Round-nosed lathe tooling?

[petertha]

Post #17 above ^^ but the groove could be roughed out with parting tool & this tool reserved for final finishing ^^

Actually I've used a X,Y step-over technique on a few occasions to obtain accurate curved profiles. Its helpful (necessary?) to have the (depth, travel) table worked out beforehand. That's another option for this radius groove question depending on accuracy - depth cut in increments, blue the stair steps, file contour to finished form.

 

[Dabbler]

Peter, the main difference between your sketch and the button tool you showed in the picture is that the carbide tool is ground to be positive rake. This takes much less pressure and helps eliminate chatter. Nice work on the stepover!

A suggestion is to make your roughing cuts using a carbide or HSS tool with a manageable radius, the do final cleanup using a button tool. OR you can just live with an accurate cut using conventional tools and then polish for finish.

- my toolmaker friend keeps saying that "there always more ways to do a job than toolmakers in the room" (!!)

 

[petertha]

Good point. No reason why rake couldn't be cut into the top face of tool steel button when the other features are being turned.

 

[Dabbler]

Peter how about making it out of a 1/2 in HSS blank, with a holder to hold it? Longer tool life than O1...

You lose the ability to rotate the cutter... Hmm makes me wonder if HSS is available in a through-hole round stock.

 

[Nogoingback]

I know that most guys probably don't own one, but the Eccentric Engineering Diamond Toolholder
can be used with round HSS steel blanks up to 1/4" diameter. Haven't tried it yet, but I might
after reading this thread.

 

[NortonDommi]

Hi Nogoingback,
I have left & right hand Diamond Tool Holders and love them I use 16 mm tools and have clamps for 6 mm-1/4" & 8 mm - 5/16" and use square bits for most turning and round when I want a nice radius. Very fast to change tool bits and set height.
Another way would be to drill a hole in the end of a bar and slot the bar then drill and tap for a clamp screw. Turn a radius on the end of a bit of tool steel the diameter wanted,(old drill bit shank),grind to the center and fit to hole in bar. This works surprisingly well and can be made from scrap.

 

[Nogoingback]

I do have a right hand Diamond tool holder that I sometimes use, and have had good results with. After writing this last night,
I thought of digging out an old HSS drill bit and cutting off the shank for a tool. Cheaper than ordering stock from Travers.

Not sure I followed you on the drilled and slotted bar. Do you have a pic?

 

[Jimsehr]

1. I went out and ground a .312 wide radius tool and cut a few grooves in a piece of steel today to see if it would cut without first roughing it out.
2. I did it on a 11 inch Logan with no problems. Got a good finish with no chatter. I will post a pic when I can. Tool made out of HSS with some top rake.

 

[petertha]

Eccentric Engineering Diamond Toolholder
can be used with round HSS steel blanks up to 1/4" diameter..

Interesting. I've heard that too but I don't see a picture of that round HSS/carbide blank configuration on their website. Does the holder have a Vee notch & the rod gets clamped in there like the square bit? Happen to have a pic of that?
https://www.eccentricengineering.com.au/index.php?option=com_content&view=featured&Itemid=101

Also, just being picky - wouldn't the projected round section cut an ellipse due to the rod's back angle clearance & probably exaggerated a bit more with any positive rake on the upper surface?

The issue with any of these 'hold the cutter' type ideas (including mine) is that the cutting profile may well have penetrate half its diameter which means the supporting tool holder doesn't have very much meat to grab onto or support without interfering with the cut profile itself. Maybe one could braze the button on like a conventional turning tool, but now its getting even more complicated. A tool & cutter grinder would make an accurate radius shape with all the requisite relief geometry directly out of HSS blank, but not everyone has such a machine.

For larger grooves the weapon of choice is a Holdridge Radii principle, but I think the smallest arc is maybe .75" or so

 

[petertha]

I've made profile cutters out of HSS. Nothing fancy but I thing correctly ground, finished, very sharp etc. They worked 'ok' in aluminum. But at a certain contact circumference then start to plow vs cut efficiently. In this example (~ 0.9" hemi combustion chamber) I then decided to make a mini Radii cutter & the cutting difference was like night & day. Feed-sweep-feed-sweep, done in no time with no fuss. I think extending this operation to steel, especially tough stuff like what tubing rollers would be made of, might be a bit more challenging yet. The problem is, this radii principle just doesn't scale well to small arc diameters.