What's wrong with T nuts? (compared to T Bolts)

Yeah, I like this too

I think ultimately, a T bolt, is a fool proof T nut, and a T nut with a damaged or blind thread is essensially a T bolt for all intents and purposes.

........... and an uncouth mechanically unsympathetic gorilla - is an uncouth mechanically unsympathetic gorilla 

Edited By Russ B on 27/03/2014 10:43:08

I seemed to have touched a raw nerve here! but it does seem that quite a few people have an abhorrence or aversion to T nuts. Thanks to those that have replied (and to MW for the PM)

My observations of the usage of T slots is that the force to break of the ears of the slot are the same where ever it comes from. Surely this can only be from a careless operator?

If something is clamped down to the table trapping the ears of the slot then they are not going to break off, but equally that particular fixing is mainly going to be used for lateral location so its no good for say, holding a vice down (the type with grooves on each side for clamping plates)

If I use a bolt or a length of studding in a T nut its very easily apparent when the thread bottoms. Even if a stud bottoms the actions of tightening the nut actually reduces pressure on the bottom of the slot.

I'm going to carry on using T nuts (well, and my two T bolts as well)

Ian P

Guess I was nit-picking. You are right, the geometric stiffness is higher, but I don't think the load will be higher, just reach a given load more quickly with fewer turns of the screw.

Regards,

Richard

Machine tables are usually made of cast iron which is a brittle material and will not stand excessive tensile forces such as those that can be generated quite easily by a stud going right through a T nut and bottoming in the slot and then having excessive force is applied. I have seen lots of machine tables damaged in this way over the years in industry due to ignorance and slap dash working methods.

Eric

When you fix any vice ,dividing head or other fixture to the machine table it is preferable to make sure it is clean and then to slide it about across the tee slots to feel that no more chips have been dislodged during handling. If you are satisfied that everything is smooth then it can have the studs and nuts fitted without lifting it and possibly dislodging some unnoticed chip. Clearly the use of tee bolts does not permit this when clamping through a hole or slot. Where locating tenons are used this check can't be used and chip marks are more likely to be imprinted on the table. So for all round use I prefer tee nuts and studs and have found threaded mild steel bar cut to suit quite adequate for the job. Colin

Hi
I've not seen much mention in this thread of bending stresses in cast iron. That is the usual failure mode of the T slots of a cast iron machine table. If your T nut or bolt is either damaged or distorted from previous use so that the inner edge of the T section touches the upper face of the T slot first you are going to produce a bending stress in the cast iron. Likewise if the T nut or bolt is significantly narrower than the T slot section you will again induce bending more readily than when using a full section width. (clear at the bottom, of course).
The main way of avoiding excessive bending stress in table clamping situations is to try to clamp through metal as with vices that have slots to take the bolt or stud and T nut. The other way is to avoid the bridging clamp type of fixing. It is this configuration that sets the middle tightening screw it tension and without any support on the table T slot this produces a bending stress in the cast iron and is when cracking and damage is most likely to occur, especially if there are long spanners or gorillas about. Sometimes its the only practical way of clamping and you may need more clamps and/or lighter cuts but it is important to keep your nuts in good order (bolt heads too)

Bob D

Only true if the stud doesn't contact the bottom of the slot. If it does, the jacking force can push everything (stud, T-nut, ear and clamped part) upwards breaking the ear in the process.

Going round in circles here.

There have been a good number of replies to this post, but I can't see a simple explanation of why it is more likely to damage the slot if a bolt is allowed to contact the bed through the T-nut. So here is my tuppence worth.

As an example think about clamping down the milling vice. In this case the T-nut is held against the underside of the slot as the bolt is turned. Directly above it is the frame of the vice, and tightening the bolt will clamp the bed and the vice together. Using even very heavy handed force will only clamp the bed and vice ever tighter together. In this situation there is no deformation of the slot, as the only force being applied is compression, and cast iron is incredibly strong in compression.

Damage could easily occur though if the bolt passes through the T-nut and contacts the bed. In this case the bolt and T-nut act together like a car jack, and will have the effect of forcing the T-nut upwards and away from the base of the slot. If enough force is applied - and it won't take much - the top edge of the slot will bend upwards, and is likely to break because cast iron is so weak in tension. And when a metal bends, tension occurs at the outside radius of the bend at the same time that compression occurs at the inside radius.

Hope this helps!

Steve

Edited By Stephen Murray 2 on 28/03/2014 00:32:08

Steve,

That is the best description of the risk that I have ever seen.

I shall continue to use my T-nuts, because I find them more convenient

... But I will remember your warning, as one more sanity-check for the list.

MichaelG.

G'day.

Following this thread with interest and it did strike me that the O.P. might have reversed the title to "What's wrong with T bolts" and gained the same answers. Given all that, I am still undecided as both systems have their merits....if used wisely at the appropriate time to suit the job in hand.

Anyway, at least I gained something from the debate as must admit I never considered the effect of offset clamping forces. Thank you all for that, .........sooooo much to learn.

Regards * Danny M *

Whilst I acknowledge the validity of the comments on perils of T nut studs creating a jacking action within the T slot I would have thought that use of overlarge adjustable spanners or even socket bars instead of suitably sized open ended or ring spanners would be a thing to avoid. Shorter tools are self limiting regarding torque. How tight does stuff have to be anyway.

regards Martin

When I fit a stud to a tee nut I always screw it in finger tight then unscrew a good half turn and would NEVER use it if the clamping nut is at all tight. When using clamps to hold a job the stud needs to be as near the job as possible and the packing as far away as the clamp allows to obtain the best mechanical advantage and therefore the maximum clamping force with the minimum necessary strain on the table slot. Colin

So many points of view and a lot of truth but some missing fundamentals.

T nuts are OK provided that you do not allow the stud or bolt to bottom out in the slot. Should this happen and you create a "screw jack" then if the nut or bolt is continued to be torqued up then yes you can easily break the slot away on the table as the force is applied to the under surface only....particularly on a Mini-mill. If you use T nuts then throw away the Gorilla in your arms. So if you use T nuts ensure that the stud/bolt dies not penetrate and come into contact with the bed.

T bolts do not act in the same way. They will only tighten when the nut and any clamping piece are in full contact at the end of the tightening process. Here is where the change occurs. You now are exerting equal compressive forces on opposing surfaces on the table slot. Under these conditions the forces cancel each other, and thus much more force can be applied with impunity before you crush the cast iron

So as always proceed with care and remember not to apply jacking forces to the underside of the T slot. Simples as the Meerkats say.

.

I too would like to congratulate Steven Murray 2 on his description of what is happening to the T slots: it accords exactly with both how I understand things and what I have read elsewhere.

I do think that there have been a number of people on this thread who have been presenting unnecessaryily highly technical reasons for this problem when all that is needed is a good dose of common sense as shown by Steve.

The answer is dead simple - do not allow anything to contact the bottom of the slot, and make sure that the clamping is a squeezing force, not a pulling/pushing force. It will not then matter if it is a T bolt or a T nut.

Regards,

Peter G. Shaw

Edited By Peter G. Shaw on 28/03/2014 10:23:33

I think this is score draw. Which ever type you use just take care and use your intelligence.

Eric

A simple solution is to 'neck' the thread of the bolt so that it breaks before too much pressure is applied and will therefor protect the table.

I never use high tensile material for holding down bolts.

Regards

I prefer T nuts because they distribute the load over a long distance, This is less likely to impose a concentrated load on the cast iron T slot which may break. I have never done so myself but have seen many T slotted machine tables with broken edges on the T slots. no doubt caused by pressure from clamping being excessive

You can mill a standard bolt head to fit in a T slot, and some will fit in as is, But the load is over a mall area, not good in my view.

You can buy professionally made T bolts with a forged head to replicate a T nut, however I have not seen them recently for sale . I have a couple "Unbreako" T headed bolts Like these.

**LINK**

T Nuts are easy to make, most of mine I have made myself out of mild steel, So for the reasons above I will stick with them. With use they tend to wear a bit allowing the nut to rock allowing it to self align with the slot if the threaded stud is not perfectly perpendicular to the table. It is easy to make a set of studs of various lengths.

I also have the ubiquitous Asian Machine clamp set you see everywhere to augment the drawer full of odd shaped hold downs and packers accumulated over time from various set ups.

Regards
john