Retracting toolholder to fit QCTP

[andyf]

For screwcutting, a chap on a Yahoo group has made a swinging toolholder for his QCTP along the lines shown in the sketch below. Toolholder A is supported by the ledge on B, and by the adjacent vertical face of B, during the cut. When the saddle is run back ready for the next cut, the work rubs on the tool-tip, raising it to keep it out of harm's way. The motor (rather than the tumbler gear) must be reversed, so that the work runs backwards as the saddle is run back.

It does suffer from two pretty major limitations. First, it will only work for RH threads; on an LH thread, there would be no side support for part A. Secondly, it won't work for internal threads, and I suppose that those are where most help is needed because one is flying blind on instruments only. I'm mentioning it for just for interest, rather than as an altenative to a proper retracting toolholder.



Andy

[Darren]

These do look nice though

[bogstandard]

Darren,

Even though I have decided it is not time cost effective to make one, that one you have shown really saddened me.

With a slight design change that could have gone from being a mediocre outside threading one, to an all singing all dancing one for both internal and external threading.

That's what comes of following other peoples designs religiously. It is OK for what the original designer needed, but not for others who follow along afterwards.

As far as I am concerned, it is only half finished.

John

[John Stevenson]

One drawback I can see with the one in Darrens pic is that bloody lever would be very close to the chuck when working on short lengths of parts with large top hat diameters.

Not my idea at all but fitted to my CVA which in turn was robbed from the Monarch 10 EE [ sighs of reverence here .. Aaahhhh ] is that the hand wheel dial has the stop built in.



This is the hand wheel removed and reversed, the dog on the right is part of the dial, the other two are just floating dogs.



What happens is that you position the cross slide close to the work and screw the small side screw in then wind in.
Then what happens is the outer dog catches on the screw, then the inner and then the fixed one which gives you nearly 3 complete turns before it all comes up tight.

You then use the top slide set over at 1/2 the angle to position the tool.

At the end of the thread you wind out, no need to look, go back to start of thread, either manually or under power, and then wind in all the way, then apply new cut with the top slide.

This means the operation is machine controlled and not reliant on a special tool holder.

Depending on machine build up it may be easy or hard to implement.

John S.

[sbwhart]

We use to have Holbrook lathes that had a quick reatract handle on the cross slide, at the end of the cut you just pulled the handle up, go back to the start push the handle down, put your next cut on etc etc don't know if it worked for internal cutting though.

Stew

[Darren]

Thanks John, I was wondering how that worked on the CVA. Quite simple really, like most good ideas.

I wonder ...... 

[geroli]

With a slight design change that could have gone from being a mediocre outside threading one, to an all singing all dancing one for both internal and external threading.

Care to expand a bit John
I tried to think but all the mods I can think of to make it in/external is very complicated???  I ask because that spesific one is on my todo list.WHAYYYYY DOWN.
Gerhard

[bogstandard]

Gerhard,

Here is a very rough C-o-C of how easily it could have been modded to allow both internal and external tools to be held, and the cam would operate in the forwards mode for external cutting and rearwards mode for internal.


John

[geroli]

Thanks John -That would work - would need to put positive stop forward as well as rear.
And it would be a bit bigger to allow it to travel without play????


Hope I remember all this when I get back to the threading tools.

Gerhard

[bogstandard]

Gerhard,

Unless you are doing very large threading work, 0.187" (4.8mm) would be more than ample movement.


John

[DavesWimshurst]

My version of a cross slide stop that stops the rotation of the screw:



The stop flips up out of the way:



If I back out more than one turn the stop is pushed up out of the way by the head of the thumb nut on the dial.

It's probably immoral but I do internal threading on the back side of the bore with an upside down tool so the tool retraction action is the same as for external threads.

It just clamps to the outside of the screw housing so is easily removed if it gets in the way, it limits the travel a little when working large diameters.



It is adjusted so that the dial reads 0 when the dial lock screw bumps the stop.  It has worked fine for me for several years now.
Hope this inspires other ideas.
Dave

[Darren]

Thanks for showing, that is so simple and looks easily implemented 

[75Plus]

Can you stand one more way to stop the cross slide?  Hope so as here is the way I do it on my 12 X 36 Asian lathe.

The tapped holes on the saddle for the follower rest are hardly used so I made use of one of them to hold my stop.



This is the stop.  It only works for external threads but that is not a problem for me as external is about all cut.



There is a little flex if pushed hard but I find the repeatability is about as good as the repeatability when using a mic. Once I got the feel of it I stopped looking at the dial each time I started another pass.

In the first picture you can see two holes in the side of the cross slide. These holes are in a position which would allow a stop to be made that could be used for both internal and external threads and would be a more rigid. I may investigate this possibility

Joe