Best way to mill a pocket in Al on a manual mill

According to a PM conversation I had with the web-maintenance lady (Katie?) some time ago, that's not completely true, Neil. Apparently, the editor can be configured so that smileys can only be added via the drop-down (thus solving the problem of, for example, quote followed by parenthesis). However users in some other forums here are deathly afraid of mice (presumably they leap up on chairs when they see one). To accommodate them, the editor is configured to insert smileys the old-fashioned way (via keyboard characters) in addition the the drop-down.

Thanks - I didn't know that.

I'm an old schooler and would be happy to stick with 'typographic smileys' and no silly pictures at all

neil

In my previous entry I said drill holes in the corners, but on thinking about it, I wouldn't bother. Just finish with a 6 mm slot drill, wind the speed up to max, and take a finishing cut right round the perimeter with a slow feed, preferably climb mill it and the chips get pushed out behind the cutter, only take a few thou off this way and there should be no trouble, even my (bit dodgy) mill will do this OK.

Ian S C

Thanks! Boring the core out of the material on the lathe is an interesting idea. Sounds like a good plan to leave the blank material oversize and clamp directly to the bed. I need to maintain a datum in X,Y, and Z for the pocket so will need to be careful the edges are "getatable" after clamping.

I found a 5mm high helix long series carbide cutter in the box so that sounds ideal for cleaning up the periphery to size.

The internal dimensions of the pocket are quite critical - it's not a box but an RF cavity. I've just tweaked the design a bit and the pocket turns out at 89.8mm x 68.8mm x 20mm. That means the walls will be just over 5mm rather than 2.5mm which will help considerably with the clamping problem.

Steve

No this was after excessively tightening up full force with supplied spanners. By that 6ft 4 beefy build blokes couldn't undo by normal means.

The bearings for the ER have been available for years from Gloster Tooling.

I'm well on with the pocket, I bored the centre out, which worked well. I then moved the workpiece over to the mill and set the stops so I had about 0.5mm shy of each finished dimension. Then moving back and forward between the stops to get rid of the rest. Once I'de done half, I then tried chain drilling out the waste (with a slot drill), again working between the stops and this did seem to be more effective than milling it away, mainly less problem with chips and faster progress (remember I'd bored out the majority on the lathe already). I used a 10mm slot drill, but could only get 18.5mm deep, so I finished up the bottom of the pocket with a 12mm 3 flute end mill.

I then changed to a 5mm long series carbide "high helix" 3 flute end mill and have taken two of the sides (between the stops) to the finished dimensions. The finish has turned out really well. The last pass was 0.05mm at 20mm deep, climb milling with some tension on the table locks. Biggest cut was 1.25mm x 1.25mm - 25% the diameter of the cutter. I've got the mill running flat out at just over 2000rpm.

Shouldn't take too long now to finish up the remaining two sides.

After that there is just a load of holes to drill and a few to tap. Two of the holes will be 3/8 x 32 tpi UNEF - all the tapping charts I can find quote 8.85mm tapping drill, but don't quote any figures for thread engagement. How does 8.8mm sound in context of a through tapped hole in this casing? I need the male thread to "bite" into the casing to ensure a good electrical connection.

On the other comment - I've never had a cutter come loose from my collet chuck (ER32) in an MT3 arbor. BUT I've never done anything particularly heavy duty either.

Perhaps it's my home-made chuck? But things are better with my new one using a ball bearing nut.

Neil

Steve, the 8.8 mm drill should be ok, are you using a taper tap? If so it will start no trouble.

Ian S C

Of course, to get in the pocket properly, you need a set of drums and a degree of skill. And who better than Dennis Chambers to show how!

Merry

The pocket turned out really well, hit all the dimensions and the 8.8mm tapping drill worked well, then managed to drill a simple straight through hole 1.5mm out - D'oh! Otherwise the whole thing is coming together really well. I'll tap the duff hole and plug it with a threaded Al rod and loctite, then try again.

So thanks for all the advice!    Must take more care over where I point that drill

Steve

Edited By Steve Withnell on 20/11/2014 21:40:36

Here you go -

This is what the pocket was for - this is a butterworth response bandpass filter, the length of the copper rods is related to centre frequency (in this case 1090MHz ). Still a couple of holes left to drill and tap for the lid and each rod should have a brass 2BA screw to provide a little bit of tuning.

The BNC connectors are the typical "single hole" mounting type which happen to be threaded 3/8 x 32TPI UNEF. Plan was to turn the threaded part of the connector down so it was the same length as the wall thickness. Which worked except that the innards are effectively crimped in place so fall apart as you turn the crimp off. I've then used PTFE as a push fit to replace the original nylon dielectric. This eliminates the need for the crimp. PTFE is a) a far better dielectric at UHF and b) will standup to the heat from soldering the inner conductors to the copper bars. I'll be using soft solder and a small tip on my propane torch to make sure I've plenty of controllable heat. The copper bar should allow me to keep the torch well away from the joint..

Steve

> a) a far better dielectric at UHF

So it will change the impedance of your connectors

very nice job - I thought you were making a gunn oscillator to fir microwave beams at the neighbour's cat.

Neil

Edited By Neil Wyatt on 23/11/2014 13:42:11

Just working my way up...

Swapping solid nylon for solid PTFE should cause the CI to move from 50 to 70 ohms, but part of the reason the connector falls in bits when you turn the end off, is that the nylon is in part airspaced (and crudely so), so hopefully the difference isn't that marked...

To be honest I hadn't spotted that - so have been doing some back tracking

Steve

I made the first filter using Jason's method, as the pocket was almost square - roughly 5:4. I've just roughed out a second filter where the pocket is 2.2:1, so I would have needed to bore two holes, resetting the work between the two holes, so just milled out the pocket conventionally. It would have easier to bore two holes on the lathe!

Thanks for the tip Jason, definitely the way to go!

Steve