suggestions for full radius on disk edge

specfab · May 25, 2015

I have a customer who needs aluminum disks of various thicknesses and diameters provided with full radius on the edge, for high-voltage gear application. I have previously provided disks from 3/8"-3/4" thk., up to about 5" diameter, and used a shop ground form tool for radius, working from each face of the part (do one side, flip the part over for the other side). Needless to say, the larger the radius, the exponentially greater chance of serious chatter. I now have a requirement for some 1" thk disks, about 5" diameter, and I'm feeling the need for a better approach. The lathe is a Clausing 5914 for the work, Aloris-clone BXA toolpost. Any suggestions for better tooling, or how to make a 1/2" radius form tool work without chatter?

I am curious about Holdridge stuff (and similar design tools), but that type of tool look like it has some limitations for access to work the back half of a disk like this.(?) TIA --

Toms Wheels · May 25, 2015

I have a Holdridge size 4 radius turning setup. On AL I doubt there would be any chatter, as you have very little stick out of the tool. To get to the back side you would need a small chuck and an arbor between centers,or just do it as you do now, flip the disk.

To do a quantity I'd suggest making a radius tool that fits the Holdridge tool holder, say a 45 degree portion of the 1/2" radius. Feed straight in, remove the radius cutter and install the PRESET single point Holdridge cutter and a couple swings and your done.

Radius in things like SS chatter, But brass bronze AL not a issue in my experience.

PeteM · May 25, 2015

Horizontal milling cutters are available with both 90 and 180 concave profiles; at least to 1" diameter. You'd probably mount the work up off a rotary table to do this in a typical vertical mill. Regular corner rounding bits would also work.

I suspect you can rig something (think Versamil) to use these cutters on your lathe, with a feed per tooth fine enough to likely avoid chatter.

It's also possible you might be able to get away with a wood shaper bit (lots of concave profiles available, cheap) if you don't feed too much at a time. Might even run on a toolpost grinder if you have one??

Also an excuse to buy a CNC lathe or a hydraulic tracer??

L Vanice · May 25, 2015

The "full radius on the edge, for high-voltage gear application" statement tells me that the radius does not need to be super accurate, but is mostly to eliminate 90 degree corners. I do that sort of work on a Hardinge lathe with T-rest and wood-turning chisels. I can get a good-looking radius by eye, or I can use a template to show me where to remove more stock. If that is not good enough, I can hit it with a form tool to clean it up.

Larry

randyc · May 25, 2015

Might try the old-timers method of using your compound to turn the radius. Will need a lantern tool holder, QC's won't work for small radii (anything under ~ 2.5 inches).

Limy Sami · May 25, 2015

Any of the simple radius / spherical turning attachments will do that job, the interweb and youface is full of them, once the radius is set it's cross slide work.

specfab · May 25, 2015

Thanks for the ideas so far - especially the PeteM one about buying a CNC lathe. I'll pass that one on to my "finance guy", and see if the 2 disks ordered will cover it..;-)

Larry -- I have given the hand method some serious consideration previously, but never tried it. I assume a hand-held chisel in aluminum would NOT want to have too much relief under the cutting edge, but would operate more like a scraper. I can see a one-man Three Stooges routine in my mind already, involving the rake handle to the chin...

Tom's Wheels -- You confirmed my gut feel for how a Holdridge tool would need to be set up. I also wouldn't expect chatter from that type of setup.

RandyC -- not sure how I would do this, but it's an interesting concept. Need to read a bit and look at the setup.

LimySami -- I have been perusing the InterTubes at length today. Many choices, some likely bad, a few good ones also.

I'm planning to do some additional work on form tool geometry, and see if my previous efforts in offhand grinding were a little rough for the requirement.

randyc · May 25, 2015

specfab said:
.....RandyC -- not sure how I would do this, but it's an interesting concept. Need to read a bit and look at the setup...

It's a forgotten method (probably never learned by young folks) but couldn't be simpler. The compound slide is slightly loosened so that it can be rotated by hand. Adjusting the cutting tool and the cross slide to the desired radius, all that's required then is to slowly rotate the compound slide by hand.

The sketch below illustrates the concept. Both concave and convex surfaces are achievable with this method but generally a QCTP is not suitable - an old lantern tool post is perfect for this job. Ideally, the tip of the cutting tool can coincide with the pivot point of the compound slide. Radii from near zero are possible and increasing the radius requires only moving the cross slide.

The illustration depicts a full radius being turned on the end of a small rod but radiusing the edge of a disc is simply a matter of relocating the cross slide and setting the tool to the correct radius.

specfab · May 26, 2015

Thanks for the info. When you mentioned this, your drawing is the concept I had in mind, and I started thinking about whether it would be possible to do the complete (180 degree) radius with this setup. That will require a fair bit of screwing around, but it's worth doing for future reference. Getting the tool and/OR compound behind the disk will simply be a matter of arranging the toolholding, it would seem.

One note: the radius size changes with the compound slide's linear motion, not the cross slide.

Sea Farmer · May 26, 2015

I gather from your OP that the disks are solid, no center hole?

I did a batch of these on 3 and 4-inch diameter aluminum disks, 1/2-inch thick, but they had center holes of 1/4" so work holding was on a custom mandrel. I can show if that will help.

Workholding solid disks, you're smarter than me. . . If you are starting with a longer piece of round stock and cutting off the disks, note the Holdridge tool (I have one) requires a decent amount of space to swing behind the work=lots of wasted material.

Onepass · May 26, 2015

If it isn't a close tolerance job, you could get by with a radius tool ground slightly larger than needed. Just cut wth only a portion of your radius. Feed the cross slide in a little, back up a little then feed in with the longitudinal a little. May have to do it a couple of times but it'll blend in pretty nice. If you been to make it look better, a file with a piece of sandpaper can make it shine!

magicmaker · May 26, 2015

I regularly do similar work on my 26" spinning lathe where the disc is merely sandwiched between the headstock driver and the tailstock follow block. Rubber sheeting glued to both drive and follow blocks helps secure things. Shaping is done freehand with one of my spinner's trimming tools. If you use a trimming tool with a round shaft, you can rotate the cutting tool into the "sweet position" and the aluminum just flies off. A centering jig can speed up the process but is not required. The alternate shaping method is by handheld belt sander.
Jim

Mud · May 26, 2015

Please forget who posted this suggestion immediately after reading it, OK?

Anyway, I've seen it done in aluminum with a router bit and router. Plop the blank down on a piece of rubber to keep it from sliding around and have at it with the router. For the second side, you might need a template to guide the router. You could even mount the router on the lathe like a toolpost grinder. A file and sandpaper in the lathe to finish.

I watched someone do it to small prismatic pieces held in a gloved hand with a laminate trimmer. Watching that inspired me to use a laminate trimmer to deburr saw cut blanks, I haven't tried the hand held bit myself.

Trboatworks · May 26, 2015

Another vote for the Holdridge- makes quick work of it.
Nice gear which can be used in a number of ways but does need some room on back side- worth it to hang disks on arbor to get the radius in one swing IMHO.

randyc · May 26, 2015

specfab said:
....One note: the radius size changes with the compound slide's linear motion, not the cross slide.

That's correct. Although I didn't say that the cross slide determined the radius, I can see how my post could have been interpreted as you did.

The cross slide and carriage are used for positioning and the cutting radius is established by adjusting the cutting tool tip location relative to the pivot. point of the compound.

EPAIII · May 26, 2015

Seem's like you could use a ball turning attachment with an appropriately ground tool to do both sides (180 degrees) in one shot.

Tony Quiring · May 30, 2015

Or make a tool holder that will securely hold a router bit and use the router bit as your form cutter.

They are available in 1/2 shank with carbide cutters.

specfab · May 30, 2015

Thanks for the additional inputs. One of the first things I thought about was some combination of router tools and usage; I have a long rich history of using routers and tablesaws in place of more use-dedicated machine tools. I think buying a 1/2" shank 1/2" radius router bit will run $30, then I still have to hold it in a substantial way to avoid chatter (or more serious failure).

I am planning to buy a 1/2" radius cutting toolbit, LH to do back and front in one workholding setup, for what looks like 40-50 bucks (MSC) just to have that available in case the rotating compound setup doesn't pan out. I have a mental picture of how to do it, but there is other work to get cleared off the machine before I start dicking with the real details.

scojen · May 30, 2015

For a tool braze a chunk of file to a piece of stock as big as your tool post will allow grind in your radius and get it done. The heftier the better.

CarbideBob · May 30, 2015

I think maybe the OP has already found that a radius of this size likes to chatter a lot. in most geometries
Making a flat tool or getting a router tip while simple just does not work so well.
Lots of contact length, fussy about rakes in both planes. pre-flats, and approach to the cut.
This is not an easy tool to make work right if you want usable speeds/production rates.
I get bar napkin sketches on ones like this all too often and I just want to no quote them.
After a few conversations I will often end up, "Fine, I'll make your tool, do not blame me if it does not do what you think it will, no performance guarantee on this one."

A radius turn fixture works great as all the problems go away. I'd look seriously at this.
Real high volumes and form tool plunge as it is much faster and worth the time to get the rakes/clearances right for the machine in use.

Big rad, just my penny's worth as a form tool maker.
Bob

suggestions for full radius on disk edge

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