Bolt fitting by knurling. Just an idea...your thoughts please?

[thomae]

Fact 1. PTG makes custom bolt bodies in different diameters to more precisely fit the Savage actions.
Fact 2. "People say that these "fitted" bolt bodies reduce "wiggle" and "slop" in the bolt-to-action fit and make things feel more precise. (IT is unclear if they actually contribute to accuracy or not.)

Fact 3. Knurling a round surface increases the diameter, but results in relatively sharp points.

So my question is whether or not it would be possible, impossible, good, bad, or indifferent to do the following:

A. Put a bolt body in a lathe and knurl the areas that are traditionally fluted.

then

B. Turn down the outside diameter of the knurling (in other words, machine off the sharp tips, so you had flat ends on the knurls (Is "knurls" the correct word for the little pointy protrusions? I am not a machinist.) and so the outside diameter would now be custom fit to a particular action.

then

C. Buff the outside to slightly round off the edges of the flats in order to reduce friction.

In this way one might have a bolt with closer tolerances and less friction between bolt and action.

What I don't know (again, I am not a machinist) is whether or not knurling would be able to increase the diameter of the bolt body for this to actually work.

I have not done this, and I don't have an extra bolt body, so I am not planning on doing this any time soon, but I was curious as to what you with much more experience than I think of the idea.

I am floating this thought exercise as a trial balloon...shoot away.

[Mach2]

Interesting.
I know little to nothing about this but if accuracy is to be affected the lugs must be knurled too.
Mauser bolts flop around in cycling too.

[CharlieNC]

Seems to me that since the bolt head is floated from the bolt body that this would not make any difference in terms of accuracy; one of the main reasons the Savage design is touted. Might make the operation smoother.

[sharpshooter]

Contrary to popular belief, making things tighter is not always better. Over the years I have made several bolt bodies that have a decreased amount of clearance between the receiver and bolt body, and despite the lack of "slop", and a better feel while cycling the bolt, the "tightening up" will only go so far until it goes the other way. Too little of clearance will cause a sticky feel, due to lack of a hard surface. The factory bolt bodies are case hardened to the equivalent of 60 Rc., which is about 20 points harder than the receiver, and would be impossible to knurl.
I have also had one rifle with a fitted bolt that shoot worse than with the factory bolt body. You have to remember that a production rifle has tolerance, and unless every part is on the money, a conflict will arise. More clearance will allow enough "float" for things to align themselves and eliminate stress.
Most custom actions have the bolt raceways wire EDM'ed after heat treat to insure straightness, and the bolt assemblies are cylindrically ground to have approx .0015" clearance. This practice stemmed from the fact that the trigger engages from the bottom of the bolt and creates a lift when the trigger is engaged. The lift creates an " out of square" situation at the bolt lug junction and typically only the bottom lug gets the lion share of contact. Decreasing the clearance decreases the amount of tip.
The Savage bolt engages the trigger on the side, and coupled with the fact that the bolt head floats, there is no tip.
The amount of "float" on a Savage bolt head is equal to 1-1/4 degrees, or about .050" in any direction as measured at the rear most point on the bolt.
According to the print, receiver bore diameter is between .700-.703", although I have seen many that exceeded that up to .712". The bolt bodies have a tolerance between .693-.695", with the majority running right at .693". I have found that if you can keep the clearance between .005 to .010", it is usually a good compromise. Actions such as the dual port could use less, because of the lack of bolt head contact in the raceways where the ports are cut from both sides.

[memilanuk]

Awesome post, Fred. Very informative!

Thanks!!!

:thumb:

[stangfish]

Thanks for getting into the topic Fred and Thomae. The only time I have seen knurling used to improve a fit was when the softest metal was knurled. Even the it was not the "recomended practice" but a patch until a better option was available. IE... an antifriction(Ball) bearing journal fit on a shaft.

[davemuzz]

I can't see how Knurling is going to make a bolt larger in diameter. I would seem to me that the process of Knurling would be to cut into the existing bolt metal.......but if it in fact "pulls" up metal to make the bolt larger in diameter, it would seem to me that such a process would shorten the life of such a bolt. Unless said bolt would be heat-treated after knurling to have the bolt metal hardened again.

Dave

[stangfish]

Dave, the knurling process(for a round outside diameter) is done by applying presure to a part rotating in a lathe with a set of wheels that are very hard with "teeth" machined in them. As the part turns pressure is applyed and the teeth press low spots into the diameter dispacing metal and making it proud. This increases the diameter slightly. There are several designs but this newer style is enough to get the idea.

http://images.drillspot.com/pimages/7544/754414_300.jpg

[davemuzz]

AHHHH.....Now Grasshopper see's. This isn't done with a cutting tool on a lathe. The metal is pushed about. So, back to my original second question if this happens......does the bolt then get heat treated again???

[stangfish]

I think it has been proven to be a bad idea due to; 1) the bolt is too hard at 60 Rockwell C scale to knurl, 2) that Savages do just fine if held between .005 and 010 clearance.

Even if it was viable there would be no reason to reheatreat since there was never an annealing process. The metal my be softened but a hardened nurling would be very similar to a file. Even if you machined it. A fine media blast might round the edges somewhat by why go to that much trouble if it aint broken. Remingtons and their clones got that covered. : )

[davemuzz]

Yeah Stang'ster. I'm not one that would see any advantage to knurling a bolt.

[FW Conch]

I think the most popular use of knurling was back in the past when we hot rudder's rebuilt our engines and the valve stems were worn a couple of thousands. We would take them to a machine shop, have then knurled to make them proud by the required amount, then machined smooth. Of course, a valve grinding job was certainly also needed at this time. The idea was to restore the valve to OEM specs, or better. But I have an idea that now, OEM valves are much cheaper than the hourly rate of a machine shop, and everyone just buys new ones, or up grades. But as far as the Savage bolt, I think sharpshooter answered the question very well. Thanks.......Jim

[Balljoint]

If you want to see bolt flop you see my Springfield bolt in my Match Rifle till it locks up in battery than tight as a rock

[davemuzz]

...because of the lack of bolt head contact in the raceways where the ports are cut from both sides.

You know....now that I think about this....my wife has said this very same thing about me in the past. However.....I really don't want to delve to deeply into the "lack of raceways" part of it.

dave

[sharpshooter]

You know....now that I think about this....my wife has said this very same thing about me in the past. However.....I really don't want to delve to deeply into the "lack of raceways" part of it.

dave

So...are you saying this is a "receiver" problem, or a "bolt" problem?;)

[stangfish]

Speaking of bolt

:bolt:




:focus:

[thomae]

Back to topic:
I apologize for having been away from this thread for several days.

Sharpshooter: Thank you for your detailed and measured response in post 4. That's exactly the kind of discussion as to whether or not my idea had any merit I was hoping to get.

By explaining, in this case, why my original idea was not necessarily a good idea, you taught me (and probably others, as well) some things I did not know. Any time I can learn something new, it's a good day.

Since my question was answered and this thread has subsequently wandered well off topic, if the mods want to close it, that's ok with me.