Soon to start the process of cutting out my cnc machine. My question is, do I pre peck the drill holes with a center end mill? Cutting 3 and 6mm aluminium sheet. The reason I ask is there will be four differnt sized holes (3mm, 4.2mm, 5mm and 6mm)in the job and I plan to use a regular jobber drill bit to drill out most of the holes. Or is it possible to just drill/mill all the holes using the 3mm 2flute hss center end mill? Excuse my terminology, I am still a nube.
I would suggest not using an endmill to drill, they are not designed for drilling and will wear/break very quickly. What I do is use a spot drill (short carbide drill) atleast 30% bigger than the finished hole on a CNC machine, on a manual machine I use a center drill to start the holes. The idea with the spot or center drill is to go deep enough to leave a chamfer (angle) 10-20% larger than the hole to aid putting stuff in the hole and to protect the edge of the hole from chipping and burrs. These keep the final size drill running straight and on location (they help minimize drill walk). The center drills are readily available and cheap. The spot drills are more expensive and more specialized. If you are just drilling a clearance hole for a screw or bolt a drill is fine, if you need a close tolerance hole I would suggest milling (circular interpolation) the hold or better yet use a boring head for very close tolerance holes for locating pins or dowels that need a specific fit.
Thank you steelspinner! I like the idea of going with the slightly larger bit to get the chamfer on the opening of the hole. Awesome idea. So you wouldn't recommend using the center cutting end mill to do the holes then? I was under the impression that these are fine for plunging into the aluminium at a rate of about 200mm/min doing 16800 rpm.....? As I said I'm still a nube. I'll look up the spot drill. The only problem being is all the differnt sized holes....... It's gonna take hours and many tool changes. Grrrr. Ok, I'm going to bore you now if I haven't already. Pardon the pun. There are only the 4x3mm holes in the whole job, these are for mounting a Nema 17. I was hoping the 3mm two flute center cutting end mill would be fine for that and kinda secretly hoping I could do the whole job with one of these if I were to be completely honest. There are around 20 or so 4.2mm(M5 tap) holes and probably over 80, 5mm holes (M5 bolts). For these I planned on using a standard jobber twist drill bit. Might go with the spot drill in these ones as their accuracy/tolerance needs to be as tight as possible. As for the 6mm and 10mm holes of which there are around 10 of each, once again I was hoping the 3mm bit would suffice. Thoughts....? My brain hurts to many numbers/feeds and speeds. Loving it though. So much to learn so little time.
You can use an endmill to make the holes a couple ways....but keep in mind you may have problems, including broken mills. Plunging a small distance (less than 1D) is ok as long as you follow it with an axial movement. Endmills are not drills, they weren't designed to be used as such. You can drill a pilot hole and then cut holes with an endmill. Another option is helical interpolation, if you have a cnc machine or indexing table. If you have a cnc machine, I can help you with the programing to make your holes. What kind of machine are you planning on making the holes with? I would stay away from jobber length drills, unless you need the extra length. Screw machine drills are plenty long enough for 6mm stock. The extra length on the jobber drills will make them much more likely to walk. Just a piece of advice.....forget time. Making things right takes lots and lots of time. If you think something is going to take you an hour, plan on 4 to do it right. When you are making machine parts, right is all that counts. A hole out a few thousand may not seem like a big deal until you find out that it is, usually too late. I don't mean to preach, I just see guys trying to cut corners everyday. It very rarely saves time. It usually only adds to the scrap bill.
Thanks steelspinner. Wise words regarding time. Went out and bought a 1/4" spot drill, hoping that this will do the trick for the 4.2 and 5mm holes? As far as the 3mm ones and 6mm ones go I'm hoping to just use the 3mm center cutting end mill to do those. I'll look into getting some stub bits to drill the holes too. So far, without adding the spot drill into the job sheetcam is calculating the job at close to two hours for an area of around 1200x500. The cnc machine that I will be using is a bit of a mongrel. Running mach3 though. It's around 4000x1500mm with a 2hp spindle. Max speed around 1500mm/min and around 25000 rpm. I'm guessing, but I figure I should be able to cut the 3mm in one pass and probably two passes for the 6mm aluminium plate. I'll let ya know how I go. I really appreciate the offer of setting up the gcode/post. I might take you up on that if I get totally stuck.
