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Mommas Don’t Let Your Studs Jack Up Your T-Bolts!

Mar 4, 2017   //   by Bob Warfield   //   Blog, Manual, Techniques  //  10 Comments

With appologies to Waylon Jennings and his iconic, “Mommas, don’t let your babies grow up to be cowboys,” I present this bad day in the shop:

Broken T-Slot

Fixing a broken T-Slot is darned near impossible…

Fixing a broken T-Slot is darned near impossible.  Your best bet is to use it as an excuse to buy a fixture plate and just cover up that painful reminder.

Over torquing your T-Slot hardware can lead to this, but the most common way it happens is when the slot is being jacked instead of clamped.

With a proper arrangement, the table is squeezed between whatever you’re holding down and the T-Nut underneath:

Proper T-Slot Clamping

Proper use of the T-Slot squeezes it…

Proper use of the T-Slot squeezes it, and it is quite strong in that configuration.

But, if the stud extends out the bottom of the T-Nut, the forces are unbalanced and pretty soon all the force goes to jacking the T-Nut up:

T-Slot Jacked Up

If the stud goes out the bottom of the T-Nut and contacts the table, the forces change…

If the stud goes out the bottom of the T-Nut and contacts the table, the forces change, and now the slot is being jacked up.  The cast iron can take a LOT less force in that direction.

A good way to prevent it is to deform the last thread on the bottom of the T-Nut so you can’t screw the stud lower.

Don’t let this happen to your machine!

And no, before anyone asks, this didn’t happen to me, it happened to another fellow who reported it in an online forum.

 

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Mommas Don’t Let Your Studs Jack Up Your T-Bolts!
4.4 (88%) 5 votes

10 Comments

  • While your picture shows the correct use of a T nut, I think that these pictures show the more typical case of the wrong use of T nuts. The threaded rod is ripping the T nut out of the table, instead of squeezing.

    https://www.tormach.com/uploads/images/Gallery/products/workholding/Clamps_Hold-Downs/31855-T-Slot_Square-MG_8197.jpg

    https://i.ytimg.com/vi/3BGPSGsHk0A/maxresdefault.jpg

    • Roland, unless the rod protrudes through the bottom of the T-Nut and contacts the table below, there is no jacking effect. You can’t tell whether it protrudes or not in the photos you’ve linked. They could actually be perfectly fine uses of T-Nuts with no problem at all.

  • Bob,
    Just about all T-Nuts you buy have the last thread deformed and I hate them. If the bolt isn’t exactly the right length it screws up the thread on the bolt and then that bad thread transfers to the next use which could be a thread in a soft fixture. It’s far better to take a 6″ scale or a piece of banding and slide under the T-Nut/bolt and make sure it clears. That was basic Bridgeport 101 when I started doing this work 45 years ago. It seems like everybody wants to idiot proof everything these days instead of teaching people to think, which in my book causes complacency and far more problems than I use to see.

    • Mike, I tend to agree. The problem with idiot-proofing is Mother Nature will simply produce a better idiot. It’s an arms race you just can’t win, LOL.

      • Bob,
        I had a tow truck operator tell me one time that the real problem is that the bureaucracy passes laws to protects the idiots from themselves and now they are breeding and the population of idiots is growing exponentially, and then they are growing up and voting for idiots… and the rest is history in the makiong.

  • Do these two styles of clamp also introduce this danger?

    U-Clamps: https://www.machineryhouse.com.au/C083
    T-Slot mini clamp: https://www.machineryhouse.com.au/C0861

    • Frank, I can’t tell for sure, but looks like the U-Clamp could have the same problem but the T-Slot Mini Clamp maybe not.

      • I thought the mini clamp would have a problem, hence I haven’t purchased anything like that yet.

        Don’t they rely on forcing a screw down into the t-slot channel to force the body of the clamp up against the t-slot inside, like your second diagram? Once in place they then use a sideways force to hold work piece. If that slips there would be the temptation to tighten the screw so it doesn’t slide.

        • Frank, I couldn’t tell exactly how the mini-clamp worked, hence my “maybe”.

    • Frank, While I would never use the mini clamp I would think you would bend or break the allen wrench tightening the set screws before you would ever break your t-slot. The U-clamp would be difficult to over tighten because the flats on the studs don’t lend themselves to a lot of over tightening and the nut at the top shouldn’t be over tightened or it could pull up on table causing breakage but that is a human issue not a mechanical issue.. The big issue is making sure the stud is perpendicular to the table and the clamp forces you are putting on a very small area where the clamp meets the table if it is right over the T-slot which is how they show it. If you put a piece of flat stock under the lower part of clamp it will protect your table from dents and increase the area of the table absorbing the clamp force. If the stud isn’t perpendicular any movement in the clamp and it will loosen completely, so I use a fully threaded stud and a nut and washer on the table pulling up on the stud and then pivot the clamp instead of sliding it out of the way to remove part. Good luck.

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