It's Stuart Hammer time......

Rethink - it makes send to the mill the bush and the gland insitu; there's a lovely straight edge on the mouthpiece which will make sure my cuts are parallel and perpendicular 

Looking at this diagram it shows an angled recess on the gland which looks to be about 40 degrees and 1/8" deep. Surely the edge cannot meeting the top as shown there must be a slight rim otherwise it would be paper thin?

You could quite happily leave a 1/32 or even 1/16 of flat rim around the edge. It is really there to just push the packing material up against the piston rod.

It is unlikely a 2mm cutter will have flutes long enough to go full depth so if you use that method open out the back corners after milling the bottom face first.

I did wonder about that, the 2mm cutter looks quite weedy. I was thinking about removing the bulk of the material with a something more substanstial and leaving the the final side passes and corners to the 2mm?

My 2mm end mill 'ain't guna cut it' - forgive the pun! 

I'm going to need one more like the one in the 2nd photo .......

As Jason has suggested, the shorter 2mm cutter that you have will form an accurate recess to its working depth, to keep the profile of the hammer shaft under control ;  and then the bit of material at the further side can be relieved to make the clearance.  A little round needle file would winkle out the scrap of material that the cutter cannot reach ... ( a bit like the old Heinekan ??  advert ...  )  I would certainly adopt your idea of using a beefier cutter to clear out most of the material beroe going in with the 2 mm one ..  Dave

As I said in the post above do what you can from the outside and then flip it over and open up the corners a bit where it can't be seen

EDIT this is how it could be done with a 3mm  or 1/8 cutter from the top

Thank you both

Just to confirm, the method described applies to both the gland and the bush and the larger diameter cutter is used on the internal facing faces of each one (see annotated diagram).

i.e the face of the bush into the cylinder is 2mm and the face of the gland facing outwards is similarly 2mm?

Yes, 2mm on the flat faces and opened up on the conical ones

Cheers thanks for confirming

I may have gone a little bit too deep when making the diagonal recess - not sure it really matters, just more packing! 

Looks like you went a lot steeper with the angle but doubt it matters.

The piston gland is machinined to size and mounted; I'm now thinking about how I flip the parts over for milling either side and keep in postion.

- The piston gland should be easier, as I can flip it over and bolt it to the mouthpiece either way up without moving the mouthpiece.

- The cylinder bush is a little more tricky as I can Loctite it in postion but still have to flip the entire mouthpiece therefore losing postion......hmmm, maybe I can use a coax indicator on the smaller hole where the bush is mounted, Loctite it in postion, mill the cylinder side then flip it over and use the coax indicator on the bigger hole

And of course both sets of shaped holes need to be in alignment........

Before I start on any of that, I'm going to make up a test piece of 1/2" x 7/16" with rounded edges as my rounding cutter has arrived. I also need to calculate what I'm going drill, then mill with a larger cutter before moving onto the 2mm.

I finally managed to JB Weld the old flange mounting holes which for some reason have been annoying me since I started on this project 

Question

I have a piece of stock 1/2" x 7/16" and am having trouble working out the where to start with the 1mm rounding over cutter! 

The dimensions are below:

With the centre of the tool directly above the edge of the work, I come away from the work 1.9mm or 0.075" (half of D1 on the diagram) then work my in and down 1mm or 0.039"??

I  usually  start with the upper corner, the one towards the shank, just a touch above the top of the part, and take a number of passes taking it to the side a bit at a time till the bottom/inner corner is almost to the side of the part. Some people do it the other way, lowering it more each pass. Potatoh, Tomattoh...

Ah okay, so 'fix' either horizontal or vertical dimension then move the other inward or downward? No need to do both?

Quote from: Sanjay FAh okay, so 'fix' either horizontal or vertical dimension then move the other inward or downward? No need to do both?

I fix one and move the other. Unless I  notice the fixed one is wrong, or I'm  notching in the curve.