[Rhodan]

Is there a ratio of gantry width to bearing spacing that can be used to figure out how long the bearing contact of the two bearings need to be for a single (centered) leadscrew to be able to drive the gantry without racking?

I'm looking at building a 2'x4' cutting area mill and I figure I can do it with one leadscrew. I have a 14" wide machine now where the side bearings are 6" total length (measure the two outside ends of the bearings themselves).

I figure its a lot cheaper to get longer 80/20 and 1/2-10 leadscrew than two leadscrews. another G201, another 270oz/in motor, couplers, bearings, etc, etc.

[stevespo]

I can't help you with the ratio. I suspect it depends a lot on the types of rails, etc. However, I did find this the other day - which may solve any racking issues with a single screw.

http://www.cnczone.com/forums/showthread.php?t=51485

Steve

[Rhodan]

Thanks, that's a great trick! Still like to know if there's a ratio and build to minimize racking without the cabling though.

[mjrandle]

Have a look at the "The Golden Rectangle" section in the following lecture notes from Slocum.

http://pergatory.mit.edu/perg/presen...20Machines.pdf

Cheers,

Mike

[stevespo]

Interesting presentation - thanks for posting that. If your new gantry is 24" wide, using the golden rectangle would require spacing your bearings nearly 15" apart. This seems unrealistic, unless you're willing to add the extra length to your table to avoid losing work envelope. Maybe that's a reasonable tradeoff. Using the 2:1 ratio may work well.

It seems to me that most benchtop commercial routers use a spacing of 6-10", and even large machines rarely go wider than 12" or so. My 40" wide gantry uses a spacing of 7", but I drilled my mounting plates so that I could space them 9" wide if I felt the need for extra width. Racking has not been an issue at 7", and the gantry is very solid.

Let us know what you decide.

Steve

[mjrandle]

The ratio of 1.618:1 is actually length to width, so it's even worse (Dohh!). For a 24" wide table the bearings "should" be spaced at a minimum of 38.83" (which is obviously impractical).

As stevepo suggests, have a closer look at the dimensions of commercial machines which drive the gantry on one side only. They probably manage to do so due to the light cutting loads, bearing clearance, low friction or compliance in the gantry. This is probably OK on a router but could be a problem for your mill (I assume that you will be cutting metal).

You may want to consider using two bearings on one side and a single bearing on the other - Google "Kinematic Design" or "Exact Constraint Design".

Also, there is a link somewhere in the CNCzone which has more detailed lecture notes from Slocum.

Cheers,

Mike