21 not closing properly?

[Mike Large]

Hey Mike, did you ever figure this out or get an answer from CRK? I have the same problem on a new small micarta, plus they messed up on the grind as well. I am waiting to hear back from them on Monday once the technician takes a look at this video I sent them. I'm hoping/expecting them to warranty a blade replacement and cover the shipping.

https://photos.app.goo.gl/zuHvJpEqp1S2kjLo1

Tyd450... I'm glad you ask man. They simply replaced the washers and pivot bushing. It is now perfect. Absolute perfection. Cost me a total of 9 bucks. Shipped it back to me for free. As well as sent me a free shipping label. Hope this helps friend. Talk to Kayla,,, she was a huge help

 

[Mike Large]

Also, they sent back the old bushing and the old washers...along with a set of 'spare washers'? From what I thought washers had to be put on at the factory to be 'fit'. Not sure if I can use these spare washers with another small sebenza or just the small i sent in...

 

[Tyd450]

Tyd450... I'm glad you ask man. They simply replaced the washers and pivot bushing. It is now perfect. Absolute perfection. Cost me a total of 9 bucks. Shipped it back to me for free. As well as sent me a free shipping label. Hope this helps friend. Talk to Kayla,,, she was a huge help

Interesting, glad it all worked out! What was the turn around time for that? Was yours doing pretty much the same thing mine is in the video I posted?

 

[TheFactor]

Tyd450... I'm glad you ask man. They simply replaced the washers and pivot bushing. It is now perfect. Absolute perfection. Cost me a total of 9 bucks. Shipped it back to me for free. As well as sent me a free shipping label. Hope this helps friend. Talk to Kayla,,, she was a huge help

That's great news !
Sometimes sending it in is your easiest solution

 

[Wanimator]

Lol that fit stuff is kinda BS. Any 1/10,000 of an inch tolerance is non existent in that part. They could superfinish their pieces with less tolerance change if they wanted.

 

[bhyde]

Lol that fit stuff is kinda BS. Any 1/10,000 of an inch tolerance is non existent in that part. They could superfinish their pieces with less tolerance change if they wanted.

Really?
How do you account for tolerance stacking in the rest of the parts? THAT is the reason they have to hand fit.

 

[bhyde]

Here is a fairly simple explanation WHY they can't send washers to you on a Sebenza but may on an Inkosi or Umnumzaan.
The example below is of an Umnumzaan from the side looking through the knife- Let's just for imagination purposes say that it's a Sebenza although the Umnumzaan is adjustable.
The blade+washer thickness MUST equal the backspacer thickness when broken in..but each one of these has a target thickness with a tolerance high and low. All parts have this tolerance or deviation from nominal that is acceptable before function is affected.
On the Sebenza, the pivot fasteners meet at the center when tightened down. Ideally, you should not have blade play or excessive tightness.
The things that control the pivot tightness at in the fastener (red parts below). Length of the contact face on the ends to the chamfer on the bottom side of the fastener. The depth of the chamfer in the scale (scale thickness is a minor function of this as keeping the depths the same have an affect when machining too)
The thickness of the backspacer is fairly well controlled. The washers are made from stock that is chemically machined..but STOCK has a different connotation because it's allowed to have variance in thickness as well. It could be + or - .001 depending on supplier. That is a huge number considering the rest of the tolerances.
The scales and the blade must be flat, but have a little deviation from nominal allowed. Perfect flatness, parallelism etc in a production environment is a bit tough to achieve.
Combine all these things and you have a reason to hand fit washers. It's not just slap it together.

I can go through some more detail a little later if needed as I understand that some of this may be a little confusing and may not make sense. Ask if you have a question and I will try to answer that for you.

 

[Wrobelan]

Here is a fairly simple explanation WHY they can't send washers to you on a Sebenza but may on an Inkosi or Umnumzaan.
The example below is of an Umnumzaan from the side looking through the knife- Let's just for imagination purposes say that it's a Sebenza although the Umnumzaan is adjustable.
The blade+washer thickness MUST equal the backspacer thickness when broken in..but each one of these has a target thickness with a tolerance high and low. All parts have this tolerance or deviation from nominal that is acceptable before function is affected.
On the Sebenza, the pivot fasteners meet at the center when tightened down. Ideally, you should not have blade play or excessive tightness.
The things that control the pivot tightness at in the fastener (red parts below). Length of the contact face on the ends to the chamfer on the bottom side of the fastener. The depth of the chamfer in the scale (scale thickness is a minor function of this as keeping the depths the same have an affect when machining too)
The thickness of the backspacer is fairly well controlled. The washers are made from stock that is chemically machined..but STOCK has a different connotation because it's allowed to have variance in thickness as well. It could be + or - .001 depending on supplier. That is a huge number considering the rest of the tolerances.
The scales and the blade must be flat, but have a little deviation from nominal allowed. Perfect flatness, parallelism etc in a production environment is a bit tough to achieve.
Combine all these things and you have a reason to hand fit washers. It's not just slap it together.

I can go through some more detail a little later if needed as I understand that some of this may be a little confusing and may not make sense. Ask if you have a question and I will try to answer that for you.

View attachment 791553

That is an amazing explanation and answer to the question.

Thanks for your insights!!!

 

[colubrid]

Here is a fairly simple explanation WHY they can't send washers to you on a Sebenza but may on an Inkosi or Umnumzaan.
The example below is of an Umnumzaan from the side looking through the knife- Let's just for imagination purposes say that it's a Sebenza although the Umnumzaan is adjustable.
The blade+washer thickness MUST equal the backspacer thickness when broken in..but each one of these has a target thickness with a tolerance high and low. All parts have this tolerance or deviation from nominal that is acceptable before function is affected.
On the Sebenza, the pivot fasteners meet at the center when tightened down. Ideally, you should not have blade play or excessive tightness.
The things that control the pivot tightness at in the fastener (red parts below). Length of the contact face on the ends to the chamfer on the bottom side of the fastener. The depth of the chamfer in the scale (scale thickness is a minor function of this as keeping the depths the same have an affect when machining too)
The thickness of the backspacer is fairly well controlled. The washers are made from stock that is chemically machined..but STOCK has a different connotation because it's allowed to have variance in thickness as well. It could be + or - .001 depending on supplier. That is a huge number considering the rest of the tolerances.
The scales and the blade must be flat, but have a little deviation from nominal allowed. Perfect flatness, parallelism etc in a production environment is a bit tough to achieve.
Combine all these things and you have a reason to hand fit washers. It's not just slap it together.

I can go through some more detail a little later if needed as I understand that some of this may be a little confusing and may not make sense. Ask if you have a question and I will try to answer that for you.

View attachment 791553

To be honest I did not understand much of that.

If a blade has side to side play on a "Sebenza" does that mean the washers are to thick or to thin?

Does excessive tightness mean the washers are to thin?

Does side to side play mean the washers are to thick?

 

[bhyde]

To be honest I did not understand much of that.

If a blade has side to side play on a "Sebenza" does that mean the washers are to thick or to thin?

Does excessive tightness mean the washers are to thin?

Does side to side play mean the washers are to thick?

Side to side play could be a few things. Could be that the fastener or pivot bolt assembly isn't tightened down OR the washers are too thin.

Excessive tightness could also be a few things. Washers too thick or the knife is not broken in. Everyone probably has a different definition of "too tight". Could also be a pinched washer.

Here..Let's add some numbers to my original explanation. The below numbers may or may not apply to actual part dimensions and they are for reference only.

Blade thickness: .140 in
Washers: .02 in x 2= .04
The total thickness of the 3 pieces between the scales should be .180 in
This means that the backspacer needs to be .180 in thick for the blade to rotate on the pivot, be parallel to the scales and to be centered in a perfect world.
The perfect world does not really exist in real life. There are always very minute variances in parts.

Let's say that the washers are from two different batches of stock from the supplier. One is at .021 and the other is at .019 on their thicknesses. It's using the extremes the supplier said the material could deviate from nominal thickness. + or - .001 inch.
Given this example, can you see what would be affected? Very likely blade centering in this instance..But the average of the two washers does still equal the backspacer thickness, so it should function OK. Now lets throw in a bit of asymmetry in the blade or some warp in the or the blade isn't flat..All of them within the specified tolerance or allowable deviation off of nominal. Just for grins, you can also add this to the scales. One isn't flat as it should be.
So assembling all of these perfectly imperfect parts gives you a problem. Individually, there is nothing wrong with the part as they are technically "in tolerance". But when the perfect storm of these parts gets together..you can have some issues.

Hope this helps to explain a bit more. It's really hard to explain the number of things that can go wrong in an assembly when everything is technically right. It also doesn't factor in parts that are out of tolerance that made it through the manufacturing process. Measuring every feature of every part in production isn't feasible..so they are typically measured when the very first part comes off the machine to qualify it, then in process measuring some parts that come off of the machine by the operator (also not usually 100% inspection) and of course, spot measuring at random intervals by an inspector. Given this, parts sometimes do get through.

 

[Mike Large]

So long story short.... I shouldn't try these spare washers they sent me on a different small sebenza?

 

[bhyde]

So long story short.... I shouldn't try these spare washers they sent me on a different small sebenza?

No idea what they sent you. Perhaps they pre-fitted washers? It's a possibility.

 

[colubrid]

Side to side play could be a few things. Could be that the fastener or pivot bolt assembly isn't tightened down OR the washers are too thin.

Excessive tightness could also be a few things. Washers too thick or the knife is not broken in. Everyone probably has a different definition of "too tight". Could also be a pinched washer.

Here..Let's add some numbers to my original explanation. The below numbers may or may not apply to actual part dimensions and they are for reference only.

Blade thickness: .140 in
Washers: .02 in x 2= .04
The total thickness of the 3 pieces between the scales should be .180 in
This means that the backspacer needs to be .180 in thick for the blade to rotate on the pivot, be parallel to the scales and to be centered in a perfect world.
The perfect world does not really exist in real life. There are always very minute variances in parts.

Let's say that the washers are from two different batches of stock from the supplier. One is at .021 and the other is at .019 on their thicknesses. It's using the extremes the supplier said the material could deviate from nominal thickness. + or - .001 inch.
Given this example, can you see what would be affected? Very likely blade centering in this instance..But the average of the two washers does still equal the backspacer thickness, so it should function OK. Now lets throw in a bit of asymmetry in the blade or some warp in the or the blade isn't flat..All of them within the specified tolerance or allowable deviation off of nominal. Just for grins, you can also add this to the scales. One isn't flat as it should be.
So assembling all of these perfectly imperfect parts gives you a problem. Individually, there is nothing wrong with the part as they are technically "in tolerance". But when the perfect storm of these parts gets together..you can have some issues.

Hope this helps to explain a bit more. It's really hard to explain the number of things that can go wrong in an assembly when everything is technically right. It also doesn't factor in parts that are out of tolerance that made it through the manufacturing process. Measuring every feature of every part in production isn't feasible..so they are typically measured when the very first part comes off the machine to qualify it, then in process measuring some parts that come off of the machine by the operator (also not usually 100% inspection) and of course, spot measuring at random intervals by an inspector. Given this, parts sometimes do get through.

o

questions on side to side play in a sebenza. I owned 2 Regular Sebenzas in the past that had side to side play and they were tightened down hard and still had play but were even and blade was not leaning. I sent them in to they replaced with the perforated washers (which I don't care for because the knife is no longer stock) they were fixed but had that weird feel of a modern Sebenza. So I finally sold them because of that.

Now if this should occur again . How can I fix this on my own and keep the original solid washers (because CRK won't replace them in any of the original Regulars any longer)

 

[blanco112]

Interesting, glad it all worked out! What was the turn around time for that? Was yours doing pretty much the same thing mine is in the video I posted?

When you think you've tried it all and you still can't fix it just send it in and they will send it back right.

 

[bhyde]

o

questions on side to side play in a sebenza. I owned 2 Regular Sebenzas in the past that had side to side play and they were tightened down hard and still had play but were even and blade was not leaning. I sent them in to they replaced with the perforated washers (which I don't care for because the knife is no longer stock) they were fixed but had that weird feel of a modern Sebenza. So I finally sold them because of that.

Now if this should occur again . How can I fix this on my own and keep the original solid washers (because CRK won't replace them in any of the original Regulars any longer)

There was play in the blade because the fasteners meet at their extents and can go no further. They bottomed out.

 

[Josh K]

Here is a fairly simple explanation WHY they can't send washers to you on a Sebenza but may on an Inkosi or Umnumzaan.

It’s not clear to me why the Sebenza is different from the Inkosi/Umnumzaan. My guess is the pivot bushing adds enough tolerance drift to necessitate hand-fitting the washers to the individual tolerances on each knife. Is this correct?

 

[bhyde]

It’s not clear to me why the Sebenza is different from the Inkosi/Umnumzaan. My guess is the pivot bushing adds enough tolerance drift to necessitate hand-fitting the washers to the individual tolerances on each knife. Is this correct?

Yes..guessing that is pretty true. On the Sebenza. The hole in the blade, pivot bushing; ID and OD, as well as the fastener is very tightly controlled. +.000 -.0002 or something to that effect to get the feel they (Chris) was after, so there isn't alot of drift on the slip fits between the OD and ID of these parts.
(OD= outer diameter, ID = inner diameter)
That becomes pretty obvious when you have problems getting the OD of the tapped fastener through the stop sleave or through the pivot bushing when you have to have nearly perfect alignment to assemble them.
Think about the scales on the Sebenza and how perpendicular they have to be for this tightly controlled assembly to come together. Now think about the thickness of the blade+washers being minutely thicker or thinner than the rear backspacer. It puts a bind on the rotational assembly because of that difference. If it's close, the washers will compensate and wear in.

On the Umnumzaan and Inkosi, I think that they took into account the variance of all these parts and simplified the design to accommodate. Give a little more clearance in one or two spots and the problem pretty much fixes itself. Pretty ingenious, albeit the heartburn some people have with adjusting the pivot puts them off.