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View Full Version : The 4-jaw blues. Chuck help.



dberndt
05-11-2006, 06:42 PM
Ok to start off with I don't have alot of time running a lathe and I'm just a newbie HSM'er.

I've been trying to turn an accessory for my mill spindle to .0005" TIR but thus far have been failing totally.

Heres a drawing of the part.

Note: drawing is 1:2 scale.
http://pad.phalnet.com/view.php?imgid=1017

The top face of the adapter seats against the top of the threaded spindle. This is what gives registration, or atleast its what I think does. I havent been holding the thread or the undercut above it to any particular tightness as I don't think this matters when the part is threaded up against the register it should seat correctly anyways?

My current order of operations is.

Center drill both ends.
turn OD to size between centers.
put in 4 jaw chuck, dial in with .001" dial indicator, With a good look at the dial I can ussually adjust to the point of no needle movement. (more on chuck problems later)
drill, bore to 20.5mm for thread.
thread using tailstock center for a guide
bore undercut
face
chamfer edges.

At this point the other end is still not faced but it has to come out of the 4 jaw chuck to be flipped to face the other end. At this point I ussually stick it on the spindle and find that it runs out .004-.010". Which is way off what I'm looking for.


A side note on my chuck, It seems to be holding things slightly crooked. A turned surface when chucked won't come out being chucked parallel to the ways, it will be off one way or the other by a few thou per inch it sticks out of the chuck. I initially didn't realize this was the case and just dialed in one location on the work piece and ofcourse got terrible runout further down the shaft. Is this normal, or should I expect the chuck to grab the work piece parallel to the ways with good repeatability?

SGW
05-11-2006, 06:47 PM
Here's an idea for you: turn the o.d. last.

Bore/thread/face the piece, all in one setting held in your chuck.

Then take it out and chuck a piece of scrap, and cut a thread you can screw the piece onto.

Screw on the piece, turn the o.d., and face the final end.

Evan
05-11-2006, 07:01 PM
Chuck it up and bore, then thread and face. Put it on the spindle of the mill and finish machining the od and other face there. Stick a tool holder in the vise.

topct
05-11-2006, 07:08 PM
If you need that .0005 here is how I would do it.

Holy crap. :D

Evan sneeks in here..........perfect solution.

CCWKen
05-11-2006, 07:16 PM
Ditto on turning the od last--On the spindle. Also, it sounds like you have chuck problems. Either the chuck jaw slots are worn or you have one or more bent jaws. You can re-grind the jaw contact area but it won't do any good if the slots are worn and sloppy. Check it out. If the jaws rock in the slots, S-Can the chuck and get a new one.

Forgot to mention: I'm confused by your dimensions. I'm wondering how you can cut 22mm x 1.5mm thread in a 10.5mm hole or in a piece that's only 16mm in diameter? :eek:

Tin Falcon
05-11-2006, 07:21 PM
SGWs logic is very sound for making the part it will remove any error in the setup.

[/QUOTE]
A side note on my chuck, It seems to be holding things slightly crooked. A turned surface when chucked won't come out being chucked parallel to the ways, it will be off one way or the other by a few thou per inch it sticks out of the chuck. I initially didn't realize this was the case and just dialed in one location on the work piece and ofcourse got terrible runout further down the shaft. Is this normal, or should I expect the chuck to grab the work piece parallel to the ways with good repeatability?[/QUOTE]
You should correct this problem. Remove the chuck from the back plate. True the back plate on your lathe. take off as little materal as possible. When you reassemble make sure there is no binding on the fasteners.

Tin Falcon
05-11-2006, 07:22 PM
.Sorry double post

dberndt
05-11-2006, 07:32 PM
You should correct this problem. Remove the chuck from the back plate. True the back plate on your lathe. take off as little materal as possible. When you reassemble make sure there is no binding on the fasteners.

I turned this backplate myself the other day and it ran nice and concentric, first thing I've ever done on the lathe that I was actually satisfied with. I'll check it out again.

The slots on the chuck are still good and tight, its a second hand chuck so maybe the jaws should be reground. More investigation.

re: turning a male screw and threading on. Well yes, this I knew but its not really the objective of the exercise. I was playing with the new to me 4-jaw and the goal was to be able to make some spindle adapters. I'm sure a 4-jaw in good condition and a good machinist could do it so I'd like to be able to do it, in about 10x the time.

Evan
05-11-2006, 07:39 PM
You do need to investigate that chuck. Still,
the best way to do the job is just the same as truing the faceplate. The adaptors should be finish turned on the spindle they will run on, even if your chuck was perfect.

japcas
05-11-2006, 07:57 PM
If I were running that part and wanted everything to run together I would chuck a piece of stock that is a little longer than needed and face, turn, bore, and thread all in the same setup. Then either part off or saw off the actual part. Then place piece back in machine and face the backside to get the proper length. Runout will be zilch if ran this way.

dberndt
05-11-2006, 08:42 PM
unmounted the chuck and checked the backplate again, its dead nuts. The chuck body runs within .001" all over (except where there are dents, scratches, gouges, etc from the not so gentle last owner).

I can wiggle all of the jaws .001" in their slots. That is to say when I grab onto a jaw by hand and pull it towards the tail stock as hard as i can i get .001" deflection. I assume thats pretty reasonable in terms of being in good shape, can't imagine the fits being much tighter really.

I do have another 4 jaw thats exceeded its useful life. Jaws are sloppy and some of the adjustment screws have cracked. Looks like the cracks started in the broached corners and have spread down the screw. Anyone have any suggestions as to other things to do with it other than a flower planter.

Mcruff
05-11-2006, 09:35 PM
If I were running that part and wanted everything to run together I would chuck a piece of stock that is a little longer than needed and face, turn, bore, and thread all in the same setup. Then either part off or saw off the actual part. Then place piece back in machine and face the backside to get the proper length. Runout will be zilch if ran this way. Ditto, if those numbers are mm then there is no reason to do this any other way. The part is only 30mm long which means you'd have less than 1 3/8" of material sticking out of the chuck. If you need more than one, when you're done parting off, bore out some soft jaws to fit the diameter and make a stop to help the length repeat . By running these between centers or taking them out of the machine to finish them on a mill you are over complicating this job.

cruzinonline
05-11-2006, 11:19 PM
When you were talking about a part being out of parallel with the ways when you chuck it up, were you holding onto say 1/2 or less of the length? I haven't seen a 4-jaw yet that will automatically hold a part square, especially when holding on a short length. All we use are 4-jaws and always put two indicators on a part, one close and one as far from the chuck as possible. Dial in on the close one and tap the end and zero out the far one. BTW, tap on the high side when its on the point of the indicator to reduce shock on the indicator. I would think that you are also using brass pads between the jaws and the finished part to keep from brinelling the part. If you are able to chuck a finish part without marking it you are not tightening up the jaws enough or you have ground the jaws which wouldn't be prudent. 4-jaws are designed to bite. The radius on the tooth is larger that the radius of the jaws in the full open position so you are really getting a "line" contact on the part and not a full contact like you would a collet. I like the idea of finishing the part and then parting it off if you have stock long enough.

ulav8r
05-12-2006, 12:15 AM
Anyone have any suggestions as to other things to do with it other than a flower planter.

Replace the screws, use it as a vise. Not as quick as a 3 jaw, but still useful.

dsergison
05-12-2006, 09:40 AM
You are TAPPING THE HOLE?

you said using the tailstock as a guide.... well if youre tapping that's part of your problem.

dberndt
05-12-2006, 10:31 AM
How would you do it. I can't get it into my drill chuck, the shanks too big. I'm just putting a dead center into the center hole at the end of the tap and running it home. Should I make a tap holder or ?

cruzinonline
05-12-2006, 10:44 AM
Can you single point, like to 80% and then tap??

dberndt
05-12-2006, 11:20 AM
Don't have any internal threading tools, its kind of holding me back. Plus the lowest spindle speed is way too fast for threading.

japcas
05-12-2006, 11:32 AM
Check to be sure your tailstock is on center. As long as your tailstock is on center their is no reason why you shouldn't be able to tap the thread in the part. I am assuming that your tapped hole is 12mm not 22mm as stated on your print as that would be impossible to do considering the o.d. is only 16mm. Single point threading the hole would work but that is a small hole and if not very experienced could cause more problems than it would help. Keep it simple! As I said earlier, do all operations in one setup and then part it off or saw it off. Then turn it around and face to the proper length and your done. If you don't mind me asking what is this part going to do?

Evan
05-12-2006, 12:02 PM
I'm just putting a dead center into the center hole at the end of the tap and running it home.

That is where the problem lies, most likely. Taper the start of the hole from major diameter of the thread over at least several mm to allow the tap to center accurately in the hole.

J Tiers
05-12-2006, 12:55 PM
If it were me, I would single-point the thread at least as a starter, and then use the tap to clean up, if at all. Bottoming taps may do better in a blind hole than trying to thread , under power at least.

Which brings up the possiblity of hand rotating for the last turns single-pointing. That way you have complete control, have the thread on-center, and all may be very well. You may have to do an angled grind to get to the bottom of the hole with the thread tool.

As far as a 4 jaw, sounds like it may be partly bell-mouthed. You can grind the jaws, but you have to load the jaw, and you can only do one at a time, since they are not synched as with a 3 jaw.

Drill a hole in the end of the highest step of each jaw, load the jaws against a ring, leaving room in the ring for your improvised grinder or TP grinder, and go for it.

If you can locate and hold the jaw at the exact bottom position, you might use the crosslide to keep it flat. otherwise just rotate and live with the curve.

A surface grinder would do it, but you might want to bias the grind so as to hold tighter at the tip by a thou or so to compensate for the movement you saw. Loading does that automatically.

(idea of drilling, using ring etc from Rich Carlstedt)

pcarpenter
05-12-2006, 12:59 PM
I agree that finishing in the spindle will allow you to make up for any other runout, but I would sure check two things:

1. What is the runout of the spindle iteself on the surface used for registration. Thus far, it appears you have only checked runout with the new part in place....which gives only cumulative runout. It might be easy to assume that all parts of a mill spindle are square to one another and have minimal runout, but the maker could have considered the part-holding portion critical while the others were held to greater tolerances.

2. If any other part (other than the face that is supposed to define registration) touches, it will influence registration and becomes an important part of the equation. Fits that involve contact in more than one dimension become a lot more complex.

Edit-- is the "chuck parrallel" issue the chuck or is your headstock not paralle with the ways? Also, do not forget that anything that hangs out more than a couple of times its diameter is going to end up tapered if you do not use a center on the other end.

Your threads may be better off loose, so that your registration surface is the only thing influencing fit if I understand the design correctly....that was what I was getting at in #2 above.



Paul

Orrin
05-13-2006, 12:24 PM
You've gotten good advice, already, so I won't make any suggestions, only a short story.

I've had similar 4-jaw problems with a brand-new "budget" chuck. I would have been much better off had I not been such a tightwad and gotten one of good quality in the first place.

As it turned out, the faces of the jaws were not parallel to the axis of the spindle. For those of you who are thinking "poor job of mounting on the backing plate," just bear with me a few more seconds. It was mounted just fine.

I mounted a test indicator on the tool holder and let the stylus ride on the jaw's clamping surfaces, one at a time. By rolling the lathe carriage back and forth, I could see that three of the jaws were almost identically "pinch mouthed" a slight amount. In other words, they were somewhat the opposite of being bell-mouthed.

So far, so good.

But, the fourth jaw was all fouled up. It pointed somewhere down toward Alabama, or in that general vicinity. There is *no way* that chuck can hold a piece parallel to the axis of the lathe. In fact, on long pieces it is rather iffy whether the chuck can even hold something tightly enough to keep it from flopping around under heavy cuts.

So, you have my sympathy. You might want to check out your chuck jaw faces.

Regards,

Orrin

Jim Luck
05-16-2006, 09:52 AM
Start with a longer piece of stock. Rough out OD and ID. Face outside end. Bore to print dimension for thread only and thread, then finish turn counterbore and OD. Also, at this point, it would not hurt to face off a couple thousandths, just to be safe that you are perfectly perpendicular!. When doing this in one set-up, you must be very carefull not to knock it out af alignment after roughing it out. Take light, carefull cuts and be smooooth in all your moves!. Seeing your chuck is bad, face off opposite end in a V-block, or just set it in the vise on your mill, make sure your mill's head is trammed- in and vise is indicated straight!. Or, if you have a surface grinder, rough it to length +.005" or +0.13mm minimum, and surface grind to finish length.

I have a cheap 4 jaw and it cannot be set perfect like a quality chuck!. That's fine with me because I bought it just to have a 4 jaw laying around for emergencies, and most work I get is not that critical. If I need a perfect part, I use my 3 jaw set-thru. And if it really need's to be perfect, I will bore out soft jaw's!!...