate things.

if your limbs are moving from shot to shot, get new limbs
if your limbs are tight and difficult to install...open up the tight area with sand paper, file etc....take a little material off- check...redo if necessary.

As precise as these machines are so long as everything lines up and the tolerances aren't too loose, you're good.

 

as long as you can get them on and off without major effort. Youre good.


Sometimes the clear coat will make it hard to get the limbs on or off. As already recommended, a little sand paper in the U and you will be fine. If the limbs are loose on the tiller bolt. Nothing to worry about. As long as the detent button is clicked in, youre good.


Chris

 

Sorry my inner-engineer couldn't step back, but that said, I am also guilty of not shooting enough, which is why I’m fiddling with my kit.

I thought I needed to dust off my Trigonometry to work through this, but all I needed was 8th grade Geometry. The dovetail is a pivot therefore angles are equal and opposite so only the ratio of the lengths matter.

Just to use approximate reasonable numbers I used 6 cm from the dovetail to the limb bolt and 60 cm from the dovetail to approximately the end of a long limb. The ratio then is 0 .02”/6 = 0.2”/60. (I know its mixed units but it just a ratio and works out the same if you do everything in metric). So if my limb notch can move 0.02” the limb tip could move 0.2”. I believe this is true because when manipulating the limb in the riser under no tension it looks like about a quarter of an inch of movement at the limb tip.

I know 0.020” doesn't sound like much but doesn't a quarter of an inch at the limb tip sound like allot? What I don’t understand yet is… will the limb tip actually move 0.2” when under load? Really isn't the only thing preventing the limb from rotating on the dovetail is friction from limb / limb bolt interface and the string in the string groove at the limb tip (which so far has been thought to be insignificant). Pulling the string to full draw will cause movement at the limb/bolt interface and vibration of the after-shot both could cause random movement at the limb/bolt interface.

Your suggestions about a “just snug fit” drove me to experiment with shimming the inside of the limb notch. I used double sided golf grip tape and a narrow strip of beer can (these days you can cut a beverage can pretty easily with a common scissors). My lower limb fit snugly, (I could install/remove the limb without moving the metal can shim). The top limb was tighter and only the can shim was needed to fill the gap. The tape added to much space. This isn't a permanent solution but sturdy enough to use for a couple of sessions. If I’m happy after that I’ll probably use CA glue and baking soda or epoxy to tighten the tolerances.

After shimming the notches I rechecked my limb alignment and center shot and found that both needed to be re-worked.

 

Well OlyShoot when I got my first rig, I did notice the loose fit of the limbs into the pocket and wondered if it's supposed to have that much play. Put that out of mind and just shot and shot for the next 11 months without a thought of it. Can't say I've noticed any inaccuracies whatsoever and when aligning the limbs, my riser and limb align are dead on straight. I've come to like the loose snap in fit with a definite click, it tells me the limbs are fully seated.

 

Your mileage may vary from mine.
Like you I thought I had a good center shot. The string seemed to align with the riser and the Beiter Blocks and didn’t seem to change all year. I was a little concerned but not worried that the front stabilizer rod was at least a ½ rod pointing to left and the dovetail sockets were right at the edges of max adjustment. I just figured it was cheap low cost equipment and I had to just live with it. After shimming the limb notch an unexpected benefit was that I could now align the front-rod to the string, the string is still centered on the riser/limbs and the dovetail alignment sockets were closer to their centers in the riser pockets. AND I had to screw the plunger IN (towards arrow) almost a full turn and it might need more. I’ll admit that I am challenged with limb alignment due to bifocals (you have to move your whole head not just your eyes), so it takes me longer to do an alignment, wish I had a laser. I haven’t had a chance to shoot yet, but I wouldn't be surprised if my arrows turn out to be more over spine-d than before, but hopefully they group better. Either way I find it sort of exciting, which is why I like archery. I’m finding that all of the tuning guidelines actually work for me. I find it the hardest to listen to what the kit is telling me, instead of the kit doing what I want it to do .
I can also tell you that the click is more satisfying as the limbs seat instead of all the loose rattles I had before shimming.
Again your mileage may vary from mine. Cheers.

 

that's why oversize dovetails are available.

 

Ok well dovetail size does matter, I have built a shooting machine, put three risers on it with all the same equipment and the results where clear. The three were the hoyt avalon, KAP winstar 2 and my own riser. KAP would not group. after looking at it i found that the limb could move in the dovetail and therefore the centre shot would change. look for posts by sniperg.