[sp1nm0nkey]

I saw a certain CNC milling machine that seemed pretty well engineered, except for one feature... they appeared to be using one truck per rail, and two rails total for the Z axis. The compromise made here seems to be to reduce cost (2 less trucks) and to squeeze out a bit more Z axis travel at the expense of some rigidity. My question then is, how good are say, THK SHS blocks in a twisting load that goes front to back, orthogonal to the axis of the rail and the axis pointing down through the rail? Is this a smart way to design a machine?

[amishx64]

Yikes! What was it supposed to mill? Balsla and pine wood? Beyond that, I'd have my doubts as far as capability is concerned. How long was each truck and what was the milling area? Got any pics?

I'd say it could be feasible if the truck were a decent length, say longer that 3", but I have my doubts about that.

[mcarvey]

Good is a relative term- it all depends on how stiff of a machine you need, and what you're trying to maximize. Generally speaking, having two trucks per rail will almost always be stiffer than just one, which is why *most* DIY and commercial machines have this configuration. However, just one *may* be sufficient depending on what you're cutting and the size/configuration of the rail. Hogging out steel requires significantly more stiffness than say, micro-machining aluminum with a 40K RPM spindle or milling foam. The "smart" way to design a machine is to determine your stiffness needs (with a quantifiable number), and then choose a rail(s)/truck(s) configuration/spacing that matches your needs depending on what you are trying to optimize (price, stiffness, development time, max working envelope, component machining/assembly time, etc) These stiffness calculations aren't usually done too thoroughly in DIY builds since most of us are only making one, our exact stiffness needs are not set in stone, and we're generally trying to maximize fun (which may or may not mean using lots of math)

You can find the stiffness of the bearings in THK's online catalog, along with a wealth of other information.
https://tech.thk.com/en/products/thk...ain.php?id=399

Page 0-8 "General Descriptions" discusses static permissible moment. The actual numbers or each rail is listed in the rail/truck's dimensional drawing pdf.

Hope this helps,
Matt

[amishx64]

These stiffness calculations aren't usually done too thoroughly in DIY builds since most of us are only making one, our exact stiffness needs are not set in stone, and we're generally trying to maximize fun

Very well put. :banana:

[sp1nm0nkey]

I did out the math and it's maybe 250 pounds I can put on tip of the longest tool I'll put in the spindle while still being alright. Figure a safety factor of 2 or 3 even and I'll break any tool I'll put in there before I hit that. Problem is, that's "permissible". I'd love to see a number that's like... rads deflected per kg.m of torque... or maybe not even a number since it's likely pretty nonlinear so say a plot? Would it have been that difficult for thk to use one of their fancy instruments that they use to get stress-strain curves to get a curve for this?

[sp1nm0nkey]

Yeah, the figure I'm looking for is rotational stiffness... instead they have the static permissible moment... which is like, how much can you torque this thing before it just flat out breaks.

[mcarvey]

Sorry- I just looked at those datasheets quickly and didn't notice that they only list the max permissable moment, and no the rotational stiffness numbers. I would think that the stiffness (rotational and should be available, but I spent a bit of time on the THK webpage/catalog and couldn't find them. Perhaps someone else knows how you can get a hold of the rotational stiffness numbers. I would be flabbergasted if THK didn't have them somewhere. It could also be the case that they only measure the XYZ stiffness and you have to back calculate the rotational stiffness (theta, psi, phi) It could be that THK doesn't directly publish the numbers in their catalog/datasheet because they don't want to confuse customers- in order to really use them effectively the customer has to really know what they are doing (and lets face it, most of their customers dont' fall into this category). They may just rely on online calculation wizards to handle the majority of customer's calculations. It appears that you can select number of rails (1 or 2), number of trucks (1, 2, 3, or 4) You can register with them (free) and get access to this, and more info here. https://tech.thk.com/ Since the stiffness depend in part on the preload (and those are options for each rail/truck) They may take a bit more digging to get (or contact THK if you really care) https://tech.thk.com/en/cat_claim/index.php

https://tech.thk.com/

Here is a pdf that discusses a bit of the technical things a bit more.
A-13 discusses stiffness, although they call it "rigidity" so you may want to search for that.
http://www.thk.com/documents/us_pdf/..._1_general.pdf

-Matt