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Abaker
07-27-2012, 10:56 AM
I have been trying to come up with a way to keep a threaded spindle lathe from unscrewing. I have a VFD on my lathe and would like to be able to use the breaking feature. I've avoided that because I don't really want the chuck coming off.

One way to keep a part from loosening is to use a set screw, but a single one could push the chuck out of alignment. So I thought it might work to cross drill four holes in the spigot of the mounting plate of my four jaw chuck so that set screws would bear on the un-threaded part of the spindle. I'd use soft face screws or brass pads to keep from scaring the spindle. This should hold the chuck in place for pretty much any breaking, and it might work OK for light cuts in reverse.

It seems that this won't really affect the accuracy of a four jaw - once it's solidly against the spindle register it's not going to move too much. Using padded set screws should protect the spindle so I don't think it's going to damage anything.

I can't be the first one to think of doing this yet I haven't seen anyone else mention it, so what am I missing? Why is this a bad idea?

Dr Stan
07-27-2012, 11:56 AM
Unless you run it in reverse there should not be any way for the chuck to unscrew.

loose nut
07-27-2012, 12:12 PM
One way to keep a part from loosening is to use a set screw, but a single one could push the chuck out of alignment.idea?

You just answered your own question. Chucks unscrewing is a long tome problem and if there was a good way to stop it, it probably would have been done. It is most likely why screwed mandrels have fallen out of favor.

The best thing you can do is make sure that the threads and all mating parts are very clean and then make sure the chuck is seated properly.

Boucher
07-27-2012, 12:20 PM
In the past there have been some examples of what you seek posted here. Probably in a thread relating to a rear tool post. My search ability is not very good.

lakeside53
07-27-2012, 12:44 PM
Unless you run it in reverse there should not be any way for the chuck to unscrew.

Oh yes there is! If your chuck is spinning fast and/or has a large work piece, and you stop the spindle quickly, the chuck can easily unscrew. Don't akk how I know.

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.

1200rpm
07-27-2012, 01:06 PM
i have a machine w/ a threaded chuck and a vfd too, i also thought about the same thing. eventually decided that there was no reason i can think of(here at home) to stop that fast and just set the decel time down to a couple seconds.
if i actually had to stop instantly i wouldn`t hesitate to go cam lock.

2 pesos

lakeside53
07-27-2012, 01:16 PM
Does it acually stop in 2 seconds though from high speed? Lathes are tough to hit a number with a vfd as the inertial load varies tremendously. Some VFD will simply extend the decelleration if the DC buss rises to max; at low speed or lightly loaded they can stop quickly, but at higher it takes much longer. I also took aggressive braking off the Emco's, and also decided a foot brake addition wasn't a good idea either for my non-collared chucks! It was interesting to see the chuck "float out" a little from the spindle then seat again. lol..

JFLingg
07-27-2012, 01:37 PM
The Sherline was my first lathe, bought second hand. One of the first things I did was spin off the threaded chuck and ding the brass ways. It was in Forward, full speed, and I spun the speed control to zero. Ding!
http://i1154.photobucket.com/albums/p523/CaseSORoof/Ding-001-1.jpg http://i1154.photobucket.com/albums/p523/CaseSORoof/Ding-002.jpg http://i1154.photobucket.com/albums/p523/CaseSORoof/DSCN0041.jpg
Every photo Itake, I can see it.:mad:
JFLingg

Abaker
07-27-2012, 02:10 PM
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?

Willy
07-27-2012, 02:13 PM
If you have sufficient room on the spindle this method is a very positive way of retaining a threaded backplate/chuck.
Spaced 180 apart two of these retainers are more than adequate to prevent spin-off.


http://i76.photobucket.com/albums/j31/250willy/chuckmount.jpg

1200rpm
07-27-2012, 02:26 PM
the above is how my "taiwanese traytop" does it.
as far as my vfd set-up, i`ve never really checked to see but i`d guess it stops pretty close to the set time- it`s on a small SB though and the biggest chuck i have is a 5", and i usually use a 4" on it so not a lot of weight.

true story- when i was playing with the vfd after initially getting it set up, i was using the 5" chuck turning a piece of about 1/4" material around 900-1000 rpm, i was using a live center in the tailstock.
found out the hard way i did not have enough clearance for the outboad spindle bearing. locked it right up. luckily no damage except my pride.
the chuck didn`t unscrew but it did break free. live center took the brunt.
it`s scary to think about it actually coming off and whipping around with that skinny little piece sticking out.

philbur
07-27-2012, 02:50 PM
Here is a German lathe manufacturers' solution to locking the chuck on a threaded lathe spindle using pins on the register:

http://i186.photobucket.com/albums/x36/philbur/chuck1-1.jpg

http://i186.photobucket.com/albums/x36/philbur/chuck1.jpg

http://i186.photobucket.com/albums/x36/philbur/chuck3.jpg

It is basically three bronze pins that bear on the spindle register. The internal taper of the threaded ring applies a very high mechanical advantage to the pins, much more than can be achieve from a threaded screw bearing directly on the register. The arrangement has the additional advantage that the force on each pin is self equalising (due to the play in the thread in the clamping ring), eliminating any tendency to misalign the chuck. It's German so of course the design is complicated, difficult to say if it's over engineered, I've never tried to spin the chuck off the spindle on purpose:eek: It does imply that screws acting directly against the register is not enough.

Phil

Dr Stan
07-27-2012, 04:45 PM
Oh yes there is! If your chuck is spinning fast and/or has a large work piece, and you stop the spindle quickly, the chuck can easily unscrew. Don't akk how I know.

Guess I've just been lucky, but then I've never tried to instantly stop a screwed on 3 jaw. All the lathes I've used that were equipped with a brake also had cam lock spindles.

alsinaj
07-27-2012, 05:48 PM
I drilled & tapped the spigot on my 8" 3-jaw chuck and inserted 3/8" setscrews that bear on the spindle threads through pads made of crushed small-diameter brass wire. I don't have a VFD, but I do sometimes start my spindle in reverse. The chuck weighs at least 30 lbs, and the motor has 5 real, old fashioned HP, so the acceleration is virtually instantaneous. It hasn't unscrewed yet. But I re-tighten the setscrews EVERY time before tempting fate.

DickDastardly40
07-27-2012, 07:51 PM
I'm not familiar enough with VFDs but believe you can adjust the braking amount so you don't experience the dead stop which could cause the chuck to unscrew.

Another thing you could try though not obviously useful for all operations is to use the tailstock with a centre onto the job to prevent any movement of the chuck.

I use a tailstock where possible if ever I have to run my lathe in reverse.

Forrest Addy
07-27-2012, 10:48 PM
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
07-28-2012, 02:56 AM
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
07-28-2012, 03:36 AM
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
07-28-2012, 04:00 AM
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
07-28-2012, 04:17 AM
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
07-28-2012, 05:09 AM
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
07-28-2012, 05:19 AM
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
07-28-2012, 07:37 AM
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/albums/member_photo.asp?a=18372&p=306929
Joe

joegib
07-28-2012, 09:20 AM
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 :rolleyes:.

Joe

joegib
07-28-2012, 09:38 AM
Here's another design:

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

Joe

Northernsinger
07-28-2012, 10:12 AM
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:

http://img.photobucket.com/albums/v227/brnstn/Weiler/Weilerchucktightener001.jpg

joegib
07-28-2012, 11:09 AM
Interesting! So, it seems that Weiler did opt to provide relieving splits in the backplate spigot.

Joe

J Tiers
07-28-2012, 11:58 AM
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
07-29-2012, 12:24 AM
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.