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Necessary precision in T-nut slots?

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  • Necessary precision in T-nut slots?

    Just got myself a Grizzly 10x22 lathe (a huge step up from my MicroMark 7x14!), and the picture on line showed T-nut slots in the cross slide. This brought up ideas for a milling fixture and custom tooling.

    My cross slide is smooth.

    The problem I have is that my mill is a mini, and after its rebuild has a slight angle just enough that an end mill leaves a ridge at the back of the cut, when it's trammed neatly on the X axis. I haven't yet figured if its origin is the Z-axis mount, or in the headstock, or spindle. Also, while turning cast iron may be an acceptable if messy endeavor, I've never milled it. Heck, I'm not sure if my mill is up to it.

    So, in general, how important is it that T slots be smooth and precise? If I'm not sure on that by the time it comes around, I'd rather send the slide off to someone here than make a messy job of it myself.

  • #2
    If you plan to use the T slots for accurate locating of fixtures, etc they need to be precise. That is why most turret mills (and other machines) have on the money 5/8 wide (or other dimension) T slots. This is also necessary to keep the fixture from moving during machining operations. Relying on just bolts or studs is just asking for fixture shifting. You really need to have pins or keys to keep them in place.
    Last edited by Dr Stan; 03-16-2011, 11:38 PM.

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    • #3
      Looking at the t-slots in the production cross slide for my Logan, I'd say they don't need to be anything special. The production cross slide is a lot thicker than the standard cross slide though.
      (No guarantee there isn't a big chunk of sand in the middle of the casting)
      Last edited by dfw5914; 03-16-2011, 11:44 PM.

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      • #4
        FWIW my money is backing Dr Stan on this one. I recently got some T-nuts for my mill but they were too large. I ground them down on the surface grinder to spot on 11.10 mm and they slit perfectly in my T-slots but bound in one area. The slots must have had some very slight burrs in that area, as I stoned the slot lightly with a diamond stone (I couldn't even tell where material had been removed) and now they slide perfectly. Just how important that is I couldn't say, but Stan's comments certainly make sense. I made new keys for my vice which are also a firm fit in the slots and it relocates perfectly. There's no doubt the slots in this machine have been very accurately machined, and I guess they did that for good reason.

        Pete

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        • #5
          misread OP, sorry

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          • #6
            I think it's important that the width of the slots at the top is even for the whole length. If you use the slot to key anything, this is the only way it will work with a fair degree of precision. The underside of the lips should be smooth at least, for the sake of the nut or whatever that will be pulled up to it when something is bolted down to the table. It would be nice if the thickness of the lips was uniform as well, although any discrepancies will be well within the range of the clamping bolts. The rest of the t-slot doesn't much matter.

            Deburring the lip corners inside the slot is a good idea. From time to time you'll have your fingers in there (if the slots are that wide) to pick out bits, and you don't need to draw blood while your fingers are coated with cutting fluid and all manner of swarf.

            On my mill, I filed the faces of the center t-slot to make the gap equal from end to end, and make sure they are straight. That's the slot I use when I need to key something for a precisely oriented mounting. After the slot was 'blueprinted', I made some keys to fit. I didn't worry about the other slots because they're less likely to be used to align something, plus that would mean that not only would the sides have to be made straight, the widths of all the slots would have to be made exactly the same.

            By the way, any filing I do on a t-slot or other machined part is done with the file held flat to the surface, and with fingertip pressure applied to the file over the center of the surface being worked on. I'm really just leveling the surface, not taking off any real meat. Knocking off the high spots, basically, and not rounding over anything. If it doesn't clean up fairly quickly like that, it needs re-machining.

            T-slot nuts- I've done this two ways. One is a square or rectangular piece of flat bar sized to fit, then drilled and tapped. The other is a strip of suitable sized bar, in which case the drilled and tapped holes are appropriately spaced for the particular fixture being mounted. The strip gives less hooking and sticking as you move it along the slot. I haven't given up on the inverted t-shaped nut, but I haven't made or used one like that for some time now. You can't rely on the upper part of a nut like that to be used as a key, and the additional threads it gives you- well I don't think you really need it
            I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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            • #7
              Alright, just went out and looked at it, and discovered that any room for the slots would make for 5/16" or even 1/4" nuts and the slot would have to be deep--it would sit in the outer edge of the dovetail. I just don't trust the itty-bitty clamping bolts those would use, under lathe forces.

              But I do have plenty room for a series of tapped holes, just would need to decide which threads to use. I even think my mill would be happier with that, and may not need re-tramming as badly.

              I will still shim the Z-axis base and check how that helps, though. Anyone know how to check where the problem is on a mini-mill, when it's just a thumb-noticeable difference between the high spot of one 1/2" pass and the low of another?

              Anyway, as least the holes would give me a clamping solution, and I can do some pins later if I need the precision. I think I'll need the clamping soon, too. I just made my first turn on it (a point to align the tailstock) and have decided to remove material from the bottom of the cross-slide to lower it at least .0625". A machine this size should not be using 5/16" tooling, IMO.
              Last edited by Deus Machina; 03-17-2011, 05:19 AM.

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