Very large radius turning attachment

[Evan]

I have been experimenting making metal mirrors and decided to try roughing the parts on the lathe so I built this very large radius attachment. The white piece on the pivot is teflon sheet as a bearing. The spring is a valve spring so is plenty strong. The rider on the cross slide is machined on the bottom to fit the same as the top slide and is locked in the same way. It also serves as a nice solid tool post when the top slide isn't needed or gets in the way.

The red glow on some parts is from my "you left the drives on" warning lamp.





The attachment produces a spherical mirror and is used to rough out the shape for the final parabolizing step on the milling machine.

[torker]

That poor ol' SB! One of these days that thing is going to get some kind of serious complex...confused cuz it can't figure out just what in the hell is next
Interesting Evan!
And no.. I'm not going to ask where you got the time...I figured it out...in Williams Lake...the days are 34 hours long. It just insn't fair!

[mark61]

How deep of a cut are you taking? You have to losen everything and move it forwards as you go deeper, right? How about just adding a bolt to the back end of the tool holder and lossening the tool bit then push it forwards with a the bolt for increasing depth of cut?
Thanks for the idea/ example of making the dish. Wondered about doing something like that a few times myself.

mark61

[Evan]

To advance the tool I just loosen the clamp bolt on the cast iron table and rack it forward as normal, then retighten the clamp. It takes much longer to tell than to do.

[nheng]

Can an SB die of exhaustion ? Is there anything yours hasn't done yet ?

Nice job and more food for thought. Reminds me of metal spinning setups ... actually, you could probably spin shallow bowels, reflectors, etc. using only a minor mod. (nose bearing or polished radius tool). Den

[DICKEYBIRD]

...you could probably spin shallow bowels...

I had some Mexican food the other night. I know all about spinning bowels!

[Dave P.]

Do you have a detail pic of how the pivoting bar slides
...say left or right at the cross slide as it swings across the part?
Is it just a stepped bushing?
Thanks
Dave P.

[Evan]

It's just a stepped bushing with essentially zero clearance. The carriage is forced to follow the arc as the arm pivots. The attachment is just as simple as it looks with no hidden parts.

[doctor demo]

That is cool Evan.
If You made the arm a lot longer and mounted the pivot off the lathe somewhere . You could turn mirrors for a large telescope, but then You would have to lap/polish them some how.

Steve

[darryl]

Interesting. I did a similar thing using the tailstock- a simple pivot on the end of the arm that I ccould mount in the tailstock chuck. That way I had fine feed by way of the tailstock handwheel.

I considered figuring out an eccentric pivot point for the arm so the cutter would trace out a parabola, but I didn't go any further with it. Coming up with leverage that would follow the radial position of the cutter to turn the eccentric pivot wasn't so hard to envision, but I lost it on the math involved to assure that a parabola would be the shape produced.