[CNC] Using The Mill To Cut Out A 4 X 9 Plate & Bushing Holes?

[countryguy]

Hi CNC group. My son and have our first real project where we make something usefull for the Plasma table from aluminum stock. A few quick Q's and feel free to just point me w/ the right terminology and I'll google away also.

1) For the holes- .25 think aluminum. Do I peck drill routine these? or just drill it as it's aluminum? what's your process w/ good results.

2) the 4" x 9" plate in image 1 is what I want to end up. How would you cut out from a larger 6061 aluminum .25 think stock?
my thoughts: I will place over wood. mount all down well, set the cutter to say .2 below the bottom. run it in say a pair 3/16 passes?

3) In image 2 I have some .625 OD brass bushings to insert into the Float mount. I'll need to undersize them by a few thou? (.003 in some threads I've seen??) and press fit them. Would you mill these or how do I go about getting an undersized 5/8 hole? It's not critical stuff here so I think just milling would be ok? Is that common or a real oh-no newbie move?

Enjoy the summer all!
CG jeff & mitch.

 

[JimDawson]

1) For the holes- .25 think aluminum. Do I peck drill routine these? or just drill it as it's aluminum? what's your process w/ good results.

2) the 4" x 9" plate in image 1 is what I want to end up. How would you cut out from a larger 6061 aluminum .25 think stock?
my thoughts: I will place over wood. mount all down well, set the cutter to say .2 below the bottom. run it in say a pair 3/16 passes?

3) In image 2 I have some .625 OD brass bushings to insert into the Float mount. I'll need to undersize them by a few thou? (.003 in some threads I've seen??) and press fit them. Would you mill these or how do I go about getting an undersized 5/8 hole? It's not critical stuff here so I think just milling would be ok? Is that common or a real oh-no newbie move?

1) To peck or not to peck, that is the question At 0.25 it's a bit of a wobbler, I might use 1 or 2 pecks just because I don't like long stringy chips.

2) I would clamp it down first, drill the holes, then use the holes to bolt it down. If the holes don't line up with the T-slots, then drill and tap the wood backer and bolt right into the wood. 3/4'' MDF is my favorite backer, but plywood and particle board work well also. I try to design parts where the holes will line up with the T-slots where possible. See one solution I came up with here: http://www.hobby-machinist.com/threads/work-holding-and-order-of-operation.35261/#post-312080 I also normally bolt the backer board to the table using flat head screws, counter sunk about 0.050 to 0.100 below the surface, that are located so as not to interfere with the work operations. I have a drawing of my table and I overlay the work drawing to set the work location on the table to be access the T-slots with some of the holes in the work. When doing a full outside profile, I normally go material thickness + about 0.020. In 0.25 material I would do 2 passes at 0.135, at about 0.010 oversize then make a finish pass at 0.270 depth. Leaves a nice finish, and normally almost burr free.

3) If your mill will profile an accurate hole, then pocketing the hole is fine. Again I would do it in 2 passes, one about 0.010 undersize and then a full depth finish pass. Maybe a 3/8 end mill? Be sure to measure the endmill to make sure of the size so you can plug that true tool diameter into the CAM program.

 

[countryguy]

Thank a ton as always Jim. I saw many of your pics here on HM where you used the board backing so that's a tip of the hat to ya. Will read your post you cited! Tia on that. Looking forward to a fun and successful project.

CG.