[DonKes]

Why do we make everything as stiff as possible and then bolt our ballnuts ,bearing blocks, and supported rods with a few 5mm screws?

Are these included in the calculations (for those who calculate)?

When does it not matter anymore?

The rows of balls in my SBR16UU's are not near optimum.

Don

[ger21]

If your components are designed to mount with 5mm screws, then their specs are based on using those 5mm screws. Provided the components are mounted to a surface that meets the requirements of the components.

When does it not matter anymore?

Rigidity will always be dictated by the weakest link in the chain.

[DonKes]

SBR16UU and TBR16UU have 5mm threaded holes. SBR16 and TBR16 supported rod drawings have 5.5mm holes (mine had none).

I have seen no mounting specs for the components.

Don

[wizard]

Why do we make everything as stiff as possible

I don't think that is accurate, you want to make a machine stiff enough to get the job done. Usually this involves a tight budget, so in the end compromises are usually made.
[/Quote]

and then bolt our ballnuts ,bearing blocks, and supported rods with a few 5mm screws?
[/Quote]
Interesting questions. One point to consider is that a lot of these components are not heavily loaded or exposed to twisting moments. For example a leadscrew and its bearing blocks should only have stresses on them associated with moving something. That something, be it a saddle or a gantry, should have its loads going to the linear bearings, you never want to expose a leadscrew to the mechanical loads of the object you are moving.

Say for example you have a saddle mounted on an axis that is under an unbalanced load ( the cutter working off one end of the saddle). That load will cause the saddle to twist some as it is running the cutter through the material. That laid should only be seen by the saddles linear bearings. Same thing for the weight of the workpiece sitting on the saddle. A leadscrew should never carry these loads. The screws holding the leadscrew nut in place should only see the force required to move the work through the cutter, which might be a few hundred pounds.

As for linear rails themselves, well ideally they would be mounted according to manufactures reccomendations. Often this requires a master rail abutted up against a shoulder of some sort. That way more than the mounting screws are registering its position. Then you have the fact that the bearings riding on the mounting rails are spread out so the load isn't just on one set of screws.

Are these included in the calculations (for those who calculate)?

I would imagine many do, especially if custom hardware is used. However the ball screw nuts are already engineered for the loads speced out by the manufacture.

When does it not matter anymore?

Simple when the machine is more than good enough for its intended usage.

The rows of balls in my SBR16UU's are not near optimum.

Don

How did you come to this conclusion? Round rails have their problems but how the bearings are designed into an axis is often as big or a bigger problem. The same mistakes made with round rails can impact profile rails.

In any event it goes back to what you expect from the machine. Apparently the machine you got isn't good enough, in other words isn't meeting your expectations. Understanding the how's and whys will lead to a course of action to fix things.

Often we get posts in this forum ( with pictures) from people that want to improve their machines. The first thought that usually comes to mind is that the machine is flimsy, that is lacking the stiffness to do the job at hand. Often this is in the linear bearings or in the gantry itself. The problem is with most people you have a hard time convincing them that machine stiffness is the first thing they should address. It is especially frustrating when their improvements will likely lead to a less stiff machine.

If you want to improve your machine it helps to define what you think is the problem. That is complete a list of your known issues. Also take the time to make realistic measurements of axis deflections and backlash. Excessive backlash is another issue that can create quality issues. In any event the idea is to find the worst parts of your machine to improve first.

[DonKes]

"How did you come to this conclusion? Round rails have their problems but how the bearings are designed into an axis is often as big or a bigger problem. The same mistakes made with round rails can impact profile rails. "

The rows of bearings in the SBR16UU are not in a symetric pattern. I am not sure they are all alike, but at least one has one row near the edge of the opening slot and the one on the other side of the slot is almost at 90 degrees from the slot. The others are an even distance between these. The pulling load is good on one side of the slot, but very bad on the other side.

I don't have any great expectations. I want to drill hole patterns in aluminum ( larger than my mill can handle) and do wood stuff. I have a CNC'd G0704 for mill work.

Don