T-Slot Torque Specs?
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  1. #1
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    Default T-Slot Torque Specs?

    Anybody have a general chart listing the tightening specs for cast iron T-Slots? If a hold-down kit uses Grade 8 studs and nuts, and hardened clamps, etc. I assume that would make the table itself the weak link...

    I'm in the process of re-cutting some teeth on a large gear and I'm getting movement/slippage of the part on the table. For this job I'd like to get close to maximum clamping force so I don't have rebraze again, but I REALLY don't want to crack-out my rotary table.

    In the absence of a full chart, if anybody has any idea what a 9/16" table slot can safely handle in terms of in/lbs or ft/lbs, that would help as well.

    Thanks!

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    Take lighter cuts!
    Way too many variables to make a realistic call here .
    Increase the friction between the part and the table...Soft sheet aluminum between the part and table. Make a fixture that locates the part directly....
    Cheers Ross

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    Or get the t-slot nut right up to the part, so a lot of the force is just compression on the slot material.

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    Are you sure you're not bottoming out on the stud threads? Limiting clamping force.

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    Thanks for all the replies!

    Here's the process I'm using: I have a center stud through the center of the gear and rotab fixed to the mill table, but obviously I need to loosen that to rotate the platter - hence using three strap clamps with step blocks around the perimeter of the gear as well to maintain my indexing during rotation (the gear and table are mounted horizontally). Basically, I'm fixing and centering the part with the center stud, tightening the perimeter strap clamps in steps, loosening the center to rotate and index each tooth, and then tightening the center when I make my cut. I checked and I have plenty of thread under my nuts. Part of the problem is the gear is 16" across and the bearing surface on the table is maybe 5" across, so there's a lot of potential rotational leverage on the part if the cutter exerts any side-load. Unfortunately, this job is getting close to the dimensional limits of my rotab and mill so I don't have a lot of room to add height or change the orientation of the part.

    My main concern is that I don't want to loose any potential clamping force that may be had by being overly timid, but I also don't want to go ape**** and potentially damage a T-slot. I looked through all of my machining texts and and manuals, and couldn't find any torque specs, which seemed like an odd omission. Going easy on the cuts is good advise that I will certainly take and I'll see if I can rig some kind positive rotational fixation as well.

    Thanks again!

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    Can you post a pic - it might help us come up with a few ideas.

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    If the rotab has a smooth ground surface, then either lay down a sheet of thin paper first, or rough up the surface a bit. Either will give more "grip" for a given force down. With the stud and nut right up to the gear, you should be able to get a good deal of force safely, as well.

    The table on one of my mills is darn near poplished,,, things like to slide. Paper takes care of that well, and dimensions are still good if paper is thin

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    Do I understand the gear is 16" OD and the rotary table is 5" OD?

    Please attach pictures.

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    I like the paper idea. I'll give that a shot. Here are some pics I took during my intial set-up work for clarity. Unfortunately, I didn't take any pics of the strap clamp set-up and I won't be at the shop for a few days. The gear is a 16" back gear for a 20" lathe, the mill is a Cinci #3 Universal, and the rotab is a 21" Troyke.

    20200101_163630.jpg
    Attachment 276221
    Attached Thumbnails Attached Thumbnails 20200106_154719.jpg  

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    You don't have much friction area. I'd be tempted to put in some blocks against that "spigot" to help hold it, or flip it and set it on a larger ring.

    But, all it has to do is work for a couple cuts, by the look of it.

    You can also clamp it anywhere else away from the repair area.... that might be good also, especially with a ring under it, or 3 or 4 blocks. Could be interesting with the small outside area for the clamps, but I think a little fab would get a slam-dunk solid setup..

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    I tried it flipped, but I didn't have enough clearance between the rotab table and cutter. Unfortunately, I didn't have 3 matched riser blocks tall enough.

    Between lighter starting cuts, the paper trick, and fabbing-up some blocks/jacks, I think I should be good. Thanks again for all the ideas!

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    A piece of machined thick walled aluminium pipe as big as you can get that will mate with a machined surface on the gear as a spacer and tightened down the gear to it might be a simple solution.You appear to have a machined surface near the centre that you have a clamp on, flip the gear over & use that surface against the surface of the aluminium pipe to support the gear & pull down tight. You would only have to face off the pipe in the lathe to the right spec to give the required height. My local aluminium supplier would cut me off a 2"/50mm piece & only charges by the weight plus a small cutting fee.
    Last edited by Cdatar; 01-30-2020 at 08:57 PM.

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    T-slots are designed to withstand the upper-limit torque of the T-bolt that was designed to fit into it. Grade 8, 9/16-12 thd, torque(dry) 154 ft-lbs, clamping load 16,388 lbs.

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    What JST said- flip it over and support on the rim. The hub contact area VS the rim contact area is what you need to take advantage of, plus the lever arm from where you are cutting to the hub is what's killing you.

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    Quote Originally Posted by Citxmech View Post
    I like the paper idea. I'll give that a shot. Here are some pics I took during my intial set-up work for clarity. Unfortunately, I didn't take any pics of the strap clamp set-up and I won't be at the shop for a few days. The gear is a 16" back gear for a 20" lathe, the mill is a Cinci #3 Universal, and the rotab is a 21" Troyke.

    20200101_163630.jpg
    Attachment 276221
    Make some standoffs to fit under the machined gear or if possible lock a lathe three or four jaw chuck and hold gear that way,

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    It looks like your problem is a small clamping radius, and a large working radius. The cutter has lots of leverage over your clamps.

    Make some supports so you can clamp on the rim of the gear. They don't have to be fancy. They can include a tapped hole and use a shcs or flange nut to support the part. You set them with a dial indicator after the gear is clamped. Put a couple of thousands of movement in the part with the support stud, and bring the part back to zero with the clamp. If the clamp doesn't feel tight enough, put a little more movement in the part with the support stud.

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    Several posters have stated the case exactly. You need to move the surface resisting twisting further from the axis of the gear. All the stud (or multiple studs) do in clamping work is increase friction by increasing downforce. With a ring or blocks supporting the gear closer to the periphery much less force from the stud will be required and "good and snug" should be tight enough.

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    Quote Originally Posted by Dank View Post
    T-slots are designed to withstand the upper-limit torque of the T-bolt that was designed to fit into it. Grade 8, 9/16-12 thd, torque(dry) 154 ft-lbs, clamping load 16,388 lbs.
    .
    most of the time the hold down strap clamps will permanently bend well before you reach max stud torque in my experience. usually you use more (many) than minimum amount of straps and stud/tee nuts for heavy loads and many use jam nuts to try for more vibration resistance.
    .
    basically I might tighten to 5 to 80 ft/lbs which is well under stud max torque. i have seen many a hold down strap permanently bent on excessive torque.
    .
    other thing to worry about grade 8 studs when bent have a habit of breaking with little warning when under load. hard to describe other than it doesn't stretch as much or as ductile as much as grade 2 hardware

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    for lathe tee slots it is recommended to take a hair off the tee nut in the middle 1/3. This means all the force is further inside the lip with no force on the unsupported lip edge.
    Bill D

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