# Question about necessary tolerance for a "slip fit"



## Building Something (Mar 20, 2013)

I'm going to cut a small slot in a piece of Aluminum this weekend, if all goes to plan.  It's a 1" dove tail slot in a 4x6 piece of aluminum.  Anyway I'm thinking of using a little gib strip to hold the piece rattle free to it's mate.

Gentlemen the question:  What do you think is the proper "over-size" for my dove tail slot to make this piece insert easily but have the "perfect fit" feel?


----------



## Ray C (Mar 20, 2013)

Depends on a few things...  How long will the cut be and how big is the piece that fits in the dovetail slot?

Gibs usually have jib screws along their length to take-up the slack.  Overshooting the size by 5-10 thou can be corrected by proper tension on the gib screws.


A couple thoughts if the geometry of the piece does not allow gib screws...  If you're doing this for the 1st time, it's a good idea to practice on a piece of scrap of the same material.  That said, aluminum tends to deform back into position so, I'd cut the desired width plus the width of the intended gib and add 0.001".  When all is said and done, if your cut is truly accurate, it will be a very tight friction fit and you'll probably need to open it up another 1-2 thou or, you could shave a little off the gib.  -Either way works.  The reason I say to cut it close, is simple.  Once you take metal off, it's hard to get it back on.  Even if you over-shoot, you can still make the jib a little thicker.

Dovetails in aluminum are tricky and keep in-mind, it might not be the best material if strength is needed.  AL wears down pretty quick and AL against AL tends to stick.

EDIT:  As you're doing this and testing the fit, try not to remove the part from the vise.  Getting back exactly in  the same position is not in your odds.

Ray




Building Something said:


> I'm going to cut a small slot in a piece of Aluminum this weekend, if all goes to plan.  It's a 1" dove tail slot in a 4x6 piece of aluminum.  Anyway I'm thinking of using a little gib strip to hold the piece rattle free to it's mate.
> 
> Gentlemen the question:  What do you think is the proper "over-size" for my dove tail slot to make this piece insert easily but have the "perfect fit" feel?


----------



## Building Something (Mar 21, 2013)

Great info thanks.  The slot is short and narrow; 1" wide by 3/8" deep maybe, by 2.5" long.  I won't have a chance to test fit the item either.  Ten thou over sounds well within my reach.  

10 thou over wouldn't feel loose or slippery when mating the dovetails would it?  It's got to have a solid feel of course, but the application is low stress, it's a drill able platform for small tools upto three lbs. Hanging off a small bench top tool.


----------



## Ray C (Mar 21, 2013)

Yes for smaller pieces like that, 10 thou will feel a little loose but not if you use set screws along the sides to adjust the jibs.



Building Something said:


> Great info thanks.  The slot is short and narrow; 1" wide by 3/8" deep maybe, by 2.5" long.  I won't have a chance to test fit the item either.  Ten thou over sounds well within my reach.
> 
> 10 thou over wouldn't feel loose or slippery when mating the dovetails would it?  It's got to have a solid feel of course, but the application is low stress, it's a drill able platform for small tools upto three lbs. Hanging off a small bench top tool.


----------



## Richard King (Mar 21, 2013)

If it is tapered you should calculate  the "Taper per inch" .  If you have the old gib take a dial caliber and set it at 1" and lock the screw.  then spray some fast dry layout ink on the gib and scribe the 1" using the ID side of your caliber.  Then measure exactly on the scribed lines to calculate the taper.  Then you can check the gap in the slide the gib will fit.  By either sliding it to the end of each side of the base slide and use your ID side to measure the Big end, and then slide you part to the other end of the bed and measure the ID of the small end.   Or use a telescope gage or gage blocks.  Many times I will bolt it to a flat piece of bar stock steel but drilling and tapping 2 hold down screws to the steel and then mill one side flat and flip it over and then use a dial indicator to set your per inch taper by tipping it in a vise.   The tapped holes can be considered oil holes when your done.


----------



## Ray C (Mar 21, 2013)

Ooooh, I like that....




Richard King said:


> If it is tapered you should calculate the "Taper per inch" . If you have the old gib take a dial caliber and set it at 1" and lock the screw. then spray some fast dry layout ink on the gib and scribe the 1" using the ID side of your caliber. Then measure exactly on the scribed lines to calculate the taper. Then you can check the gap in the slide the gib will fit. By either sliding it to the end of each side of the base slide and use your ID side to measure the Big end, and then slide you part to the other end of the bed and measure the ID of the small end. Or use a telescope gage. Many times I will bolt it to a flat piece of bar stock steel but drilling and tapping 2 hold down screws to the steel and then mill one side flat and flip it over and then use a dial indicator to set your per inch taper by tipping it in a vise. The tapped holes can be considered oil holes when your done.


----------



## Richard King (Mar 22, 2013)

Richard King said:


> If it is tapered you should calculate  the "Taper per inch" .  If you have the old gib take a dial caliber and set it at 1" and lock the screw.  then spray some fast dry layout ink on the gib and scribe the 1" using the ID side of your caliber.  Then measure exactly on the scribed lines to calculate the taper.  Then you can check the gap in the slide the gib will fit.  By either sliding it to the end of each side of the base slide and use your ID side to measure the Big end, and then slide you part to the other end of the bed and measure the ID of the small end.   Or use a telescope gage or gage blocks.  Many times I will bolt it to a flat piece of bar stock steel but drilling and tapping 2 hold down screws to the steel and then mill one side flat and flip it over and then use a dial indicator to set your per inch taper by tipping it in a vise.   The tapped holes can be considered oil holes when your done.




Once you have the gib made and if you want to get it better, you need to check to be sure the gib surfaces are flat and straight.  Even though you are making a tapered gib, each side needs to be straight and flat. *  I recommend you lay each side down on a know flat surface which can be debated to what is flat, but for our benefit now lets say we have a granite surface plate that is 6" longer then the gib.  After you have the gib machined as close as you can by the measurements you you made, file off the sharp and rough edges, lay the gib on the plate and see if you can slide a .001" feeler gage under it all around the edges.  

Then swivel it; hold the right side lightly holding with your thumb and forefinger and push the gib forward 1" and then pull it back 2" and observe where the pivot point is.  then it is flat it will pivot or we call it rebuilding terms "the hinge point".  When it's flat it will hinge at approx. 30% from each end.  We can debate where those rotation or Airy points are, but for out of the laboratory use lets say 30%, note that point.  Then reverse hands and do the hinge test again.  Note where the hinge is.  Flip it over and check the flatness on the other side.  Now comes the fun part.   Many of you know that when you machine steel or cast iron the metal is stressed and it will bend.  One side will show high in the middle and the other side is high on the ends.  

Now I straighten the side that rests against the positive side which is the side that rests against the machine side that does not ride or a surface that slide by it.  Lets say it is high in the middle, you have to bend it straight and I usually bend it by hand with small gibs, but laying on the edge of a table with it sticking out 1/2 way and then pushing down on it with about 20 pounds of pressure, then check it on the plate and repeat if needed. (there are other more precise ways, but to save time lets do it this way now).  I then scrape that side flat so it has contact as close as 100%.  and hinges good.  You can also tap on the top of the gib with a finger or small soft blow hammer and listen for the hollow / solid noise.  In the real world you would know that the surface the positive side of the gib rest against you know it is flat too. but to speed up this message we can discuss that another time.

Now flip the gib over and check the hinge on the riding side.  We would like to see it hinge on the end and we can not slide a .001" feeler gage under it.  You have to not press down hard on the gib when making this test or you could bend it.  If it is good, then slide the gib into the machine and check each end with a feeler gage.  If it is off you can machine it again, put it on a magnetic sign and grind it or scrape it so the gap on each end is the game.  When you are within .001" and want it better you will use a dial indicator as shown in the picture below.  until you have both ends the same.  I only show testing one side, but you would check one end with the gib loose, so you would get say .002" slop or gape.  You would push one way and let go, remember the number and then pull it back and let go and remember the number.   Now loosen the indicator rod and move the dial indicator and repeat the push pull. Then you know what th taper is and if you want it better you would scrape it.   I plan on adding pictures to this thread after we do the Scraping class in GA in a few weeks.   But for now I will just add the following 2 pictures I took at a class in Taiwan back in 2010.   Still awake?   LOL


----------

