Problem- Machining 6082T6 aluminium, drills don't work, advice please

[Blakelidge]

Hi,

I am currently producing some bar mounts using 6082T6 aluminium round bar, basically, the piece is turned down to 50mm and I drill out the centre to 18mm and then, using a boring bar, I increase the central hole to 25 mm (the size of my rail) The peice is then parted off at 12mm wide (bit like a metal donut if you will) The part is ultimately slit across the central axes so I have two semi-circular parts (similar principle to a scaffolding clip) I then need to drill through the part so that the two halves can be bolted back together with 3mm x 40mm panhead bolts so the part can be clamped onto the rail.

Here is an image of something almost identical (but without the arm and tray) my part is v.similar to the lower section:



http://www.radarbusters.com/radar-de...lebarmount.cfm


I have already machined several of these on the lathe but have so far ruined three of them due to the following problems:

Problem 1. The 2.5mm hole commences in the centre of the part at the top, but is somehow wandering off track during cutting, regardless of rpm or pressure applied, and comes out off-centre at the other side, I am using a cobalt 2.5mm drill bit mounted in my mill. This problem ruins the part. I guess the bit is flexing in operation causing this issue, is a drill bit the wrong thing to use here? I found a place online that supplies a 2.5mm endmill but it looks like it would only bore to a max depth of 12mm. What can I use that will accurately track a 2.5mm hole through 40mm of aluminium?

Problem 2. The 7mm hole to accommodate the bolt head on one side of the part also fail to drill correctly, the drill bit flexes as it commences its cut on the curved surface of the round part, it always tries to follow that curve instead of tracking straight down the pilot hole I made, again this ruins the part. Is there such a thing as a 7mm endmill or boring bar that will accurately commence a cut into the side of a curved surface? The depth of this is less of an issue, as I only need to accommodate the depth of the bolt head, about 9mm

Thanks for taking the time to read this, I hope there are some suitable products available (and where would I buy them?) as drill bits clearly won't work for this job, or I'm doing something grossly wrong, but I'm no engineer so look to you for some guidance please.

Thanks
Dave

[Guldberg]

As you noticed, its quite hard, if not impossible, to drill on a curved surface. I would use an end mill instead, then followed by a drill the correct size. You would want the end mill to be as short as possible, as you should be more conserned about the ridgidness than the depth, since you are going to drill it to the correct size afterwards.

With 2.5 mm center hole there is also the possibility to just make a spot with a center punch, this should keep the drill bit in plate

[mactec54]

Hi Blkelidge

You will have to use a endmill say 3mm or as big as you can to put a flat spot on the side of your round part then spotdrill & drill if you have a screw head then make the endmill a little bigger than the head of the screw to put the flat spot on the side of your part