Straight Linear Track

[jdm2008]

I need to make a test fixture for some parts that need to test for perpendicularity. I am going to mount a drop gauge dial indicator on a linear slide and carriage(or plate or trolley) and run the indicator across a short surface.
So in this case I need the rail to be very straight(within .001"-.003" around that neighborhood because the test is +-.030"). None of the tracks I have a straightness call out
Does anybody have places that guarnatee the rails to be straight within this amount?

[RICHARD ZASTROW]

Generally the straightness of a rail is dependant on the straightness of the surface(s) the rail is mounted to.

For example, the bottom of the rail would be mounted to a surface ground plate and an edge perpendicular to the bottom aligned to a shoulder in line with the axis parallel to the linear axis.

I often place a keyway into the flat surface then insert a key into the keyway. The key provides the shoulder to locate the edge of the linear rail. The keyway/key must be deep enough to clear the bottom of the linear bearing.

Dick Z

[harryn]

Thank you for the reply, that is similar to what I have heard in the past, although the use of a key is new for me.

If the rail is drilled for top mounting, then naturally the next step is to drill and tap the holes into the plate for the screws to go in. Do you use the same setup to drill these hole locations ? The reason I ask, is that it seems like you could easily move the rail off of the machined edge with only minor offsets of the the screw placement.

Thanks

Harry

[RICHARD ZASTROW]

There are many methods of pushing the rail against the locating edge. I've even used a few flat head machine screws to wedge the rail tight against the key.

Most linear way manufacturers have those methods in the tech sections of their on line catalogs.

Dick Z

[harryn]

Hi Richard, thanks for that input. Maybe I just don't have enough experience with these rails to explain my question properly:

- 1) machine a flat straight edge for placement
- 2) mechanically push the rail into this edge

Now, assuming that this is for example hiwin rail drilled for putting the screws in from the top to hold it down, since the screw holes aren't perfectly placed compared to the rail edge, won't that move it away from that edge when I put in the screws ?

In other words, how much tolerance is there for screw hole placement relative to the edge before I didn't gain anything from making the edge ?

[keebler303]

In other words, how much tolerance is there for screw hole placement relative to the edge before I didn't gain anything from making the edge ?

What is the difference between the clearance hole size in the rail and the diameter of your screw? That will tell you the clearance. It may be wise to drill the mounting holes at the same time the "step" is machined so that you know they are parallel. You will have at least 5-10 thousandths play in your screw holes, you should be able to get the holes lined up inside that amount.

If you find that the shank or head of the screw id pulling the rail, you can grind down the diameter of the head or the shank where it would contact the rail to gain some additional clearance, while leaving the part which needs the threads intact.

Matt