# Need formula to measure angle of machine dovetail slide



## todd774 (Mar 11, 2017)

Does anyone know off the top of their head how I can find  a formula to measure a milling machine
dovetail way angle so I can make a straightedge for scraping (for example). It seems to me there is a formula that can be used with micrometers & dowel pins.

I found some info. on formulas if you know the angle already. I'll admit to taking a shortcut by asking someone else. I'm just spending too much time trying to figure it out. I don't want to reinvent the wheel so to speak so
I don't want anyone to go out of their way too much if I can do the same. I just thought somebody might be able to pretty easily point me in the right direction.
Thank you

  Todd


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## Dave Paine (Mar 11, 2017)

Feels like this is simple trigonometry.  Measure the height call it something like "A", called the "Opposite" in trig.  Measure the width, call it something like "B", called the adjacent.

A / B is the tangent of the angle.  Take the value and look it up as arc tangent.  There are many sites with trig calculators, like this one.

http://www.rapidtables.com/calc/math/Arctan_Calculator.htm


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## Bob Korves (Mar 11, 2017)

The straightedge for marking up dovetails for scraping needs to have a smaller angle than the dovetail.    The straightedge needs to be tipped so it rubs on the dovetail surfaces accurately.  A 60 degree dovetail would use a 55 degree or thereabouts straightedge to mark up the dovetail surfaces, one at a time. The actual angles are nominal and are pretty much meaningless.  The only important things are that all the sliding surfaces end up precisely parallel and with the contacting surfaces fit so they have contact over the entire surface, with perhaps 20 points per square inch contact.  Dowel pins and various precision measuring tools are required to keep the spacing of the dovetails accurately parallel as you proceed.  If you want to really understand all this, read Connelly's book, Machine Tool Reconditioning, the bible for that work and a free download on this site:
http://www.hobby-machinist.com/threads/connelly-on-machine-tool-reconditioning.41802/


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## Bob Korves (Mar 11, 2017)

I don't know your skills, Todd, but I might add that scraping dovetails on a milling machine is not where to start learning scraping skills.  It is pretty fussy work, not for a beginner.  If you do not already have the experience to do it, you will need to start learning on simple flat surfaces and slowly work your way up to more difficult projects like reconditioning machines, where there are lots of other considerations, like adjusting or maintaining the geometry of the machine as you proceed, and starting from the correct reference surface to get the correct final result.  It is not really difficult, just needs the correct plan and the required skills.


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## willthedancer (Mar 11, 2017)

If you're just comparing the mean on the dovetail from place to place, there are some considerations to keep in mind.

If there is a curvature in the flat weight bearing areas, it will measure as additional wear in the dovetail.

All you need are ground pins that give you tangent points on the surfaces you want to check, and have the opposite side of the pin outside of the dovetail far enough out to get the mic on it. The pins don't even need to be exactly the same diameter.

Sent from my Moto G Play using Tapatalk


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## todd774 (Mar 11, 2017)

Wow. That's a lot of good information. You guys are alright. Thanks for the link to the book Bob, & the other info. I downloaded it into a folder. Lindsay sold a similar book (if not the same) a few years back for big bucks.
I always regretted not getting it. Yes Dave, that appears to be a tangent problem to solve. I seem to remember the famous Indian now SOH CAH TOA . Tangent / Adjacent.
I'll get back to this later when I can concentrate on it. 
I'm setting up in my mind how I am going to increase the travel of my seig x2 cnc milling machine that I just built. Always trying to learn & increase my skill level.
Thanks to you too "willthedancer"
Todd


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## Bob Korves (Mar 12, 2017)

The book retails for about $100 in hardcover.


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