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| Swingup external threading tool |
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| andyf:
Hi Anthony, Hmm... I think it might have to be a bit more complicated, and rather more like Cleeve's version, to make it rigid enough to work with a boring bar-like tool for internal threading, which would (on your sketch) have to stick out to the left, rather than point straight up. Andy (in :coffee: and wet blanket mode) |
| bramley51:
Andy and John,I've just finished my self retracting tool,and I'm chuffed to bits.It works like a charm,even for an ijit like me. :).Thanks for presenting the design,single point threading no longer scares the cr*p out of me. I can highly recommend this tool to anyone to anyone who's never tried single point threading 'cause it's "too difficult". Make this tool,and people will pay good money to see you at work :D Thanks again,guys. :nrocks: |
| troup:
Must be something in the water here in NZ, over half the people I know of from the interwobble who've run with this idea have been kiwis. To whom I have now added myself, and will post more details, but I have a WARNING for those with a VFD who feel inclined to take the idea to the logical conclusion, and add some sort of automatic reverse. What I'm talking about is a sort of self-acting "canned cycle", so you can thread pretty much like an NC lathe for the price of a switch or two. Going directly from "full steam ahead" to "full astern both" used to be the preserve of turret lathes (eg Herberts with massive 4 speed motors and 'plug reversing') or the likes of John Stevenson's Tos, but now with VFDs it's as simple as adding a single changeover limit switch (a normally open pair AND a normally closed pair of contacts) or two simple limit switches. If either the switches or the dogs can be relocated, your lathe can automatically and accurately cycle through the threading zone ad infinitum, leaving you to simply increment the cut. (Runout grooves can be narrower, threading speeds higher > better finish, quicker job completion: all very enticing....) HOWEVER this silver lining does have at least one cloud: particularly when commissioning the reversing facility for such a setup, be aware that repeated reversals can cause a scroll chuck to self-open, to the point the jaws fly out. I learned this last night by almost having it happen. Yikes! The scroll is the flat disc with a single spiral groove which engages with the teeth on the backs of the jaws, and also with the bevel gear pinions with the square recesses for the key. The mechanism of the self actuation is that the scroll (at least on a bigger chuck) has a lot of rotational inertia if you're doing say 250 rpm in one direction. It wants to continue doing so even when the lathe suddenly switches to 250rpm in the opposite direction. Similar to the situation on a screwed spindle nose, where the chuck will happily unscrew when you go suddenly from forwards to reverse. In the case of the scroll, it's switching from reverse to forwards where the problem occurs. In my case, I was commissioning, with nothing clamped in the chuck, and trying to see how repeatable the reversing position would be (about 0.5mm before any attempts to refine the mickey-mouse lashup). The only symptom was a funny noise which sounded as though it came from the front spindle bearing. Picture yourself leaning down, (putting your head inadvertantly in the line of fire) to try and pinpoint the noise, as you flick the fwd-reverse switch with your thumb.... The jaws may open a considerable distance on each occasion, about 5mm in my case, and spinning at this speed they're essentially invisible. When I stopped the trial, they were almost fully out. This was a heavy industrial chuck and each jaw weighs about as much as a 1/2" Jacobs drill chuck. Apart from the commissioning case or, say, threading between centres using the chuck only as a catchplate, when the jaws are not clamping anything, it's possibly even worth considering this scenario when you decide how tightly to do up the jaws of a job you'll be threading with rapid reversals. I'm thinking particularly of threading, say, thin walled tubing, where there's little resistance to allow building up a decent amount of clamping pressure, and it seems to me the jaws could very easily release the work. If I do jobs like this I'll either dial in longer decel and accel parameters, or make a closefitting plug for the work. (Note to self, DO NOT FORGET!!!) |
| John Stevenson:
But this is with nothing in the chuck. I use my swing up tool all the while now and I do a lot of screwcutting. Because the TOS is a dream to use with this tool I basically screwcut everything and finish off with a die if I have one or use the Coventry die inserts. Last week for example I did 24 special M18 x 1.5 pitch bolts 60 mm long. At no time have I had the chuck jaws come loose because of the quick reversal. John S |
| troup:
I'm relieved to hear that, John Just to clarify, though: that contingency (inertial loosening when a lightly gripped part is reversed) was not the main thrust of my warning, nor was normal screwcutting. What I wanted to alert people to primarily was the situation which might arise during COMMISSIONING or troubleshooting of any instant reverse facility: lots of reversals, dry running (so pushing the envelope speed wise) with nothing gripped in the chuck. |
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