Setting rails straight, and keeping them straight - how?

[gerald_d]

Hi All

One of the most fundamental points of CNC machines is that the axes are straight. But, with good measuring tools (eg. autocollimator), we will see curvature, bumps, twists, dips, etc. in any rail. All of us have probably fiddled with at least our long x-rails and have gotten to the point where we decided, "this is straight enough".

Now that we have a very stiff gantry, we can see that our x-rails are not as straight as we had thought - at some spots, one flank of a v-roller loses contact with the rail. (I can slip a 0.002" feeler gauge in). A more flexible gantry simply bends to accommodate that small imperfection, and maybe the stiff gantry will cause the high spots to wear off....?

Has anyone got some ideas on getting really straight rails? Let's keep it to our SB-styled "sharpened" plate/angle edges, and not get into linear bearings. (Unless we look at the beds for linear bearings, which also have to be straight - the bearing just bends to fit the bed).

For, our gantry lengths, we can use a local guy that has a "bed way grinder" on which he repairs lathes and other machine tools, but our wide table won't fit on his grinder to do the x-rails. I am trying to think of a way of re-grinding the x-rails after they are bolted/welded down......

[marshawk]

I just had a picture in my mind of a cutter/grinder attached to the Y cart, grinding the rail as it travelled down it's path. The first 2.5 feet would have to be done manually...

Just a thought...probably garbage...

Chip

[gerald_d]

No Chip, not garbage. We can start with rails that are too long, and cut off the "bad" points later. Snag is, how do you run the y cart down a straight path?

One could lay down temporary rails, provided that one can find straight temporary rails. Have thought of alu extrusions, but their specs aren't all that great either.

[gerald_d]

Just to illustrate the problem of "straightness".....

Let's say we had a perfectly straight (theoretical) steel round bar 2.5" diameter, 8' long. When that bar is supported horizontally, held by its ends, the center droops about 0.02" under its own weight. Of course someone will say that the bar is not straight - put a trestle under the center of the bar and you can bend it 0.02" the other way. This illustrates the myth of the straightness of linear bearings - they are only as straight as the bed that you made for them to lie on.

[marshawk]

Well, A pair of "calibration" rails attached to the "working" rails would probably do the trick. If you used them only a few times a year, they would not be worn down appreciably for several years.

Retractable wheels could be mounted on the Y cart to run down the calibration rails, and lift the y cart off of the working rails.

Checking level and true would only require a string stretched tightly across the top.

There would have to be a solid, adjustable, metered mount for the grinder, that can be attached to each side of the Y cart.

What better way to ensure a true wheel path than by using the machine that runs on it?

You're right...the idea wasn't garbage...

Chip

[dingwall]

Gerald, just a thought here. For setting and checking our guitar necks for straightness we use an aluminum beam with a stand-off at each end and a dial indicator in the middle. In our case the standoffs are two 1/4" allen bolts with jam nuts to secure them. The dial indicator is zeroed on our flattest machine table.

Then to fine level our fingerboards we use an aluminum I-beam with 3M stickit grit.

The face of the I-beam was previously trued by rubbing it on the machine table using stickit as an abrasive.

Of course this would only take down the humps. For the dips, would it be possible to hold an anvil against the outside of the rail and lightly tap the inside of the rail with a ballpeen hammer to displace the metal a bit and force the edge upward?

[rhfurniture]

I have put hardened bwc rails on bright steel angle on my benchtop Y axis and the results were excellent. I havn't applied high tech measurement, but the movement is way superior and the rails much straighter visually and every other way I can devise. I did use the next size up though.

[gerald_d]

There will be ways to force an already machined rail into position - hammer blows, shims, weld/heat shrinking. But this will probably leave smaller wobbles that are still best removed by a final skim.

A big part of the problem is to actually measure the straightness of the 2 individual flanks of the ^. I don't think that string is anywhere near to good enough for this. Sure, a string will ensure a straight enough line for a cut board, but the snag here is that a stiff gantry forces the roller to climb up the rail, throwing out the z-height. (The climbed-up roller also doesn't seat firmly and that gave us some of our "chatters")

rh, wow, you are blessed to have bright angle iron over there! Increasing the wheel size would reduce the wear that we see on our soft rails.

[gerald_d]

To understand what a collimator does, imagine mounting a laser pointer on a short wheelbase car that ran down the rail, with the beam pointing like a headlight down the rail. But 30' away there is a target for the laser to shine at. If the rail was straight, the laser would stay steady on the target as the short little car rolled down the rail.

A collimator is a telescope looking at a mirror mounted on the short car. Looking in the telescope you see in the mirror the face of the telescope dead center, unless the mirror car has tipped or swayed. This is put very crudely to illustrate the principle only.

[dirk]

Here's an earth moving idea:
Start with long rails, maybe 2 to 3x longer than what you finally want to end up with. Make a grinding jig with a long wheelbase to ride the rails. every pass should get better and better. On the same principal as a motorgrader used to finish grade moving soil. The reason they're so long is any humps the front wheels run over is a lot smaller hump transfered to the blade. The next pass you've eleminated the original hump and now have a small hump in a different location. You could even get a little high tech by using a laser to adjust your grinding wheel height just like a lot of motorgraders use.
Dirk