Max length for an acme threaded rod. End support or fully support the frame?

[dlpaul]

What is the longest axis that a 1/2-10 5 start acme rod can drive? I'm sure that there are many determining factors to this question. Could someone possibly enlighten me?

Thanks.
Paul

[ger21]

Depends how fast you want it to go. The faster it spins, the more tendency it has to start whipping. The slower you need it to spin, the longer it can be.

[dlpaul]

I'm not looking for a lightening fast machine. I do, however, want to have the most accurate machine possible and am not willing to sacrifice that for a longer axis.

I also have a few more questions.

-Is using a 72'' rod on a 62'' axis out of the question?
-Will the material I want to cut affect the accuracy of the machine on an axis that long?
-Is there a point in the axis that won't be as accurate due to the rod deflecting under cutting forces?

Thanks.
Paul

[ger21]

If you need 62" of travel, than it's unlikely to work, but depends on the design of the machine. On my machine, I get 46" of travel from ~66" screws.

No.

No.

Machine deflection is more likely than the screw stretching, which is what would probably have to happen. If using plastic nuts, then heavy loads could cause the nuts to have some play.

Be aware than most acme screws have a tolerance of ±.009/ft, and could be off as much as .045" in 60" and still be within their tolerance. This is very unikely, though. If you want extreme precision, you need to buy more precise screws.

Can you tell us what you'll be doing and what precision you're looking for?

[dlpaul]

Gerry,

Thanks for the quick replies. Solidworks is showing me 62'' of travel with the weight of the gantry dead center of the carriages. I've attached jpgs and as you can see the x axis is fully supported, so I don't think machine deflection will be an issue. I have 1/2-10 5 start precision rods from Mcmaster-Carr and will be using dumpster anti-backlash nuts. I'll be using the machine to cut molds for rc sailplanes -- some MDF, but mostly tooling board.

Thanks again,
Paul

[jsheerin]

Your x rails are fully supported, but your bed is not. So while your rails will be pretty stiff, your bed will deflect under any downward force applied by your cutter. This could lead to different depths of cut at the middle of the bed versus the ends, as the gantry will always be in about the same place vertically while the bed will deflect more in the middle than at the ends. If this turns out to be a problem you could easily skin the top and bottom of the bed to stiffen it up.

It also looks like your gantry uprights are pretty small (thin) and not tied into your gantry cross bar very well, although you don't specifically show that area. Adding some triangular bracing between them and the gantry cross bar on the outsides of the uprights would help prevent the entire structure from folding under load.

Also, when you make the steel rail on your gantry that much larger than the supporting 80/20, you introduce more deflection. The minimal 1/2" overhang is more desirable.

Of course your router will work as is - these are just suggestions to improve it while you're still in the design stage and it's easier to change things. I'm currently redesigning my router to make it stiffer, so I've been thinking about all this stuff.

I'd personally go with rack and pinion for 4' of travel or longer. It's especially easy if you're using cncrouterparts components although you'd have to lose the center drive, add another motor and flip your x rails to the outside, so it would add some cost. Of course if you went with a G540 it already has 4 drives.

[dlpaul]

jsheerin, I haven't added any braces to the gantry, it was just something to test with, but I will definitely change the rail to 4'' stock. Thanks.

I was certain that anything over 48'' of travel would be ambitious with an acme screw, thus (for now) I will build something within that range. Is this a poor design, should I move the supports to the ends and move the rails to the out side? I don't see many machines built the way I have design this one. I thought surfacing the top with the machine once complete would flatten it.

Paul.

[jsheerin]

To clarify, it doesn't matter that it's an acme screw - a ball screw with the same diameter and pitch would have the same issues. And honestly, a 48" travel 1/2" diameter screw can work. It's what I have on my machine (1/2-10 5 start). You'll just be limited on the max speed you can go to around 200-300ipm. If you get a large diameter screw you can spin it faster before whipping, but it gets more expensive and anti-backlash nuts are not as readily available in larger sizes.

Your design isn't bad - I'm just pointing out where the weak spots are so you can look at them and decide if they'll be problems or not. If you surface the top it will flatten it, but if you then cut a part in the middle of the table that requires the router to drill down into the part, the table will flex under the part and your cut might not be as deep as you want. This will happen with any design - the question is how much will it deflect. So you stiffen the machine until the deflections during usage are small enough to not effect the parts you want to make. The tricky part is figuring out what these levels are.