When checking bolt-lug face surface % with a sharpie...

[Susquatch]

RE: over-travel; I'm in the low to none camp SO LONG as the inertia is low. If the pull weight is low, the mass of the trigger itself is low, and the trigger travel is very short, then the impact - if you can call it that - of hitting the stop is somewhere between insignificant and non-existent.

As soon as there is any inertia involved - as there probably should be in a field grade trigger, then I can see where over-travel would be greatly beneficial. Delay hitting that stop until after the bullet has left the barrel.

At one time someone like Volquartson sold an over-travel stop screw that had a rubber nose on it. Creating a soft stop instead of a hard stop. Never tried it, no idea how well it did or didn't work.

Your rationale agrees with mine except in one significant way. And that means we disagree on the outcome.

There can be no doubt that higher pull forces make over travel a bigger concern. But, while I agree with your assessment that lower pull weights reduce the need for overtravel, I cannot ignore the findings if world class shooters like Ratigan, Boyer, and Tubbs who all STRONGLY advocate lots of overtravel. These characters also use very low pull weights! So I have to conclude (mostly on faith and less on science) that it's important to have lots of high overtravel on a target rifle and EVEN MORE IMPORTANT on a hunting rifle!

But like I said, that's mostly faith based. One cannot ignore the possibility that the world champions are wrong.... LOL!

 

[ntsqd]

I highly doubt that they're wrong for themselves. They could be wrong for others. The particular rifles they use may dictate specific triggers that can not hit the goals that I stated as what I think are necessary for a no over-travel set-up, and that drives them to a long over-travel set-up. I've no idea what they use, how they use it, and if any of that applies to me.

Still, their expert suggestions should be tried on for yourself to see how they fit.

EDIT: Occurs to me just now that all of my no over-travel conditions could be met statically and it would still be desirable to go with long over-travel due to how recoil affects those static conditions.

 

[Susquatch]

I highly doubt that they're wrong for themselves. They could be wrong for others. The particular rifles they use may dictate specific triggers that can not hit the goals that I stated as what I think are necessary for a no over-travel set-up, and that drives them to a long over-travel set-up. I've no idea what they use, how they use it, and if any of that applies to me.

Still, their expert suggestions should be tried on for yourself to see how they fit.

EDIT: Occurs to me just now that all of my no over-travel conditions could be met statically and it would still be desirable to go with long over-travel due to how recoil affects those static conditions.

Excellent point! I have no doubt whatsoever that recoil has a major impact on all such things. As far as I know, Tubbs is the only one who uses any significant recoil shoulder pressure, but he shoots prone and the other two shoot from a bench. All three describe the same rationale though. They all say, (in my words) never allow the trigger finger to reach a hard stop before the bullet leaves the barrel.

FWIW, my info comes from their advice to others...

Edit - given the relatively long lock time vs bullet time, it's not hard to imagine how easy it is for the trigger to reach a hard stop before the bullet is gone.....

 

[Alex Wheeler]

Peak chamber pressure happens just after the bullet enters the throat (Benchrest shooters are loading in the 60-70k psi range). At this point the lugs are seated. Its hard enough to get 100% even lug contact with clearance around the bolt, it would be about impossible with no clearance, which is the case when the triggers lifting the bolt. A poor bedding job is enough to twist an action and unseat a lug. Lug contact is very important to accuracy and is not a given on any action, even a custom. Lapping is ok to fine tune once things are very close. It is not a way of "truing" lugs that dont touch, as lapping lugs puts taper in the lug seats, no matter how its done you cant avoid that. Many bolts have a reduced diameter in the center to allow for more clearance when cycling. You can turn down the center of a tight fitting bolt to get the best of both worlds, I used to do this on a particular Benchrest action that wasnt the smoothest and it helped some but a good design with tight fit will run smoother than a sloppy one.

 

[ntsqd]

Assuming that the thrust applied to the bolt while lapping the lugs isn't excessive I'm not seeing how they would be lapped to a taper. Lapping compound is fluid enough that it isn't going to build up thicker in one region over another, and if you're not bending the lugs from excessive lapping thrust then I don't see the mechanism to make this happen. I can see the lug seat corners nearest the bolt body getting slightly rounded over from lapping.
Not saying it doesn't happen, just that I'm not understanding how it can happen.

Assuming that lug seat taper does happen, the lugs will match them. Is that bad? Wouldn't 100% contact before ignition on a very slight taper be preferable to 80% or less contact (before ignition) on perfectly square lugs & seats? It would change each lug's force vector's direction, but if they are both tapered then opposing Cartesian forces are generated, canceling each other out. This will add some Hoop Stress to the action while reducing the lug seat shear stress slightly. Those opposing forces will also try to center the bolt between the lug seats. If the action was machined accurately then centering the bolt between the lug seats centers the bolt in the action.

 

[Susquatch]

Assuming that the thrust applied to the bolt while lapping the lugs isn't excessive I'm not seeing how they would be lapped to a taper........ Not saying it doesn't happen, just that I'm not understanding how it can happen.

Assuming that lug seat taper does happen, the lugs will match them. Is that bad? Wouldn't 100% contact before ignition on a very slight taper be preferable to 80% or less contact (before ignition) on perfectly square lugs & seats? It would change each lug's force vector's direction, but if they are both tapered then opposing Cartesian forces are generated, canceling each other out. This will add some Hoop Stress to the action while reducing the lug seat shear stress slightly. Those opposing forces will also try to center the bolt between the lug seats. If the action was machined accurately then centering the bolt between the lug seats centers the bolt in the action.

Somebody here is thinking...... I like that! It's always good to challenge such things.

If you can accept the fact that the amount of lapping is proportional to the time spent lapping, then you will see that the lug surface is not lapped evenly. The face of the lugs closest to the lug ramps will get lapped all the time, and the portion of the face furthest from the Ramps will only get lapped a few percent of the time. Of course, the lapping force on a given section of the face also gets reduced as the area increases, but this effect is less than the amount of lapping time. That's why I like to lap only at the far extreme of lug rotation with just a tiny little bit of rotation. This doesn't eliminate the effect, but it does minimize it.

I don't have an opinion (yet) as to whether or not a slightly sloped lug face is harmful. It may do as you suggest. However, I do think it is directionally wrong to increase stresses in any way and therefore I prefer to play it safe and keep things flat and square.

In my personal opinion (not based on any facts) stresses increase the odds of changing the initial conditions from shot to shot. This dependency is generally the result of hysteresis, friction, or just statistics. Ideally, if we have the identical initial conditions, then we get the same results with each shot.

It's a lot easier for my mind to accept that we are more likely to achieve the same conditions (or at least closer to it) with flat square lug surfaces than we are with any slope.

 

[ntsqd]

In lapping you're basically grinding the high spots down to the low spots. You should probably stop once they're all the same level. Assuming no deflection due to excess thrust the two mating surfaces will arrive at the average of their combined previous forms. If they were basically square to start with they should be basically square when you're done. Any other result would be the fault of an improper process.

A well mated bolt and action are more likely to go to the same position every time than a poorly mated bolt and action. We may not want angled lugs & lug seats from other perspectives, but from purely a return to exact battery position I don't see any reason why it would be more or less successful than square lugs.

The force components from a .473 case head operating @ 50 ksi would be 8786 lbsf along the action CL and with perfectly square lugs & lug seats 0 lbs. in the perpendicular direction. Assuming that the lug seat face angles changed 1° each, which I think would be a lot, puts ~153 lbsf total outward on the action (~77lbsf per lug seat) and increases the force along the action CL to 8787 lbsf.

 

[DSheetz]

Isn't the point of lapping the bolt lugs to get more surface area to make contact and balance that contact between the bolt lugs so that they share in the load bearing when the round is fired and to evenly align the round in the chamber before it is fired as well as to make the bolt easier to open and close by removing the rough surfaces that are left from the machining process ( how ever smooth they may seem )but yet not so polished as to make them sticky when lubrication is applied between the mating surfaces ?

 

[Susquatch]

In lapping you're basically grinding the high spots down to the low spots. You should probably stop once they're all the same level. Assuming no deflection due to excess thrust the two mating surfaces will arrive at the average of their combined previous forms. If they were basically square to start with they should be basically square when you're done.......

I agree and disagree. Yes, lapping wears off the high spots first. But as the lapping progresses, it will wear off the high spots next to the ramps first - for the same reason I outlined earlier. The end result will not be a flat square matting surface. It will be a taper or ramp.

Allow me to try to explain this again. For the sake of simplicity, let's divide the lug faces into 10 elements. Element one is next to the ramp, and element 10 is at the far end of engagement. As the bolt is turned element 1 will be in contact before elements 2-10. As the bolt is turned a bit more, element two will be in contact and so will element 1. As the bolt is turned a bit more, element three will be in contact and so will element 1 and 2. And so forth and so on until the bolt reaches element 10 and then reverses direction. At this point, one can observe that the leading edge of the lugs and the ramp edge of the abutments gave been in contact 10 times more than the trailing edge of the lugs and far end of the abutments. After many such cycles, the result will be uneven wear with element 1 worn 10 times as much as element 10. So basically, the high spots wear down much faster at one end than the other.

Another way of looking at this might be to realize that there is a difference between lapping lugs and lapping a continuous surface. That's because lugs have a discreet start point and a hard stop end point. Regular lapped surfaces do not have this start and stop condition.

It's a bit analogous to lapping in a screw and threaded hole. If you apply compound down the length of the screw, turn it in and out 10 times, then ad Vance another thread and turn it in and fully out 10 times, etc etc. The result will be a tapered hole. However, if you insert a pin in a hole and push it all the way through and then back and forth so that the entire shaft stays engaged, the result will be a cylindrical hole.

I hope that helps. If not, then I am not going to be able to help much more and we will simply have to agree to disagree.

 

[ntsqd]

I think that I see the disconnect here. That would assume that the bolt rotation is taken to full possible every lapping cycle. I have not lapped the couple of bolts that I have done that way, nor would I suggest it. I only rotate the bolt maybe a 1/4 of it's total rotation when lapping the lugs & lug seats. Precisely for the reason why that you outline. I want to maintain as much full lug contact as possible.

Then I lap the caming surfaces, never taking the lugs into even partial lock-up, except for a couple of the last strokes to knock off any corner 'flash' or burr that might exist there.

Last I lap the extraction cam surfaces, but I just kinda take a pass at those and don't get to hard into them.

 

[Alex Wheeler]

The leading edges of the lugs and abutments will wear at the fastest rate, no matter how little you rotate the bolt. Its unavoidable. Headspace changes as the bolt is closed when taper exists. I have measured as much as .005 taper in factory lapped custom actions. When you make surfaces too unique you are asking more out of them to mate up. 2 flat parallel surfaces is preferred to me because they will mate up under pressure so long as the action bore is straight and square, bolts have clearance and you are lapping them to fit in only one way. This applies to lapping lugs as a "truing method" (Edit: not how lapping is meant to be done, which is after good machine work.)

 

[ntsqd]

The leading edges of the lugs and abutments will wear at the fastest rate, no matter how little you rotate the bolt. Its unavoidable.

I disagree. The more that you rotate the bolt, the more that this is likely to happen but within a small window you're maintaining enough minimum overlap of the lugs and seats that it can't happen due to that overlap. Lapping is essentially accelerated wearing. How can it wear more in a margin narrower than it ever achieves?

 

[Susquatch]

I think that I see the disconnect here. That would assume that the bolt rotation is taken to full possible every lapping cycle. I have not lapped the couple of bolts that I have done that way, nor would I suggest it. I only rotate the bolt maybe a 1/4 of it's total rotation when lapping the lugs & lug seats. Precisely for the reason why that you outline. I want to maintain as much full lug contact as possible.

Then I lap the caming surfaces, never taking the lugs into even partial lock-up, except for a couple of the last strokes to knock off any corner 'flash' or burr that might exist there.

Last I lap the extraction cam surfaces, but I just kinda take a pass at those and don't get to hard into them.

No disconnect, in one of my previous comments, you will see that I don't rotate the bolt all the way either. I keep it at the far end of its rotation.

My explanation was deliberately exaggerated to explain what happens. It doesn't matter though. ANY amount of movement (even just 5 degrees) will result in some portion of the lug unlapped while the rest is being lapped and will cause a ramp to form. It's impossible to avoid unless both surfaces are continuously engaged - which is virtually impossible in this situation.

That is why I only lap enough to smooth/polish the machined surfaces a wee bit to reduce friction. I don't lap to improve fit.

I don't pretend that it doesn't affect headspace. I headspace after all machining and minor lapping of the receiver and Bolt is complete. And I always grease my lugs with a high quality antiseize grease.

You began this exchange with not understanding how a ramp could form,

Assuming that the thrust applied to the bolt while lapping the lugs isn't excessive I'm not seeing how they would be lapped to a taper. Lapping compound is fluid enough that it isn't going to build up thicker in one region over another, and if you're not bending the lugs from excessive lapping thrust then I don't see the mechanism to make this happen.

I tried to explain the mechanism behind how it can. That has evolved into a debate about degrees, marginality, and caveats. I really don't wish to do that because it goes nowhere.

I hope we can simply agree to disagree if you don't accept my explanation.

 

[ntsqd]

I began this with not understanding that you were talking abut a ramp that is 90° to what I was picturing. No, I do not know why I thought that.

I see what you're saying and I can see how that can happen. I would say that if a ramp is formed that the whole process is in error and that the root of the problem is not the lapping, but is the lack of good surface finish from the machining that caused there to have to be excessive lapping. Lapping is not a substitute for good machining, it is a final polishing step.

I suppose someone would debate the change in head-space due to lapping the lugs. I wouldn't. I would contend that if you are lapping correctly that the change is minimal to of no consequence unless there was a preexisting problem, but that it absolutely does change.