Center an Oval Mechanicaly

Can you take a photo! I have some ideas in my head but without seeing I'm not sure I totally follow!

Picture, sketch, something to describe scale and what you are doing. Remember, you have the parts in front of you, we don't.

What feature do you use for center target? Is it a hole? A boss? An X?

If it's an oval ring you could use a cross shaped locator picking up the X & Y axes. How oval is it?

Well, although I still don't understand what the OP is trying to do, your solution makes sense, even to me.

I'm still trying to cope with "gluing" two pieces of steel together and chucking that in a lathe.

GA from me, too, newcomer.

And welcome to CR4.

GA and welcome from me, too! A couple of things: I would suggest two half-cones, and they must be truncated (tips cut off) such that they don't quite touch the base plate when the oval is in place.

To the OP: there is more than one shape that can be considered an oval. Generally, when I think of an oval, I think of two equal semicircles joined by two equal straight lines (a racetrack), but eggs are are also considered ovals, as are ellipses. As others have indicated, a sketch or photo would clarify.

Thanks for all of the replies, The oval shape is a tricky answer, sometimes the rings come from heat treating as a perfect oval (equal dimensions on the same plane) other times I get them as egg shape. The image below is similar to what I am now using. The out of round varies from .005 to .040.

If there is that much variation in shape, you can't even define what 'centered' means, and have all ovals 'centered' within ±0.002".

You should improve your precision by using 2 smaller half-cones, which will locate using the minimum radii of the ovals, rather than a single larger cone, which will locate using the maximum radii of the ovals. I'm assuming this is a manual placement, where the operator can jiggle each oval into position...

Is this slightly oval ring of relatively constant thickness? If so, could you center using the OD instead of the ID?

If not, what is the subsequent machining operation going to do? Make it round? Is enough material removed to make the accurate (.002) centering necessary?

The term "oval" is ambiguous. It can mean an ellipse (symmetrical about both axes); an obround (the race track shape described by dkwarner in post 6, also symmetrical about both axes); or an egg shape, tapering from end to end and symmetrical only on one axis. The first two have a clearly definable center; the egg does not.

If the shape in question is consistent, a generalized cone of that shape will be able to center it. If the centering jig must accommodate various length/width differences, it will take something like two cross-configured wedges to do this. The dual cones in post 4 (GA) are an excellent approach.

If you are trying to correct an off-round shape into a circle, this will be difficult, because if you press a cone into it, it will spring back some.

I am not sure that this is significant but, I glue the part to the pallet because the lathe chuck actually clamps onto the pallet ant not the part directly. The reason for this is because I have a power open close chuck that only accommodates a small range of diameters. The parts in question vary greatly in diameter, anywhere from 1.25" to 20" OD.

"Spring back" as described has been noted. The jig has a counted weight to prevent the cone from being forced down.

I am not expecting to be able to locate 100% of the parts I just need to improve what I have. My current scrap rate is @ 30% if I can lower it down to 15% I would be happy!

I know that I accomplish what I need to do electro-mechanically but I am looking for a simpler approach.

As noted previously none of the parts are similar. Each part has a slightly different shape, this is a major part of my problem. I have tried many shapes for the "cone" all have worked to some degree but none has been acceptable.

I hope this helps!

I'm getting the impression that these are rolled and welded rings, and that your Roll/Weld supplier is leaving out the post-weld heat treat and final roll. With a 30% scrap rate, I think you need to tighten the acceptance tolerances on incoming parts, or else add a bit of thickness to the stock. A little more machining time should be quite acceptable if it reduces your scrap rate significantly.

Thanks Dk for your input The rings are actually rough machined parts. We have tried for years to get our supplier (our own sister company) to add material to the parts but they claim they can't. So I am left to deal with this situation. Yeah I know it's an internal logistics and political nightmare. Sometimes I am amazed as to how screwed up some American companies really are.

The rings are not welded, they are cut from a sheet then machined to a rough dimension. heat treated then shipped to us for final machining. We hold tolerances to within .0005" on all dimensions and part sizes.

he more I look into this I am thinking I am going to have to go with some type of automated position system that can locate the exact center of an odd shape part then place the part on the pallet.

I kind of figured doing it mechanically only was going to be a very difficult challenge.

Sometimes what appears to be a simple problem is actually a complicated one.

A variation of the 'split' clamp: using 'stretchers' that take advantage of the self placement/alignment of roller bearings. That is that the roller bearings will find the maximum interior width of the oval. Your accuracy will depend then on the tolerances of the bearings & their adjustments (equal distancing from the center). Perhaps use different size or sets of bearings to maximize centering capacities. The size of the bearing matching (nearly) the inside minimum radii (the 'ends' of the oval). Carlos

D, How about splitting the centering die, adding matched expansion (think dining room table geared rails), so that it will locate the 2 foci of the 'oval' mechanically before attaching it to the pallet.

Here's hoping my exaggerated rough drawing will attach.

CJM

What operation are you going to do once the part is located on the machine? Can the same operation be done by any other method?

First of all thanks for everyone's relies. I got some really good ideas.

I have come up with something that may work let me know your thoughts. Below is a sketch.

The way it works, G, is pushed down on F at it's center. F can pivot on G. Part G then pushes part on E which pushes parts A-D outwards. When any part A-D makes contact with the surface of the ring the opposing part can continue to travel through part F till it makes contact. If A-B make contact before C-D the part can be manually rotated to make contact at all four points.

I am thinking about spring loading parts E with enough travel so the ring will not have to be turned manually.

PS: we have tried so many variations of the cone and nothing has come close. We tried splitting, half and quarter. the problem is the radius varies in ovals and a cone has a fixed angle of different diameters. No circle we have used has ever produced satisfactory results.

I'm now at a total loss! Are these parts also out of true across the face! I'd imagined they were only out of round! Let me get this right, you are given some wonky looking bits of metal that are so stressed out, they're bent and deformed to a 30% scrap rate! (If that was my sister, I'd kick her out!) Reject the lot and get the sister company to get it's act in gear! You can't turn a donkey into a race horse by pumpin steroids into it!

If I only had that choice but when you are dealing with the idiots I am you have to be creative!

How about this?

Two almost identical arms have bearings mounted on pins at both ends. The pins are equidistant from center, and spaced so the bearings just fit the correct circle ID. The lower arm is fixed to the main shaft, and the upper arm rotates on the main shaft. The upper arm has extensions downward so all four bearings are at the same height. Extension springs pull the two arms together.

In use, the two arms would be moved, stretching the springs, until the pairs of bearings touched. The tool would then be lowered into the ring, across the longest dimension of the ID. Releasing the springs, the arms would move apart, and jiggling the part would move it as close to centered as possible.

I've presumably exaggerated the eccentricity of the oval for illustration purposes. There could be more than one set of pins and bearings, so the tool could be used for more than one size of ring. It might be preferable to use a torsion spring, and surely there are other changes that might be beneficial.

Give it a try! Dick

What are the parts like before heat treatment? Could you do your operation before the heat treatment?

I was thinking that if a journal had to be turned concentric to the bore or OD to keep a certain wall thickness then I would have thought maybe it would be better if that process was done first before heat treat! Problem is, we don't know what is the machining operation being undertaken nor do we know anything about the part! All is just guess work! So far we have a distorted ring that needs clamping on a lathe as close to centre as possible to do some form or machining! Is he facing, cleaning up the OD, ID, sticking journals on it, chamfering edges, putting grooves-recesses! Any of these holding methods may hold the part but will that address the problem of producing quality part which can be reproduced time and again without throwing money away on scrap metal and manufacturing time? (Sorry, all in one breath !!) Maybe he should be facing them first so at least he has a flat datum somewhere on the job! Or as mentioned before, stress relieve the parts first! The list goes on, I really think work holding is not the solution to this particular problem! Maybe a rethink of the whole process is in order! Never know, it may give him a 30% profit!

Like your tag line says " it's kinda fun to do the impossible" I have a situation like that here.

I built the device that I showed and so far I am able to consistently hold a .003 variation to center on parts that vary from .005 to .030 out of round either egg shape or oval and what ever junk I am receiving. Somehow the parts are all very close on the face .002 or less.

We ran 115 parts today and the scrap rate was reduced by 68%. I will know more as time goes by and we run more of these junk raw parts.

I think I can get more out of my device with some more tweeking. I am going to look at pressure on the parts next.

That's good news dt! These are the types of problem I enjoy trying to solve but it can get frustrating if you can't see the job and get your hands on the machine! Would love to be over there in Florida sorting this one out with you, but alas, I'm now living it up in Spain! Anyway, keep us all updated because I'm sure there are more people here who find this sort of thing touches their button so to speak!

Cheers Andy

I wanted to give thanks to this old thread and all the advise I got. After many months of trial and error I was finally able to develop a device and process that solved my problem. I am able to locate the center and place almost any shape smaller than 22" cu. to within .0002" of it's center.

Although no single suggestion solved the problem, collectively I was able to get enough ideas to develop the solution. So thanks for all the ideas!

That's wonderful! Can we see what did finally work?

Turned out fine again!! Love to see some pics too!

I will post some photos as soon as I can get clearance. The Co. that I work for has or is in the process of applying for a patent.