Pocket milling, those pesky corners!

[Walt]

My project, a bike light, entails the cutting of two pocket holes into an aluminum block. I'm getting a consistent flaw in the cut, the corners almost always end up being cut deeper than the rest of the pocket. (This may have been discussed previously here, but searching on "pocket hole" and variations isn't getting me useful info)

How do you mill corners so all 4 sides of the slot are even with the corner radius?

I am milling slots in aluminum mold bases to put inserts into for a modification. The inserts need to seat flush on some sides as they will be in part of the wax cavity. My problem is that when I bring my cutter to a corner on the [FONT=inherit !important][FONT=inherit !important]x [/FONT][FONT=inherit !important]axis[/FONT][/FONT] and then change to the y axis cut it leaves an indent. I am using a bridgeport with a DRO and plotting my dimensions from the center of the slot so my cut is x-4/x4. If I bring my cutter to x-4 it still leaves the indent. I even tried a plunge into the corner on the proper coordinates and back milled on the x then continued on my y cut. Tomorrow I am going to strap this beast to a bport with [FONT=inherit !important][FONT=inherit !important]ball [/FONT][FONT=inherit !important]screw[/FONT][/FONT] leads on the x and y and see if there is any difference.

Quoting:

"You might try climbing the finish,or stop .05 away move .05climb in conv.out,you might need to stand next to a practiced mill hand my friend to get it,OR,do like the kids and buy cnc."

Can someone translate this for me?

It seems like you would end up going in and out on the same wall ("climb in conv. out"), then have to do the reverse sequence (conventional in, climb out) for the opposite wall. This would seem to defeat the purpose of climb milling both sides of the corner and preventing the mill from torquing itself into the wall.

What does he mean by, "stop .05 away move .05"? On which axes?

I'm posting for help here, because everyone there seemed to get it and I just don't.

Walt

 

[DMS]

They are basically saying to take a very light pass to finish (5 thousandths). The problem is that you are running into backlash issues, and since you are (most likely) using conventional milling, the rotation of the cutter is pushing pulling itself into the far side of the piece. You can try a light pass to finish, but that may not work if you gibs are too loose. Try snugging them up, better yet, lock the axis that you are not moving (always good practice). Even better would be to set up manual stops. This has the added benefit of making the whole process faster anyway.

CNC fixes this problem (well, if you have anti backlash screws, it's not really the CNC part that makes this go away), but you run into other issues on corners. Basically, it's not a cure all, it's just trading vices.

 

[Syaminab]

Drill the Corners first, then approach from the side to finish, if possible with a smaller radius cutter than the Drill bit used for the corners. Lots of oil when machining the final pass, also if possible, use a six helical endmill with a high angle for removing the swarf, just be sure to lock your drill advance, as high angle helicals tend to push down and bury on the parts, then the bottom will be your headache.
You are having this problem, because you are loading twice the endmill on the corner, so it grabs and bends when changed direction. On CNC, we approach this with what is called Trochoidal moves, in which we basically cut and recut approaching from bigger radiuses, and always use an endmill smaller than the corner radius. Something similar to doing spirals for approach.
Regards.)

 

[Walt]

They are basically saying to take a very light pass to finish (5 thousandths). The problem is that you are running into backlash issues, and since you are (most likely) using conventional milling, the rotation of the cutter is pushing pulling itself into the far side of the piece. You can try a light pass to finish, but that may not work if you gibs are too loose. Try snugging them up, better yet, lock the axis that you are not moving (always good practice). Even better would be to set up manual stops. This has the added benefit of making the whole process faster anyway.

CNC fixes this problem (well, if you have anti backlash screws, it's not really the CNC part that makes this go away), but you run into other issues on corners. Basically, it's not a cure all, it's just trading vices.

I appreciate the thought, but wouldn't the problem be worst in the 0,0 corner, and best in the opposite one? Because I'm not seeing that, at least not consistently. I think the issue is like discussed in the link, it shows up from driving the mill into the corner and having it dig itself in from torque load on the mill cutter. I do lock the non-moving axis (usually!).

Thanks,
Walt