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Round-nosed lathe tooling?

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Harvey

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#1
I need to produce some large-radius bends in some 5/16 x .035 4130 tubing and thought I'd do so by cutting some round-bottomed 5/16 wide grooves in one end of my Harbor Freight's sheet metal slip rollers. However, I can't seem to find any round-nosed lathe tool bits. Why is that?

Harvey
 

Jimsehr

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#2
It would take about 5 min to grind one out of high speed steel
or carbide.
 

Bob Korves

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#3
Make your own from HSS.
 

Dabbler

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#5
I regrind those cementerd carbide tools for this sort of work. They don't work well as they come, and they are perfect for the kind of thing you want to do!
 

Harvey

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#6
Thanks, Guys, for your replies. I've seen blank tool stock before and always figured that it's intended for cutting custom shapes. However, I wouldn't think that a simple 5/32rad would be uncommon. Also, making something "by eye" doesn't seem (at least to me) to follow the machinist's "code of perfection". :)

Can a tool blank be formed with a "common" bench grinder or belt sander? The tool doesn't need to be perfect but I don't want to end up with something crappy either.

Harvey
 

Glenn Brooks

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#7
Harvey, yes, use a bench grinder and medium to fine wheel. A belt sander probably won’t give you enough control to properly shape the tool. Lots of videos and instructional threads on grinding HSS tooling out there. Just as in grinding drill bits, takes a bit of practice to make the correct shape. But it’s not a big deal to learn. Use a set of concave radius gauges to control the shape.

Glenn
 

rgray

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#8
Use a radius gauge. Compare your grind to the gauge to see your progress in making the tool.
No radius gauge? Poor man's/emergency 5/32 radius gauge...drill a 5/16 hole in some sheet metal and cut the hole in half.
Now you have a gauge.
 

mikey

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#9
If I were to grind a round nose tool, I would use HSS and use a radius gauge like @rgray said. I would mark the top of the blank with a Sharpie and scribe a line to define the shape and then grind it on a belt sander. In my opinion, a belt sander is the ideal tool for lathe tool grinding. Set the tool rest at 12 degrees to give you adequate relief so the tool cuts instead of rubbing and hone the top to remove any burrs.

Use a slow speed and feed manually. This tool will have a LOT of surface contact and will chatter if you go too fast.
 
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Harvey

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Thanks Guys!

Harvey
 

benmychree

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#11
If I were to grind a round nose tool, I would use HSS and use a radius gauge like @rgray said. I would mark the top of the blank with a Sharpie and scribe a line to define the shape and then grind it on a belt sander. In my opinion, a belt sander is the ideal tool for lathe tool grinding. Set the tool rest at 12 degrees to give you adequate relief so the tool cuts instead of rubbing and hone the top to remove any burrs.

Use a slow speed and feed manually. This tool will have a LOT of surface contact and will chatter if you go too fast.
12 degrees is quite excessive and would result in a weak cutting edge; 5 deg. would be more than enough, unless you were using Armstrong style tool holders, in that case, it would not be enough.
 

mikey

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#12
12 degrees is quite excessive and would result in a weak cutting edge; 5 deg. would be more than enough, unless you were using Armstrong style tool holders, in that case, it would not be enough.
A matter of opinion, perhaps.
 

mikey

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#14
I certainly don't have that kind of experience in the trade but I do have an interest in HSS tool grinding. It has been sort of a pet focus for me for the last 30 years but I grant that I am not in a production environment. Still, I've ground a bunch of form tools over the years and have found that 12 degrees works well on a round nose tool like this (as opposed to the 8-10 degrees that is normally recommended). If the nose radius was narrower, I would increase the relief angles to 15 degrees to lower cutting forces even more to reduce chatter potential and improve finishes.

As I said, a matter of opinion.
 

Dabbler

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#15
hey, we all do what works of us. A matter of how you use it as much as how it is made!
 

pds65

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#16
I was looking for inserts like this and Kennametal has some inserts, also micro100 has half and full convex radius bits, too but @ $40 for something I can grind from HSS blanks, I think I'll try that first too as Jimsehr mentioned above
 

petertha

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#17
I'm going to be faced with this exact same situation making steel rollers for a mini tubing bender. It will see both imperial & metric tubing +/- their particular OD tolerances. From what I've read, a good finish & dimensional accuracy on the radius groove is somewhat important. The other issues (depending on the bender design) is not just making one roller wheel, it may have to be a matched set.

I've had moderately decent luck making simple hardened parts out of O1 tool steel just using torch & (oven) tempering. If I turned a dedicated cutter disc looking profile from O1 tool steel, I could make the appropriate cutting diameter + relief angle + hold-down hole in one turning operation. Or maybe even get fancy & also turn an edge relief on the upper lip. That just leaves a tool holder which could be a blank of steel, tapered on the business end & threaded hole for fastener. I'm not sure I would trust it to make the entire groove in one plunge operation if it large-ish radii, but the groove could be roughed out with parting tool & this tool reserved for final finishing.

General idea sketch shows a 0.375" cutter disc x 3/16" thickness with 6-32 button head bolt. I'm not sure how far you could scale it down before the fastener gets too tiny. At that point I guess break out the cutter grinder machine LOL
 

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Bob Korves

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#18
I'm going to be faced with this exact same situation making steel rollers for a mini tubing bender. It will see both imperial & metric tubing +/- their particular OD tolerances. From what I've read, a good finish & dimensional accuracy on the radius groove is somewhat important. The other issues (depending on the bender design) is not just making one roller wheel, it may have to be a matched set.

I've had moderately decent luck making simple hardened parts out of O1 tool steel just using torch & (oven) tempering. If I turned a dedicated cutter disc looking profile from O1 tool steel, I could make the appropriate cutting diameter + relief angle + hold-down hole in one turning operation. Or maybe even get fancy & also turn an edge relief on the upper lip. That just leaves a tool holder which could be a blank of steel, tapered on the business end & threaded hole for fastener. I'm not sure I would trust it to make the entire groove in one plunge operation if it large-ish radii, but the groove could be roughed out with parting tool & this tool reserved for final finishing.

General idea sketch shows a 0.375" cutter disc x 3/16" thickness with 6-32 button head bolt. I'm not sure how far you could scale it down before the fastener gets too tiny. At that point I guess break out the cutter grinder machine LOL
That tool looks like a likely chatter producer on steel due to the large radius. Find or make one that is really rigid, and then mount it as rigidly as possible.
 

petertha

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#19
That's what I suspect/fear too Bob. The commercial tool I have with round carbide insert has a milled pocket. I thought about adding that recess feature on tool holder but I'm not really sure how much the that might be contributing to stabilizing the cutter. It might have more to do with cutting edge repeatability? My only experience with this tool is making the valves on my radial engine. It was also roughed out & then the circular cutting tool made the final tulip shape. But the difference is I could creep in on it so minimized all that lip contact which is where chatter rears its ugly head.

I thought about a mini Radii cutter which is a way better way of making these profiles... but not at these small diameters.
 

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T Bredehoft

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#20
NO one has addressed the difficult part of the cut. removing most of the material.

Use a cut off tool to remove 80% of the metal
 

petertha

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#21
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.
 

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Dabbler

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#22
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

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#23
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.
 

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Dabbler

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#24
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.
 

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#25
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

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#26
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.
 

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#27
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?
 
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Jimsehr

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#28
  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

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#29
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
 

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petertha

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#30
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.
 

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