Which thread for T nuts

I have always found the sizing of Tee nuts and slots rather confusing and can see why the subject in this thread is getting slightly complicated.

Whilst there may be DIN or other standards for slots and nuts, there are a lot of machine manufacturers who seem to make them to whatever dimensions they feel like, that fact in combination with the variety and age range of machines in home workshops means that one can only really be sure of the nut-in-slot fit by trial and error.

I have made an assortment of nuts of different lengths, number of holes and the thread sizes. Mill is smallish with slots 11mm wide so I have metric and imperial ranging from M10 to 1/4" UNF with few M4 and M3. 99% of the time I use M6 as I have lots of different length M6 capheads.

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.

As an aside, most of the nuts I make now have the top faces 'undercut' so that the pressure that the nut applies to the slot ears is concentrated at the widest part of the slot. My logic being that the pressure is being applied to the shortest 'lever' possible. The lever in this case being the cantilever that is the ear.

Ian P

I agree about the number of different sizes and accepted long ago that it was better to make them myself to fit the machine in question.

How do you “undercut” your T nuts Ian and would you care to share some pictures.

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.

I have used two methods to relieve the top faces of the nuts, One I used on existing nuts was by using an endmill smaller in diameter than the width of the top faces to cut a few thou deep recess either side of the centre leg. I made some nuts from new by tilting the cutter about 1 degree. It means the part of the nut that sits in the slot has tapered rather than parallel sides, not that it matters.

I suppose that it would be easy and quick just to create the relief with a file.

Pictures to follow

Ian P

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

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

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.

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.

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.

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

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

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....