#181
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Kobus,
No shimming at all? I was sure you would take the opportunity to use up some rum and cola? |
#182
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His rum is in a glass bottle and his cola in plastic - real classy guy from around Charlize Theron's original hangout.
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#183
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I am a straight and level guy...
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#184
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Wanna hear a little secret? MM #5 has no shims either. I aligned everything laterally, and left the vertical to chance - It actually worked out fine for the first year in production. When I moved the machine I did shim just a little, but nothing serious.
All those pictures of really nice acrylic and such cut, were from the original setup! I guess when you have a great design - everything works like it's supposed to. Sean |
#185
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After over a year operating mine, I regrated spending so much time on shimming...
I recon with a good vacuum hold down, all sheet stock will trun out as true as the surfaced spoil board. |
#186
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My major aim in getting the rails reasonably level is so that the 4 rollers of the gantry share the load evenly, but the bit of twist flex of the gantry mostly takes care of that.
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#187
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This may be off topic but while we are at it, what will happen if we increase the height of the gantry by lengthening 1020456?
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#188
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To keep the 4 rollers equally loaded, eventually you'd have to increase the 'wheelbase', or reduce the speed? Think of the 'weight transfer' (as the racers call it) of a topheavy SUV braking or accelerating hard.
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#189
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SUV don't have a spring to hold it on the road...
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#190
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True, but there's also the issue of lateral forces (along Y). If 1020456 is taller, then 1020451 and 1020452 need to be taller to match. That may induce flex between the gantry tubes and the wheels, and require making 1020451+2 out of thicker steel, or perhaps adding stiffening ribs (along with the resulting weld distortion).
I'm not an engineer, so I haven't (and won't) calculate all the possible forces here. But it's important to remember that it's a system of parts, and changing one part can have many side effects. |
#191
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Springs (or rear spoilers) holding it down are secondary. If where the drive force is applied is low (near the road or the table), and the center of mass is high, then any acceleration induces a torque about the center of mass, just like pulling on the top of the gantry would. That torque changes the loading on the wheels. The bigger the lever arm for that torque (the taller the gantry, or the shorter the wheelbase), the stronger the effect. Build one out of lego if you still doubt this.
(I trust Gerald that equally loaded rollers actually matters.) |
#192
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Under cutting conditions the rollers will not have equal loads. But we have to try our best to get even or balanced loads. What I am trying to say is that we don't want a roller to lose pressure contact with the rail, because then the stiffness of the whole machine suffers.
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#193
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Interesting...
Unfortunately, it is not easy to determine the cutting force acting on the gantry. This just lead to a dead end. For all I want to do is that extra 50mm or so of extra gantry clearance, maybe its too much to ask. |
#194
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Ken, remember that if you lift the gantry up 50mm, then the z-slide has to extend 50mm more from the Y-car (for cutting the same job).
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#195
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That is true, but what if I don't venture into that extra extension?
If I have a thicker table board, with vacuum plenum on it + bleeder board. That extra 50mm clearance will come in handy. & at most I'll extend another 25mm... From that an additional 20~35mm extra extension. Not really a lot... I hope... |
#196
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I don't think it is a lot either.
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#197
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A little bird told me that in a few hours the rails will be straight to within 7 thou, vertically and laterally.
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#198
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...and at the cutting tool bit that is almost undefinable. Great work.
Sean |
#199
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A couple of photos of the shim procedure -- cards and Guinness .
The paper cutter came in very handy, especially for the spots that needed multiple card thicknesses. It uses cutting wheels, and one can make a perforated cut. That's ideal for folding the cards, without the usual bump that results from folding something this thick. |
#200
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The gantry is now rolling on the rails .
It's NOT rolling well . I've got about 1mm of diagonal wobble (propellor twist). I tried bending it down against the table, as Robert M did in his build thread. I ran out of thread on my clamps before I got it bent far enough to make a difference. Next attempt: pull up instead, using my gantry crane. I clamped three corners to the table, and used the crane to lift the fourth corner to twist the gantry the other way. Didn't work... I ended up lifting two legs of the table clear into the air . The other headache is that the rollers are not contacting equally along the length of the table. I have more wobble at one end than the other. Hmph. Any suggestions? |
#201
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Congratz! Now the work begins. . . . . .great job
Tim |
#202
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Insert car jack into last photo
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#203
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That is a lot of fun!
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#204
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1. Guinness and cards... can peanuts and dartboards be far behind?
2. Untwisting the gantry. The steel has to stretch past its elastic limit, for the untwist to become permanent. The pull-down attempt says that the deflection must exceed a few mm. The pull-up attempt says that the force must exceed the weight of half the table, about a ton. So how about this? A vertical wooden frame, a foot or two tall, in the xz plane, midway along the gantry's y, braced. Like a spreader for a mast. Run webbing or rope or chain around the whole thing (mast analogy again: like shrouds or standing rigging), mostly in the yz plane but angled to effect an untwist. Tighten it with a spanish windlass or come-along. The spreader goes above or below the gantry, depending on which way you want to twist it. Like a beam reach with no reefs, leeward rigging slack, windward rigging like a piano. Last edited by camilleg; Fri 22 April 2011 at 09:47.. |
#205
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#206
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Faster to adjust and measure than using my suggestion. Mille grazie, G.!
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#207
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My simple method : tie two ends on good side to channel, on the other side put thick and long plank to serve as lever, pull it down (two men weight works right ) inspect the bend or apply it more or on other sides. done
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#208
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Two-man weight = 300 lb. Force required exceeds 2000 lb.
So mechanical advantage needed exceeds 6.7. Gantry width = 1.5 ft. 6.7 x 1.5 = 10. Melissa, got a 10 ft plank that's strong enough? (I think C-clamps would be safer, for a monster this big. Danilo's way is faster, for smaller gantries.) Last edited by domino11; Wed 13 August 2014 at 10:14.. |
#209
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With our last few gantries, we found a mini trolley-jack (plus 2 clamps) to be the easiest to use and control.
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#210
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Right... tightening a C-clamp that already has tons of force on it may be asking for trouble. That's what hydraulics are for, either trolley jack or bottle jack.
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