T-Slot Size vs Clamp Size?

[Susquatch]

Just for info only, mostly for the benefit of anyone who likes to gronk on t-bolts, here is a number to remember.

Very roughly, it takes just 10 ft-lbs of torque on a 1/2 inch bolt to generate over 1000lbs of tensile clamping force. The actual number depends on the thread quality, pitch, and condition, and the presence of grease/oil on the threads.

To visualize this, think of 1000 plus pounds of force pushing your vise against the table at each T-Bolt. Two 1/2 inch T-Bolts is over a ton of force. That vise isn't going anywhere. Yes, bolts are very strong and can apply huge amounts of force. We often underestimate just how much force we can generate through the magic of thread leverage. As Archimedes said, "if you give me a lever and a place to stand, I can move the world!"

What happens when you gronk on the bolt? 100 ft-lbs is quite easy to apply. But 100 ft-lbs generates well over 10,000 pounds of compression force per bolt. It becomes very easy to see why non supported loads or bottomed bolts can so easily break a T-slot.

My main message here is that you don't have to gronk on those bolts to secure a vise (or anything else) to the table.

If you have a fully contiguous compressed column of metal without any gaps, you can freely add more torque, but it isn't needed. For most situations, a 10 pound pull on a foot long wrench is plenty.

 

[mbond]

The hazard of bottomed bolts is probably a load applied by that bolt between the bottom of the T slot and the underside of the slot based on the useless screw pressure? Useless because there is no overburden (useful clamping) on exactly that spot on the bed. The overburden (useful clamping) happens at a distance - where the metal of the bed will resist it, but the metal of the T slot doesn't.

Another method to avoid that is to have a very short length of thread cut into the bolt

 

[CWret]

Another method to avoid that is to have a very short length of thread cut into the bolt

Good solution, i like it - most bar clamp kits come with rods threaded on both ends. Simply shortening one end would be easy.

 

[Susquatch]

The hazard of bottomed bolts is probably a load applied by that bolt between the bottom of the T slot and the underside of the slot based on the useless screw pressure?

I think the big risk for bottomed bolts comes from the fact that it even happens with a proper compression stack but isn't noticed until "SNAP".....and it's too late.

Confession time.

Personally, I prefer fully threaded T-Bolts and Nuts. Usually I buy them that way, but I have threaded a few that had not been threaded through. I don't recommend that practice to others. But for myself, I prefer the rigour of checking the bolt length each time. It isn't hard to do and doing so builds good habits. I'd rather depend on good habits and knowledge than a safety net that could fail someday when I happened to use a fully threaded part that was too long and I didn't notice it.

Confession over.

 

[Dabbler]

@Mcgyver there is an alternate method for staking T nuts I have used in the past. It hasn't failed me yet....

Place the T nut in a good vise with the bottom (to be staked) facing up.

Using a cold chisel (I use a 1/4" one for my 1/2 NC nuts) and radially stake the last thread at about a 45 degree angle. I do this about 6 times round the last thread.

Thjis way the threaded rod or stud gets almost full engagement, then hits a hard stop that cannot be overcome by (moderate?) torque alone. It does not seem to harm the leade on the thread.

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I agree totally with @Susquatch comments regarding torque versus hold. Most people gronk too hard on the wrench when tightening down on hold downs, etc, Better to use an extra hold down if you are squeamish.

 

[CWret]

Dreaded tear-out on mill table:

Here’s another method to avoid bottoming out the clamping bolt (or stud) on the T-nut slot (other than staking the bottom of the T-nut or using a bolt with short threads or carful measuring)

My mill vise hold down clamps are used for the vise only. The clamps have a dedicated bolt which was shortened such that it fully engages all of the T-nut threads but cannot reach the bottom of the mill T-slot. This is pictured (referenced) in my post #18 above but it was not obvious nor had I mentioned that the bolt lengths had been modified.
Here’s a couple pics showing the mill vise clamps with the modified bolts.


(To make sure they don’t get separated and when I get aroundtoit I’ll paint the bolt, T-nut and L clamp)

 

[Dabbler]

@CWret I store hold down hardware, including dedicated T nuts, clamped in the vise jaws when storing them. (I lose things too easily if I don't)

 

[Susquatch]

Here’s another method to avoid bottoming out the clamping bolt (or stud) on the T-nut slot (other than staking the bottom of the T-nut or using a bolt with short threads or carful measuring)

I've already told you I like what you did. But for the benefit of your readers, let me add that the relieved ears of your T-Nut place the load that the T-slots see in total compression at both ends - the free end and the end under the vise. This is an ideal outcome of your design. Without those ears (or feet) your tie down bolts could compromise your vise hold down system.



This is a problem that all of us with modular vises like the Gerardi must face because they don't have columnar mounting ears. Your solution is very well thought out and well executed.

Since I hate painting, I love @Dabbler's idea of a baggie. Zip lock baggies are cheap.

 

[ChazzC]

@Mcgyver there is an alternate method for staking T nuts I have used in the past. It hasn't failed me yet....

Place the T nut in a good vise with the bottom (to be staked) facing up.

Using a cold chisel (I use a 1/4" one for my 1/2 NC nuts) and radially stake the last thread at about a 45 degree angle. I do this about 6 times round the last thread.

And I thought I came up with that, even down to using a 1/4” cold chisel! Although I only stake in two locations so I’m always pushing against a vise jaw: I always screw the stud in by hand so that seems adequate. Oh well, great minds, etc.