Umnumzaan sample variation theory

[Ante Vukorepa]

I've mentioned in https://www.bladeforums.com/threads/another-umnumzaan-joins-the-fray.1748148/ that my 'zaan was extremely gritty and exceedingly hard to open (painful with both hands) until broken in.

From multiple threads i've read while preparing for purchase, it seems like there's been a lot of sample-to-sample variation both over the years and from within same batches - some people had smooth Umnumzaans straight out of the box, some broke in in the first hour of use, some had issues for weeks until either persisting through it or sending it back to CRK.

So here's my theory, and i apologize if this was covered before - it's the damned ball. It's an inherent source of tolerance issues. What most people call "tight tolerances" is actuall the fit (tightness of fit and allowance). Tolerance is the variation from sample to sample. For the most part, CRK knives exhibit tight tolerances wherever it makes sense, and rely on other classical techniques of ensuring alingment and fit, like self-centering screws / countersinks.

The ball, is the one exception.
This is what the crimping looks like up close:

Does this look like an inherently repeatable process?
It doesn't to me. A bit more force on one side vs. the other and you'll end up with a very slightly differently aligned ball.

Here's the track it inscribed in my 'zaan's blade:

If you look closely, you'll notice it's slightly off center from the hole. This causes the lockbar to flex sideways slightly when the knife closes and the ball falls into the detent hole, requiring a slight additional force to kick the blade out of the detent on this particular sample.

This is one source of the variation, but there's another one.
The ceramic balls are not perfectly spherical (not just in 'zaan's case).
In fact, look closely at mine:

It's a bit hard to see, but make sure to switch the video to HD and pause occasionally. My ball has a ridge right smack dab in the middle of the interface with the blade. This causes it to bite deeper into the blade when locking, but more importantly in this particular case, when opening and closing the blade too (since the slight ridge goes all the way around).

My bet is - on a 'zaan with a more... uh... spherical sphere... the friction and break-in period would be much faster than on mine.

So there ya go.
What do you think, am i completely off base, or is there something to it?

 

[Evany]

I don't remember where I first heard it (so I might be wrong), but supposedly one of the reasons for CRKs switch to the ceramic ball interface was to eliminate hand fitting of the lock up action in order to streamline production. If that's the case the design probably allows for slightly looser tolerances and a bit of misalignment.

 

[XL883N]

I think it's just learning how to use the knife. Rest your thumb against chamfered titanium then follow the curvature pushing past the thumbstud. Blade pops right out.

 

[Ante Vukorepa]

I think it's just learning how to use the knife. Rest your thumb against chamfered titanium then follow the curvature pushing past the thumbstud. Blade pops right out.

This isn’t about the technique, but differences from sample to sample. Some are easier to deploy, some harder, out of the box.

I don't remember where I first heard it (so I might be wrong), but supposedly one of the reasons for CRKs switch to the ceramic ball interface was to eliminate hand fitting of the lock up action in order to streamline production. If that's the case the design probably allows for slightly looser tolerances and a bit of misalignment.

That seems about right, from what i’m seeing.

Adding a very slight chamfer to the detent hole on the blade would probably negate it. The way it is, for slightly misaligned balls, especially if they’re also not very round, you need to wait for the ball to cut/wear itself a slight ramp at the edge of the hole before the knife can be opened one-handed.

Depending on the amount of misalignment and the smoothness, this can take ages. In my case, the ridge helped, as it’s sharp enough to cut at the edge (and form a groove in the blade too) fairly quickly.

 

[Evany]

You can always send it back to CRK and perhaps they can make it perfect. You might have one that slipped through QC. It may not be relevant but from my experience with 21s and earlier models, they tend to be on the stiff side when new. We all want our CRKs to be just right from the start. My take is that they were conceived as hard working tools designed to last a lifetime or two (or three). If that means that they are a little hard to open when new, I can live with that.

 

[Ante Vukorepa]

Too complicated, i'm not in the US.
It'll wear in a ramp in the detent hole.

Like i've said, in my case it just so happens the ridge (equator if you will) of the ceramic ball is oriented so it helps cut it, so it's going fairly fast.

 

[Alsharif]

Interesting observations, thanks for sharing!

 

[marthinus]

The lines you describe other than the detent track is most likely the washers.

Depending on how the “crimping” is done around the ball it can easily become a repeatable process with the right jig and an arbor press.

 

[Ante Vukorepa]

The lines you describe other than the detent track is most likely the washers.

Since washers are softer, that means there really was grit (steel debris from grinding or sharpening maybe?) out of the box in there.
Good thing i disassembled it and cleaned it immediately after unpacking it, i guess...

Depending on how the “crimping” is done around the ball it can easily become a repeatable process with the right jig and an arbor press.

Agreed, should be fairly easy to tweak the process.
Right now it looks like it's crimped first on one side, then the other side (in two steps) with a shear-like tool.
Just making it a one-step process would probably greatly improve the tolerances.

Or, if adding another step to the machining isn't a problem, as i've mentioned previously, just chamfering the hole would likely make most of the complaints about non-worn in knives go away.

 

[Heirphoto]

One variation I have noticed is lock bar tension. On my first Zaan it was easy to release the lock and close the knife. My second one was considerably harder, uncomfortably so if I was just fidgiting with it, opening and closing multiple times. Maybe I am just used to how easily my 21 and 25 lock bars release but it kind of dampened my enjoyment of the Zaan

 

[Ante Vukorepa]

It's not just you nor your sample, mine is very hard to release as well. Again, in my opinion, has to do with ball vs. blade detent hole alignment - if they're slightly misaligned, you need to fight not just the lockbar tension, but also slight sideways lockbar flex, and increased friction between the detent hole and the ceramic ball.

I don't think it'll ever get as easy to unlock as the 21.

As an aditional data point, 21s have the back side of the lockbar flexing/pivoting point milled a little, i imagine in order to make unlocking a tiny bit easier, and to control the failure mode when the lockbar breaks. Umnumzaan does not.

 

[blanex1]

I still think 21s have a Moore solid lock up, most likely do to the hand fitting process ! I do own a Inkosi but my thought is I really wish CRKs would go back to the hand fitting method, takes Moore time/work but is this not what made CRKs in the 1st place, only my thought .

 

[Ante Vukorepa]

It's still the same process - the face on the blade tang is ground off until it opens and locks.
The difference is, the tolerances involved can be much lower, and it likely takes way less iterations to do it, since you're not mating two surfaces along a 2D curve, but a surface and a sphere at a point.

Holding an Umnumzaan and a 21 in hand, i can't say i'm noticing any differences in the solidity of the lock. I get that the 'zaan's locking action might *feel* less solid, but that's due to the sound and tactile feedback more than anything. I can't back any of this up with solid data, though, so i guess it's just a "feel" again