# How to round a corner to a wanted radius ?



## compact8 (Feb 26, 2022)

For a corner or arbitrary angle. What is the way to do it ? It doesn't have to be very accurate. I have got a hobby-grade Emco indexing table ( no ball bearing ) which can be used as an unprecise rotary table but I can't figure out how to set it up.


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## Nutfarmer (Feb 26, 2022)

Find the center of t!he radius. Then center that point in the center of the rotary table. Next off set the spindle by the amount of the radius.  If there is a hole in the center of the radius ,put a shaft through the hole that has a slip fit . Put the piece in the vice with the shaft  resting on the top of the vice jaws. Make a series of cuts moving the part a little each cut around the shaft axes. It will produce facets that can be smoothed over with a file


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## compact8 (Feb 26, 2022)

Nutfarmer said:


> Find the center of t!he radius.


This is the hardest part. If the workpiece does not have a hole, how can the center be found ? The angle to be rounded off may not be a right angle.


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## Winegrower (Feb 26, 2022)

If I understand your question, it’s a geometry problem.   If you have any two lines, say the edges of your part, you want to draw a circle or arc tangent to both lines.   The center of that arc is placed over the rotary table and proceed as Nutfarmer fully explained.

Draw the lines, extend to where they cross, then move back the same distance on each line, draw perpendiculars and where the perpendiculars cross, that’s your center, and from there to the lines is your radius.   If you don’t like the way it looks, move back either more or less and repeat.  This works no matter the angle the edge lines intersect at.

If there’s no hole, then the challenge is in figuring how to hold the part on your rotary table.   That depends a lot on what you have available and how clever and inventive you need to be.


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## compact8 (Feb 26, 2022)

Winegrower said:


> The center of that arc is placed over the rotary table and proceed as Nutfarmer fully explained.
> 
> If there’s no hole, then the challenge is in figuring how to hold the part on your rotary table.


Finding the center of the arc is no problem on the drawing as I use CAD. The challenge is to locate the center on the workpiece and place it at the center of the rotary table.     If the radius is large enough may be I can scribe some lines on the workpiece, align the center of the rotary table with the spindle and use a pointed tool to line up with the spindle with the arc center marked on workpiece. However if the radius is relatively small that will not be doable. 
The workpiece can be held down with normal clamps so thats not a concern.


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## AGCB97 (Feb 26, 2022)

I don't get it. What is the shape of the workpiece? Is it round? Do you have an edge finder?


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## compact8 (Feb 26, 2022)

AGCB97 said:


> I don't get it. What is the shape of the workpiece? Is it round? Do you have an edge finder?


The workpiece is a polygon plate. Some of the angles are not right angle.


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## Lo-Fi (Feb 26, 2022)

Grab a pin the size of radius you want, put it in and accurate chuck. 
Center the table under the spindle. 
Indicate a parallel in on top of the table some distance away from where the workpiece will be and clamp it.
Place the workpiece on the table and bring the pin down to just above it. 
Use an adjustable parallel between your fence and workpiece to bring one edge inline with the edge pin. 
Slide it in the other direction to bring the other edge inline with the edge of the pin. 
At this point, the pin should be perfectly describing the radius you want to create.
Clamp your workpiece.
Offset your table in the same direction as the fence an appropriate distance for the cutter you're using, remove the adjustable parallel and have at it. 

The key here  is having that fence on the table, it gives you a zero degree ref to place the part against and the rest falls into place. Joe Pi has some great RT videos that go into more depth.


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## AGCB97 (Feb 26, 2022)

So put it on your rotary table (location is not critical) and attach a DTI to the locked spindle or mill head. Now sweep the DTI along one edge using X or Y travel and turn the RT till there is no deflection in the DTI. Mill that corner. Move to the next edge and repeat.
Aaron
EDIT  You could also save a little time by using the parallel as Lo-Fi stated above if there are very many to do.


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## graham-xrf (Feb 26, 2022)

From about 6 days ago, Quinn has been showing how on this very thing. She was after losing most of the setup time.


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## compact8 (Feb 26, 2022)

Lo-Fi said:


> ..... Use an adjustable parallel between your fence and workpiece.....


I like the idea. Have heard about adjustable parallel but didn't think I need it until now. Will explore in that direction.


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## Flyinfool (Feb 26, 2022)

It depends on the needed radius, If the radius is small enough a corner rounding end mill is fast and easy but works best on square corners.

This is a part I had to make. It is .250 thick Stainless.
There are 3 outside radii and 2 inside radii all radii had to be tangent on both ends.
I then used a corner rounding end mill to put the nice radius all around the top edge.
This project sounds a lot like what you are doing.

In this case I had to make 28 of them so I made a fixture that went into the #2 Morse taper in the center of my rotary table.
I did all of the calculations and layout in cad to design the fixture.
The fixture was a flat plate with 5 sets of mounting holes to mount the part. Each set of mounting holes would put the center of the radius exactly on the center of the RT. The mill DRO was zeroed on the center of the RT for the full process.

First OP was to cut blanks out of bar stock
Drill & C'Bore the bolt holes for a very tight fit on the screws so I could use them for locating on the fixture.
Drill and tap the center hole.
Flip the part and using the first set of holes as reference, drilled and C'Bore the 2 pockets for the O'Rings.
Back to the band saw to knock the 2 corners off so I had less stainless to turn into chips.
All of the outside profile was done with a 3/4 dia 2 flute indexable cutter, all of the rest of the work was HSS.
Then I had to mill one of the straight angled edges to the center of its R, rotate the RT to cut the first R, Remove the part and bolt it down to the next position, rotate the appropriate degrees and so forth till done.
Then came back and did it all again with the corner rounding end mill.
I then made a pass with the fly cutter to give it a nice look on the top surface. The tram was kicked very slightly "off" in the direction of cut to get just a semi circular pattern as the customer (ME) preferred that look to the pattern you get with the tram set perfect.
Last thing was to open up the bolt holes to a standard screw clearance.


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## MrWhoopee (Feb 26, 2022)

Belt sander


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## C-Bag (Feb 26, 2022)




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## machPete99 (Feb 26, 2022)

Joe Pi has a series of good videos on rotary table use and the geometry involved:


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## BGHansen (Feb 26, 2022)

Excellent videos by Joe Pi.  I think the OP is asking how to setup his rotary table and subsequent work for cutting a radius.

First step is centering your rotary table on the mill's spindle.  I'd start with a dial test indicator in the spindle with just the rotary table on the mill table.  Turn the spindle by hand sweeping the DTI around the center opening of the rotary table (or around a pin in the center of the table).  Zero out your hand wheels.  Your rotary table is now centered on the mill's spindle.

Second step is centering your work's radius center point on the center of the rotary table.  When you layout your project, you should know where the center of the radius is.  Center punch it.  Put a wiggler with a braille (sharp) point in the mill's spindle.  Set your work on the rotary table and eyeball your center punched hole to close to center.  Fire up your mill and true the braille point.  Bring the wiggler point down into the center punched hole and retract it.  If it's spinning off center, tap the work on the rotary table to better center it and repeat.  Once you can lower/raise the wiggler into the center punched hole without the wiggler going off center, you've got your part's radius center point centered on the mill's spindle.  Clamp down the work (best to have it lightly clamped to start with, then snug it down).  Repeat the wiggler check to confirm nothing moved.  

Now that the part's center is on the spindle/rotary table center, you can move the mill's table in X and/or Y to your radius dimension (plus the cutter's radius).  You can also do this with an edge finder with a pointed end.  The wiggler or edge finder are under $20.

If the tolerance only requires about +/- 0.010" accuracy, I'd use an SDA laser center finder (~$140 or around $40 occasionally off eBay).  They make at least three variations of these:  Center dot only, cross-hair and concentric circles with center dot.  The circle one is really handy for getting within 0.010" (or better) of an existing hole or boss.  Move the spindle up/down to change the size of the circle(s) to match your feature, then move the table until the circles line up.  I'd use the laser to locate the rotary table center on the spindle's center.  Set the work on the rotary table and move the work to pick up the work's radius center.


Bruce


Edge finder with a center finding tip




SDA laser/center edge finder



Doesn't show very well, but it's projecting circles on a dividing head indexing plate



Caveat on the laser is the line is wider than a scribe line.  You can adjust the intensity to get a finer liner at the cost of visibility.  I turn off the shop lights when using.


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## compact8 (Mar 1, 2022)

After tring varies methods, I found that having a hole on the workpiece at the center of the arc is the most convenient. At places where such holes are not available. Gluing one on works quite well.  A thick washer of the wanted radius with a center hole is made. Then it is positioned over the workpiece as shown in the photo,  super glue is sparingly applied to a few points along the base of the washer to hold it in place.  The washer can be snapped off easily without heating after cutting is done.


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