Workpiece becomes loose on centers when using dead center

[Pete F]

Stop talking out your clacker Bill, a dead centre is effectively just a plain bearing and can be lubricated with anything at hand. Some things are definitely better than others, but anything is better than nothing, so if grease or bar oil is what you have at hand and you can keep it on the centre, then it's fine.

 

[Trboatworks]

Brother- I need to stop reading this forum.
I have been happily turning with a carbide tipped dead center in a 9" manual lathe.

Do I now 'need' a $250 buck Concentic live center....?

 

[morsetaper2]

Do I now 'need' a $250 buck Concentic live center....?

They are nice to have for the exact reason this thread exists. Got mine on ebay for a fraction of new.

 

[creedtown182]

These work piece were about 15" in length and about 1.500 in diameter. They were lightweight shafts. The reason why I was using a dead, half center and not a live center was because I was to face the finish face the ends square. These are Navy Shafts and I they want the faces to be square to the O.D. within .0005 or .0002, I can't recall which one.

I'm going to try that method of using the tailstock hand wheel imbalance to keep pressure on the work piece, and try not using the quill lock. I did look in my book "How to Run a Lathe" by South Bend for the leather lace idea. Also I looked in my book "Running an Engine Lathe" by Fred H. Colvin. All I saw in both books about the leather lace is, like Mike C. said, about holding a drive dog and shaft back against a faceplate and headstock center while using a steady rest.

I did turn on a dead center again last night and didn't have trouble with the work piece becoming too loose between centers. The shafts I was running last night were about the same size, however, these shafts didn't have the very small center hole in the one end, they had a larger center hole. In the book by Fred Colvin that I was reading it says, "Size of Center - If it is tool steel and is to be hardened, make the hole and the center as small as can be done with safety, but not small enough to cut the lathe center or prevent the piece being held steadily." In the one type of shafts I was working on, I think the center holes were too small, therefore it was cutting into my soft, headstock center.

 

[Robert Campbell Jr.]

not a bad idea to re-adjust a rotating centre as well (live centres are what's in the headstock ) as the work heats up, especially on long skinny work....that force has to go somewhere. I set it so the work can be moved by hand but doesn't floppy around. No clanging or clatter

100% agree dump the dead centre and get a rotating one .....and get a good one. They are good because the bearings are high quality. What work are you doing on a lathe that the run out of a quality rotating centre is too much?

Only legit reason imo for a dead centre is if the bulkier rotating centre is in the way, or if you are using a half centre.

When your "live" center is removed from the chuck and transferred to the tailstock, do you remame it? What do you ask your apprentice for when you'd like him to retrieve it from the tool box for you? Does whatever you call it at that moment ever confuse your apprentice? What am I saying? Of course he's not confused, you also tell him where you intend to to put it this time, right? Giving him a fighting chance to run through your descriptive list variations......

Most of us think that the description of a center applies to the center it'self, not to where you may shove it on any given whim.

On the street or in my garage, I always call my truck a..... truck.

I do realize that over time, regional jargon and text books may disagree on tool identification but in my library, the majority of referrences to a "live" center, show a multi-piece center with bearings stuffed between them, while they commonly call a one piece center being placed in a spindle or not, a "dead center".

The first-nit picker doesn't stand a chance..... and seldom is the responding nit-picker left alone either.....

Bob

 

[Mcgyver]

When your "live" center is removed from the chuck and transferred to the tailstock, do you remame it?

don't be daft. The names of my centres, Charles, Reginald, Frank, Bartholomew don't change when they're moved for the live centre position to the dead centre position

The first-nit picker doesn't stand a chance..... and seldom is the responding nit-picker left alone either.....

Bob

looking through craiglist today..... 3/4 of the world seems to erroneously uses 'vintage' to mean old. Should we all? Just a lone eruditic voice in the wilderness, unperturbed by the opposing numbers

 

[Pete F]

These work piece were about 15" in length and about 1.500 in diameter. They were lightweight shafts. The reason why I was using a dead, half center and not a live center was because I was to face the finish face the ends square. These are Navy Shafts and I they want the faces to be square to the O.D. within .0005 or .0002, I can't recall which one.

I'm going to try that method of using the tailstock hand wheel imbalance to keep pressure on the work piece, and try not using the quill lock. I did look in my book "How to Run a Lathe" by South Bend for the leather lace idea. Also I looked in my book "Running an Engine Lathe" by Fred H. Colvin. All I saw in both books about the leather lace is, like Mike C. said, about holding a drive dog and shaft back against a faceplate and headstock center while using a steady rest.

I did turn on a dead center again last night and didn't have trouble with the work piece becoming too loose between centers. The shafts I was running last night were about the same size, however, these shafts didn't have the very small center hole in the one end, they had a larger center hole. In the book by Fred Colvin that I was reading it says, "Size of Center - If it is tool steel and is to be hardened, make the hole and the center as small as can be done with safety, but not small enough to cut the lathe center or prevent the piece being held steadily." In the one type of shafts I was working on, I think the center holes were too small, therefore it was cutting into my soft, headstock center.

In post #18 I suggested this was most likely the problem. It's not that they're "too small" it's that they're not deep enough. The tip of the centre should sit in the pocket formed by the tip of the centre drill. If it's not drilled deep enough the tip will instead bottom out in that pocket, the centre both won't run correctly and will loosen with use. From what you initially wrote I'd suggest the centre drill you were using was WAAAAAY too big. It will depend precisely on the type of work you're doing, but extrapolating from the tolerance figures you mentioned I'd probably centre drill shafts this size with a #3 (??), you may even be able to get by with a #2. I think from memory you were using a #7!!

I don't want to tell you how to suck eggs, but there is a "correct" depth to drill a centre; not too shallow and not too deep. If you want a smaller centre hole, you don't just use the same centre drill and drill less deep (as it sounds as if you've done), you use a smaller centre drill. Drill down until you're approaching the centre drill diameter, but don't over-drill it so that it's boring the hole. If you do that you'd have the opposite problem and the centre will ride on the rim of the hole.

I'd consider it highly unlikely it was your headstock centre causing the problem. Admittedly both headstock and tailstock centres should be drilled correctly, but any problems normally manifest themselves at the tailstock end.

As mentioned, pretty much any lubricant can be used but some are better than others. White lead was, I believe (before my time) traditionally used and I understand was excellent for this purpose. There's no good reason to use EP type lubricants as the temperatures and pressures in this application won't be sufficient to activate the EP additives which require localised high pressure/temperatures to work. If I really wanted to give a toss what I used I'd probably go for something with a graphite additive, as it works well at these low pressure and temperature applications. Fortunately I don't really give a shit, do work just fine using dead centres, and so far no two headed babies have been born in my household as a result.

Anyway, the bottom line sounds as if you need to invest in a set of decent "center" drills (because you guys can't spell). I'd strongly suggest you don't cheap out on them either. Buy good quality ones as below about a #3 the drill diameter (ie the tip part) gets pretty small and they're easy to snap off, especially if you're drilling tougher material. If it snaps off you'll often scrap the work. Not that, errr, I'm speaking from experience in that area

 

[Greenwud]

Glad you finally managed to extract a small measure of genuine wisdom from all that effort...treasure it.

Pot-kettle-black.
Glad someone is making an effort to address the basics and what really matters.

 

[creedtown182]

In post #18 I suggested this was most likely the problem. It's not that they're "too small" it's that they're not deep enough. The tip of the centre should sit in the pocket formed by the tip of the centre drill. If it's not drilled deep enough the tip will instead bottom out in that pocket, the centre both won't run correctly and will loosen with use. From what you initially wrote I'd suggest the centre drill you were using was WAAAAAY too big. It will depend precisely on the type of work you're doing, but extrapolating from the tolerance figures you mentioned I'd probably centre drill shafts this size with a #3 (??), you may even be able to get by with a #2. I think from memory you were using a #7!!

I understand what you mean about the center holes being drilled to the correct depth. I'm sure the #14 bell center hole was drilled to the correct depth because the entire "bell edge" was visible and came up flush to the end of the shaft; it was not drilled too deep to where the "bell edge" was below flush with the end of the shaft. The reason why the other end of the shaft had a 5/8" center hole was because the O.D. of the shaft was a good bit larger at that end. This O.D. of the shaft was the largest part of the shaft, but it is only .375" in length. Not that it really matters though, just letting you know to give you an idea of what these shafts looked like. It almost looks like a flange at one end of the shaft.

 

[Pete F]

Decent dead centres still have their place at table, nuisance and all. So do 'economy' grade live centres. Try one. Upgrade as you need to do.
Bill

Perhaps if you actually read what people write instead of your constant patronising BS, you wouldn't have missed when the OP clearly stated WHY he was using a dead centre.

"The reason why I was using a dead, half center and not a live center was because I was to face the finish face the ends square. These are Navy Shafts and I they want the faces to be square to the O.D. within .0005 or .0002, I can't recall which one."

I can't think of anyone who is going to stop the lathe, remove the work, smear some more idiot punter's lubricant in to the centre hole (since the idiot punter's lubricant is a grease there's no point in smearing it on the outside), replace the work, and continue for just one more pass before repeating. Indeed just using any old tacky oil I have handy I most definitely do not have to "replenish the charge" of lubricant" after each pass and if it's not much work to be done I'll do the whole job with just the initial brush of "don't give a shit" lube. Yet again you're talking crap Bill, and nobody, and I mean NOBODY would do what you claim.

I understand what you mean about the center holes being drilled to the correct depth. I'm sure the #14 bell center hole was drilled to the correct depth because the entire "bell edge" was visible and came up flush to the end of the shaft; it was not drilled too deep to where the "bell edge" was below flush with the end of the shaft. The reason why the other end of the shaft had a 5/8" center hole was because the O.D. of the shaft was a good bit larger at that end. This O.D. of the shaft was the largest part of the shaft, but it is only .375" in length. Not that it really matters though, just letting you know to give you an idea of what these shafts looked like. It almost looks like a flange at one end of the shaft.

Ok I get the picture. I do however feel you're drilling overly large centres for the job, at least as I picture it. Another thing to check if you're finding the work is loosening excessively, is the condition of the centre hole after it's been drilled. If the centre hole isn't clean after drilling it, the centre will basically burnish the imperfections down when you first start using it. Again, that's another reason to use smaller centres as the bearing surface is smaller and tends to be less affected by this type of thing.

Hopefully that's of some use and you're able to solve the problem ... in spite of the pointless distractions from someone in the peanut gallery

 

[Greenwud]

Get real!

Your mini-lathe and chain bar oil Bu Ci just are NOT a good example.

I'd like to see you turn a locomotive axle or the armature for a 100 HP DC motor for a Joy continuous mining machine with but one application of your '"don't give a shit" alleged lube. There are precise diameters at both ends and in between, shoulders, grooves, and on the motor shafts tapers and threads as well.

"NOBODY" BTW, was three shifts a day, five days a week for years on-end. Dunno how many USWA machinists Galis averaged, but at the time of the 1965 layoff, 30 or 31 of them were seniour to me. Didn't bother to count those Junior - recall was based on seniority.

Anyone clueless enough about turning 'tween centres to keep flogging chain bar oil and one-time-only application just does NOT understand serious turning.

Not a surprise the OP knows more about it than you do. He studied. He has not stopped learning.

As to the client insisting on flat end-faces and to spec? No sweat. MOST jobs require that.

Just run the rest of the fits on live center, face to short of the centre and a tad long, support the work (sling off the traveling crane in our case), swap-in a half-dead-centre, clean up the end.

Very little time or stress on the DC or its lube. Drilled center will not have moved. That's why we use them.

Bill[/

More irrelevant ad homineum........
Remember that next time you turn a locomotive axle, folks. One would take it that those that do don't need to be told, and a loco axle isn't the piece in question; iirc it is 15" long.
No-one mentioned the application of a notched center for shafts where the end requires accurate facing. Old tech but allows the face to be cut while rotating on the center instead of supporting with the steady rest.
Hey, they even work on 24" half shafts using moly sulphide paste as a lube.

 

[Pete F]

Off the meds again Bill? WTF do locomotive axles have to do with what the OP's 1.5" shafts? Oh you da man when it comes to lathes Bill, if only you had one that ran. Never mind, let's revisit your gems of wisdom you bestowed upon the word in regard turning between centres shall we;

Keep studying on it. It should come right.

Meanwhile .. If there is space enough on the shaft for a dog, there is space enough to grip that end in the chuck. 4-Jaw. of course. You want it centered. And a live centre in the TS might help as well.

And then there is the 'small' shaft. They can flex under cutting pressures..

Bill

Wow Bill, you really nailed that one didn't you. So your brilliant wisdom you so condescendingly granted the gentleman was to use a 4 jaw because "of course" you want it centred. FFS

No Bill, I don't turn locomotive axles for a living. Lucky for both myself and the OP as nobody was asking about that. On the other hand I've certainly done work of the size the OP was asking about with what sounds like similar setups, hence why I did my best to try to help him out based on what I've learnt while doing so.

 

[PDW]

No Bill, I don't turn locomotive axles for a living. Lucky for both myself and the OP as nobody was asking about that. On the other hand I've certainly done work of the size the OP was asking about with what sounds like similar setups, hence why I did my best to try to help him out based on what I've learnt while doing so.

Personally, I use Trefolex as a dead centre lubricant.

At least, I did the last time I used a dead centre.

Which was maybe in 1988 or so.....

And yeah, I understand just fine why the OP used a half centre, perfect reason for one.

FWIW I was taught that with a dead centre, you can guarantee some friction and some elongation so if you don't want to burn off the nose of your dead centre *regardless of lube type* you back off a touch on the TS handwheel then reapply force until it feels just right. Hard to describe but the feel is obvious.

PDW

 

[specfab]

Of peripheral relationship to the OP's questions:
I did some taper turning between centers recently (first time for me), and ran into the question of the center-drill geometry vs. the ~1.5 degree angle and its required offset. Seemed "intuitively obvious" that the smaller the center drill hole, the better, but I am interested in other's opinions about this topic. I also used a dead center in the headstock spindle taper. The item being made was actually a plastic tube about 12" long with (different) tapers on about 3.5" of length at either end, so cutting stresses were not that great. The tube was held between shoulders on some aluminum endcaps. I also tied the drive dog (actually a drive bar attached to the endcap fixture) to the drive plate bolt with a nylon cable tie (not a leather bootlace...). Got the job done, but have been wondering if there is something I need to look at more carefully in this offset taper turning arena.

 

[dian]

 

[Robert Campbell Jr.]

Having run across a few versions of this: "..some people make the mistake of just drilling less deep with the same centre drill. This is a mistake and will cause the centre to bottom out," just gotta' ask, what wand are you Merlins using to make that happen, and why ever would you?

Please forgive me if that was satisfactorily answered, if so, I missed it.

Assuming here that "centre" (which looks to me as if it should be pronounced "sentry") is just one of those Old English oddities, means the same and is pronounced the same, (with regional deviations) as center, not something altogether different, thereby contributing to my confusion.....

Point in fact: Center drill - Google Search

Bob

 

[David Utidjian]

Bob,

I was wondering about that also. Only prob I can see is if the hole is so shallow that the pilot part* (nipple, if you will) of the drill does not go full depth and, at least, start on the 60 degree conical part of the hole. As soon as the conical part begins to form and barring the work metal being upset a proper sized center will never bottom in the hole. In fact just about ANY sized center will never bottom as long as the OD of the center is larger than the ID of the pilot.

*According to various images in Bob's link it appears that the 'pilot' diameter == drill diameter and 'OD of the center' == body diameter.

-DU-

 

[awander]

Yeah, I don't get it either.

Having run across a few versions of this: "..some people make the mistake of just drilling less deep with the same centre drill. This is a mistake and will cause the centre to bottom out," just gotta' ask, what wand are you Merlins using to make that happen, and why ever would you?

Please forgive me if that was satisfactorily answered, if so, I missed it.

Assuming here that "centre" (which looks to me as if it should be pronounced "sentry") is just one of those Old English oddities, means the same and is pronounced the same, (with regional deviations) as center, not something altogether different, thereby contributing to my confusion.....

Point in fact: Center drill - Google Search

Bob

 

[Robert Campbell Jr.]

Bob,

I was wondering about that also. Only prob I can see is if the hole is so shallow that the pilot part* (nipple, if you will) of the drill does not go full depth and, at least, start on the 60 degree conical part of the hole. As soon as the conical part begins to form and barring the work metal being upset a proper sized center will never bottom in the hole. In fact just about ANY sized center will never bottom as long as the OD of the center is larger than the ID of the pilot.

*According to various images in Bob's link it appears that the 'pilot' diameter == drill diameter and 'OD of the center' == body diameter.

-DU-

Well....maybe David but what sort of rank amateur believes that the cone of a center drill and the matching cone of a center is an accident? The cones are designed to engage in a solid fit, with or without a lubricating film between them, with the tip of the center dangling in a void.

Just in case there was any confusion, like substiting a pilotless spotting drill for a center drill, I posted the link. Shows lot's of center drills and identifys them as such.

It's not an accident that hole and center fit. I have built a ball center system for turning tapers that depends on that fit. It features a tapered roller bearing in a housing, that holds an abreviated center and loosely seats captive around a hardened ball set in a boring head. If I couldn't depend on a solid fit of the center to the part, there would be two points of pivot @ the tailstock and it would be worthless. There is also a captive* ball centered on the dog plate/spindle line. *got tired of tossing out my balls with the swarf......

I know, "without pix it's a lie." Just don't want to be guilty of another hijack.

Off-set turning between 60° centers is so crude as to be useless to me. Mount the lubricated workpiece and fire it up. After endless re-tightening of the tailstock and long before the centers have gouged a modified cone into each end of the workpiece, marring the centers and digging down into that important clearance hole, the lube is gone, along with any dialed-in accuracy*. It's like micrometer clamps and screwdriver chisels only worse.

*Off-set taper turning requires some math, one given being the length between centers, that disappears during the gouging routine. It's nice to have the part centered too, but it's a crap-shoot when using gouging to determine where center will end up. Round and round it goes, where it stops nobody knows.......

Bob

 

[Spinit]

Re do every thing or one thing and then the other to isolate the issue. It is safe to say you are at that point here. That is what we have to do from time to time. If I were there working it out I would have started that right away after looking to see if anything is obvious. I would not list here all the things to check and why because I do not want to and feel you just have to kind of roll up the sleeves and get into it. I have used Dead centers frequently and had very few issues. I did use lube and I recall I did not wish to generate a lot of heat and tool pressure while cutting. If you are using a inserted tool with a 1/16 nose radius it may not like that much. The machine makes a difference as far as if it is a good machine or a bad one. If it is a good machine we use then it behaves like it. Good luck.