cutting a concave radius suggestions

[werowance]

im at work right now wishing i was in the shop and thinking about a project i am working on. i am about to start my first engine the bj cicada and for the front bearing it calls for a .200 radius to be cut. just trying to picture the best way to do this? pdf plans here :http://modelenginenews.org/plans/BJ_Cicada.pdf sheet 2 for my question.

here are my ideas and questions i have on each of them.
1. mount the 1x1 block to my rotary table and use a ball nose end mill. problem here and questions - wouldnt i need a .400 ball nose mill to do a .200 radius? if so then scratch that cause that would be hard to find. and actually ive never owned a ball nose mill before so i may even be wrong on the radius of a .400 mill = .200 r

2. use my boaring head (for the first time) and cut almost down to the radius then switch to a custom ground form tool in the boaring head to get the radius.

3. ball turner on the lathe - really dont want to because i dont have that much material sticking out of the chuck to hold it and still get my ball turner in there.

i realize the radius isnt that criticle to the function of the engine. but wanted to try and not modify the dimensions if i can help it.

 

[Cogsy]

How do you plan to make the rest of the part? From a quick look at it (and I'm far from an expert) I would probably start with round stock in the lathe. From there I'd turn the front spigot, then form the radius with a specially ground piece of HSS. Then I'd reverse it, cut and face it to length and turn the back of the part. Then it'd be off to the mill to square the required centre section.

Doing it that way would mean you can have plenty of material in the jaws at all times. Like I said, I'm no expert but that's how I'd tackle it. I'm sure other, more knowledgeable builders will chime in.

 

[werowance]

How do you plan to make the rest of the part? From a quick look at it (and I'm far from an expert) I would probably start with round stock in the lathe. From there I'd turn the front spigot, then form the radius with a specially ground piece of HSS. Then I'd reverse it, cut and face it to length and turn the back of the part. Then it'd be off to the mill to square the required centre section.

Doing it that way would mean you can have plenty of material in the jaws at all times. Like I said, I'm no expert but that's how I'd tackle it. I'm sure other, more knowledgeable builders will chime in.

my plan (in my head anyway) is to take a pice 1x1 by about 1.50 in the 4 jaw chuck, face the back side first and make the .875 radius cut for the back and drill/ ream the .250 shaft hole.

then mount it on the mill with front bearing pointing up and centered with my mill spindle and either use a boring head or end mill/rotary table to make the shaft bearing or "snout" as i keep wanting to call it. once that is done ill finish up by cutting the snout to final length with mill or slitting saw etc. might even rechuck it in the lathe and just part it off.

i dont have the round stock in the material i need but i do have it in square stock.

what do you think of this plan?

 

[Jasonb]

For a start the drawing says 0.200" DIAMETER so you only need to be cutting a 0.100" radius.

Easiest way is to just grind the radius on the end of a HSS toolblank, if you actually make the end "D" shaped so the end is a half round it will be a more useful tool, something like this.

Its not a critical size so anything from 3/32" to 1/8" raqdius will do. You could if you wanted use one of the indexable tools with teh 6mm dia button inserts but HSS will do the cjob and cost a lot less

J

 

[WOB]

There is no need to make form tools for a simple decorative radius cut. Marv Klotz has a simple ball cut calculator program to generate an X-Y cutting table for any radius.
See http://www.myvirtualnetwork.com/mklotz/#shop Scroll down to Home Shop Software and look for "ballcut.zip" on the left. It's easy to use once you do the first one.

WOB

 

[werowance]

For a start the drawing says 0.200" DIAMETER so you only need to be cutting a 0.100" radius.

Easiest way is to just grind the radius on the end of a HSS toolblank, if you actually make the end "D" shaped so the end is a half round it will be a more useful tool, something like this.

Its not a critical size so anything from 3/32" to 1/8" raqdius will do. You could if you wanted use one of the indexable tools with teh 6mm dia button inserts but HSS will do the cjob and cost a lot less

J

doh!:wall: you are correct, it is diameter not radius.

well with that stupid mistake out of the way, what do you think of my my setup options to cut the whole thing?

 

[Jasonb]

It can all be done in the lathe

If starting with 1" square stock I would clock it tru ein the 4-jaw and face off one end, while its there machine the 0.875" spigot that goes into the crankcase and drill & ream the 1/4" hole all in one setting so they are concentric.

Change to the 3-jaw and hold by teh 0.875" dia with something between teh jaws and work to protect the surface, you can then face to length and then use the round ended tool to create the 0.400" nose leaving the 0.100" flange and forming teh radiused inner corner all in one go.

If making from round stock I would do it in the same order with the final op being squareing up the flange using the rotary table on the mill

The same tool can also be used for the prop driver so even more reason to make one

 

[Swifty]

I would make it the same as Jasonb, except I would use a centre in the tailstock when turning the nose end as extra support. The radius is not important, just grind a form tool somewhere near it.

Paul.