[How-To] Beginner Question: How do I Turn a 5/8" radius on the Lathe?

[robotwizard]

I'm going to take a wild guess here, should I use a turning tool with a round insert that's 5/8" in diameter, or is there another way?

 

[markba633csi]

Well, if you had what's called a ball turning tool you could adjust it to produce a concave radius
Otherwise yes, a round insert would work although you may have quite a bit of chatter- use slow speed and plenty of cutting fluid
With soft metals like aluminum you might be able to do it by swinging the compound; I wouldn't try it with steel though

 

[extropic]

I'm going to take a wild guess here, should I use a turning tool with a round insert that's 5/8" in diameter, or is there another way?

It depends . . .

The title says 5/8" radius and the OP says 5/8" diameter. Please clarify.
What kind of lathe are you using? Is it stout enough to handle a large round insert/cutter?
A lot of cutting edge in the cut means a lot of cutting force and potential damage/chatter on the workpiece.
Is the desired feature on the face of the workpiece or on the OD? or ID?

The simple answer is to get a radius turning tool.

Suggested reading:

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[robotwizard]

It depends . . .

The title says 5/8" radius and the OP says 5/8" diameter. Please clarify.
What kind of lathe are you using? Is it stout enough to handle a large round insert/cutter?
A lot of cutting edge in the cut means a lot of cutting force and potential damage/chatter on the workpiece.
Is the desired feature on the face of the workpiece or on the OD? or ID?

The simple answer is to get a radius turning tool.

Suggested reading:

[How-To] - Ask a Better Question

Asking better questions will get you better replies. By asking a question, you're asking others to do some work for your benefit. Please put in the effort to ask a better question so the replies can be more focused, useful to your issue and show respect for the contributors. A good question...

www.hobby-machinist.com

The stock is stainless steel. It is a pulley for a capstan winch that will use 5/8" diameter rope. The inside corners need to be rounded as is tradition, so it is on the OD. The lathe is a PM 11 x 27 . It's not a super precision cut, but needs to be smooth when finished. If that has to be done later with emery cloth so be it.

 

[extropic]

The stock is stainless steel. It is a pulley for a capstan winch that will use 5/8" diameter rope. The inside corners need to be rounded as is tradition, so it is on the OD. The lathe is a PM 11 x 27 . It's not a super precision cut, but needs to be smooth when finished. If that has to be done later with emery cloth so be it.

OK
I think I would grind two 3/8" HSS blanks. One with the left hand 5/16 R and the other with the right hand 5/16 R.

Cut as much material (rough in the radii) as you can with more conventionally ground tool bits, then try to finish the radii with the 5/16 R grinds.
Some people might try a 5/8 diameter carbide router bit, in a special holder, such as:

Core Box Router Bits: Extra Deep and w/ Upper Ball Bearing Guide

Amana Tool carbide tipped core box router bits ready to ship today. Large selection of core box router bits to choose from.

www.toolstoday.com

Let us know what worked for you.

 

[extropic]

The more I think about it, my gut sense is that a 5/16 R cutter fully engaged (90°) in the cut is going to be a heck of a lot for that lathe = crazy chatter. If you could get 5/8" diameter round inserts, you could grind a flat on it (remove some of the cutting edge circumference) and rotate the insert so only 10°-15° are in the cut. Then rotate the insert 10°-15° and cut that portion. Repeat until you achieve you full R. YMMV

 

[Jim F]

Joe Pie has a video about this.

 

[mickri]

I recently had to turn an elliptical radius to make a sheeve for a genoa lead block on my sailboat. First I used Sketchup to make a drawing of the sheeve. I then drew lines the same width as my parting tool. Next I measured the depth of each line from centerline to the edge of the radius. Sketchup will measure to .0001. Using these measurements I make cuts with the parting tool about .005 short of the dimension. This left a bunch of steps that needed to be smoothed out to complete the radius. I ground 3/8" HSS to roughly the shape of the radius. I used this tool bit and a half round file to smooth out the steps.

 

[mickri]

You guys are going to ask how did I grind that elliptical radius on 3/8 HSS. I flattened the ellipse to 3/8" and carefully took out the steps using different parts of the edge.

 

[Jimsehr]

I'm going to take a wild guess here, should I use a turning tool with a round insert that's 5/8" in diameter, or is there another way?

The size and type of the pulley can make a big difference. A one inch od pulley and a six inch od is like night and day. Same as 303 stainless versus 316.