[How do I?] measure linear rail flatness

ok...so one long lunch later I'm feeling better about things. I took the rail onto the surface plate and the total deviation was under .002" cool...it's as flat as I could ask for.

So I put the rail back on the base and tried Ken's method. Indicator right next to the bolt head, and starting in the middle of the rail and worked outward.
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All but one of the heads was within a few tenths, and the other one was about .001 low. I kind of knew it had a low spot there as when i stoned the top the lubricant would always pool there. Cool...so after adding the appropriate shims I was curious and went back to the repeatometer trick. The whole rail is within .001 now. Cool! Starting to feel confident that it's actually flat.

so now onto measuring parallelism to the slave rail!
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I swept the surface and found the high spot, set my zero and went to work. Not the best results I'm afraid. It's low as much as .010 in one area! But I feel a little lucky, I stuck a straight edge across the top of the master rail and measured the change in height to the high spot on the slave and it's within .005". Well within NSKs advertised limit. So I guess I have some shimming to do, and then onto the X axis
 
Before you jump over and shim up the slave rail by .010", measure the height of the surface to the center portion of the slide you have there and see if the mounting surfaces are at the same height. If they are, that means in my book that you are ok and don't shim the rail. The NSK bearings will have a slight tilt side to side. Not easily notice at the rail, but when reach out over the slide base as you show in the picture, it's multiplied by the length of the arm of you dial indicator mount. Wild guess, if that's 10" reach there, then 0.001" movement at the NSK bearing, that's .010" drop at the dial indicator. Think about it a bit. Ken
 
Ken, maybe I'm not following your logic.

I've tried to measure the height difference from the top of the master rail to the top of the slave rail the best I could. this was around .005 difference. I did this at the highest spot on the slave rail in order to be able to shim the rest of the rail up. So once I did that I zero'd the indicator at the high spot and swept the rest of the rail to get the deviation from that point. My thinking is if the master is straight, and I pick a point on the slave, any deviation from that point is the change in height from the master. Or am I overlooking something?

I also did check to see if the bearing was tilted a bit. To measure the height from the master to the slave I placed an Al extruded bar (I know I know, it's the best thing I had long enough) on top of the master block and measured from the top of the bar to the top of the master rail near the bearing, and then again to the slave rail. I then took off the bearing, flipped it around 180* and measured again. Only a few thou difference. This tell me the bearing is pretty darn flat, but I suppose I could still have a twist in the rail.
 
but I also see your point. If my master rail still reads .001 off in spots it will certainly exaggerate the reading at the slave side.
 
If you look at the photo of the rails on the ways, with the indicator and stand over the top, do you see the flat areas between the dovetails with the numbers written on it? That is the original, unworn, untouched factory reference surface. They made that nice machined area for a reason. Lightly stone any burrs off the surface, and then use a depth mic to measure from there down to the ways. You are over thinking this. Direct measurements are always preferred. The introduction of intermediate stuff just clouds the issues and offers more places for errors to accumulate.
 
You might be measuring Twist in your primary when you see vertical deviation in your secondary.
 
If you look at the photo of the rails on the ways, with the indicator and stand over the top, do you see the flat areas between the dovetails with the numbers written on it? That is the original, unworn, untouched factory reference surface. They made that nice machined area for a reason. Lightly stone any burrs off the surface, and then use a depth mic to measure from there down to the ways. You are over thinking this. Direct measurements are always preferred. The introduction of intermediate stuff just clouds the issues and offers more places for errors to accumulate.
Yeah, you're right. This is what I ended up doing and saw only .0005 difference from left to right side. Although interestingly enough there was a slope from front to back. Not a problem as I can shim the column to get square.

I wish i had a surface plate large enough to fit the whole base, but unfortunately I do not.

So looks like that issue is solved and I'm back onto the retrofit!
 
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.
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so as i slide the blocks down the rail the indicator will measure deviation from the block with the indicator to the other block.
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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....
Large precision granite square or straight edge and thin shim stock or feeler gages will get you close. Then a flashlight to finish. No kidding. A 24" of mediocre quality granite straight edge is only a few hundred dollars. I have a small granite square I use for checking perpendicularity, but have not yet picked up a longer straight edge for checking flat.

A large granite surface plate might work, but you would have to 4 corner level the base first before trying to measure flatness. I assure you that cast base is not likely to be perfectly perpendicular between its precision top surfaces and its feet.
 
yeah, I was a little hesitant to use the top surface just for that reason. But in the end, i'ts the best I can do without a surface plate. I also filled the base with epoxy granite, so I'm hoping that will help resist any deformation in the base when I bolted it down.
 
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