Model A - clutch spring screws

 Now that the primary chain guard is finished we'll start on some lathe jobs, finishing the clutch dome when it's screws are made. This will need a change from the three-jaw chuck to the ER25 collet which we will do in a while, but first we'll use the chuck to hold the hexagon stock we need to make the clutch spring screws and perhaps the battery strap nuts.

We have four screws but we need six, so here we go. First job is to turn down the bar stock to a 1/4" diameter for threading, copying one of the existing screws.

After a small problem with a blunt die, I was delighted to find that the torque upgrade that we did on the lathe some months ago enabled this 1/4" thread to be cut under power.

Once you're set up it's dead easy to knock these out. In 10 minutes or so I have two of these ready:

The next step, and for this I need the ER25 collet, is to shape the head to that low radius the original has; then we can think about cutting the screwdriver slot and making the washers.

The raised head is formed using a specially ground tool. It's important to keep this sharp and feed it slowly to avoid chatter marks in the work:

Nicely raised heads:

I used the compound slide adapted for vertical operation to put the slots in the screws. They are cut with a 1/16" slitting saw.

Here's the result, with some old screws for comparison:

A nice little job I think. Still need the washers - those standard ones are too small. The originals are special and are quite tight on the screw.

Mini-lathe - slitting saws and lightweight milling attachment

 Why do I always choose the difficult path? All I needed was some slots in the heads of the clutch spring screws and the clutch dome screws which I suppose I could have made with the hacksaw but no, there must be a difficult way, a better way, a more challenging way - a Simon way. 

The screws looked quite good when they came off the lathe: 

I've had a slitting saw and a suitable arbor for quite a long time, bought for some long lost reason. The problem with slitting saws in the mini-lathe is that is difficult to get work into the right place to use them. Screw heads can of course be slotted with end mills or slot drills or hacksaws but actually a slitting saw is the best way - an end mill or slot drill is always going to be a delicate creature at that size and a hacksaw is not going to produce a very tidy result. 

I thought about setting up a jig and using the milling table somehow but it never really gelled until I thought about holding the work in a tool post and using the compound slide vertically. I thought I could do this with a simple bit of angle so I bought a chunk of 3" x 3" black steel angle, 4" long and set it up on the cross slide with a few holes:

It works rather well and could be arranged to put the work in the right place relative to the slitting saw and to be able to span the slitting saw with the stroke of the compound slide.

You use the lead screw to drive the saddle and get the work centralised - but this reveals that there is a lot of end float in the lead screw which will need to be adjusted out.

I used a piece of scrap hex as a test piece, and you will notice that the chuck has been replaced by the ER25 collet holder which will not only grip the slitting saw arbor more effectively but also allows considerably more space to approach the work.

You set the depth of cut with the cross slide, and make the pass with the compound slide:

The result of the first test was a nice slot that didn't go across the middle of the hex, despite my efforts to line it up properly. The problem is clear once you strip off the tool post and put a square on the lathe bed:

Well you remember I said it was a bit of black mild steel, not known for keeping it's dimensions! I put a shim in the appropriate position to square the tool post to the lathe bed.

For this next test I have a 1/4" CEI hex head bolt in the workholder that I made earlier, centred accurately.

Here's a close up. The slitting saw is set to cut downwards pushing the work into the lathe bed to minimise movement of the saddle, the cross slide, and the assembly holding the compound slide:

This results in very nice cut, perfectly centred. This is actually made with three passes, each of 0.5 mm depth.

The next step is to make a similar cut in one of the real clutch spring screws. This saw is 3/32" wide so will be fine for the clutch centre screws but I think it will be too much for the clutch dome screws.