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  1. #61
    Join Date
    Dec 2005
    Posts
    231

    Gantry slides smoothly

    Well. The modifications are complete. The new bearing holders are well set although I have concerns about the MDF compacting from the clamping bolt pressure over time. May well be worth making up a larger and thicker clamping washer to spread the load. But that's an MDF issue more than a design flaw.

    So the two pictures show the View down the Y axis with the new bearing truck holders clamped in place. Once the entire router is complete I'll do up some G-Code and make units that have better hole tolerance.

    Now the key here is that I used the original trucks and MDF holder for the aluminium angle. All I did was drill out the holes to fit T-Nuts and following the suggestion from someone else on the CNCZone used a forstner bit to break through the hard MDF surface so the teeth of the nut would bite and the nut would sit below the surface of the MDF.

    So anyone can do this modification if that part of their JGRO is stock.

    John
    Attached Thumbnails Attached Thumbnails DSCN4566.jpg   DSCN4565.jpg  

  2. #62
    Join Date
    Dec 2006
    Posts
    177
    Very interesting John, I think my gantry without the Sears router came in around 45lbs so using the pipe was the optimal choice. But even with using the pipe I get enough sag that the cuts near the midsection always became deeper. Most people said they didn't notice until I pointed it out but I knew right away.
    Anyways, you will most definetley need some type of anti-sag device.
    Keep us posted,
    Tom

  3. #63
    Join Date
    Dec 2005
    Posts
    231

    Table to hold JGRO

    Not much progress on the actual JGRO since the last posting. It's been taken apart and squirreled away into various places. Hope I can find everything that I put in places where I'd be sure to remember.

    Anyway. I've made a base from 2x4 and 2x6 pine and spruce lumber. Full mortise and tenon joints that 2.25" deep and 1/2" thick. Some of them fit a bit loose so I'll be using dowels and thickened WEST epoxy to hold it together.

    Trying to decide if I should cut dados in the rails and posts for 1/4" thick panels. That would keep dust out but might make it resonate and be much louder. Don't know yet. Still not glued together yet so I have time.

    Anyway, here's a picture.

    John
    Attached Thumbnails Attached Thumbnails BaseFrame1.jpg  

  4. #64
    Join Date
    Dec 2005
    Posts
    231

    wibbly wobbly lead screws

    It's been a while since I've posted. I made up a list that had 14 items on it. The last item was take the JGRO to the dump and go and buy a real CNC router.

    Anyway, I'm having huge lead screw issues. Rather than throw good money after bad trying yet another piece of acme rod I thought I'd just design a belt reduction drive system for it.

    To give you an idea of the time and energy invested into this already I'll explain what I've built. My last CNC table used the stepper motor bearings to serve as thrust bearings. Not the best plan.

    So this time I ordered a bunch of angular contact bearings and cast and turned some housings for them. The photo shows the bits and pieces, the 3D rendering shows how it goes together.

    The problem is that first I don't have straight Acme rod. Second, being that it's crooked it must have been a tad bent where I held it in the collet in the lathe and although I did my best with an outrigger support, I suspect the threaded rod wasn't turning concentric to the lathe axis. So my 10mm section that is a smooth almost zero clearance fit into the bearings ends up not being on axis with the bearings.

    That makes the lead screw want to scribe an arc. Add to the fact that it's bent and I haven't been able to improve it for straightness means it is strong enough to shake the entire gantry at 60 ipm. And at slow speeds it makes the router cut a zig zag line down the mdf.

    I'm guessing at this point if I had used the 1/4x20 tpi all thread it's weak enough and flexible enough that it will move and not the gantry. Also since the coupler allows the lead screw to shift relative to the motor, again the gantry isn't moved.

    So buy over designing, I've actually created a larger problem. As I see it I have two options. The first is to just go to belts. The second is to turn a coupler that allows the lead screw to move relative to my thrust bearings. Or at least so the drive porition doesn't introduce wiggle.

    Keep tuned.
    Attached Thumbnails Attached Thumbnails StepperMountAssem.jpg   DSCN4605.jpg   DSCN4597.jpg  

  5. #65
    Join Date
    Apr 2009
    Posts
    144
    Quote Originally Posted by jcdammeyer View Post

    So buy over designing, I've actually created a larger problem. As I see it I have two options. The first is to just go to belts. The second is to turn a coupler that allows the lead screw to move relative to my thrust bearings. Or at least so the drive porition doesn't introduce wiggle.

    Keep tuned.
    It always sucks when a plan doesn't work out.... I know this from experience. lol

    What about just cutting off your machined end and grabbing some dumpsterCNC adapters or lovejoy couplers? maybe you can get the screw straight enough to salvage that way?

    Or, is your screw long enough to cut that bit off and re turn it on the lathe?

    I am running mine with no thrust bearings at the moment, but am rebuilding my gantry and plan to just use normal bearings as thrust bearings. They will be attached to the acme with locktite(lol) and pressed into the gantry and again, secured with some sort of adhesive.

    BTW: Nice table

    JGRO Complete - G540, 380oz Nema23s, 1/2-10 ACME, 30"x14", Craftsman router
    Joes 4x4 R&P in progress

  6. #66
    Join Date
    Dec 2005
    Posts
    231

    Bent acme screws

    I decided before I plunge into belts I'd try 5/8"x5tpi ball screws from ROTON. I ordered a 72" length and two ball screws. The ball nuts are just small enough to fit where the plastic nut assembly sits. Had I more room I might have gone to something thicker.

    I'm still concerned about whip at higher speeds but I'm also positive that I can't find straight enough ACME 1/2" without spending at least 1/2 as much again as the ball screws.

    And if they don't work, I have an XY table that I can retrofit with the screws cut much shorter or use one on the Z axis and swap over my Gingery Lathe to ball screw.

    I'll post the results.

    John

  7. #67
    Join Date
    Dec 2006
    Posts
    177
    John,
    Sorry to learn of your problems so close to the finish. At this point you have alot of time and modifications invested in this project to just chuck it but I too know the frustrations that occur when modifying projects.
    Anyways keep us posted as to the chucking or the throwing more time and money at it.

    Dustin,
    I know you know of VXB bearings anyways I was just looking at some nice ball bearing thrust washers for my modded JGRO and to have 6 delivered to me will cost about $20 bucks.
    The problem with using radial bearings on these types of applications is the force is always applied at 90 degrees to the way they are designed to operate thus leading to sloppy bearings after awhile... you could probably double the radial bearings and get better performance but eventually they both would get sloppy and need replacing.
    I think optimally if you used both a thrust bearing and radial together (kind of like I do on some of my RC helicopters) you would achieve a very robust and smooth setup.

    my 2 cents,
    Tom

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