Stopping a threaded lathe chuck from unscrewing

[Forrest Addy]

If you try to stop it in 1 second it's almost certain to unscrew but for a normal sedate decel, you might be jumping at shadows.

When you screw on the chuck, seat it with a sharp thump. It should stay put for any reasonable braking duration like say 3 seconds or 5 seconds which is what I have mine set for. It's a normal stop which allows me a moment to reach for the chuck key or a micrometer.

Consider: a VFD set for 5 seconds of braking motors down the speed from 60 Hertz more or less linearly to zero. Slow from 30 Hz and the braking time is 2 1/2 seconds. 15 Hz etc. Accel/decel time is roughly proportionate to the Hz setting on the control pot. Just to confirm, set the accel/decel parameter for 15 seconds at 60 Hz and check the times from different control pot settings.

Braking from top spindle speed would be the worst case for your lathe. Let's say the chuck turns 900 RPM or whatever at 60 Hz. Set the VFD for 10 Hz and do a start and stop several times. The chuck will see the same decel but for a briefer time.

Therefore, a brake cycle at 10 Hz means the chuck will brake from 150 RPM. If the chuck tries to unscrew there's not enough inertia in its rotation to unscrew very far. Take the jaws out and place a piece of carpet on the ways just in case. Do some stops at several decel parameter settings until the chuck unscrews. Then set the decel to double the decel time and go on your merry way.

[Bryan B]

What about cutting? For instance on my camlock lathe I find it very convenient to turn short tapers in reverse, with the compound wheel facing me. Be nice to have that option on the threaded lathe too. I have an idea that involves a left hand thread and a taper. Easier to draw than describe.

[lakeside53]

Exactly what I want to avoid, except that 6" 4 jaw has a bit more mass. My luck the ding would be on me. I have a pretty robust cranium (runs in family) but still, that's gonna hurt.

Lakeside: Thanks for that idea about splitting the stub and then clamping it with a collar. Did you ever try to work in reverse with it?

I used reverse a lot... No issues with the clamped chuck; plenty without.

[Evan]

The chuck shouldn't unscrew. My South Bend has the original "Instant Reverse" motor which will reverse from full speed to full reverse in a couple of seconds at most. It never unscrews the chuck. If the chuck unscrews that easily the fit is poor. The spindle or corresponding parts of the chuck may be dinged up or simply not fitted well.

The chuck should be difficult to remove although if done properly is isn't a problem. I lock the chuck in back gear and use a longish wooden handle in the chuck jaws to unscrew it without smacking it. No chance of breaking gears if done that way but it should take a hard pull to loosen it. The only time I have ever had it unscrew was when trying to reverse in back gear a one inch morse taper drill from a bore in which it was stuck tight. That was because I had tried to step drill a slightly smaller hole in 7075-T6 aluminum which is notorious for grabbing drill bits.

[Blackadder]

Unless you run it in reverse there should not be any way for the chuck to unscrew.

not true

they will unscrew when rotating in the normal direction if you brake /stop the spindle , the spindle/chuck is rotating anticlockwise looking at the chuck therefore if you stop the spindle the chuck tries to keep going ( some bodies law a body in motion tends to keep in motion ) as its going anti clockwise it will unscrew beleive me i know it will happen

[joegib]

I've had an couple of Emco lathes. They have a locking split collar as part of the backing plate. That clamps onto a shoulder on the spindle. There are two models -one was integral with the backplate; the other (later) was just a collar that compressed the backing plate stub nose (it was split into 4 with a saw). This is preferable to the set screw idea - you will push your chuck around and I doubt they will lock well.

Schaublin also supplied locking rings with their screw-on chucks. This is simply a split collar that applies compression to the backplate spigot where it surrounds the spindle register. Clive Hartland over on the Model Engineer website posted a picture of one here:

http://www.model-engineer.co.uk/albums/member_photo.asp?a=18372&p=306929

Joe

[Paul Alciatore]

Just spit-balling here, but how about a drawbar with a left hand screw and a Tee shaped plug that fits the drawbar like a collet at the chuck end to hold it in. The cap of the Tee would be larger in diameter than the hole through the chuck so it would bear on the face of the chuck. For normal turning, the spindle thread would hold it. If breaking or running backwards and the chuck starts to unscrew, the Tee plug would tighten harder on the left hand thread and hold the chuck on tighter. Removal would be two steps, first remove the Tee plug with the drawbar and second, unscrew the chuck.

This is a simple, two or three part device. No modifications would be needed on the spindle or on the chuck for this to be installed. No set screw marks would be made on any parts. All would be returned to factory condition when it is removed. And with the possible necessity of individual Tee plugs, different chucks could be accommodated with the same accessory.

[philbur]

I can't see how the clamp functions, it seems to be on the outside of the backplate spigot. I can't see how it clamps on to the spindle register. What am I missing?

http://www.model-engineer.co.uk/albu...18372&p=306929
Joe

[joegib]

It's a straightforward split collar with a tangential capscrew spanning the split (unfortunately the inset for the capscrew head is not too clear in the photo). Some people first encountering this device start querying whether the backplate spigot also needs to be split. The answer is no — as long as the backplate spigot is well-fitted to the spindle register, tightening the collar via the capscrew applies enough compressive force to lock the chuck (well, Schaublin think so and I'm prepared to trust their judgement).

OTOH if you're of the school of thought that doesn't fit the backplate spigot closely to the spindle register, this device won't work for you .

Joe

[joegib]

Here's another design:

http://www.model-engineer.co.uk/forums/postings.asp?th=47650

Joe

[Northernsinger]

One of my Weiler lathes (an LZ 330) has a threaded spindle and this is the manufacturer's lock ring for it, pictured with a six inch three jaw:

[joegib]

Interesting! So, it seems that Weiler did opt to provide relieving splits in the backplate spigot.

Joe

[J Tiers]

The ONLY way I have managed to get mine to unscrew when not wanted is an interrupted cut taken in reverse......

That said, I suggest that you DO NOT try any of teh clamp setups which attempt to hold the chuck in by a drawbar or other similar means..... if there is any real energy attempting to unscrew the chuck, it has a LARGE mechanical advantage over the drawbar, and something is probably going to "give".

I don't doubt that some have done that and "had no problems".......... people do lots of things and "get away with it". Doesn't mean YOU will.

A direct hold on the spindle that stops the unscrewing, like the pins, or a circumferential clamp etc, is a more direct solution with forces that are directly opposing the unscrewing motion itself, instead of trying to stop it AFTER the mechanical advantage of the thread is added.

[Abaker]

Thanks to everyone for some great ideas. I knew there was better way - now I know what they are.

I like the clamping collar method. If I get the chuck done right it might not even need to be split to grab on to the spindle.