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oxford
11-20-2013, 09:46 PM
This is certainly one of those it would be better to buy it than make it projects but I figured I would ask for some input. My wife is wanting a mandrel for making bracelets on. These are around 12" long, tapered, and also oval. I first told her I couldn't do it, but I think I can do it.

First, I don't have a taper attachment for my lathe, I am pretty sure I can get the correct taper with tailstock offset. This will be the deciding factor if I move forward on this.

Now onto the oval part, it doesn't have to be machined completely perfect, I could do some blending with a grinder if need be. I was watching a wood working show one time where a guy was making an oval hammer handle. If I remember correctly, he laid out his center marks on the ends and then also made some center marks off center. He turned the big diameter of the oval first with the centers then put the piece back in on the offset center and turned one half on the oval, then repeated with the other set of centers for the other half of the oval. He then blended it all in with sand paper.

I don't see why this shouldn't work on the metal lathe while cutting it on a taper at the same time. Anyone else have a different way they would attempt this? Here are some dimension of a commercial one: 12" long, small end is 1"x1.375", big end is 1.875"x2.125"

She also wants this in stainless steel, so a lot of material removal on a not that fun material. :rolleyes: If it keeps her happy though, it keeps me happy

Ries
11-20-2013, 10:04 PM
This is a forging project.
Relatively easy to forge with a power hammer- I make tapers all the time.
And stainless is no big deal to forge. In fact, I am working on a project right now that has 24 pieces of 1 1/4" 304SS round, forged to a taper at both ends. Due to time restraints, I just forged the sample, and had a friend of mine who has a much bigger industrial forge do the production run, But I have forged dozens and dozens of similar tapers.
Easy enough to ovalize a round taper after its forged- just heat it up again, and hammer it a bit.

Now, obviously, YOU dont have a forge and power hammer and years of experience forging stainless- but, hey, if you did, this would be about a one hour project, including the final grinding and sanding. Some things, historically, were made with different tools than a lathe...

RussZHC
11-20-2013, 10:14 PM
Very basic question, is it necessary to make this shape a solid? I have no clue as to the forces needed for making bracelets.

IF not, I would be tempted to try some sort of weldment...split the overall shape into 4 pieces (mirror of each other?), literally beat the really heavy gauge sheet into shape, not sure but heat may become part of this equation...it may also be needed to add reinforcements say top, bottom and 2 splitting the remaining length...

Edit: the above could be a really bad idea, forgot about the stainless parameter...

aostling
11-20-2013, 10:33 PM
If I remember correctly, he laid out his center marks on the ends and then also made some center marks off center. He turned the big diameter of the oval first with the centers then put the piece back in on the offset center and turned one half on the oval, then repeated with the other set of centers for the other half of the oval.

I'm not seeing how this works. Here is the arc method of drawing an [approximate] ellipse, if that is close to your desired oval:

http://i168.photobucket.com/albums/u183/aostling/ScreenShot2013-11-20at83009PM_zpseafdf0c8.png (http://s168.photobucket.com/user/aostling/media/ScreenShot2013-11-20at83009PM_zpseafdf0c8.png.html)

The two big arcs might be turned on two different centers. But then when you tried to turn the small arcs on different centers you would eat away into the desired profile.

Rich Carlstedt
11-20-2013, 11:03 PM
Using Allen's presentation above, I would do it in the mill with an index head and offset centers using a higher tail-stock
You must really love her !

Rich

darryl
11-20-2013, 11:08 PM
Just another idea- no machining involved- lay out a tapered pattern on some stainless sheet, roll it into a cone and weld the seam. Then pinch it into an oval shape. Take a piece of thin-walled tube of the largest diameter that will still fit through the small end of the ovalled cone and epoxy that to a piece of mdf, making sure it's vertical. Then mix more epoxy and smear that around the base, then stand the cone over it, centered at both top and bottom. Once cured, choose a filler mixture and fill the space between the two parts about 3/4 of the way. Let it cure.

Break off the base and belt sand this small end to an acceptable finish. Now you can belt sand the big end to a nice flat finished edge, then stand it upright again and top up the filler. You might make a stainless end cap to finish off the look, and you might use that to hold some T nuts that will become embedded in the filler. Use those as mounting points for a stand of some sort- or make the end cap and stand as one piece. I think I'd prefer a separate stand, as that will let you belt sand the lip around the end to clean it up and give the 'bracelet arbor' a finished look.

1-800miner
11-20-2013, 11:58 PM
My clapped out lathe cuts oval tapers all the time.
But never on purpose.

Look at Rich Carlstedt's piece at the end of the thread about turning a radius.
There is you quick and dirty taper attachment.

The more I look at the diagram,I would use a shaper to tackle this project.
Taper it on the lathe then move to the shaper and use your offset centers to do the oval.

But the forge does seem the easy way.

J. R. Williams
11-21-2013, 12:49 AM
Turning an oval is easy with a lathe. many years ago I worked in a shop that made oval forms for the canned milk industry. The oval was made by using a chain drive , a two to one ratio, from the back of the lathe's chuck to a jack shaft mounted on the back of the lathe. The back shaft was connected to a crank that moved the cross slide as the work rotated. The back crank was adjustable to be able to change the stroke. The product was a forming horn used to hammer the seam.

winchman
11-21-2013, 01:58 AM
Does it really need to be smooth? If not, you could make slightly smaller forms for each end, and use 1/8" SS welding rods in between. They would touch on the small end, and have slight gaps between them on the large end. Weld them together at the ends before removing the forms.

Another way would be one piece of SS tube alongside a second piece that's been trimmed and flattened to give the profile you want. Weld the laps, and grind them smooth.

Unfortunately, those wouldn't stand up to any beating and framming, but I don't know if that's required.

macona
11-21-2013, 02:44 AM
4th axis on a a cnc mill.

Winchman, yeah, it will need to be smooth.

winchman
11-21-2013, 07:06 AM
Assuming you want to make it out of round bar....

Drill and tap both ends of the round bar with for a largish stud. I'm thinking at least 3/8", but 1/2" is probably better.

Scribe the desired outline on each end.

Get two short pieces of angle that you can secure to the mill table, and make a vertical slot in each for the studs in the round bar.

Get four sets (concave and convex) of spherical washers to fit the studs. One set goes on each stud between the round bar and the angle on each end, and one set goes between the angle and the nut on each end. The spherical washer sets will allow you to tighten the nuts without shifting the position of the bar, and you'll be able to get enough clamping force to keep the bar from moving while you're milling.

Align the scribed patterns with the tops of the angle at each end, and secure the nuts. Mill that facet of the bar. Loosen the nuts, rotate the bar a few degrees, align the pattern again, tighten the nuts, mill the facet, repeat....

Jono
11-21-2013, 07:20 AM
Use a toolroom lathe with form relieving attachment.

Seriously though, Rich Carlstedt has the right of it. Reminds me of a training exercise we did, though that was concave on a rotary table.It's not a true ellipse, not that it matters.

vpt
11-21-2013, 08:04 AM
Cut a round stock in half, insert flat tapered wedge in the middle, weld the seems, smooth, done.

Stepside
11-21-2013, 10:25 AM
Most of the Bracelet Anvils I have seen were Cast Iron and highly polished. The Iron was a thick walled oval, tapered tube. You need the mass of the Iron to work well as an anvil and you need the fine finish to keep from imprinting a rough surface on the inside of the bracelet. The stainless might be too soft to make a decent anvil and a thin wall might make it ring like a bell. My temptation would be to buy one and find something else to build for her shop.

Barrington
11-21-2013, 11:56 AM
The trick is to use multiple large radii to create the 'pointy' ends:-

http://i564.photobucket.com/albums/ss82/MrBarrington/oval_zps8227b166.png

Using only three centres it may need a lot of blending, five is better.

Cheers

.

jackary
11-21-2013, 12:02 PM
If the ovality is fairly small just place a round bar just protruding in a 3 jaw chuck with a piece of sheet metal under one of the jaws. The thickness of the piece of sheet metal will determine the amount of ovality. Then centre drill the end of the bar producing an off centre, centre drill dimple. Now slide the bar out of the chuck without rotating the bar to the offset tailstock centre and reclamp the chuck jaws still keeping the sheet metal spacer in place and turn off the outer surface until about 180^ of the outer surface is removed. You can also do this with a four jaw chuck and if the taper is too much it may be better to machine the bar taper concentrically first then put in the offset centre drill. The final outline can be finished off with a few file strokes. Good Luck
Alan

aostling
11-21-2013, 12:41 PM
The trick is to use multiple large radii to create the 'pointy' ends:-

Using only three centres it may need a lot of blending, five is better.


That is a good diagram, makes it clear what technique the OP was referring to.

oxford
11-21-2013, 04:19 PM
Thanks for the replies, it gives some things to think about. This doesn't have to be solid but like mentioned commercial ones are a very heavy wall. From my understanding there is some beating on this and it is used as a form tool. Barrington, that is the procedure I was talking about. Your right that 5 centers vs. 3 would make the blending a lot easier.

We will see if I move forward on this project, like I said this is one that would be better off just buying. I am sort of at the point with it that I just want to see if I can do it.

dian
11-22-2013, 03:58 AM
If the ovality is fairly small just place a round bar just protruding in a 3 jaw chuck with a piece of sheet metal under one of the jaws. The thickness of the piece of sheet metal will determine the amount of ovality. Then centre drill the end of the bar producing an off centre, centre drill dimple. Now slide the bar out of the chuck without rotating the bar to the offset tailstock centre and reclamp the chuck jaws still keeping the sheet metal spacer in place and turn off the outer surface until about 180^ of the outer surface is removed. You can also do this with a four jaw chuck and if the taper is too much it may be better to machine the bar taper concentrically first then put in the offset centre drill. The final outline can be finished off with a few file strokes. Good Luck
Alan

how exactly is that going to produce an oval?

jackary
11-22-2013, 05:55 AM
It will produce a tapered out of round which can be filed into an oval. i.e two radii blending.

dian
11-22-2013, 08:35 AM
it might be better not to remove "180" and drill an other hole and turn down the other side. (o.k., thts what barrington said, i believe.)