Which thread for T nuts

[Neil WyattModerator @neilwyatt]

Posted by AdrianR on 13/05/2019 13:37:56:

Sorry the OP was confusing, I had already identified the slots as M10 as per Howard Hall's Data book i.e. 12mm wide etc.

I am impressed JasonB can remember the make, it is a BMD-16 from Engineers Toolroom.

I may just have to be pragmatic. Having forgotten to buy M10 studding in Doncaster and having 30m of M8 studding, I guess I will tap them M8.

Despite the assurance that bolts prefer to shear than strip, beware cheap plated studding, in the real world under tension it can stretch and then strip with consumate ease. Use stainless studding for clamps, its much less likely to give way.

Neil

[Ian PParticipant @ianp]

Posted by Neil Wyatt on 13/05/2019 15:54:08:

Posted by Ian P on 13/05/2019 14:58:47:

I have never quite understood the logic of the various methods of preventing a bolt or stud touching the bottom of the slot. Allegedly it to prevent the slot ears being broken off, as I see it the same damage would be the same whether the nut was being pushed from below or pulled upwards from above.

Two reasons:

The main risk if the thread bottoms out without the object clamping and you don't realise you might keep tightening trying to sget some grip and jack the nut up to break the slot.

For heavy clamping ideally the object being clamped pushes down to the t-slot so it is squeezed rather than just pulled up. When applying upwards-only force you need to be careful of over-tightening especially if shock loads can be expected.

If someone tightens a bolt/nut/stud with enough force to break off the slot lugs then I would say they were not using a force commensurate with the job in hand. One has to have some mechanical understanding and sympathy (or use a torque wrench all the time).

Squeezing the slot projections does give by far the strongest fastening but it is not always an option. The machine vice I use has grooves down each side so can only be held down with clamp plates.

Ian P

[Nigel Graham 2Participant @nigelgraham2]

I concur with Howard Lewis' warning – whether studs, set-screws or bolts (I chose to suit the task), the fastening must not bear on the floor of the slot.

The commercially-available clamping sets sold by various model-engineering suppliers seem to be 3/8" BSW rather than M10, but anyway their nut threads are staked to prevent the studs passing through them.

For the delicate T-slots on my EW lathe's vertical slide and boring-table I made full-length T-nut strips, rather than separate nuts, with 1/4"BSW threads.

[Paul KempParticipant @paulkemp46892]

If you are tapping your own tee nuts, use a taper tap and don't go all the way through. Easy way if you use all thread as the studs to stop the stud winding right through the nut, can keep the nuts nice and tight on the stud too if required.

Paul.

[JasonBModerator @jasonb]

Posted by Nigel Graham 2 on 16/05/2019 22:30:17:

The commercially-available clamping sets sold by various model-engineering suppliers seem to be 3/8" BSW rather than M10, but anyway their nut threads are staked to prevent the studs passing through them.

In my experience most sell both metric and imperial, the M10 sets will fit newer machines with 12mm slots and they sell the 3/8" ones for people using old iron with matching nuts for 1/2" slots. Other suppliers just sell metric sets.

The metric will fit both size slots with slightly less bearing surface but you won't get a 1/2" nut into a 12mm slot.

[Nicholas FarrParticipant @nicholasfarr14254]

Posted by Ian P on 13/05/2019 16:13:25:

Posted by Neil Wyatt on 13/05/2019 15:54:08:

Posted by Ian P on 13/05/2019 14:58:47:

I have never quite understood the logic of the various methods of preventing a bolt or stud touching the bottom of the slot. Allegedly it to prevent the slot ears being broken off, as I see it the same damage would be the same whether the nut was being pushed from below or pulled upwards from above.

Two reasons:

The main risk if the thread bottoms out without the object clamping and you don't realise you might keep tightening trying to sget some grip and jack the nut up to break the slot.

For heavy clamping ideally the object being clamped pushes down to the t-slot so it is squeezed rather than just pulled up. When applying upwards-only force you need to be careful of over-tightening especially if shock loads can be expected.

If someone tightens a bolt/nut/stud with enough force to break off the slot lugs then I would say they were not using a force commensurate with the job in hand. One has to have some mechanical understanding and sympathy (or use a torque wrench all the time).

Squeezing the slot projections does give by far the strongest fastening but it is not always an option. The machine vice I use has grooves down each side so can only be held down with clamp plates.

Ian P

Hi Ian P, it is the realisation that the bolt/stud is bottoming out that is the important point. You can get a situation where you are tightening the top nut on a stud and the stud turns in the T nut rather than the top nut turning on the stud. You may find that the clamping is still loose, so you give it a bit more and you may not feel the resistance that is capable to break cast iron slots. If this situation where the clamping is not being achieved, the stud will not yet be in full tension, but the bottom of the stud will be in compression, which can be greater than you realise.

Caution is better than a blunder.

Regards Nick.

Edited By Nicholas Farr on 17/05/2019 08:46:57

[Ian PParticipant @ianp]

Posted by Nicholas Farr on 17/05/2019 08:38:54:

Posted by Ian P on 13/05/2019 16:13:25:

If someone tightens a bolt/nut/stud with enough force to break off the slot lugs then I would say they were not using a force commensurate with the job in hand. One has to have some mechanical understanding and sympathy (or use a torque wrench all the time).

Squeezing the slot projections does give by far the strongest fastening but it is not always an option. The machine vice I use has grooves down each side so can only be held down with clamp plates.

Ian P

Hi Ian P, it is the realisation that the bolt/stud is bottoming out that is the important point. You can get a situation where you are tightening the top nut on a stud and the stud turns in the T nut rather than the top nut turning on the stud. You may find that the clamping is still loose, so you give it a bit more and you may not feel the resistance that is capable to break cast iron slots. If this situation where the clamping is not being achieved, the stud will not yet be in tension, but the bottom of the stud will be in compression, which can be greater than you realise.

Caution is better than a blunder.

Regards Nick.

I do follow the logic of preventing anything projecting out of the underside of the T nut. Personally I prefer the nuts fully tapped and rely on myself not damaging the slot.

One reason to fully thread (for me) is that I frequently use T nuts in places other than T slots when I am setting up a job on the mill, typically they are useful as plain nuts when using angle plates as they bridge the (sometimes disproportionally) wide slots that many cast examples seem to have.

Ian P

[not done it yetParticipant @notdoneityet]

In many instances I use a bolt instead of a nut, but combined with a nut. I can screw in the bolt until it stops, then retract it say three or four turns, apply a spanner to the bolt head, to prevent it turning, and tighten down with the nut. That way, there is no chance of breaking out the slot casting by that mistake while making sure the bolt is theaded as deep as practicable into the T nut threads. T nuts are staked, but better safe than sorry….

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