Making Round forms on a metal lathe

I'm a machining beginner and i'm working on a harmless first major project a chess set.

I picked it because all but one of the chess peices is symetrical about an axis of rotation (all but the knight) the project in theory shold be simple.

I'm using brass mainly because i have a source of spare round rod and becaue its soft and forgiving on newbies like me

the only question i have is for doing arcs or round ends is lathe filing the only way to round them off? or is there a method and tooling i'm missing? I'm not using CNC if it wasn't obvious =) thanks

Brent

Reply to
Brent
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There's a thing called a radius turning attachment. It is often used to make the round knobs on the end of handles and the like. A number of different designs have been published in such magazins as Home Workshop Machinist and Projects in Metal. You might be able to find some pictures with a Google search. They are not cheap attachments.

Jon

Reply to
Jon Elson

Go to ballcutter.com & look at the Ball Cutter. I made one just like it using a carbide tip, out of odd bits of metal in my shop & it cost peanuts to make.

Reply to
Salamanda

Hand grind the desired radii in HSS blanks, then store them for future use. That's common practice in manual machining. It takes a little practice, but there's no reason why you shouldn't be able to produce something that will serve the purpose. Use a standard set of radius gages as a guide for grinding. Remember to provide proper clearance, and, for this case, no rake. Any rake will alter the form the tool cuts unless you can accomplish the rake with a chip breaker that follows the radius without changing its relationship to center. That takes considerable skill----likely beyond the ability of someone learning to sharpen toolbits.

If you have a small lathe, you may have trouble with tools with a large cutting area, so consider the above suggestion carefully. It takes a serious machine to run form tools. They are more forgiving in free machining brass.

Harold

Reply to
Harold and Susan Vordos

Or another option is to grind a concave form tool. Seeing as you need to make 16 identical pawns this might be a reasonable way to form the ball on the tip.

You can also make one set from the brass, and the other from a free machining stainless or aluminum.

Reply to
Roger Shoaf

There is a work around for not having form tools or a radius or ball turning attachment. I hesitate to suggest it, because a beginner can get into trouble with it, so be careful! On a lathe with a compound (the part that sits atop the cross slide and can be adjusted for angles), you can leave the compound somewhat loose and rotate it by hand as you make light cuts. You need to set the tool away from the part center line by the radius you want to turn. If you can make some sort of handle sticking out on the opposite side of the pivot from your cutting tool, it will give you more leverage and control. Be careful not to get into the chuck!

This is not accepted lathe practice. Cutting forces are very high and it is easy to screw up. Google radius attachment or ball turning attachment and look at how they work. You may find one you want to make.

Reply to
Ron Thompson

There's a simple-to-do, but not simple-to-understand method described in "The Machinist's Beside Reader" that uses step cuts to approximate a ball, then simple bluing and filing to take out the steps. It takes much longer to understand the math and learn the method than it takes to actually do.

Or... you could run down to your local radius-turning-tool store and buy a Holdridge Radii attachment. (got one... it's a nice rig)

LLoyd

Reply to
Lloyd E. Sponenburgh

Greetings Brent, It all depends on what kind of tools you have. But I'll describe two different methods you can use. For concave curves you can make a too,that cuts on the tangent. See this link for good pictures and descriptions:

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way is to make form cutters out of either mild steel or tool steel. File, drill, grind, etc. the desired profile mirror image into the steel. Then, if using mild steel it can be case hardened with some stuff called "Kasenit". If using tool steel then follow the directions that come with the steel. The heat treating can all be done with a propane torch. Since it's brass you are machining the tools don't need to be super hard. This makes the heat treating more forgiving. When making the form tool clearance must be provided so that the tool doesn't rub. A way to visualize this is to imagine a hole going through a plate. If you pushed something through this hole it would rub the sides all the way. But if the hole is conical with the smaller diameter at the top then something pushed through would only contact the hole at the edge. When making the form tool you should file or grind this into the tool. A third method is to make a template out of something easy to shape, like sheet metal, and then file each part to fit the template. ERS

Reply to
Eric R Snow

Harold, I thought of form tools as well, but was hesitant to suggest it, being a newcomer - I covered them at school as part of the lathe theory (but, havent been taught tool grinding yet, so its still all theory to me)

Why no rake? - is it because of the material, ie brass, and would it be different for, say, steel or aluminium?

And wouldnt you be limited to the size that you could fit in your tool holder? - unless you got a wide, flat bit and didnt worry about the overhang out of the holder? - just curious..

Andrew VK3BFA.

Reply to
Andrew VK3BFA

Guy Lautard's method in "The Machinist's Bedside Reader" uses a parting tool. I found it far easier to use a plain turning tool and come into the part from the right.

The math takes understanding the formula for a circle, but once you know the formula, you can calculate the cuts. I based my numbers on the circle being in the second quadrant.

The radius end was perfect and took very little time.

-Bruno

Reply to
Bruno

There's a number of different ways of doing this, form tools are one way. Grind a tool to the concave/convex radius you want and feed it straight in. If you don't have a tool and cutter grinder, it'll take some skill to get the tool ground to the right radius. On a chess set, the tolerances probably aren't going to be that tight, though. Bigger radii call for wider tools and a more rigid machine to avoid chatter.

Guy Lautard wrote a set of books called the Machinist's Bedside Readers, one of them has directions for doing manual coordinate turning, basically generating a table of X-Y coordinates for cutting spherical surface of a desired radius. After cutting a set of stepped cuts, you then smooth off with a file or abrasive cloth.

Here's one sort of radius cutting attachment:

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company has other types for more money. This is a perennial home shop project, back issues of Model Engineer, Home Shop Machinist and others have basically the same idea with more or less complication. Some are manual, some have worm drive to turn the cutter around the axis. Accurately setting cutter protrusion can be a problem, most designs have a socket for a setup spud located right on the pivot axis, you then measure protrusion from the point of the spud.

If you have no particular requirements other than the end be rounded off, you can rough it out with the cross-slide movements, then file it by eye to shape in the lathe. If you do this, make absolutely sure you have a handle on the file, you can get some serious injuries from bare file tangs if the file gets loose with the workpiece under power.

Stan

Reply to
stans4

That's the problem-----or at least one of them. Rarely does one encounter a situation where that idea works. Being able to locate the tool properly as it relates to the pivot point of the compound is usually not possible without altering something. Simply adjusting the compound slide isn't enough.

Harold

Reply to
Harold and Susan Vordos

No. I made mention of the reason in the post, but I'll repeat it here. When you alter a tool by grinding rake, you also alter form, albeit in a small way. As the form moves away from center, it sees the material differently, and cuts accordingly. If you're trying to establish a radius, or other form, rake changes what you think you have. That's one of the reasons you should NOT grind rake on a threading tool. It also minimizes a tools ability to cut if a general back rake is ground. By grinding a chip breaker that parallels the cutting edge, which is ground with zero rake, the tool receives proper rake angle and still cuts the desired form. Because of the large area in contact, rake that would serve well for steel would likely work fine in brass, too, assuming the machine is robust enough.

Yep! You're limited to the size of the tool, unless you can be satisfied with a portion of the circle. Often that's all that's needed, so it's a solution to the problem. Look at it like this: Form tools are troublesome to use----so by the time you've reached the limits established by the size of a tool, you've probably passed, by a generous margin, the ability of most lathes to run a form tool that large. As I suggested in the previous post, it takes a serious machine to run form tools successfully. Small, home type machines tend to lack, woefully, the type of rigidity required.

Harold

Reply to
Harold and Susan Vordos

==================== called radius turning attachments and ball turning attachments.

For bolt and go solutions see:

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Ball turning attachments, knurl tool holders and rear mounted cut-off tool holders are traditional home shop machining projects. If you want to roll your own see
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(about 1/2 down page)
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For your purposes and skill level a simple unit like the first one listed [LMS#1970] should be more than adequate.

Unka George (George McDuffee) ............................. I sincerely believe . . . banking establishments are more dangerous than standing armies, and that the principle of spending money to be paid by posterity, under the name of funding, is but swindling futurity on a large scale. Thomas Jefferson (1743?1826), U.S. president. Letter, 28 May 1816, to political philosopher and Senator John Taylor

Reply to
F. George McDuffee

ballcutter.com doesn't seem to exist, neither by trying to go to it or doing a "whois" search.

I'd like to see the Ball Cutter, could you check the site name again?

Thanks,

Jeff

Reply to
Jeff Wisnia

Greetings Harold, I can't count the times I've turned the part by hand in the lathe to remove chatter when using a form tool. And it seems that no matter how massive your lathe is there is always that part that pushes the limits of that particular machine. ERS

Reply to
Eric R Snow

Hey Eric,

Yep--been there---done that. If you have a large enough machine, most of the problems seem to evaporate. I recall with fondness my first job outside the missile industry. I was assigned to a 17" Axelson on which we turned some cast steel pulleys for C size belts. It was a job that repeated, a part of the product produced by the corporation. We'd cut one side of the groove, then the other, using a form tool. Believe it or not, no chatter, but you had to know when to get out of the cut. I never mastered it as well as old Ozzie Gallagher, who had been employed there for years. His grooves were masterful, a great finish and no chatter. On a scale of 1-10, I'd rate those old Axelsons about 11. Wonderful machines.

Harold

Reply to
Harold and Susan Vordos

I've always wanted to try this from a different approach. Get someone with a CNC lathe to cut out maybe as few as four types/blanks out of brass and aluminum and then modify them each at home with a shaper , lathe, mill, files, ect... That way they would at least be close to the same size as the rest without taking forever.

Reply to
Sunworshipper

Look on eBay for "lathe radius turn*" and you will find the following auction (among others): # 260035622642

Sometimes, commercial radius turning fixtures show up, sometimes things like this which can be scaled to the size of your machine.

Another trick is to mount the stock in a chuck on an index head, set it to an angle from the table on a mill, and hold a reversed boring bar in a boring head in the spindle. Lower the rotating boring head until the boring bar is cutting around the end of the stock, and then start cranking the index head to rotate the workpiece. This will make something similar to the ball on a bolt handle on a rifle. You'll need to bring it to the lathe and use a file to finish up the center if you want it to look like a ball all the way through the axis.

Indeed so.

Enjoy, DoN.

Reply to
DoN. Nichols

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