Gantry

[alexis221]

Hello. I am in the middle of constructing my gantry....and I have a few questions. Having never built a cnc router I dont know what to expect in the building process. The gantry is 36 inches long and 20 inches high on rails with two trucks on each side. I have it pretty much mocked up....Also using two 4.5 inch wide x 36 inche long aluminum cross members which will hold my rails. The question is with everything mocked up when I push one side of the gantry it seems that the other will kinda lag behind. There seems to be no binding on the rails and it does track well for just being mocked. Is this to be expected? My ballscrew will be below the gantry and in the middle.( if I push from the center then it will track evenly on both sides.) Does this sound normal for not have the bottom assembly attached? Thanks.

[bearwen]

Yes this is normal you will also notice that most DIY machines even welded ones and some of the Factory ones will do this even after you get the bottom on this is called racking if you search the forum you will find it ALOT the big thing is once you get the bottom on adjust the bearings for the snuggist fit that doesnt bind and then put a clamp or something in front of one side and push or pull the other side and see how much it racks this should be minimal also depends on what you mill and how deep you go in a single pass as to wether or not you will load it enough to rack it when your cutting.

good luck and have fun.

[alexis221]

I did clamp a piece of wood where my under carriage would go and it did stiffen the racking a bit. But still if I push one end with a fast pushing motion it does rack a bit before both sides slid evenly. When I push in the center of the structure I have no racking at all. Again my uprights are 20 inches tall and my bearings on each side are spaced 11.5 inches apart....Again I am using steal uprights welded to 12 inch feet if you will with two aluminum 4.5 inch wide x 36 inchs long going to the other side. I will have pictures soon. Thanks.

[ger21]

This is why a lot of machines are driven from both sides. It's difficult to build a gantry that will have no flex.

[mxtras]

A general thumbnail in engineering for determining bearing spread is the 2/3 rule. If the bearing spread on each side is 2/3 of the span, it will eliminate as much racking as the structure will allow. In other words, if your span is 6' you space the bearings 4' apart on each side. Any racking is then likely a product of structural inadequacies. This considers only guiding forces, not forces incured by use of the machine.

This thumbnail has proven reliable for me over the years but obviously is not the answer to all situations. It's a decent thing to keep in the back of your mind when designing, though.

Scott