Shaping an external (semi)circular profile

[kd4gij]

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

Box it up and send it to be. f it is bigger than 6" then disregard.

 

[Eddyde]

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]

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]

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:




...and the bottom:




Here's the disc sander table with mitre slot:

....and how the jig fits the table:


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


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

-brino

 

[savarin]

heres how I did it, https://www.hobby-machinist.com/threads/the-giant-binocular.55688/ message 21.
It would be a simple matter to make a clamp for your part and do the same thing.

 

[brino]

@savarin posted the other one I was looking for!

Here's a direct link:
https://www.hobby-machinist.com/threads/the-giant-binocular.55688/post-520251

-brino

 

[RJSakowski]

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]

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.

 

[spike7638]

Wow! What a great collection of solutions! Thanks, everyone!

 

[brino]

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