Calculate Error from uneven rail and reference

[mekanik]

You stated in your previous post that you had a length of heavy duty aluminium 1500mm long and an acurate machined surface that was straight to within 0.004mm if memory serves, use this to adjust your rail, i did explain how to do this some time ago as this question has been asked countless times, start fixing the rail from the center work your way alternately towards each end, you will then in your case have a section of rail 1500mm straight to the given accuracy of your reference gauge, you then use your secured section of rail as a datum for setting the remaining unsecured section of rail.

[feldah]

Sorry when I asked the question again. I had to give back the Aluminium. As my rail is set now. But my question was more out of theoretical interest.

[John McNamara]

Hi all

One quite simple way to set rails straight is with a stretched wire. Together with a cheap digital microscope (Get one with electronic cross-hairs) off the net. Alternately an optical microscope can be used with a cross-hair reticle. Mount the microscope on a rail slider on the already positioned rail beside the stretched wire you have mounted in your machine frame already positioned in line with where the rail will be placed, best you offset it say 20mm to allow access to the rail screws.

The procedure is simple. just work your way along the rail centering the microscope cross-hairs on the wire as you go.

You will need to make up some sort of mounting for the wire to position it accurately. One end will be fixed one end will have a large weight attached to it.
The weight is normally calculated to b about .75% of the breaking strain of the wire. http://www.roeslau-draht.de/fileadmin/user_upload/Downloads/music3B_Zb_2.pdf

Thinner is better mine is .17 Round not hex.

This is not a new technique Wire alignment goes way back in time. It is still used to this day.

https://www.google.com/search?rlz=1C...iz.tTYd1bOv-T0

Accuracy of 0.001" is obtainable over long lengths.
Gravity will affect the vertical plane, to check the flatness of your rail mounting surface the Catenary sag of the wire will need to be calculated for various positions along the wire. Think of sagging power lines. https://www.google.com/search?q=cate...hrome&ie=UTF-8

Over less than a metre the sag is very small.

Cost
https://www.google.com/search?q=usb+...w=1008&bih=537

You do not need or want high power about 40 is fine.
A handy tool to have anyway
100 feet of wire
https://www.fortepiano.com/index.php...usic-menu-item

Regards
John

[AndyGuid]

Thanks John, I love the simplicity and low cost of that idea!

[mekanik]

Hi John
Do you have a link to a cheap digital microscope, we used to use the Taylor Hobson kit @ work but there are no cheap options that i know of.
Regards
Mike

[John McNamara]

Hi mike

I also have a Taylor Hobson alignment telescope, not the digital just the older optical model, It is quite a fiddle to set it up and once set if it is knocked it will go out of alignment, having to set the targets and keep checking is slow.

I restored a lathe a while back and used wire, it was a Heidenreich and Harbeck 20 Ro 2.4 metres between centres. to check the straightness of the bed I attached the wire along the side of the bed level with the top of the V's and used an optical microscope. not fancy just a student microscope. the wire was firmly attached and did not need to be constantly checked it never went out of alignment being firmly bolted in position. The microscope was attached to a stage that was guided by the V ways was fixed over the wire and positioned with a Mitutoyo micrometer stem (They are available to be built into an instrument). error readings were taken with the micrometer after positioning the microscope over the wire. it worked very well.

I later purchased a cheap USB microscope About 40AUD it came with software. regrettably I lent it out and it never returned! Hmmm.

So I also need to buy a new one too with cross hair measurement software.
The trouble is there are hundreds of models available for a few quid.

Maybe someone in here has purchased one and can report on it?

Regards
john

[mekanik]

Hi John
Thanks for your reply, since i have been thinking about the rail alignment issue i have wanted to go with optical alignment as this seems the best way to do it, but the cost of a Scope/targets ect makes it a none starter. if i had still been working i could have borrowed the kit from work. will look into USB microscope idea.
Regards
Mike

[John McNamara]

Found a photo of the setup I mentioned, it was just a lashup for a one off, it worked well. Note the bolted on wire support and adjustment unit