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

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

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

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

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.

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

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

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.

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