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View Full Version : "T" nuts. Longer, or shorter?

Buckshot
05-23-2004, 06:15 AM
.......I bought a dual toolpost cross slide for my 11" Logan. It has 3 "T" post slots. The one closest to the operator is about 4" long, going across the bed. The other 2 are on the opposite end and run parallel to the bed, and are a bit over 3" long.

Using some 1x1 inch HRS I milled full length "T"s to fit into the 3 slots. However I began to wonder of thses full length T-nuts were the best deal. I mean, if you applied 100 lbs of force to the attaching bolt and the "T" was 4" long you'd in essence have the 100 lbs of force, but spread over more area.

OTOH, with a 1" sq "T" nut and the same 100 lbs of force you'd have it concentrated. I'm thinking the longer "T" nut would be the more likely to slide.

Is this right, or am I thinking too much :-)?

Thanks,
Rick

Ian B
05-23-2004, 06:24 AM
Rick,

My guess would be that it's only the part of the tee nut within 1/2" or so either side of the stud that's in contact with the underside face of the tee slot - I think (especially with there being a hole through for the stud) that the long nuts will flex.

Ian

Ian B
05-23-2004, 01:46 PM
Rick,

If you really want to stop the long tee nuts sliding (even when you slacken the bolt off), why not drill & tap a couple of holes through the tee nut, either side of the bolt and put Allen grub screws in to bear down on the bottom of the tee slot? Unlikely that the tee nut will move then.

For belt & braces, you could even drill a couple of dimples in where the grub screws bear down...

Ian

Paul Alciatore
05-24-2004, 02:01 AM
Rick,

My training in physics has taught me that the friction between two real, nominially flat surfaces in a real world situation is almost entirely proportional to the perpendicular force that is holding them in contact and almost completely independent of the area of that contact. I'm sure that some one here will dispute this and will have some example to prove his point, but it will almost certainly not be anything like your T nut situation.

It is therefore unlikely that the length of the T nut will have any influence on the slippage.

Another thing to consider is that, as Ian said, the real world T nut will flex under the stress and only the area near the stud will actually be in contact.

If sliding is a problem, a better solution would be to add additional studs and nuts. Or one long nut could have two or more holes for multiple studs. This may sound contradictory to my statement in the first paragraph but when you consider the fact that the long nut will flex, what you actually have is two or more separate areas, each of which having it's own area and clamping force so their frictional forces will add together.

OTOH, I have seen T nuts distort and crack from the stress of tightening. I do believe a longer design of a single hole T nut would help with this problem. I always make my T nut designs somewhat longer than standard practice.

Paul A.

winchman
05-24-2004, 02:45 AM

I'd be very careful about using additional screws which bear aganst the bottom of the T-slot. You can develop a lot of jacking force, which just might deform or crack a cast iron table.

Roger

sandman2234
05-24-2004, 10:28 AM
I agree that using jackbolts could do harm to the t-slots, by puttin too much pressure on them.
However, if you decide to go ahead with it, use some of those screws that have the plastic tips, so you will not marr the bottom of the t-slot. Frequent use of them would tend to make a lot of markings readily visible from the next buyers angle.
I still wouldn't use them.
David from jax

JCHannum
05-24-2004, 10:39 AM
Don't use jack screws, there is a good potential to damage the T slot by mis-directed pressure.
An adequately sized T nut will supply plenty of gripping force, as it is also pulling the toolholder down on the cross slide, and that friction is what holds things in place. Make the nut about twice as long as it is wide, and nothing will move under normal use.

Ian B
05-24-2004, 03:48 PM
I understand the caution being advised about putting jack screws through a tee nut; overtightening certainly could crack a tee slot.

However, is the force that this produces on the top lips of a tee slot any different from the common setup whereby you have nothing directly above the tee nut? I'm thinking about the clamping kits with step blocks, long studs and a bar bridging the step block and the work. One pushes, the other pulls, but it looks similar to me...

Ian

JCHannum
05-24-2004, 04:40 PM
Ian, with the setup you describe, for a mill, the force is put on the T slot, and if improperly applied, will pull the T slot out.
It is worth noting that the better T nuts for such set ups leave the last thread undersize or uncut so the stud will not pass through and act as a jack to further damage the T slot.
T slots on milling tables are usually heavier that those on lathes to take the added abuse.

Ian B
05-25-2004, 03:12 AM
JC,

Yes, you're right about the tee nuts not being tapped through - I found this with my Chaiwanese clamping set, and I thought they hadn't been properly tapped. Luckily, I read this somewhere else before trying to run taps through them all!

There's no doubt that, once tightened, a tee nut is unlikely to slide in its slot; my mention of the screws was to hold the tee nut in place when the main bolt is slackened off.

I have a few tee nuts for the mill like this, and find them a handy way of holding studs in place when setting up. It's much more controllable than screwing the stud through the tee nut, as with this method you have no idea of how much of the torque applied to the nut is being transmitted to the table.

I usually tighten the screws with the long end of an Allen key - no real force, but enough to stop the stud moving.

But it's good that you & others warn about breaking tee slots - I've never broken one, but I can imagine the sickening feeling when it happens.

Ian