Thanks: 0
Likes: 0
Needs Pictures: 0
Picture(s) thanks: 0
Results 31 to 45 of 67
Thread: chuck troubles
-
18th July 2012, 12:06 PM #31GOLD MEMBER
- Join Date
- Nov 2007
- Location
- melbourne australia
- Posts
- 2,648
I don't see how this can occur. When the first two jaws contact the workpiece they won't "load up" at all, they will just push the workpiece towards the third jaw. Only when all three jaws are in contact with the workpiece will any of them start to load up. Geometry (from a book).
-
18th July 2012 12:06 PM # ADSGoogle Adsense Advertisement
- Join Date
- Always
- Location
- Advertising world
- Posts
- Many
-
18th July 2012, 12:19 PM #32.
- Join Date
- Feb 2006
- Location
- Perth
- Posts
- 27,798
You guys have got the real problem a bit askew
" if by some chance two of these jaws come into contact with the workpiece before the third jaw " . . . . the piece will simply travel perpendicularly in the vice until it meets the other jaw. That's what a self centring means.
However, if the piece meets the third jaw at a non-normal angle then not as much radially pressure can be applied to the contact surface of the piece as the other two jaws so the third jaw does not hold as much as the other two.
It's not that one jaw is overloaded as much as one jaw being underloaded relative to the piece which leaves the piece prone to movement.
-
18th July 2012, 12:24 PM #33
As you are most book machinists it probably matters not.
Have you ever set a job up in a three jaw chuck using a fixed steady and had the job either walk out or walk into the chuck.This is caused in a way by the chuck gripping the material and the steady.
If the material being gripped by the chuck is not held securely by the three jaws along the full length of the material in the chuck the material will have the tendency to walk in or out as it rotates in the fixed steady,it causes a spiralling effect.
Have you ever attemted to hold black bar with scale on it in a three jaw chuck and had the material move on you,most times workshops if they do intend to hold black bar or forgings,castings etc in a three jaw chuck will have an old chuck for this purpose.
The three jaws of a conventional three jaw chuck work together,hence the term self centreing,if by some chance two of these jaws come into contact with the workpiece before the third jaw the other two jaws will load up before the third jaw grips,this in itself will create excess load on the scrolls of the chuck causing them to apply unequall pressure to the job.
As a geometric question it should be self explanetry.
I should point out to all that I am not trying to oppose anybody in this matter but I am trying to get an answer that is based on genuine scientific / geometric principals.
if by some chance two of these jaws come into contact with the workpiece before the third jaw the other two jaws will load up before the third jaw grips,
When I place stock in the chuck I do up the jaws until they are just touching and then slightly rotate the work back and forth whilst slowly tightening the chuck. This is to align the work to the length of the jaws and to find the best position. You can feel if there is a problem.
I get the impression that some are taking this personally. If you are then you have misconstrued the meaning of posts. I for one am on this forum to further my knowledge and maybe pass some knowledge on, as I have said before.
Dean
-
18th July 2012, 12:56 PM #34Distracted Member
- Join Date
- May 2010
- Location
- Lower Lakes SA
- Age
- 58
- Posts
- 2,557
Try this thought experiment: Rotate your 3 jaw so one jaw is at 12 oclock. Place a short piece of roundish stock on the lower 2 jaws, so it can sit there on its own. Now start closing the chuck. What happens?
- The stock stays where it is and is gripped only by the lower 2 jaws?
- The stock is lifted by the lower jaws until it meets the upper one?
-
18th July 2012, 01:19 PM #35.
- Join Date
- Feb 2006
- Location
- Perth
- Posts
- 27,798
Maybe this will explain what is going on.
Attachment 216152
On the left the forces applied to the round piece by the jaws are ALL equal and meet at the radial centre of the piece. In the middle and right drawings the forces applied to the round piece by the jaws to the surfaces are NOT ALL equal because the surfaces are not all perpendicular to the jaws.
This in turn means the tool does not experience equal back forces as the piece goes around, which is why piece slippage can occur under these conditions.
The case on the right is the worst since the centre of the 3 jaw back pressures does not even meet at a single point and the chuck in effect creates a back torque on the piece which is probably why some pieces can walk their way out of the chuck.
BTW I have only minimal machining experience so you may not wish to pay any attention to what I am saying
-
18th July 2012, 01:31 PM #36GOLD MEMBER
- Join Date
- Jun 2007
- Location
- sydney
- Age
- 64
- Posts
- 3,566
As was said earlier and I agree with if the stock is not bright or peeled you shouldnt use the 3 jaw.
By all means if a book says you can go for it.
If it works for you go for.
Dont ever complain if a 3 jaw runs out more than stated after using it this way.
A book machinist is a machinist that relies on books for most of there knowledge,there appears to be quite a few on this forum with out much practical experience.
-
18th July 2012, 01:40 PM #37GOLD MEMBER
- Join Date
- Nov 2007
- Location
- melbourne australia
- Posts
- 2,648
-
18th July 2012, 02:09 PM #38Senior Member
- Join Date
- Jul 2012
- Location
- Griffith NSW
- Posts
- 257
I have got some experience in this problem through work. At school, we trialed he use of hot rolled stocks for some time to save some coin, but it would regularly work itself loose in chucks. It most often occured when the stock was inserted all the way in the chuck, centre drilled, then extended and supported with a live centre. My thoughts on the matter were similar to the person who mentioned the fixed steady, because the centre isnt necesarily on axis, the work was forced to occilate in the chuck, causing it to move in the jaws until it had abraded enough metal away to become loose.
The problem was nearly completely eradicated when we went back to bright stocks. Replacing the chucks made it better again, the previous work done on the backing plates was pretty average and they had to be remade and trued on all of the mating surfaces.
It didnt damage the chucks though. The runout of a given chuck (of the old chucks) stayed pretty well the same for the years that I was at that school. The new chucks occasionally saw some hot rolled metal for various odds and ends and they never showed any signs of changing either. I think the damaging the scrolls or the guideways for the jaws argument is a bit of a stretch.
-
18th July 2012, 03:00 PM #39Distracted Member
- Join Date
- May 2010
- Location
- Lower Lakes SA
- Age
- 58
- Posts
- 2,557
Bob, thank you for trying to shed light instead of heat. I don't really understand why the force lines change given that the jaws are constrained, but I'm in over my head on that one and I know you are a man of science so I'm inclined to believe you. But I thought the question was about the amount of force on each jaw? Does that also change?
-
18th July 2012, 03:31 PM #40.
- Join Date
- Feb 2006
- Location
- Perth
- Posts
- 27,798
I agree the jaws are constrained but it's whats happening at the contact surface that is important since this determines the final radial force applied.
If a force X is a applied at 90º to the surface of an object then the object experiences a force X. (diagram on left)
If the force is not applied at 90º then the force experienced by the object is <X or on my drawing Xv. The non-perpendicular geometry also generates a sideways or in my drawing horizontal force (Xh). Because the chuck jaws are constrained it effectively means the chuck ends up applying, and the piece ends experiencing the sideways force. The vector sum of the sideways and vertical forces must add up to the original force X so Xv will always be less than X.
It's this sideways forces that can cause out of round pieces to turn the pieces to turn under load
I hope that helps.
-
18th July 2012, 04:13 PM #41
Bob this certainly explains some things but what I have been discussing is slightly out of round stock. I have pointed this out in my posts.
If you were to rotate the stock in the 3rd photo as I have mentioned while tightening it would settle into a far better position than it is at the moment.
All of these situations are possible but you have to remember that the first cuts are for truing up and need care and checking of the chuck tightness.
A book machinist is a machinist that relies on books for most of there knowledge,there appears to be quite a few on this forum with out much practical experience.
Maybe someone can point me to some technical reference about this.
Dean
-
18th July 2012, 04:21 PM #42.
- Join Date
- Nov 2008
- Location
- Perth WA
- Age
- 71
- Posts
- 5,650
Peter,
I imagine there are quite a few of us who are book machinists, I'm certainly one of them.
Very seldom have I been shown how to correctly perform the majority of tasks I perform these days in the shed but I think I get by. I've learnt by trial and error and there have been plenty of the latter.
This forum has been a great source of knowledge gleaned from those with far greater experience than I will ever have. The other source has been my books. Most of us participate here because we wish to learn and share ideas and experiences.
And for a lot of us this is simply a hobby. I can imagine the frustration a proficient machinist must have when he or her view some of our efforts but what can we do but heed advice and try and improve.
Bob.
-
18th July 2012, 04:29 PM #43GOLD MEMBER
- Join Date
- Jun 2007
- Location
- sydney
- Age
- 64
- Posts
- 3,566
Any remarks I make whether diparaging or not are based on what I read here.
Work at present is as a train guard,I blow a whistle open and close doors and answer questions.
-
18th July 2012, 04:34 PM #44
-
18th July 2012, 04:38 PM #45GOLD MEMBER
- Join Date
- Jun 2007
- Location
- sydney
- Age
- 64
- Posts
- 3,566
More a whistle blower,and its my own.
Similar Threads
-
2 pack troubles
By old_picker in forum FINISHINGReplies: 8Last Post: 8th October 2011, 01:22 AM -
Turret Troubles
By Anorak Bob in forum METALWORK FORUMReplies: 38Last Post: 11th July 2011, 08:46 PM -
Gearbox troubles
By bitza500 in forum THE HERCUS AREAReplies: 1Last Post: 6th January 2009, 04:51 AM -
Makita Cut Off Saw Troubles
By under thumb in forum METALWORK FORUMReplies: 13Last Post: 29th June 2007, 05:08 PM