There's only so much moping about you can do before you need to get a grip and start doing something constructive. So, now that I've got my work situation sorted for now, I need to give some thought to getting my hands dirty again. One upside of this working from home lark is that I am not having to work away from home during the week so I now have my evenings (and more of the weekend) for workshop time - not something I was anticipating any time soon, as we weren't expecting to be moving house for some months. Notwithstanding the need to keep the Domestic Manager happy by doing some work on the bathroom refit, that boils down to cutting metal in some form or another.
Not being one to finish one job if there is the possibility of starting another, my mind has been flitting about trying to identify something to get stuck into. I think I'm probably going to try to stay focused on the Bridgeport conversion Z axis assembly in the end but if I'm feeling like getting stuck into something more traditional and manual such as a bit of lathe and mill work, there are some options I've had on the back burner for a while.
High speed threading:
Just before I got the CNC conversion bug in Canada, I was putting the finishing touches to a threading clutch system for the Bantam. This is similar to the system fitted to Hardinge HLV lathe and others. Because the clutch is running at spindle speed, you don't need to worry about closing any half nuts at the wrong position and screwing up your partially cut threads. This is particularly relevant when cutting metric threads on an imperial machine, for instance. I guess an exception might be if you are cutting a multi start thread but we don't need to be worrying about that any time soon.
My system is pretty much complete but for the fact that it became evident that some clot (no, not The Stupid Fat Bloke) had clearly crashed the gear train and bent the output shaft from the headstock gearbox - the one that drives the start of the change gears. Presumably they had had the sense to replace the sacrificial shear pin with something more solid. This was an ex-school machine from what little I know of it. Bottom line is that said shaft has a fine wobble on it, despite my best efforts to straighten it. I've not managed to find a replacement so far. Anyway, that seems sufficient excuse not to actually fit the thing to my machine.
Retracting cross slide:
Once you've finally got yourself one of these high speed threading clutches installed, you'll find yourself wanting to withdraw the tool from the thread while returning the saddle to the start of the thread, before dialing it back in for the next cutting pass. That's where the retracting cross slide comes in. These are usually use an an eccentric to cause the cross slide to withdraw 5-10mm, then return, by means of flicking a small knob in and out (fnaaaarrr).
Here are some examples:
https://www.hobby-machinist.com/threads/tools-to-simplify-threading.20867/
http://www.primrose-engineering.co.uk/raglan-retracting-cross-slide/
Of course, while the principle is reasonably simple, the implementation on a particular machine requires careful thought, as the space is often rather limited. So here's what the saddle / cross slide setup looks like on my machine. There's quite a lot going on here, which limits the room. Certainly, with the cross slide in its fully retracted position, it pretty much touches the micrometer dial. This brings any thoughts of fitting an operating knob on the top (where the oil nipple is in this photo) to a halt.
Whipping the micrometer dial off:
Removing the two hex socket bolts releases the leadscrew. The leadscrew is splined to the hollow splined shaft that runs in the housing I'm holding here. The pinion is drive by the apron and provides the power cross feed when engaged.
You can see the driving pinion if you peer through the hole in the apron:
In the exploded view below, the worm wheel #27 is driven by the long shaft along the front of the machine. This drives worm gear #14, which also incorporates a std pinion gear that is in constant mesh with gear #10.
The cross slide power feed is engaged by pulling the selector knob (#12) towards the operator. This brings gear #10 into mesh with the pinion on the cross slide leadscrew. To engage the longitudinal feed, the knob is pressed in, away from the operator so that gear #10 meshes with gear #9 instead - this drives the pinion that meshes with the rack on the bed.
This cutaway shows how the cross slide leadscrew and the drive pinion are assembled:
An exploded view of the saddle assembly gives a different perspective:
I forget who gave me this but these photos show what you find if you get into the saddle. This is the power feed selector knob with its sliding gear (#10)
Here the selector knob is fully pushed in so that the feed is connected to the longitudinal rack and pinion. Or would be if it were mounted on the machine.
This is the overload knock-off mechanism that automatically disconnects the power feed when the saddle or cross slide hits a stop. It results in an increased axial force on worm wheel (#27) which eventually overcomes the detent stop (spring loaded pin) and causes the straight cut pinion to disengage from its meshing gear (not shown here). Simple but apparently quite consistent and safe.
Design concepts for the Bantam?
Space is tight, with the power feed selector knob, thread indicator dial (#16, telling you when to close the half nut) and hand wheel crowding for space. And that's no forgetting that the cross slide covers the handwheel "keep" (#19, extension nose thingy) when the cross slide is fully retracted. This is looking challenging...
Let's think on't before getting into this....
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