dardeshna
Plastic
- Joined
- Dec 10, 2015
I'm getting an odd wall taper when trying to walk in a bore for a needle roller bearing. The bore is 1.02" OD, 1.18" deep, and the wall thickness around the top 0.8" of the bore is only 0.15" (the part is a hub of sorts).
I'm trying to achieve a press fit of -0.0009" to -0.0015" on the diameter, however the best I could do was a "bowed" wall. At the top, it was -0.0002", in the middle it was +0.0004" and at the bottom it was -0.0018". No matter how I walked it in, the taper remained pretty similar.
Specs:
Machine - VF2SS
Material - 7075 Al
Cutter - 1/2" 3FL 2"LOC
Roughing Speed - 12K RPM, 72IPM, 0.002IPT
Finishing Speed - 5K RPM, 22.5IPM, 0.0015IPT
Cutting Strategy:
I'm trying to achieve a press fit of -0.0009" to -0.0015" on the diameter, however the best I could do was a "bowed" wall. At the top, it was -0.0002", in the middle it was +0.0004" and at the bottom it was -0.0018". No matter how I walked it in, the taper remained pretty similar.
Specs:
Machine - VF2SS
Material - 7075 Al
Cutter - 1/2" 3FL 2"LOC
Roughing Speed - 12K RPM, 72IPM, 0.002IPT
Finishing Speed - 5K RPM, 22.5IPM, 0.0015IPT
Cutting Strategy:
- Rough out to 0.02" radial stock remaining
- Finish to 0.002" radial stock remaining with 0.007" stepover, climb cutting
- Walk in to desired dimension using cutter comp (~2 attempts, climb cutting, spring pass on each)
- Tool deflection: This the primary suspect in my opinion. The tool is 4x in length compared to diameter. This doesn't explain how the bore gets wider before getting narrower however.
- Climb vs conventional: The general consensus seems to be that climb cutting deflects the tool perpendicular to the feed direction while conventional might help when finishing because deflection is long the feed direction?
- Tool tip wear: I'm trying to break the habit of roughing at 1-1.5x DOC, 0.05x WOC, but maybe this is contributing to why the bore is smaller at the bottom.
- Thin part wall: 7075 isn't the stiffest material and the part wall is only 0.15" for most of the depth of the bore. Perhaps the wall is deflecting at the top?
- Feed rate / chip thinning: I've read that taking too shallow of a cut can cause the tool to rub and deflect more. I tried cranking the finishing feed rate up to 40IPM and then 60IPM on the second attempt, but it didn't do much other than worsen the surface finish.
- Tool runout: I'm using a hydraulic tool holder and the tool is relatively large so I don't believe runout should be a huge contributor. Admittedly I have not checked tool runout.
- 3/4" end mill: I think this one is kind of a no brainer but I just ran out of time. I'd probably have to be more careful of slowing down the feed rate since it's getting close to the diameter of the bore.
- Machine the bore first: If I machine the bore first before roughing the outside, then the wall thickness shouldn't be an issue. I do have a slight concern about the bore warping as the outside is then machined away.
- Conventional mill: I'll give this a shot when walking in the bore and see if it helps the taper.