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Shaping an external (semi)circular profile

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spike7638

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#1
Suppose I have a strip of 1" wide, 1/8" thick flat stock --- let's say stainless because it's what I'm mostly likely to need this for --- and I want to make it into something like a tongue-depressor or popsicle stick: basically, I want the end of the piece to look like a 1"-diameter semi-circle.

I can do this in a sort of crude way, by scribing the circle, using a bandsaw to cut near it, and then finishing off with a belt-sander to come pretty close to the line, and then doing a bit of filing to remove any burr or scratches. But what I'll get won't be a very accurate semicircle; nor will it be very pretty, given my hand-tool skills, but I'm working on those.

Is there a way to do this task more precisely on basic machine tools (like a lathe and mill, but without CNC)?
 

tweinke

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#2
Rotary table is my first thought. But that ruins the basic tool stipulation.
 

RJSakowski

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#3
A rotary table would be the easy way.

If you have a DRO on the mill it is possible to mill a series of stepped moves to rough out the profile and then hand finish. My Grizzly DRO has a program that calculates the position of each of the steps. It loads them into the way point table and gives the distance to go to hit the next way point. You have to make sure to move x and y in the proper sequence so you're not cutting into the part and it is PITA if you want fine steps but it can be done.

Another way to do it is to make a jig with a pivot and a clamping method for your strip. Place the strip in the jig so the center of the adc you wish to cut is concentric with the pivot and clamp it, Rotate the part and jg on the pivot to cut the arc. I had done this years ago to cut a curved slot in some sheet.
 

rock_breaker

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#4
Handwork on a lathe face plate. It would require some serious dial indicator work and a Dremel grinding tool to get the finish you want and absolutely no power in the lathe motor.
Have a good day
Ray
 

Janderso

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#7
I have a new to me Rotary table, not sure what to do with it yet.
 

rock_breaker

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#8
There was a youtube video, tips and tricks, Dale used drill stock as a pivot in the center on top of the mill vise, he spun the flat steel stock to make a radius. I'll see if i can find it. Her it is ....
Enjoyed and appreciate your corner rounding video.
Have a good day
Ray
 

RJSakowski

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#9
There was a youtube video, tips and tricks, Dale used drill stock as a pivot in the center on top of the mill vise, he spun the flat steel stock to make a radius. I'll see if i can find it. Her it is ....
However only works if there is a center hole. I don't believe that the OP indicated so.
 

homebrewed

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#10
Awhile I made a fixture to put a radius on a carbide scraper. It's possible it could be adapted for your needs. It consisted of a holder which had a radius at one end (your choice on how to make THAT -- the pivot scheme would be fine). The other part of the system was a table for my grinder, which had a low fence on the side closest to the grinding wheel (the fence can't be any higher than the thickness of the holder). The work is attached to the top of the holder so it extends past the rounded end of the holder, and brought up to the grinding wheel. Grind at all angles until the rounded end of the holder is in full contact with the fence. Done. The resulting radius should end up as smooth as the radius on the holder.

I didn't need a highly precise radius on my scraper (or a small radius of curvature) so this worked OK for me. Getting a precise radius of curvature, or a small radius, would be a different can of worms.
 

Eddyde

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#12
I can do this in a sort of crude way, by scribing the circle, using a bandsaw to cut near it, and then finishing off with a belt-sander to come pretty close to the line, and then doing a bit of filing to remove any burr or scratches. But what I'll get won't be a very accurate semicircle; nor will it be very pretty, given my hand-tool skills, but I'm working on those.
You might be surprised just how nice a radius you can get that way. Of course it won't be super precise but for most applications that is my go to method.
 

brino

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#13
I have never done it, but I have heard multiple times about a simple pivot pin for the work piece that is rotated by hand.
I believe that's what Bob alluded too above.
something like this:

Warning, this will be considered totally unsafe by many people....but there it is.
1) give yourself as long of lever handle as you can for better control
2) do not take deep cuts

If you do not have a centre hole to pivot on, then you need to come up with something that clamps to the bar and does have a pivot.

-brino
 

brino

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#14
I had to run out to the shop to get photos of a circle sanding jig I made for my little disc sander a few years back.
It makes rounding ends of things fairly clean and easy, if you can put up with a pivot hole.
I suppose you could make a work clamp that holds the work piece and pivots on the pin, so no hole required.

It is mostly MDF with a wooden runner to fit in the sander table mitre slot.
The aluminum channels and nuts are from Lee Valley:
http://www.leevalley.com/en/hardware/page.aspx?p=61986&cat=3,43576,61994,61986
http://www.leevalley.com/en/hardware/page.aspx?p=45161&cat=3,43576,61994,45161

The mitre-slot channel is epoxied between the two MDF pieces.
The t-slot aluminum channel is captured by a nut on the hex head bolt you see on the bottom.
Loosen that bolt to set the radius, then lock down for sanding.
The set-screw is used as the pivot, and is short enough that you don't need a thru-hole on the work-piece.

Here's the top of the jig:
top1.jpg

top2.jpg

...and the bottom:
bottom.jpg

bottom2.jpg

Here's the disc sander table with mitre slot:
disc_sander_table.jpg
....and how the jig fits the table:
adjusted.jpg

...and a close-up of the channels and t-nuts:
t-nut_close-up.jpg

I don't use it often, but it is quick and easy to use.

-brino
 

RJSakowski

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#17
Brino, In my case. I used a screw and a nut which was able to pivot to make a crude lead screw system which allowed me to control the rotation. The piece being slotted was fastened to a movable plate controlled by the lead screw. That was around thirty years ago. I should still have the mechanism but for the life of me, I don't know where. If I find it, I'll shoot a picture.
 

Charles Spencer

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#18
I used to make chisels that attached to the sides of cutting dies to split the excess material and prevent it from binding. We rounded the back end of the chisel. Since the back wasn't critical - basically it just looked neater - we laid out the curves and ground off the excess material by hand.
 

brino

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#20
Brino, In my case. I used a screw and a nut which was able to pivot to make a crude lead screw system which allowed me to control the rotation.
Thanks for the clarification, Bob.
Your method is both safer and precisely controlled.
-brino
 
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