# Kurt doesn't use an acme thread ???



## woodchucker (Apr 6, 2017)

I just viewed a D688 on ebay, I notice it doesn't use an ACME thread... What the ???

I would think that for strength they would, as a regular thread is not as strong.  And with that big crank of a lever, certainly you are putting some heft into  it.

Was I seeing things?


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## JimDawson (Apr 6, 2017)

Nope, no Acme thread on a Kurt.  It's about a 1 inch - something, left hand, V- thread.  I'm too lazy to go out to the shop and actually measure one.    But the nut is about at least 2 diameters long, so plenty of engagement.


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## woodchucker (Apr 6, 2017)

Reminds me of my palmgren drill press vises. I like them, but find it hard to believe that they are not acme..
So what your saying is, I shouldn't worry about it.. And I won't... but for a minute there I went WHAT???


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## markba633csi (Apr 6, 2017)

I wonder what the reasoning behind that is? Acme is standard on practically every vise I've ever seen.  I think you guys have been duped.  Better sell all your Kurts before the factory admits their BIG MISTAKE
Mark
ps just heard Don Rickles passed so I'm being as silly as possible in tribute


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## Randall Marx (Apr 7, 2017)

I'll take them off your hands so you don't have to worry about the value dropping!


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## Redmech (Apr 7, 2017)

I'd suspect reasoning is you will get more clamping force with a finer thread.  Being all things equal, same force exerted on the same length lever. Most anything on heavy equipment that needs max holding will have fine thread versus course or even more course like acme.


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## Silverbullet (Apr 8, 2017)

Wouldn't a buttress thread or square be stronger? Just asking.


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## Redmech (Apr 9, 2017)

I do believe acme would be a stronger thread, but with proper design fine thread is completely adequate. 

Like Jim Dawson said, the nut is about 2" long, do there is plenty of surface area for the screw to engage the nut, and with properly hardened hardware, and a 1" diameter screw, and the screw is protected (guarded) from impacts or damage from something, the fine thread, will never be an issue. 

The fine thread versus any other thread equals maximum amount of clamping power.  Which is a great quality to have in a vise. 

Ross,


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## Redmech (Apr 9, 2017)

One last thought of fine versus course, with diameter of threaded rod being equal, and length of nut as well. Fine thread will have more surface area engaging than course thread will,


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## 4GSR (Apr 9, 2017)

I can show you some 4 pitch 60 degree vee style threads that will hold up the left side of the World, too.

Back when I learned about threads and thread stresses,  it's true, the more wraps you have in a given length creating more cross section area at the pitch line, the more strength you will gain in the thread design.  It gets more technical than this.  I've seen threads destroy themselves by rolling over at the pitch line.  I've seen them shear out, the nut thread, at the major diameter of the screw and that one is neat.  I have one of them around here somewhere.  Of course, 99.9% of all of the threads I deal with daily, you won't find them listed in any standard book.  Most of them are static loaded, a few are dynamic or static loaded with torque.  Threads are interested when you get above the text book variety one's most people use everyday.

As Jim mentioned above, the nut is nearly 2" long.  Actually the thread on the screw is around 2" in length.  The nut has much longer thread in it from what recall on my vise. The nut is also cast iron, so it is kind of sort of self lubricating which helps in transmitting torque into force to clamp something in the vise.  The thread used is about 1-1/8" OD x 6 or 7 pitch.  It may even be metric.  I haven't measured one to verify that.  Again, from the number of turns of full engagement and crunching the numbers, you'll be surprised how much the thread will handle before it hits its yield limit in shear.  It will be greater that the screw cross section at root of the thread.   Got to remember, there is two areas that have to be calculated to determine which one is weaker in a male or female thread.

Ken


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## T Bredehoft (Apr 9, 2017)

4gsr said:


> creating more cross section area at the pitch line



4gsr almost has it. The finer thread, the larger the pitch diameter, thus a larger area to resist shear.


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## Redmech (Apr 10, 2017)

I'm a heavy equipment mechanic, we use a lot of 1 1/4 fine threaded rod for puller bolts. That puller rod Caterpillar says is rated at 60 ton, I'm 99% sure that's right. I'll look at my books in the morning. Very strong.


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## kd4gij (Apr 10, 2017)

I am pretty sure Kurt knows what screw is the best for the job.


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## 4GSR (Apr 10, 2017)

This is a little off topic.  Here's a sheared out thread on some composite material we did a test on a few years back.  Don't remember the load applied, it was in the 60K lbs./force range.  Ken


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## whitmore (Apr 11, 2017)

Silverbullet said:


> Wouldn't a buttress thread or square be stronger? Just asking.


There's more frictional torque in a Kurt vise when it's tightened, 
than if it had an Acme or square or buttress thread.
Probably you want that friction.   

My wood vise uses a buttress thread; the main benefit is that
the 'nut' is not expanded under load.  
In the wood vise (it's a quick-release type) the buttress thread makes
the half-nuts not disengage under load, because the surface pressure is
axial force, not the right direction to push the nut open.


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## whitmore (Apr 11, 2017)

4gsr said:


> This is a little off topic.  Here's a sheared out thread on some composite material we did a test on a few years back.  Don't remember the load applied, it was in the 60K lbs./force range.  Ken



That's typical of failures in dissimilar materials; the stretchiest one takes
no force on the back threads, so the front threads start to shear.   Then
the 'front' just moves along the total engaged distance.

I've also seen this on a bicycle rear (drive) hub, where there was more than
one 'standard' for the thread.   24 tpi(US, GB, Italy, etc.) and 1mm(French) 
are very close, and the penalty for guessing instead of measuring is ... as I recall, a lot
of time spent rebuilding wheels (and a few bucks on the alloy hub part,
which failed, of course).


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## 4GSR (Apr 11, 2017)

Here's my favorite one I pulled on a test.  The screw material is 110K psi minimum yield material and the nut material was about 35K psi class 40 gray iron.  I have 56K for the shear load it took to shear out the cast iron.  This was right at the minimum needed for the product.  I had to go in and increase the amount of thread engagement by four threads.  BTW, this was a 1.000-8 NC-2 thread.  Ken


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