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

[jason udallParticipant @jasonudall57142]

Added to that
.in the jacking situation
1 all that force doesn’t improve clamping..
2 when ADDED to the pull from above means a stud that couldn’t break the slot alone sees potentially double that force which might exceed the slots yield strength.

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

‘Stiffness’ of bolts/studs is not much relevance to breaking of T slot edges .

I agree that untrained monkey syndrome is reponsible for most breakages .

On the more general subject of bolts though there is some interest in what Jason U mentioned .

Any stack of engineering components held together with bolts is a system of springs and what happens within the components under load is not always obvious at first glance .

In case of a car engine with cylinder head and gasket bolted down to engine block the most bizarre things can occur .

With the normal use of long studs done up tightly the load in the studs can remain the same , increase or sometimes even drop when the cylinders are pressurised during firing stroke .

Lots of experimental work had to be done to solve problems with very long studs doing weird things on radial aeroengines .

Food for thought .

Michael Williams

Edited By MICHAEL WILLIAMS on 27/03/2014 14:24:59

[LambtonParticipant @lambton]

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

[colin hawesParticipant @colinhawes85982]

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

[jason udallParticipant @jasonudall57142]

I take from this. Don’t allow your bolts/studs to bottom in the tee slot.
Don’t leave scarf/chips between mating sufaces when clamping
Avoid unsupported pulls on table..where clamp or stock help prevent pull out.

Let me add…tee nut bar…over long with regard to normal tee nuts..spread any loads over more tee slot….

[Robert DoddsParticipant @robertdodds43397]

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

[Gone AwayParticipant @goneaway]

Posted by Ian Phillips on 27/03/2014 12:27:56:

If something is clamped down to the table trapping the ears of the slot then they are not going to break off

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.

[SteveMParticipant @stevem36008]

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

[Michael GilliganParticipant @michaelgilligan61133]

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.

[Danny M2ZParticipant @dannym2z]

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 *

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

In 61 years and about 40 years in industry I have seen zero T slot breakages caused directly by ‘wrong’ choice of T nut/bolt .

Breakages occur when people don’t take proper care .

Much more common causes of T slot breakages are :

(1) Crushing the T slot lips by overtightening .

This is a common problem on older and some hobby lathes where the T slot lip isn’t thick enough .

(2) Distorting the T slot lips by using poorly shaped T heads/nuts .

(3) Distorting the T slot lips by having large pieces of swarf trapped in .

(4) From using inadequate numbers of clamping points or inadequately tightening down .

Either means that a kick or dig in when machining can impact a chunk out of T slot lips .

Lots more .

Engineering is both very simple and very complex – art is to wade through it all and decide what is relevant .

Regards ,

Michael Williams .

[Martin KyteParticipant @martinkyte99762]

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

[colin hawesParticipant @colinhawes85982]

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

[MadMikeParticipant @madmike]

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.

.

[Peter G. ShawParticipant @peterg-shaw75338]

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

[LambtonParticipant @lambton]

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

Eric

[IDPParticipant @idp]

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

[John McNamaraParticipant @johnmcnamara74883]

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

[Jerry WrayParticipant @jerrywray14030]

You could always give a British manufacturer your business. Especially as they are local to me!

Look at the range they produce **LINK**

JerryNotts

[Gone AwayParticipant @goneaway]

Posted by John McNamara on 28/03/2014 12:08:27:

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

 

KBC have various kinds of Teco T-slot bolts that are not bad.

 

Edit: Ooops … sorry John, I thought you were in Canada – must be thinking of another John. I'll leave the link though for anyone that's interested.

Edited By OMG on 28/03/2014 16:07:53

[Rick Kirkland 1Participant @rickkirkland1]

Am I really reading this? Neck the bolts so they break! Whhaaat? I refer the forum to yesterdays comments made by the honourable Oompa Lumpa. I rest my case your honour.

Rick.

PS, if it was a joke then ha ha hee hee, thats a good one!

[Nigel McBurney 1Participant @nigelmcburney1]

When I started work as an apprentice,1958, practice in our works was to use T nuts and their use was general in industry, the threads were were Whitworth 3/8 for small machines ,1/2 inch for medium machines, and I never saw a T slot broken as quite frankly we were trained in the use of machines and did not pick things up as we went along.In larger organisations trained tool setters set up machines for the monkeys just to wind handles and fiddle the bonus.

Where I worked ,a small company ,the owner kept an eye on things and was very particular about good workshop practice,He would not allow the use of long spanners,did not like studs and nuts on mills and bolts were used with t nuts on all machines. The whit thread is coarse so the jacking force is smaller than modern metric threads ,and the risk to table t slots was less, to select a bolt for a vice you just put the vice on the table making sure there is no swarf trapped then position the hole/slot in the vice base over the T slot and select a bolt that will hold the vice securely and not touch the bottom of the slot if it was close to the bottom then place a washer under the bolt head,then slide a t nut under the vice and bolt it down, the same method to select bolt length can also be used on clamps higher than the table which span the workpiece and a suitable block. If Tee bolts are used say to hold down a large vice ,then the vice has to be lifted up high and fiddled over the bolts with the risk of trapping fingers and trapping a bit of swarf between vice and table resulting in a ding in the vice and table,far easier with tee nuts. On small machines ie Myford cross slide ,to reduce the risk of damage long tee nuts can be made the full length of tee slot in the cross lide. with a row of tapped holes in the nut.

When I see photos on the web of various owners set up on milling machines ,its amazing how many have left spanners ,vice handles ,spare clamps on the machine table, our shop had plywood boards about 10 inches square which sat on one end of the table and and any tools in use had to be left on the board, What is the point in having precision machine flat tables and then putting rough tools on them which over period put dings and burrs on the table and so risk the accuracy of the work,

One point back to clamps, when holding work on the table if the work moves under the side thrust of the cutter,do not over tighten the clamps,and risk the tee slots or distort the table,use thrust blocks to take the sideways thrust,the block is clamped via bolt and tee nut, the face in contact with the table must be flat and smooth to avoid spoiling the table surface and place a piece of paper between table and block to increase the friction and stop the block slipping.

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