The backstop fits in the lathe mandrel and provides a axial location for thin workpieces to be held in the chuck. Without it, it is difficult to get a thin disk of material running truly square and faced off parallel in the chuck.
It is another Hemingway kit, which also provides for a number of accessory heads to assist in making small turnings such as custom nuts and washers. It is described in G H Thomas's book The Model Engineers Workshop Manual, and includes a drawings which are not fully dimensioned, but had I needed to, I'm sure I could have figured it out. However, the kit came my way.
It is a good exercise in accurate turning. The usefulness of the tool depends on being able to turn parallel shafts to be a close running fit in reamed bores. Otherwise it is pretty straight forward. The kit did however contain a couple of surprises.
The sleeve that fits in the headstock has an MT2 taper, and I understood that the kit would contain a piece of round bar from which to turn it. It did - but it also contained a blank MT2 arbor. Not to look a gift horse in the mouth, I decided to use the arbor. The existing drawbar thread had to come out and a larger thread for the connecting tube cut, and a hole right through the centre drilled and reamed. Somewhere along the way cutting got very tough. There seemed to be a hard spot which caused first the drill and then the reamer to complain loudly when close to breaking through. Nothing is visible in the finished bore or thread, but it did cause some concern.
The tube that connects the sleeves at front and rear of the mandrel was supplied in the form of a piece of very soft aluminium tube, which had to be threaded at both ends. One thread came out well, but the other was simply chewed up by the die. And this was an almost new die. I suppose I should have turned the thread on the lathe, but I didn't. Fortunately I found another piece of tube, in a somewhat harder aluminium alloy, in the scrapbox. That explains why it looks so tatty in the photo, but it threaded successfully and does the job.
The kit and drawings as supplied are for the Super 7. The ML7 has a shorter headstock, and all the relevant dimensions have to be reduced by 1.5 in to compensate. There is nothing in the instructions to tell you that, I just had to figure it out for myself.
Nick Baines Model Engineering