Greene King (S15) Expansion Links
Another job for my brother's S15! He doesn't have a rotary table so I said I would part machine the expansion links for him.
The links are cut from 3/16" thick gauge plate and I machined the slots by mounting the blanks on the rotary table using the length of brass plate that I keep for this operation. It's getting a bit tatty and hole riddled now so may have to be replaced soon!
The first job was to mark a centreline down the plate and mark off the centre of the radius and then the centreline of the curved slot scribed with dividers. The centres were then lightly centre popped to give a reference point.
Next the position of the hole in the tail of the link was marked out and drilled and reamed to take a silver steel pin to accurately locate each blank. Two blanks were then cut from the gauge plate and drilled and reamed for the hole in the tail and also two more holes along the rear edge for 5BA bolts to fasten the blanks to the plate. The blanks were drilled together as a pair so that the holes would be identical in each one. One of the blanks was then fitted onto the locating pin in the plate, clamped in position and the two mounting holes spotted through onto the plate. These were then drilled and tapped to take the bolts.
After centreing the rotary table on the milling machine (using a centre in the table and a needle in the mill chuck), the plate was bolted to the table using the needle in the chuck to line up the centre pop on the plate with the centre of the table. The table was then wound across until the needle aligned with the line scribed where the centre of the slot in the link would be. The dial on the leadscrew was then zeroed so that it could be returned to the centre when necessary. This also made it easy to move the table an equal amount either side of the centre point to produce a slot of the correct width and with edges equally spaced either side of the centreline.
Lining up the axis of the mill with the centre of the slot to be machined
One of the blanks was bolted onto the jig and a series of 1/8" holes drilled along the centre of the slot using the index collar of the table to space them equally, firstly using a centre drill and then an ordinary drill.
Centre drilling the holes for the slot
Drilling the holes
The holes were next milled out with a 1/8" carbide endmill to form a continuous slot which was then finished slightly undersize.
Slot milled but not yet to width
The 1/8" endmill was replaced with a 1/16" diameter and the slot carefully cleaned up to final size. This is where you need to concentrate as it's very easy to turn the table the wrong way and take a bit out that you don't want to! I cheated and wrote it down on a bit of paper to keep reminding myself which way to turn the table handle!
When the slot was finished I went back and widened it slightly at the top and bottom to get rid of the rounded corner to the slot. You can of course file the corners square but it's as easy to do it this way. It's just to allow the die block to go right to the ends of the slot.
My brother was going to shape the outside of the links by hand but I thought that while the blanks were set up I may as well mill the front edge to shape and also partly mill the rear edge, just leaving the easy bits for him to do. The front edge was done with a 1/4" carbide mill but the rear edge was done with the 1/16" to leave the part with the fixing bolts in place.
Rough shaping the blanks
That was the links finished as far as I was concerned so the next job was the die blocks. I made these from my favourite material, bearing grade PEEK, as it seems to wear well and is dead easy to machine. Two blanks were parted off a length of bar and the faces cleaned up on carborundum paper. The rod used allowed two die blocks to be machined from each blank so this would make one die block plus a spare for each link. As it happened, each slot in the links were exactly the same width and I could have got away with using just one blank. The fact the slots were identical was more by luck than judgement! I didn't expect them to be the same.
The brass plate was drilled and reamed for two more 1/8" diameter pins spaced 3/8" apart and the PEEK blanks treated the same. The two holes were spaced using the rotary table again to ensure that each blank would fit onto the two pins in the plate. The blanks were then pushed onto the pins and roughly cut to shape using the 1/16" endmill. They were then very carefully skimmed down to a good push fit into the slots in the links. You have to be careful here to keep the hole in the link central in the die block and not to one side. I make the blocks a good push fit to begin with and after they are cut to shape, carefully fit them by easing them with a fine file until they will slide nicely up and down the slot but still a little tight to allow for bedding in
Cutting the blanks to shape
'Finished' links and die blocks
My brother can cut the die blocks to final size. Don't want to do everything for him!