Which thread for T nuts

[AdrianRParticipant @adrianr18614]

The mill I have has M10 T nut slots, but the mill is quite small. The table is only 150mm wide with three slots. M10 seems a bit of over kill and large, I am wondering if I should make M10 nuts but with M8 studs.

Any thoughts?

Adrian

[AdrianRParticipant @adrianr18614]

[Tim StevensParticipant @timstevens64731]

The nuts need to fill the slots with a little clearance for swarf, etc. Which studs you use – as long as they fit within the slots – is up to you, so M8 would be OK but not so rigid, should you need it, as M10. Or, if you like, use UNF or BSF threads – but if you have a variety of thread choices, stamp the size on the T-nut so you don't get cross threaded, literally

Cheers, Tim

[SillyOldDufferModerator @sillyoldduffer]

You could, but I'd make studs to suit whatever you happen to have that needs bolting down.

The purpose of the stud is to stop things moving about on the table so generally the fatter the better. If your milling vice can take 10mm, use 10mm. If it takes 8mm, go 8mm. It's not that smaller diameter studs aren't strong enough, it's the reduced grip of the washer and nut plus the extra opportunity for bendy leverage that makes movement more likely. I have two different sizes on my mill because not all my accessories can accommodate full-size bolts.

Dave

[Clive FosterParticipant @clivefoster55965]

Stick with M10 to hold the vice and, if you have or will have one, rotary table. Whilst on the subject of vices take a look at the recent thread on aligning vices and decide what method for rapid, adequately accurate alignment will suit you and your work best. For now. Of the quick and simple methods using M10 on special, fixed stud, Tee nuts would work with my pull back against the slots method. Going down to 8 mm would suit Vics version with an inverted Tee nut locating in the vice. Hafta say that if I were starting over I'd follow Vic version. Something better and more engineered comes later when you feel the need.

For general purpose Tee nuts on a machine of that size I'd go down to 6 mm, maybe even a few 4 mm ones too. The smaller sizes give you more options for passing studs through small jobs whlist still leaving room to machine things. Also let you use very short clamp bars. When I first had my little BCA the most used clamp bars were less than 2" long with integrated jack screws rather than the usual step blocks. Your relatively large slots take up a good deal of room on the small table. Consider making some clamp down straps with integrated 10 mm studs able to pass down into the slots. Often much more convenient than trying to bridge the slots so that ordinary step blocks can be used.

Something to consider in future is whether a grid of tapped holes plate will work better for you at holding things down than the Tee slot table. For example bought an aluminium breadboard with M6 tapped holes from Thor labs **LINK** to use with my, now long departed, BCA and found it well worth them money. Making would have been cheaper but I was relatively time poor and cash rich at the time. Tapping umpteen holes is boring too. Correct material for DIY would have been tooling plate which is relatlively expensive eating into DIY savings. If the size is right a double density version might be even better **LINK**. DIY means you can do what's perfect for you but it is more work.

Clive

[jason udallParticipant @jasonudall57142]

I have some tee nuts…long version longer to spread load..

 

If you made long nuts tapped say 10 in the middle and say 6 mm and 8 mm either side..thus , as has been said you have the flexibility to use the thinner studs…

 

Edited By jason udall on 09/05/2019 18:13:28

[JasonBModerator @jasonb]

Your table size falls between an X3 and a SX2.7, I've used M10 on my X3 for about 12 years and that suits the SX2.7 too.

On the odd occasion where I may need to get in close with a small cutter I have a few larger tee nuts threaded M6 so can use the old clamps etc from when I has a Unimat3

[AdrianRParticipant @adrianr18614]

Hmm, glad I asked, some dam good ideas here, it has got me thinking.

I have an old Abwood vice, it is about as wide as the table and 1/2 as long, very well built but wont take M10. So M8s for that a few other small sizes for specials and a full set in M10 sounds like a plan.

Multiple holes sounds good too

[Howard LewisParticipant @howardlewis46836]

You can produce quite an axial load with M6 nuts and studs (To my shame, I cracked a Cross Slide for a ML7 by overtightening 1/4 BSF nuts!)

M8 should be quite adequate. If the studs protrude through the nbottom of the Tee Nut, they may well mark the bottom of the slot. In an extreme situation, the top of the slot could be cracked.

My advice is to use studs, rather than setscrews or bolts.

I always adjust the studs so that they do not quite reach the bottom of the T Nut, and then centre punch them, around the edge, to stake them into place..

Howard

[steamdaveParticipant @steamdave]

GHT used 1/4" BSF threads in his Tee nuts on 3/8" clamping bars. This was on a Tom Senior mill.
If it was good enough for him, it would be good enough for most others, even converting to the metric equivalent!

Dave
The Emerald Isle

[Nick HulmeParticipant @nickhulme30114]

If used correctly the threads should be acting purely in tension, a good M8 bolt will exceed your requirements, to say anything else is to demonstrate a lamentable lack of basic engineering knowledge.

[VicParticipant @vic]

My box full of clamping components has both M8 and M10 sizes for flexibility. I did this because I already had lots of bolts and studding in both sizes.

[Anonymous]

Posted by Nick Hulme on 12/05/2019 17:12:38:

If used correctly the threads should be acting purely in tension……………….

The bolt might be in tension but the threads themselves will be in shear. For many materials the shear strength is greater than the tensile strength. That's why a bolt shank will break before stripping the thread.

Andrew

[Neil WyattModerator @neilwyatt]

I'm with Clive, I use M6, M8 and M10 on my X2.

Neil

[Robin GrahamParticipant @robingraham42208]

I've always assumed that the shear forces (ie the forces parallel to the plane of the bed) when milling were carried by friction between the workpiece, or vice, or whatever you clamp to it, and the bed. In which case the shear strength of the fastener is immaterial, except in the sense that Andrew mentions. Is that wrong?

I recall reading of Colin Chapman saying – ' you can hang a double decker bus from a quarter inch bolt'. If that's true, given that the coefficient of friction between steel and steel is about 0.1, I agree with Nick that M8 will be more than sufficient.

Robin.

Edited By Robin Graham on 12/05/2019 22:47:52

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Robin Graham on 12/05/2019 22:46:42:

I've always assumed that the shear forces (ie the forces parallel to the plane of the bed) when milling were carried by friction between the workpiece, or vice, or whatever you clamp to it, and the bed. In which case the shear strength of the fastener is immaterial, except in the sense that Andrew mentions. Is that wrong?

.

Your assumption is correct; and Andrew's very concise correction of Nick's statement is spot-on

MichaelG.

[John HaineParticipant @johnhaine32865]

It wasn't entirely clear from the OP, but I assume that the slots are 10mm wide – M10 is I think normally used as a thread description? An M10 stud would then need a 10mm clearance which would bring the crests uncomfortably close to the flanks of the nut. My Novamill has 10 mm wide tee slots, all the tee nuts that came with it and I have bought subsequently are M8. My VMB on the other hand has 12 mm wide slots, the standard 12 mm tee nuts are threaded M10.

[JasonBModerator @jasonb]

I think Adrian has a BMD-25 mill which is what Engineer's Toolroom called the X3 so 12mm slots often described as "slotted for M10."

[Nicholas FarrParticipant @nicholasfarr14254]

Posted by John Haine on 13/05/2019 09:15:32:

It wasn't entirely clear from the OP, but I assume that the slots are 10mm wide – M10 is I think normally used as a thread description? An M10 stud would then need a 10mm clearance which would bring the crests uncomfortably close to the flanks of the nut. My Novamill has 10 mm wide tee slots, all the tee nuts that came with it and I have bought subsequently are M8. My VMB on the other hand has 12 mm wide slots, the standard 12 mm tee nuts are threaded M10.

Hi John, so would you call a standard nut for an M10 bolt/stud, M17.

Regards Nick.

[John HaineParticipant @johnhaine32865]

Of course not! The M prefix refers to threads.

[AdrianRParticipant @adrianr18614]

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.

[Ian PParticipant @ianp]

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

[VicParticipant @vic]

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.

[Neil WyattModerator @neilwyatt]

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.

[Ian PParticipant @ianp]

Posted by Vic on 13/05/2019 15:31:08:

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.

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

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