Critical Tolerances for Lockback Knives

N

NeoClassic

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First time posting in this subforum.

I'm considering creating a mid-lockback folder for my own personal enjoyment and am researching what are the most critical components within the mechanism to prevent lock rock (i.e. talking more up-and-down movement than side-to-side bladeplay, though I'm certainly intent on avoiding the latter as well).

I assume the face of the lock-bar itself has to be among the most important aspects. But beyond that, here are some questions I've had:

- How crucial are tolerances for the pivot-point on the lock-bar and the blade pivot itself? Do they have to be infinitesimally engineered or is there some "leeway" in terms of the level of decimal points or thousands one needs to watch for these components?
- In regard to the above, how does one balance enough clearance to allow the lock-bar (and blade) to pivot freely enough for ease of use, versus making it as "tight" as possible for better lockup?
-Is a mid-lockback more challenging in regard to avoiding lock rock than a traditional lockback?
-What role does handle and pivot-pin material (and heat treatment of those) play?

I understand introducing a stop pin such as on Cold Steel's Triad Lock greatly improves the integrity of the mechanism. Short of that, I'm curious what can be done preemptively in a traditional lock-back's construction to ensure that lock rock/bladeplay does not develop over time (or worse, creep in from the outset).

Maybe I'm overthinking this, but my line of inquiry stems from various posts on different forums about users claiming to have encountered lock rock on a variety of lockback knives from reputable manufacturers, including Buck, GEC, and Spyderco, to name a few.

I realize these are topics that could fill many a dissertation, so I'm not expecting full-on data dumps here. Rather, I'm just hoping to get some general guidance and be pointed in the right direction for what to focus on.

Any insight is welcome and appreciated, as are recommendations for books on this topic.

Thank you for your time and interest.
 
Last edited:
I am afraid you may suffer the same fate as I…make such a long winded post, with the questions buried and scattered, that folk look but cannot even be bothered to write..TLDR.


Chris Crawford Knives. Does tutorials and patterns.

Lock Back Folder makers

When making the blade notch do you leave the the notch with 90 degree angles or do you put in an angle at the rear of the notch? Gearing up to make lock backs and getting mix info. What seems to work best for you?
www.bladeforums.com www.bladeforums.com
 
C_Claycomb said:
I am afraid you may suffer the same fate as I…make such a long winded post, with the questions buried and scattered, that folk look but cannot even be bothered to write..TLDR.


Chris Crawford Knives. Does tutorials and patterns.

Lock Back Folder makers

When making the blade notch do you leave the the notch with 90 degree angles or do you put in an angle at the rear of the notch? Gearing up to make lock backs and getting mix info. What seems to work best for you?
www.bladeforums.com www.bladeforums.com
Click to expand...

I'm not sure if the reason for no responses is as you say (my questions are prominently bulleted, though admittedly surrounded by lots of other text), or due to an unwillingness to share what some might perceive as competitive advantages.

Either way, I'm sure I'll find my answers elsewhere and through my own trial and error.
 
Chris Crawford sells an Allen Elishewitz tutorial on lockbacks. Another book you might look for is Don Robinson's "My Way" on his method of creating a lockback from a machinist's POV. I doubt either will discuss mid-locks, though either will give you a good sense of what are critical tolerances and which are less so.

I'd also suggest it unlikely knife makers on here would have an unwillingness to share. Not what this place is.
 
Some of your questions are difficult to give exact answers for, as you mentioned. Tolerance and fit using exact numbers can be a huge rabbit hole to go down. It’s not all just diameters but there is lots of geometric tolerance as well. Meaning parallelism, perpendicularity, straightness, roundness, concentricity and on and on.

The reason most production knives have lock rock and blade play is because it’s much cheaper to manufacture the pieces with a little extra clearance to insure easy and consistent assembly. Comes down to the type of methods used to manufacture the pieces. Quaility control is one of the most expensive parts of manufacturing, so dropping the quality slightly can make the cost drop significantly.

If you want to make a knife for yourself though and you are more concerned with quality, as it sounds, then what you need to do is just fit the pieces together at assembly. You aren’t concerned with interchangeability so the exact dimensions you end up with don’t really matter that much. It’s all relative to the fit of one piece to the other.

I’ve never made a mid-lockback but the principal is the same. It mostly just comes down to good practices and well set up tools/machines. Make things as square, parallel and flat as possible.

Hone the pivot hole after heat treat until the pivot just fits. Those adjustable brass hones with some honing compound work great. Just hone a bit, check fitment, hone some more and so on. Also remember that a tight fit won’t necessarily go together easily. So be diligent when checking fitment.

Basically you just need to take your time when fitting things together and be aware you may have to assemble/disassemble many times. Experience will teach far more than reading about it.
 
N NeoClassic
I beg your pardon. I had not noticed the little "-" in front of each line of questions, and I see now that while the forum displays Lists, it does not have the shortcut button for producing them

phpBB • BBCode Ordered Number List

www.phpbb.com www.phpbb.com
I have started a number of threads, with posts much like yours, lots of thought into what I am asking, lots of words, lots of leading multi-option questions....and more often than not the response has been disappointingly sparse. Shorter posts, shorter sentences, more open questions do seem to work better for starting conversations, which is what forums are about. Also, saying less when you are at the beginning of learning means you don't leave people thinking "oh boy, where do I start" ;)

While I don't post much, I have been here a long time, and folk are very helpful, but threads started with very specific, very technical questions can be slow to get started.

What's your background for making stuff like this? Might help folk tailor answers. What kind of tools do you have access to, or are you trying to work out what you need in order to achieve an acceptable result?

While I have not made a lock back I have tinkered with making replacement blades for a couple of axis locks and a frame lock, my day job is aerospace mechanical design.

As a starting point, better tolerances are always going to be better, there are just diminishing returns, but if one has to ask, it is unlikely you are anywhere near that point. So reaming holes, not just drilling, maybe some barrel lapping in the holes. Flat lapping surfaces, micrometers to measure thickness, a design that lets you sneak up on fit-up rather than have to hit it dead-on straight from the gate. Just as Contender Machine Contender Machine says!

Stick at it here, there is a ton of expertise :D

  • How crucial are tolerances for the pivot-point on the lock-bar and the blade pivot itself? Do they have to be infinitesimally engineered or is there some "leeway" in terms of the level of decimal points or thousands one needs to watch for these components?
  • In regard to the above, how does one balance enough clearance to allow the lock-bar (and blade) to pivot freely enough for ease of use, versus making it as "tight" as possible for better lockup?
  • Is a mid-lockback more challenging in regard to avoiding lock rock than a traditional lockback?
  • What role does handle and pivot-pin material (and heat treatment of those) play?
Click to expand...
 
i will give you a bit o friendly advise
#1 buy a cheap lock back and take it apart ... this allows you to see the construction of the design
i will say very accurate holes are a must ...Drilling undersize and reaming to the measured size
do not ASSUME anything is the size someone claims it to be ... Measure it
i will also say that true flat (Precision Ground) is not a OPTION it is a Requirement...
on a lock back the fit of the lock into the blade is a very small allowance to sneak up on ....and very easy to overshoot
 
fitzo said:
Chris Crawford sells an Allen Elishewitz tutorial on lockbacks. Another book you might look for is Don Robinson's "My Way" on his method of creating a lockback from a machinist's POV. I doubt either will discuss mid-locks, though either will give you a good sense of what are critical tolerances and which are less so.

I'd also suggest it unlikely knife makers on here would have an unwillingness to share. Not what this place is.
Click to expand...
Thanks very much for the suggestions on the books. I'll check them out. And I happily stand corrected regarding the sharing of information on this forum.
 
Contender Machine said:
Some of your questions are difficult to give exact answers for, as you mentioned. Tolerance and fit using exact numbers can be a huge rabbit hole to go down. It’s not all just diameters but there is lots of geometric tolerance as well. Meaning parallelism, perpendicularity, straightness, roundness, concentricity and on and on.

The reason most production knives have lock rock and blade play is because it’s much cheaper to manufacture the pieces with a little extra clearance to insure easy and consistent assembly. Comes down to the type of methods used to manufacture the pieces. Quaility control is one of the most expensive parts of manufacturing, so dropping the quality slightly can make the cost drop significantly.

If you want to make a knife for yourself though and you are more concerned with quality, as it sounds, then what you need to do is just fit the pieces together at assembly. You aren’t concerned with interchangeability so the exact dimensions you end up with don’t really matter that much. It’s all relative to the fit of one piece to the other.

I’ve never made a mid-lockback but the principal is the same. It mostly just comes down to good practices and well set up tools/machines. Make things as square, parallel and flat as possible.

Hone the pivot hole after heat treat until the pivot just fits. Those adjustable brass hones with some honing compound work great. Just hone a bit, check fitment, hone some more and so on. Also remember that a tight fit won’t necessarily go together easily. So be diligent when checking fitment.

Basically you just need to take your time when fitting things together and be aware you may have to assemble/disassemble many times. Experience will teach far more than reading about it.
Click to expand...
Your thoughtful response confirmed some suspicions I had regarding mass production versus handmade. I very much appreciate your insights. I'm at the very outset of this journey so every little bit helps. Thank you.
 
C_Claycomb said:
N NeoClassic
I beg your pardon. I had not noticed the little "-" in front of each line of questions, and I see now that while the forum displays Lists, it does not have the shortcut button for producing them

phpBB • BBCode Ordered Number List

www.phpbb.com www.phpbb.com
I have started a number of threads, with posts much like yours, lots of thought into what I am asking, lots of words, lots of leading multi-option questions....and more often than not the response has been disappointingly sparse. Shorter posts, shorter sentences, more open questions do seem to work better for starting conversations, which is what forums are about. Also, saying less when you are at the beginning of learning means you don't leave people thinking "oh boy, where do I start" ;)

While I don't post much, I have been here a long time, and folk are very helpful, but threads started with very specific, very technical questions can be slow to get started.

What's your background for making stuff like this? Might help folk tailor answers. What kind of tools do you have access to, or are you trying to work out what you need in order to achieve an acceptable result?

While I have not made a lock back I have tinkered with making replacement blades for a couple of axis locks and a frame lock, my day job is aerospace mechanical design.

As a starting point, better tolerances are always going to be better, there are just diminishing returns, but if one has to ask, it is unlikely you are anywhere near that point. So reaming holes, not just drilling, maybe some barrel lapping in the holes. Flat lapping surfaces, micrometers to measure thickness, a design that lets you sneak up on fit-up rather than have to hit it dead-on straight from the gate. Just as Contender Machine Contender Machine says!

Stick at it here, there is a ton of expertise :D
Click to expand...
I definitely take your point about brevity and will keep that in mind going forward. Thank you.

Interesting you mention replacement blades as that's something I've considered too, mostly for Spydercos (don't like the blade hole) and Victorinox (would love one in 14C28n or similar). In fact, the latter might be a good starting point for me, come to think of it, as--to answer some of your questions--I'm starting from literal ground zero with no experience whatsoever in knife-making. I'm a writer/editor/journalist by trade so my skill is reporting, learning, researching, triangulating. So I guess my initial shotgun blast of a post was to just move off center and see if I could start somewhere and figure out where that somewhere could be. Replacement blade for a 30-year-old Victorinox Recruit might be it as I will be figuring out angles and fitment of things, even if not as complex as a lock-back.
 
Pinoy Knife said:
i will give you a bit o friendly advise
#1 buy a cheap lock back and take it apart ... this allows you to see the construction of the design
i will say very accurate holes are a must ...Drilling undersize and reaming to the measured size
do not ASSUME anything is the size someone claims it to be ... Measure it
i will also say that true flat (Precision Ground) is not a OPTION it is a Requirement...
on a lock back the fit of the lock into the blade is a very small allowance to sneak up on ....and very easy to overshoot
Click to expand...
Thanks a lot for your advice. Your first suggestion has been on my to-do list for a while; I've watched many disassembly videos of kickbacks and studied photos for their guts at length, but it will definitely help to have a few in hand, as you suggest, to tear down.

When you say "true flat" is this in reference to grinding/polishing the surface yourself even if it's claimed to be flat versus just taking a manufacturer's word for it?
 
Regarding “…“true flat” (Precision Ground)…”
There is more engineering than art in making folders, and in engineering there is something known as GD&T for Geometric Dimensioning & Tolerancing. Within which there is Flatness, Parallelism, and Perpendicularity which are all key to having a folder work. Separately there is Surface Finish.
Flat is the first side of your blade or spacer. You can do a reasonable job with a disc sander, followed by abrasive paper glued to a granite slab, or thick float glass.
Parallel is what you need the opposite side to be.
When you make your hole, you want it to be perpendicular (square) to the surface.

I screwed up the frame lock replacement blade, after having the RWL34 blank all profiled, heat treated and surface ground like it was 600 grit, by not triple checking that the mill spindle at work was dead square to the surface against which the blade sat for drilling and reaming.

One can buy “precision ground” flat stock that looks like it was sanded at 60 grit. It might be flat, but it isn’t smooth enough for a surface that will see friction, like from washers around a blade pivot. Also, things move when heat treated, so flattening needs to be checked, tuned or redone post HT. good news is, you don’t need a big area flat. Bad news is that small areas are harder to keep stable for hand flattening.
 
NeoClassic said:
Thanks a lot for your advice. Your first suggestion has been on my to-do list for a while; I've watched many disassembly videos of kickbacks and studied photos for their guts at length, but it will definitely help to have a few in hand, as you suggest, to tear down.

When you say "true flat" is this in reference to grinding/polishing the surface yourself even if it's claimed to be flat versus just taking a manufacturer's word for it?
Click to expand...
yea when you understand the geometry required it helps.. seeing it on the sample/taken apart knife gives you a visual reference..
be warned though this is a very expensive deep rabbit hole to jump into..
Precision is a very tough thing to pull off ... Measurement tools that are accurate to 1/2 of 1 thousands are not {Harbor Freight } type items
same with a good Drill Press, Bits, Reamer sets then the steel has to be precision ground {final holes sized after heat treat} Carbide only!!
life is a Adventure !!!! or i'm just Crazy ..... (he he)
 
One thing I'd add to that is don't let the tab on the lockbar bottom out completely in the bottom of the blade notch. Since there's a bit of an angle on one side of the notch the tab acts as a wedge dropping tightly into the notch. Leaving it slightly oversize so there's a bit of space at the bottom allows for some wear. In this scenario any wear on the tab or notch allows the tab to drop a bit deeper still ensuring a tight fit due to that wedge effect. Hope that makes sense.

Eric
 
ea42 said:
One thing I'd add to that is don't let the tab on the lockbar bottom out completely in the bottom of the blade notch. Since there's a bit of an angle on one side of the notch the tab acts as a wedge dropping tightly into the notch. Leaving it slightly oversize so there's a bit of space at the bottom allows for some wear. In this scenario any wear on the tab or notch allows the tab to drop a bit deeper still ensuring a tight fit due to that wedge effect. Hope that makes sense.

Eric
Click to expand...
I think it does and thanks very much for this. Under this premise, is it safe to assume also that the "hook" -- for lack of a more technical term -- on the blade tang that the lock bar "grabs" also should have some clearance between it and the lock bar?
 
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