[How do I?] measure linear rail flatness

cs900

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Ok, so a little background info. I'm in the process of converting my mill ( a PM45) over to linear rails, and i'm to the point where i'm getting ready to install the master rail for the y axis on the base. So I'm trying to measure the flatness of the master rail so i can then measure for parallelism on the slave rail.

So on to the good stuff...I think i've come up with a good method for checking flatness, but would like to hear you guys opinions. In my efforts i think i've come up with a poor mans repeatometer.
XJKwHhI.jpg


so as i slide the blocks down the rail the indicator will measure deviation from the block with the indicator to the other block.
FY7Hnqi.jpg


so if I'm thinking this correctly, i'm getting a fairly consistent deviation of -.002 to -.003 along the rail which tells me my rail is bowed down at a fairly consistent rate. So thoughts, is there a better way to do this? Am i completely off my rocker?

guess i should also add, that my surface plate isn't large enough to fit the base...and also the base is about 180lbs so moving it is a bit of a chore....
 
Gauges like that one (and the Rahn Repeat-O-Meter) do not show flatness or level. They simply show changes from one area to another (whether the path is moving more up, more down, or staying the same), which must then be interpreted. They do not know level or flat. To get a bit of understanding, imagine the rails were warped into a perfect vertical circular hoop, 50 feet in diameter. The gage would move along with a steady needle all the way around, but the surface would not be flat, correct? Gages like that one need to be used in conjunction with another type of measuring tool. To use your surface plate example, it would typically be an autocollimator, which compares angles very accurately. Together, and with charting all of the results, the readings suggest the actual 3D topography.

Still, very nice job of rolling your own test rig! Now you need the poor man's autocollimator...
 
Thanks for the reply bob. Yeah, i tried to word the errror i was seeing to reflect that i measured a convex bow in the rail since each "step" i took away from the back side showed a negative value, that just happened to be fairly consistent along the length.

So after thinking about it for a while i think i came up with a better approach, but absolutly would like to hear feedback still. My reasoning was that if the base is indeed bowed i could loosen all but the back bolt which would allow the rail to become straight again. So i did, and using the same method as before verified no real deviation from point to point on the rail. I then went back and measured the deviation using the rail as my straight reference. 20180119_004810.jpg
Sweeping to the end net me about .004 deviation, which seems more realistic than what i was seeing before.
20180119_004820.jpg
So this confirms the convex bow in the rail i was seeing before, and i think i can just shim the rail based on the deviation measured from this method. Thoughts?

And im absolutly going to look up the autocollimator and see what i can rig up, haha.
 
Can I ask where you got those rails? I have a job going on now that needs a heavier set than what was given to me.
 
the rails are NSK LH20. I actually got these used off ebay, and was super fortunate as they look like they were never used. They also have these cool lubricating strips sandwiched under the endplates on the blocks (the little tan pieces) that according to NKS are all the lubrication the blocks need.
For what it's worth I got them from seller athomemarket
 
One thing you didn't do when you laid the rail down on the bed. You need to take your feeler gages and check for gaps under the rail and the bed. Your bed is more than likely not flat from wear in areas causing the readings you are seeing. Linear rails are extremely flat and straight right out of the package. You will probably have to shim up under the rails with different thicknesses if shim stock from 0.0005" to 0.0040" in thickness. If you know how to scrape and fit slides, you can scrape the bed to a flatness using the rail as a straight edge.
 
One thing you didn't do when you laid the rail down on the bed. You need to take your feeler gages and check for gaps under the rail and the bed. Your bed is more than likely not flat from wear in areas causing the readings you are seeing. Linear rails are extremely flat and straight right out of the package. You will probably have to shim up under the rails with different thicknesses if shim stock from 0.0005" to 0.0040" in thickness. If you know how to scrape and fit slides, you can scrape the bed to a flatness using the rail as a straight edge.
that's a good idea. I'll try checking it with feeler gauges tonight. I tried shimming it this morning based on the error I saw, but doing the ghetto repeatometer test again showed the same bow in the rail after shimming.

I have the blueing compound and a carbide scraper from when I fitted the gibs to the mill maybe I'll give that a go.
 
Another thing to do along with what I said above.
Start in the middle of the length of the rail. With all of the cap screws loosen, Place your dial indicator next to the cap screw, on top of the rail. Zero the dial with the cap screw loosen. Next tighten the cap screw and note the amount of movement. You should get somewhere around.004" as you indicated above. Place a .004" shim under the capscrew between the bed and bottom side of the rail. Repeat tightening the cap screw. You should get 0.000" reading. Get it within .001" and move down the the next cap screw and check.

When tightening cap screws holding a rail down. Start in the middle of the length and work outwards to the ends, just like you would do installing a head on a engine. This should get you the best of accuracy and alignment of the rails when done.

Ken
 
If you can get access to a surface plate, do so, and check for flatness of the backside of the rail. Easy to do, place your indicator at the middle of the length of the rail. Press down on the rail and see if the pointer on the indicator moves. Let's hope it don't. If it does, you have to take the readings in account to fitting to the bed. Don't try to straighten a rail! Your best to buy another one to take its place.

Don't rely on the bed of your lathe to be flat!!!! Just looking at the pictures, part of it is machined, the old ways could be scrape to a known flatness? I would not trust it without verifying with a know flat straight edge. And even then, does it have a twist in the bed? A lot to think about here. Take it a step at a time. You're doing something I've wanted to do is build a CNC lathe using linear ways. And using a existing bed is a good start as long as you know it is straight and flat without a twist.

Let us know how you make out. We're here to help.

Ken
 
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interesting. The NSK installation manual indicated you should start at one end and tighten the bolts in a row.

I do have a surface plate, and I'm going to check the rail on my lunch break.
 
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