[mnelson78]

Before I found this site I purchased 55" long 20mm round (unsupported) linear rods. I'm wishing I'd purchased the fully supported rails. Will this create a problem? What kind of weight are these rated for?

What is the maximum Y axis you would recommend for being driven by 1 ball screw?

I plan to use this machine for mainly 2D cutting, mostly MDF and plywood, but don't want to limit my options too much, but also want a stiff gantry. What height would you recommend from the sacrificial deck to the bottom of the gantry?

[ger21]

Before I found this site I purchased 55" long 20mm round (unsupported) linear rods. I'm wishing I'd purchased the fully supported rails. Will this create a problem? What kind of weight are these rated for?

Well, they're going to flex. I don't believe that those shafts are rated for any specific weight. The bottom line is that they will flex, regardless of how much weight is on them.

What is the maximum Y axis you would recommend for being driven by 1 ball screw?

If you mean gantry width, it depends somewhat on the design. Assuming your running a crossmember under the table to toe the two sides together, then the larger the crossmember, the wider you can go. But another downside to a single screw, is that the entire table is suspended from the ends, which can have an impact on rigidity.

What height would you recommend from the sacrificial deck to the bottom of the gantry?

The lower the better. It depends on how thick the material you plan on cutting will be.

[wizard]

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Before I found this site I purchased 55" long 20mm round (unsupported) linear rods. I'm wishing I'd purchased the fully supported rails. Will this create a problem? What kind of weight are these rated for?

Basically 3/4" over more than 4 ft. Yes I would say that is a problem. As for weight there are ratings for bearings but you would need to structurally analyse the rods for the spans they are working with. For more information about the bearings contact the bearing manufacture and get a data sheet.

What is the maximum Y axis you would recommend for being driven by 1 ball screw?

There are many factors to consider. We don't even know for sure if this is moving table or moving gantry design.

I plan to use this machine for mainly 2D cutting, mostly MDF and plywood, but don't want to limit my options too much, but also want a stiff gantry. What height would you recommend from the sacrificial deck to the bottom of the gantry?

[jueston]

i had a machine where both the X and Y axis were unsupported steal rods and i couldn't stand it, they would flex all over the place and i could never get consistent quality results... sent that machine back to its maker and now i will build a machine with something fully supported

[ger21]

As for weight there are ratings for bearings but you would need to structurally analyse the rods for the spans they are working with.

I was assuming that he was asking about how much weight before the shafts start to sag, rather than the load capacity of the bearings.

One way to make the shaft behave as if they were more rigid, is to use two mounts at each end, with few inches of space between them. This prevents the ends from flexing, which makes the shaft stiffer along it's entire length. You will lose a bit of length, though.

[mnelson78]

Thanks everyone, great information. This will be a moving gantry design, and yes I was asking about weight on the rods before sagging became a problem, basically how heavy can I make my gantry before sagging was significant.

Based off the information everyone provided, in designing my base, I plan to have two box beams running under the base top, along my x axis approximately 50mm under the front and back edge. I will mount my unsupported rods to this (using 2 brackets on each end), and if I get too much sagging I can install supported rail along this same box beam.

I was thinking a gantry 6" above the sacrificial deck would give me tons of options. Even 2" would probably cover 95% of what I want to do. Does a gantry 6" above the desk seem reasonable?

[wizard]

Thanks everyone, great information. This will be a moving gantry design, and yes I was asking about weight on the rods before sagging became a problem, basically how heavy can I make my gantry before sagging was significant.

They will sag some that is simple physics. The problem is the vibration and deflection you get out of the unsupported rods.

Based off the information everyone provided, in designing my base, I plan to have two box beams running under the base top, along my x axis approximately 50mm under the front and back edge. I will mount my unsupported rods to this (using 2 brackets on each end), and if I get too much sagging I can install supported rail along this same box beam.

Again sagging is only part of the issue. In any event why not go with supported rails right off the bat! If you are building in provisions for supported rails why hold off?

I was thinking a gantry 6" above the sacrificial deck would give me tons of options. Even 2" would probably cover 95% of what I want to do. Does a gantry 6" above the desk seem reasonable?

6" isn't too bad. Given a rigid gantry beam and well designed vertical supports it makes for a versatile machine. 2" of clearance might give you more stiffness and a bit more stroke on the X axis but it can be very limiting basically forcing you to to sheet and thinner pieces of stock.

I'm not sure what your intentions are but let's imagine one use for this router. Say you have some 2" or so rough cut stock you wish to flatten on one side to run through a planner. It is easy to see that by the time you shim it up and retain it in place that you won't have enough clearance to run the gantry over it. So it is easy to find situations where 2" of clearance simply isn't enough. I would suggest that you go with at least 4" of clearance and ideally a bit more. Note this is Y axis saddle clearance.

The other part here is Z axis stroke length. Ideally you want enough stroke length vertically to completely clear the cutters you are most likely to use. 4, 6 or whatever inches of Y saddle clearance is useless if the cutter can't completely clear the work. When doing your design work don't forget to consider how your Z will work with the gantry clearance you design in.

[louieatienza]

I think the levering action of the gantry on the unsupported linear rails will cause you grief. As Wizard mentions, forget flex - just the chatter and vibrations alone at that length would be enough to cause grief (and ruined work and damaged bits.)

There are a couple options for your Z carriage. One is to have the carriage retract past the gantry. Unless you have a pretty low Z clearance/travel, I'm not a big fan of this (rails on the saddle, bearing blocks on the carriage) because it requires the saddle assembly to sit way above the gantry bridge, and the bearings of the carriage move way far from the work. In other words, the bearings in relation to the collet bottom is at a worst-case-scenario, regardless of what spindle height. Works great on a column mill since the column never moves. It is easier to build. The other way (my preference) is to mount the rails on the carriage and leave the blocks on the saddle. This keeps the bearing blocks in relation to bottom of the collet at it's lowest possible position at all times, which to me is the most ridged setup possible and actually the format most large commercial machining centers use. The downside is that you need taller gantry legs, which usually means wider leg bearing spacing, but in my opinion, this is easier to do than having to reinforce the spindle carriage and reinforce the saddle. Another issue is that it means a more complex saddle construction.