Operation Order for maching a part feature

Jay Fleming · Mar 22, 2017

I've got the part below I will be making soon. It is a planet carrier. The overall width is 1.5", material 6061 or 7075, pocket width 1.030", pin hole .375". My question is regarding the gear pin hole and gear pocket. Would you drill and ream the hole all the way through, then machine the gear pocket? Or vice versa? I'm leaning towards the first method because of the machine it will have to be made on and how I have to hold the part. I don't have coordinate rotation, so getting the pockets aligned properly after machining to then drill the holes would be a nightmare in my feeble mind. I'm also worried about the drill and reamer walking between holes.

Opinions based on practical experience would be appreciated.

Jay

Hazzert · Mar 22, 2017

Practical experience says stay away from 7075 if you can.

I would turn the OD, rough turn the ID, put in on a mill and put in the 3 bores undersize at a nominal dimension, then put in the pockets and then put it back using the pins for location and finish the bores to size then lightly skim the inside bore to size to remove any burrs. If burrs aren't a worry then machine the pockets and then finish the thru pins in case it warps.

Tonytn36 · Mar 22, 2017

I would also suggest doing the 3 pins and center bore last after all other material is removed. It is going to move around when you pocket it out and I doubt the holes line up after that. Rough them out, yes, finish them...no.

Jay Fleming · Mar 22, 2017

Hazzert said:
Practical experience says stay away from 7075 if you can.

I would turn the OD, rough turn the ID, put in on a mill and put in the 3 bores undersize at a nominal dimension, then put in the pockets and then put it back using the pins for location and finish the bores to size then lightly skim the inside bore to size to remove any burrs. If burrs aren't a worry then machine the pockets and then finish the thru pins in case it warps.

Tonytn36 said:
I would also suggest doing the 3 pins and center bore last after all other material is removed. It is going to move around when you pocket it out and I doubt the holes line up after that. Rough them out, yes, finish them...no.

I want to run this by you both since you basically had the same response. I can get 6061-T6511 tube with a 3.50 OD and 2.00 ID, which are my nominal dimensions. Going this route, would you make any changes to your recommendations?

Hazzert · Mar 22, 2017

No because your issues will be with the burrs inside the pockets and the thru bores and any associated warpage with the large material removal. If nominal OD and ID are acceptable I would probably still lightly skim the OD or ID for a round datum surface to work off of.

Jay Fleming · Mar 22, 2017

Hazzert said:
No because your issues will be with the burrs inside the pockets and the thru bores and any associated warpage with the large material removal. If nominal OD and ID are acceptable I would probably still lightly skim the OD or ID for a round datum surface to work off of.

Thanks for the info. Do I need to be concerned about the other three holes warping? They'll have pins that slide through loosely upon final assembly. I would also need to use these holes for locating after machining the pockets.

lowCountryCamo · Mar 22, 2017

Why stay away from 7075? And are you allowed a change in material?

Jay Fleming · Mar 22, 2017

lowCountryCamo said:
Why stay away from 7075? And are you allowed a change in material?

It's an in house product, so material is still up in the air.

Hazzert · Mar 22, 2017

In my experience 7075 machines great but creeps so his bores might line up today but not tomorrow. Mind you my experience has mainly been plate, maybe it's a different beast machined from round bar.

Regardless, I wouldn't change the process between the two materials just the expectations.

Ridethelighting · Mar 22, 2017

Turn od, and Id. Put in through holes and counter bores. Make a triangle fixture to mount the round part by By the three holes in previous op. Use triangle to index around to your three pockets. Will index as accurate as your plate is. Mount down with same plate/fixture to drill your reamed pin holes in final op if your worried about it moving.

I personally would roll on with all 6 holes first op. Finish part on trianglur fixture and call it a day. I just don't see it moving unless it's ran through the ringer with cutting stresses.

deadlykitten · Mar 23, 2017

maybe you are not allowed to flip the part

maybe you are not allowed to index the part

i don't know ....

depends on tolerances

you can just use a drill and finish with sandpaper until that part fits

Jay Fleming · Mar 23, 2017

Hazzert said:
In my experience 7075 machines great but creeps so his bores might line up today but not tomorrow. Mind you my experience has mainly been plate, maybe it's a different beast machined from round bar.

Regardless, I wouldn't change the process between the two materials just the expectations.

Is there a more stable aluminum variant you would recommend? Or just go with 6061 and be careful when machining?

Hazzert · Mar 23, 2017

6061 is fine, 7075 might be fine in anything besides plate. My experience has been trying to maintain .002" flatness on a 18" x 6" x 9mm thick pieces of 7075 and having it come off the machine after roughing warped 3mm and requiring bending and careful facing to bring it back to flat.

Jay Fleming · Mar 23, 2017

Depending on local availability, I may give both a shot and see how difficult they are to get right.

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