Welding my X gantry (Part 1 of 3)

Hello Shopbot Friends. I had to break this into 3 posts, with 4 pictures being the limit for any one post.



Part 1
Last September, in a quest to improve the rigidity of my X Gantry, I replaced the factory supplied aluminum struts with rectangular steel tubing (1.5” x 2.5”).


With the X gantry in position on by PRT alpha (60 x 144), I measured the distance between the two “motor carriers”, carefully cut the steel tubing to length, removed the aluminum struts, clamped the steel tubing in place, and took great care to make sure that the rectangular steel tubing was exactly perpendicular to the 3” square steel tubing of the X1 “motor carrier”.


After welding the gantry together, and then mounting the Y rails to the X gantry,(again taking great pains to make sure both Y rails were perpendicular to the X1 motor carrier’s 3” steel tubing, I realized that I should have made sure that the Y rails were perpendicular to the X rails. But of course they would be,,, why wouldn’t they be??????


Horror of horrors, I found in fact that at the far end of the Y rails (relative to the X1 motor carrier) was out of square by about 1 inch,,,,,,,, or enough that no amount of egging of holes, or re-locating the tapped holes that attach the Y rails to the X gantry rectangular tubing, could make the Y rails be perpendicular to the X rails. How could this be, when in fact the Y rails were perpendicular to the 3” square tubing of the X1 motor carrier?


If you look at picture A, and you will see that the “Motor carriers” at both ends of the X gantry are each made of two pieces of steel, the “long angle bracket” and the “3” square steel tubing”. As it turned out, the long angle bracket and the 3” square steel tubing were NOT aligned as I thought they were. I “ASSumed” that the long faces of these two parts would have been co-planer in both the long dimension (X axis) and in the short dimension (Z axis). And they were not co-planer in either dimension…. (This is perhaps hard to explain/understand,,, but when looking at this picture (that was taken after the fix was made), the top of the long angle bracket was tipped back slightly, and one end of the long angle bracket was not even with the same end of the 3” square steel tubing.


This out of whack alignment of the parts of the “motor carriers” was not a problem with the original aluminum struts, because with the strut bolts loose, it was easy to just slide everything around until the Y rails were perpendicular to the X rails, then the bolts would be tightened to lock everything in place……. Easy……


So, to fix my welded, out of alignment X gantry, I had to grind off all of the welding to separate the motor carriers from the rectangular tubing. Then, with LOTS of grinding, and using big wedges, a Sawz-all, a big hammer and a grinder, I was able to separate the long angle brackets from the 3” square steel tubing. (Picture B). And I had to flip the 1.5”x 2.5” rectangular steel tubes over, so that I would move all of my drilled and tapped holes onto the bottoms, where no one would see my mistakes...


Then I wanted to do a dry mock-up, to make sure everything would be OK BEFORE welding the X gantry back together a second time……. So I placed long threaded rods inside of both the rectangular tubes, which then allowed me to bolt the X motor carriers to the rectangular steel tubing. (Picture C). And I bolted the long angle brackets to the 3” square steel tubes. Because of the precision cutting of my rectangular steel tubing, I was able to square and tighten the basic gantry assembly itself. (Picture D).
http://www.talkshopbot.com/forum/att...id=32960&stc=1http://www.talkshopbot.com/forum/att...id=32961&stc=1http://www.talkshopbot.com/forum/att...id=32962&stc=1http://www.talkshopbot.com/forum/att...id=32963&stc=1

Part 2
And then because thelong angle brackets were attached by bolts to the bottoms of the 3”square tubing, I was free to move the X gantry until it was in factsquare to the X rails. (Picture E).

With full confidencethat with everything tight, my alignment could be made good, Iun-bolted the long angle brackets from the X motor carriers, and putthe remaining X gantry on to the bed of my Shopbot. (Picture F). This assured that this entire assembly would in fact all lie in asingle plane. Then I squared up the rectangular tubing of the gantryto the 3” square tubing of the gantry, assuring then that thegantry was fully squared, and all lying in a single plane.

(Note,finding an accurate, large square isn’t easy, at least not for me. I have lots of framing and “speed” squares, but when I laid outa BIG 3-4-5 triangle on a large sheet of plywood, I could detecterrors, that I could not correct (using a punch in the corner of theframing square, does change the squareness of the square,,,,,,, but Ionly succeeded in making mine worse…..) It dawned on me to tryusing large, 16” square vinyl floor tiles, and on my big 3-4-5triangle, I found they were dead on accurate. (Picture G). I usedthose, and was delighted, to have lots of great squares to check all4 corners of my gantry simultaneously…… really a greatdiscovery as this greatly increased my confidence, by not having tomove one questionable square around all 4 corners, one at a time….. And after the gantry was fully welded, (Picture H) I was able todo a sanity check, of nipping off the corner of a floor tile, andthen place the tile in each corner of the welded gantry, withouthaving the weld get in the way of the square. I wouldn’t want tocut off the corner of a good framing square, so I could place it overa weld in the corner of the gantry…..)


http://www.talkshopbot.com/forum/att...id=32964&stc=1http://www.talkshopbot.com/forum/att...id=32965&stc=1http://www.talkshopbot.com/forum/att...id=32966&stc=1http://www.talkshopbot.com/forum/att...id=32967&stc=1

Part 3
Then I put the X carparts back on my Shopbot X rails, and fumbled around to get the longbrackets bolted on to the 3” square tubes, to restore my nowadjustable X motor carriers. (Picture I). Then using a full sized3-4-5 triangle, where the height was the length of my X gantry, I wasable to very carefully bring the rectangular tubing of the X gantryto exactly 90 degrees away from my X rails. (Picture J). Then Imounted the Y rails onto the X gantry, located all the holes, drilledand tapped the mounting holes for the Y rails. Then using washersand shims and straight references, I carefully made the Y railsperpendicular to the X rails, made the Y rails perfectly level, andperfectly straight. (Level and straight do not happen byaccident,,,,, lots of trial and error to get the dips out of the Yrails, and making them straight.)


Maybe there is noone left who would want to strengthen their old PRT alpha gantry, asthose who are willing to do this, did it long ago. I will say itis NOT easy if you have never done this sort of thing before, and Ihad not. Be willing to undo it all and do it over if it doesn’tturn out right,,,,,,,, otherwise, why bother?


I have carefullyreassembled the Y gantry and the Z axis. But I haven’t gotten theZ axis fully tweaked yet. When I got close, the weather got nice,and I have been working in the garden and other outside projects…….. So I can’t tell you if this upgrade helped or not……. I’llwrite up my observations on the Z axis, as that was also not as easyas it might seem,,,,,,,,,, at least, when I plowed ahead to get itdone, I could tell things weren’t right, and I had to start over,and think about what I was doing first,,,,,, then it made sense and worked.


Thank you Brady forencouraging me to do this. I know I have learned a lot about mymachine in the process, and it will be worth it in the end.


Chuck
http://www.talkshopbot.com/forum/att...id=32968&stc=1http://www.talkshopbot.com/forum/att...id=32969&stc=1

Good job Chuck. The beauty of steel is that if you mess up or need to change something, it's a matter of getting out the wheel, hacking off the afflicted and welding it back up. Besides steel is much stiffer than aluminium which means less sag in the middle of that 5' Y. I think I welded mine up in few hours after 2" making riser blocks... which is real handy because I have 10+ inches under the gantry for things like Newell posts etc.

Thank you Brady!

Speaking of "riser blocks", while I have of course welded the 1.5"x 2.5" rectangular steel tubing to the 3" square steel tubing, I did not weld the long angle iron pieces to the 3" square steel tubing. Those connections are made with bolts. I wanted the ability to put in "riser blocks" at a later time, as I have had a couple of times where I needed an extra inch of clearance.

Thanks again, Chuck

I admire your desire to make your shopbot a better machine. You've been working hard on this project for a while. I'm glad it is coming together for you.

Thank you Dave. I appreciate your positive feedback. Best regards, Chuck

I did this to mine about 7 cnc machines ago. That along with adding bracing to the table to stop it from shaking. The tricky part was, every time you weld something the metal pulls, so you need to allow for that. I had to heat and pull things back straight after welding in some spots.

Hello Kenneth. I am still fiddling with some other parts of my Bot, so I haven't yet been able to test out my welded gantry. When you did your gantry welding, how would you rate the improvement? Beyond the added bracing, did you make any other structural changes to your machine which you would recommend? Thanks, Chuck

I would say it improved about 50% toward where I wanted to be. Diagonal bracing on the lower frame helped a lot.

Thank you Kenneth....... I'll have to look at my frame,,,,, but it never seemed to me that mine wobbles much. Maybe I run things too slow............ Chuck