|Ian P||27/03/2014 07:42:14|
2594 forum posts
Many times in the past I have seen dire warnings about the danger of T slot damage caused by bolts in the nuts hitting the bottom of the slot and producing enough force to break off the edges of the table slots. This subject has recently emerged again on a Boxford forum I visit and prompted me to seek clarification here.
What I don't understand is why anyone could think that the force required to break off the lugs is any different in the two situations.
Various suggestions are repeatably made such as, 'Deform the bottom thread of the nuts' (so bolts cannot project), but I have also seen, 'Never ever use T nuts'!
In my opinion T nuts are far more useful and versatile than bolts. If somebody told me I could only use bolts I would be seriously hampered.
|Mike Clarke||27/03/2014 08:20:08|
94 forum posts
I always finger-tighten the stud and then remove the T-nut from the slot and check there is plenty of unused thread. I made my own t-nuts - some suggested damaging the bottom of the threaded hole to automatically prevent the stud being screwed in too far, but I don't think I'd like to rely on that.
As for the tee-bolt - I guess they avoid the top of the slot being physically jacked up from the base of the slot? And when clamping surfaces to the mill table, it is guaranteed to be simply clamping.
6382 forum posts
A screw thread can produce enormouse force so it only takes one half wit in a commercial workshop with a big spanner to misuse it and they can break the machine. Hence the reputation is born.
|Les Jones 1||27/03/2014 08:44:06|
|2261 forum posts|
23058 forum posts
Like Ian I can't see the difference in a bolt jacking the nut up from below or overtightening the nut on the top of the clamp, as far as I can see both will exert the same force on the tee nut unless somebody can explain why bottoming the thread puts more upward force on the slot.
943 forum posts
I think it is generally assumed that the work/tooling will be immediately above the T nut/bolt when tightened. That way as the thread is tightened, the thread axial load pushes the work down onto the edges of the T slot and reacts the upward pull of the bolt underneath. This does not happen when the nut is jacked up from the bottom of the slot.
The obvious flaw with the 'general assumption' is that you don't always have the bolt underneath some work or tooling that can immediately react this load (as when you are using clamps), so the load is working against the edges of the T slot in exactly the way as happens when you jack up a T nut.
So, I agree with Ian in his original post.
|2002 forum posts|
I can see the concern that with a T-Nut the bolt might be forced through the bottom and jack everything up - whereas a T-Bolt cannot do this. I guess it could happen - although never to me.
My understanding has always been that "best practice" is to make sure that the T-Nut/Bolt actually fits the T-Slot well and (where possible) is also supported on the top of the T-Slot. This clearly happens where (for instance) a flat block is bolted directly to the table and the base of it directly covers the top of the slot. However, it is also possible to clamp indirectly by the use of an auxiliary table or (for instance) the rather nice 1-2-3 blocks I got for Christmas (still finding different uses for them JS!). Both of these will help support the top of the T-Slot and have the added benefit of preventing distortion on a slotted lathe cross-slide for instance
However, I think the best practical advice is to use as many clamps as possible, don't over-tighten them and rather than go heavy with the clamps - just take a lighter cut. If you are worried enough to want to really tighten up your T-bolts/nuts - then perhaps it's time to stop and re-think your clamping arrangements.
|Oompa Lumpa||27/03/2014 09:27:00|
|888 forum posts|
Just proves the old adage, common sense isn't.
I put people into two classes (shallow I know) :- Mechanically Sympathetic people and everybody else. I firmly believe that mechanically sympathetic people have a far higher understanding of the world around them.
Not on topic and Philosophical for nine thirty in the morning, but hey, I feel philosophical today.
|Phil H 1||27/03/2014 09:29:01|
|128 forum posts|
I think I can see part of the concern.
I can imagine a 'bottomed' stud being tightened into the table causing stress round the slot whilst for example, the component is still loose because of a sticking or bottomed top nut (perhaps the top nut has been run down the thread onto the plain part of the stud?). The temptation - if you are half asleep - might be to keep turning the spanner without clamping the component putting more and more stress on the slot.
I think I need a picture but that might be the concern.
|Mike Clarke||27/03/2014 09:33:23|
94 forum posts
If you put a bolt in a tee nut in a slot and tighten it - as an example - surely that is trying to force the nut through the top of the table. That's what I imagine the concern to be anyway (when something is in contact with the table directly above, i.e. vice, rotary table, fixture etc).
Edited By Mike Clarke on 27/03/2014 09:36:44
|Nicholas Farr||27/03/2014 09:37:47|
3421 forum posts
Hi, a couple of things to consider are that while you are tightening the studs in a T nut, it can turn and bottom out in the T slot and you will have the clamping force of the work you are holding plus the jacking force of the stud bottoming out, so you can have two independant forces being applied to the same area of the slot.
The other thing about tightening bolts or studs is that they in them selves have and elastic range in thier use and of course if you get heavy handed with the spanner damage can occur. When a stud is in a jacking situation, the stud does not compress to the same extent as a stud will stretch and therefore the force can be much greater.
The same thing can happen when studs are allowed the bottom out into a blind threaded hole although the tightening torque may not have been reached.
Edited By Nicholas Farr on 27/03/2014 09:42:13
|Russ B||27/03/2014 09:37:57|
|615 forum posts|
Surely this is a lesson in good practice - I'd never jack my T nut up using the bottom of the slot, and my T nuts, both made and bought all have 2-3mm clearance.
I screw the stud in, then I lift up and down to make sure I still have 2-3 movement, then I crack on with the clamping.
If the Nut is tight on the stud so much it can't be held by hand, then stud or the nut's threads are damaged, bent or stretched etc. etc. and go straight in the scrap bin to be reborn another day.
- I would expect the whole clamp assembly to be tightened finger tight, then receive a 1/2 to 1 turn to tighten, so even if it did tighten completely in the T nut, it still wouldn't stick out more than the 2-3mm gap under the T nut.
|dave greenham||27/03/2014 09:43:24|
|100 forum posts|
Surely the simple answer is to make your own studs
but only thread the bottom so far ensuring that it doesn't protrude
through the bottom. The bare part of the stud stops it going through
so the problem doesn't arise .
|Russell Eberhardt||27/03/2014 09:45:49|
2751 forum posts
I just make my T-nuts with a thread going about 2/3 of the way through. Having said that I have never had any problems with commercial ones but then I am no gorrilla and I use short spanners
6382 forum posts
I believe that when Whitworth defined his standard threads he also defined standard ie maximum lengths for the spanners to reduce the ability of monkeys to overtighten them.
I have some T bolts in my scrap bin, half inch I think, that have the treads distorted to sawtooth shape at the point where the nut was.
|jason udall||27/03/2014 09:57:49|
|2031 forum posts|
|Might I suggest the difference is this..|
Given that the protrusion of stud is short relative to the length above the table and that the "tensile modulus" of said stud is less than the compressive "modulas"..
It means the jack situation is much stronger than the pull ...consequently there is more force available ( before yield in the stud) when your tame gorilla swings on the spanner.
In production ws I see torque wrenches uses specifically for tightening ..
|jason udall||27/03/2014 10:14:45|
|2031 forum posts|
|What I am trying to say is..the give is the stud in tension is much larger than in compression. .thus "feel" is much less forgiving in the jack situation than the pull|
|Rick Kirkland 1||27/03/2014 10:24:52|
175 forum posts
Oompa lumpa! Right on! Yeahhh!
|Gordon W||27/03/2014 10:27:10|
|2011 forum posts|
Two things to consider- Stud could jack up the nut and break the table slot, If a bolt is used in T nut there is no way to tell if the work is clamped or just bottoming out. This was hammered into me when a youth. Do not use bolts.
943 forum posts
I remain unconvinced by this. If it were the case, surely tensile & compressive moduli would be quoted for steels. They are not, but in many composite materials, tensile & compressive moduli (as well as strengths) are quoted because the materials have radically different values. In tests on my degree course at university we established that the E for the steels we were given (EN24 & EN16) was the same in compression as it was in tension to better than the experimental error of 1%. Quite why we needed to prove this at the time, I have no idea, and this is the first time since (30 odd years) I've ever had the chance to refer to it.
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