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Sharp corners on 3d o-ring groove shape


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Hello,

Apologies in advance if this is the wrong channel. Looking for some advice on putting an o-ring groove on the top of an unusual 3d shape. An engineer insists on putting sharp corners inside of the groove. Below is a hand sketch of an isometric view of the groove on the top surface. It seems inevitable that a flat end mill would result in the effect shown in the other photo, where the two corners are dragging along the geometry. This is on a 5-axis head/head machine. It seems the only way to achieve this is to rough it with a ball end mill, and finish the corners with a tiny ball end mill. Any thoughts would be highly appreciated.

3d_oring_0.png

3d_oring_1.png

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Tell him to go back to school and learn how to design something that is manufactureable. What is the max radius allowed in the corners? Had a company years ago ask the place I was working for to machine some rails for a robot system. They were 40 feet long and they wanted us to hold .001" tolerance for flatness, positions and other things on these rails. I told them $1,000,000 per rail there were 12 of them and it would be a 48month lead time and still could not say we could hit those tolerances. Engineer was not happy left and went to 40 different shops and every single one of them no bid the project. The owner came along the 2nd time he came in and wanted to understand why we wanted so much, but also why where the only ones willing to even bid the work that everyone else called impossible. I told him it was impossible in 6061-T6 aluminum to even think about trying to hold that kind of a tolerance. I then had to teach them both the engineering 101 stuff about how temperatures and other environmental factors make even checking the part almost impossible. They both looked at me like didn't think about that. I then asked then what is the systems form, fit and function? Once that was said and done I showed them and extrusion that came in 10' lengths that with some holes added to it would do everything they needed. The extrusions were about $120 each. Machining the holes would be about $200 for each one and they could do everything they needed for the system with a +/-.100" tolerance over the 40 feet.

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I've got one.  Datum A is a flat plane.  Datum B is a cylindrical hole through that plane.  Datum C is another cylindrical hole through that plane, about 3/4" away, with .002" position to Datum B.  Features are specced to .005" position relative to these datums, some over 12" away from them.

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Thank you All - great community. Yeah there's no convincing this engineer. He says there's no allowable radius. I proposed having an end mill made with a very subtle radius as shown below in the hand sketch - still no reply. Any thoughts on this idea would be highly appreciated. The 40ft rail story is classic - insane but not surprising. Thanks for sharing that great story.

custom_oring.png

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Well this a way to give insane one what is he asking for. You would need rough it with an endmill and then send it our for sinker EDM. Need to talk to your local sinker house and ask them how much stock they want left to burn it in. Slap the engineer with that cost and then see if he still doesn't want to budge.

Different story along these lines. Had a part that had a 6" bore going through an irregular shape. The engineer decided that a o-ring was needed to seal the tube to the adjacent part. They show me the drawing and I laughed and said .002 profile tolerance for an o-ring groove over 22" that looks like a race track is going to be almost impossible to hold. That and it needs to be done with a 20" Long Right Angle head on a 5 axis machine. They thought okay quote this. I did they said too high we will do it ourselves. I was like sure knock yourself out. After 3 weeks of trying I get a call from the VP of Manufacturing. Ron remember that Right Angle project we had you quote can you still help with it. I came in an told them their setup wouldn't work the part needed to be standing up to machine the feature. They all looked at me dumbfounded and I had to explain to them that just because this is a 5 Axis machine doesn't mean that feature is machinable setup like that. The only way to make it happen would be to have the Right angle head move in relationship to the head along both axis to keep the tool normal to the bore. By standing the part up we will be using the C axis to spin the Right head keeping the tool normal. The way it was setup would have required a 6 Axis machine with the ability to spin the right angle head in the spindle, but still keep it locked. Then the light clicked and they got it setup the way I recommended. Machine was a limited C Axis travel. I programmed it with the head prewound as it started in the cut. I unwound the head from top to bottom back to the top and then wound it back up take each depth of cut down the center of the o-ring groove. The the last pass was to make a finish pass on each wall never once pull the head out of the cut. I had told them from the very beginning I cannot be sure we will hit that tolerance of a .002 profile. Maybe we hit a .005 profile tolerance, but think .010 profile would be more attainable. They said do your best and we got it cut and it was nice looking motion and it cut much better than I expected. They ran 4 lefts and 4 rights and everyone was happy. I got the inspection report back and the worse one was .0016 Profile tolerance. I was surprised they came out that nice.

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I didn't commit to anything - just threw the idea out there. A colleague of mine says that Robb-Jack might be able to make something like this. I don't have experience working with sinkers but I'll look into that too.

Unfortunately I do not have the authority to turn him away. Managers here are just as clueless - bad situation. Blind leading the blind.

Once again great story! Race track is a good analogy. You bring up some interesting points about 5-axis features. Reminded me of some issues I've had maintaining perpendicularity between the machine head and the part surface on head/head machines. I had to sort of brute-force my way through the process. I've also experienced that same C-axis winding issue.

I'll probably make some separate posts about these issues with more detail.

 

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5 hours ago, tilikoom said:

I didn't commit to anything - just threw the idea out there. A colleague of mine says that Robb-Jack might be able to make something like this. I don't have experience working with sinkers but I'll look into that too.

Unfortunately I do not have the authority to turn him away. Managers here are just as clueless - bad situation. Blind leading the blind.

Once again great story! Race track is a good analogy. You bring up some interesting points about 5-axis features. Reminded me of some issues I've had maintaining perpendicularity between the machine head and the part surface on head/head machines. I had to sort of brute-force my way through the process. I've also experienced that same C-axis winding issue.

I'll probably make some separate posts about these issues with more detail.

 

Post them up. Part of the reason I share the stories is to hopefully give ideas to others. We all have different experiences and part of helping each other is talking about them. 

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