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Alguy
08-24-2007, 03:28 AM
I was trying to square up some stock in the lathe , it is clamped to the compound, here is the setup
http://i61.photobucket.com/albums/h48/alguy50/basementshoppics048.jpg
it is cutting ok and the finish is good the problem is that it acts like a mill that is out of tram.the first part of the cut is down when it gets to rear part of the cut cutting up it is cutting deeper about .002 ,
http://i61.photobucket.com/albums/h48/alguy50/basementshoppics050.jpg
what is causing this. what am i doing wrong or what wrong with my lathe?
thanks for any help

Davek0974
08-24-2007, 03:52 AM
Its a well known problem but i cant remember the cause. Most lathes are designed to cut from the centre out or from the edge in, you would not normally allow the tool past the centre point, in which case the fault would never show up.

The only suggestion i could offer is to increse the swept diameter of the tool so it does not cut on the back stroke. It is probably caused by some minor inaccuracy in the saddle alignment or may be designed-in on a lathe. How about a milling cutter in the chuck and a vertical slide?

Sorry i cant help more.

DAve

Alguy
08-24-2007, 04:00 AM
I have just read in chap 2 of Tubal Cains "milling in the lathe" that a lathe cross slide is not exactly at right angle to the mandrel axis perhaps .002 003 per foot, maybe this is it,,?

Carld
08-24-2007, 10:08 AM
Are you sure the work is parallel to the chuck?

If you think your cross slide is not square to the bed, that is parallel to the chuck then chuck up a short piece of round stock about 3" dia. Take a cut down the side of the round stock and then face it. The side should be square to the face cut. If not you have a problem and now know why you are getting bad results.

A lathe should hold very close to square for face cuts.

Doc Nickel
08-24-2007, 10:15 AM
It doesn't matter whether the work's parallel to the chuck, as the cutter will make it parallel to the table travel.

The only two issues I can think of would be the work slipping in the hold-downs, or the cross-slide having some slop in it, so that the upstroke moves the slide slightly, in a way the downstroke doesn't.

On that same note, I suppose I could see a loose headstock bearing doing the same thing- allowing a slightly different positioning as the cutter goes from the upstroke to the downstroke.

Either way, the fix would be, as Dave noted, to increase the tool swept area, and only cut your workpiece with the bit going one direction, not both.

Doc.

NickH
08-24-2007, 10:32 AM
The problem IS as described in the Tubal Cain book, he states that a lathe when correctly set up takes slightly more off at the center than the edge,
Regards,
Nick

Your Old Dog
08-24-2007, 02:10 PM
The problem IS as described in the Tubal Cain book, he states that a lathe when correctly set up takes slightly more off at the center than the edge,
Regards,
Nick

Oh boy! Here we go. :D

I don't buy that. I can see that scenario if the tail stock were higher or lower then the quill but that's the only way.

daryl bane
08-24-2007, 03:09 PM
As I know alittle about this, Mr. Cain is correct. It is accepted practice of scraping in the lathe carriage in mfg. to "throw" the crosslide. A facing cut should be about .0002 to .0003 concave. The idea is that the facing cut stock should be able to sit flat on a smooth surface. I think the .002 error is alittle much but a good indicator and a parallel in a four jaw should illuminate the problem.

Peter N
08-24-2007, 03:18 PM
Absolutely right.
Lathes are set up to face ever so marginally concave so that the the faced end doesn't wobble like a weeble.
On Myfords for example, the carriage is set to face concave by 0.001" in 12".

Peter

Carld
08-24-2007, 03:39 PM
I must have been brunk and inpoxipated from my arish kofie dish mourning when I sad the wark pees had to be paraell witf da charles, ah, chuck. The rest of my disertation is true or so I thought, I mean think.

I'm not so sure if I agree with Mr. Cain but I see no reason to argue, unless, naw, hell, it ain't worth the effort.

GadgetBuilder
08-24-2007, 03:41 PM
No need to guess, it's easy to determine whether your cross slide has the slight angle suggested in Cain's book.

Make a small circle near one edge of the face plate, mount an indicator on the cross slide and indicate the circle when it is at about center height on the near side and lock the carriage. Then rotate the faceplate 180 so the circle is at center height on the far side and crank the cross slide out to indicate the center of the circle again. The difference in readings divided by the distance between measuring points is the "error" per inch.

Unclear whether it is an "error" or not, depends on your point of view :D

John

lane
08-24-2007, 07:17 PM
Yall are correct. Tighten up the set screws on the gibs on that S outh Bend That will help . been their done that . Your cross slide is tee toddering as it moves across Had same problem once.

Carld
08-24-2007, 08:35 PM
It appears there may be some validity to Mr Cain's way.

I got out the accuracy test record for my lathe and looked up the facing cut part. It starts out by stating, "To face hollow or concave only on 12" diameter" and went on to give the dimention achived. While I don't always believe the dimentions written in factory accuracy test sheets, the fact that they test each thing gives insight to what to look for.

Sooo, apparantly the lathes are set up to face concave as Mr. Cain states.

Al Messer
08-24-2007, 11:08 PM
Pick up a copy of "Milling Operations In the Lathe" by Tubal Cain and he explains it quite clearly. It is the way lathes are normally designed and made that makes it do thusly.

Alguy
08-24-2007, 11:27 PM
I did as suggested mark the faceplate and rotate 180 while measurin with an indicator mounted on crossslide it was about the same maybe a tenth or 2
(0001)difference,, also while the indicator on faceplate about 3 inches from
centerline i pushed and pulled the edge of faceplate towards the tailstock about 30 lb push and pull and could measure about .003 deflection is this too much?
thanks for all the answers allen

Evan
08-25-2007, 02:55 AM
Yep, my SB9 was set up to face slightly concave and I had exactly the same problem with the same type of fly cut. What I did was to shim the headstock at the left end very slightly to bring it in direct alignment. I then realigned the rest of the lathe to match. I have left it that way for some years now and it causes no problems for other work.

In manufacturing you never try to hit an exact spec. You try to stay inside the error band allowed. In the case of facing it is far more desirable for the face to be concave if it isn't "flat" than for it to be convex. For that reason the specs always call for flat, plus zero, minus N. Since you can't hit flat every time if you try the manufacturer doesn't aim for it.

Carld
08-25-2007, 03:24 PM
If the facing operation results in a concave surface then when you place it on a mag table to grind both sides flat and to a specified thickness it won't rock on the table.

The moral is, there is a place to rock and roll and a place to not.

caveBob
12-12-2012, 10:25 AM
Yep, my SB9 was set up to face slightly concave and I had exactly the same problem with the same type of fly cut. What I did was to shim the headstock at the left end very slightly to bring it in direct alignment. I then realigned the rest of the lathe to match. I have left it that way for some years now and it causes no problems for other work.

In manufacturing you never try to hit an exact spec. You try to stay inside the error band allowed. In the case of facing it is far more desirable for the face to be concave if it isn't "flat" than for it to be convex. For that reason the specs always call for flat, plus zero, minus N. Since you can't hit flat every time if you try the manufacturer doesn't aim for it.

Sorry for reviving this old thread but it looks like the answer I have been searching for (but now, perhaps not wanting to hear :) ) ala Google. Anyway, from what I remember of assembling my Atlas after cleaning... it was a pretty tight fit, hope there is enough wiggle room to shim.

Just so I have it clear... indicate how much discrepancy I can discern @ 180 - front to back - then shim thickness will equal 1/2 of that?

I have noticed a slight crowning while facing in the past, but it wasn't enough for me to be concerned until now. The plan is to make a small (1 1/2" and one @ 2 1/4") face mill cutter out of stress-proof hex stock with 3 cutters, so milling without the crown would be peachy. From what I've read so far... a face mill cutter with only one cutter is called a "flycutter"? :)

As usual, all help is appreciated...

Forrest Addy
12-12-2012, 12:18 PM
That lathe looks like it has some time on it. Cross slide wear hour glass. Facing cuts tend to follow the loaded dovetai so naturally they'll get some exit drag.

I'm surprized you don't also get a little undulation from the crossfeed screw. Run a stone pr a smooth file over the milled face a couple strokes to see if you get a repeat pattern telegraphing the crossfeed screw's pitch.

Steve Seebold
12-12-2012, 12:25 PM
It looks to me like ways on your cross slide are not square to the Z axis ways.

Paul Alciatore
12-12-2012, 11:24 PM
I have done some milling in the lathe (SB9) and can confirm the slightly concave scraping of the cross slide. The way I have heard it explained is that it is impossible to have it completely square so if there is going to be any error, it is best to have it in the direction of a slightly concave facing cut. How much this may be probably differs from manufacturer to manufacturer, form model to model, and from lathe to lathe. Mine is probably off by 2 or 3 thousandths in a foot and this can easily cause problems if trying to make a surface flat.

Like Evan, I hit on the idea of shimming to correct this, but I did it on the carriage, not the headstock. You do not normally need to move the carriage during a milling cut and I just find it easier that way. One or several thicknesses of aluminum foil usually works well. Use equal thicknesses on diagonally opposite sides on the Vee rail as this will just raise the front of the carriage a bit. Be sure to take it out after the milling is done.

darryl
12-13-2012, 12:06 AM
Part of what Paul said is what I would have said- if there's going to be any error in the squareness of the cross slide to the spindle axis, it should be in the direction where a facing cut will be concave and not convex.

I get that, but I have two problems with it- first, maybe I actually want the face I'm turning to be flat. I can always move in a bit and take off another thou if I need the face to sit flush to something with the entire edge touching. It's my machine, and maybe I'd like to be able to face an accurate flat- if the machine is off, intentionally so, I don't have that capability. Hmm.

Second problem I have with that is how do the chinese machine a cross slide with that slight of an error in the correct direction? Do they care to even? Why wouldn't they simply machine the carriage with the ways at 90 and be done with it- would be easier and faster and who would complain?

Anyway, I realize it's not a chinese machine we're talking about, but the 'problem' seems to be universal. 1 thou over 12 inches is a pretty close alignment, and it would seem that great care would have to be taken to assure that the 'error' was 'perfect'.

In any event, from Alguys picture it would seem that the error is a lot more than that, and is too much. The next step for him might be to go through a headstock alignment check, just to see whether a slight adjustment might improve both things.

Evan
12-13-2012, 03:25 AM
You may want your facing cut to be flat but are you willing to pay what it takes to have a lathe manufactured to the degree of accuracy that it will be so close to flat that the difference cannot be measured in tenths? That is what it takes to ensure that a faced cut will not be measurably convex. Even a few ten thousandths of an inch convex will produce a face that rocks noticeably on a surface plate and no machinist will accept that.


Why wouldn't they simply machine the carriage with the ways at 90 and be done with it-

Because it is not possible. There is always error and always an error band. Ignore that concept at your peril.

Evan
12-13-2012, 03:37 AM
There is a common source of error that produces non-perpendicularity of the carriage to the turning axis. Wear of the carriage way receivers is not symmetrical. Because of working loads on the tool the effect is to try and twist the carriage. Usually, much more cutting is done toward the chuck than away from it, especially when threading. This cause a wear pattern that produces what is called "fishtailing" of the carriage. When it is cranked toward the chuck it twists the carriage clockwise and when away counter clockwise. The result is that the carriage will not be square to the turning axis and the error will depend on the direction it was cranked before locking it in place for a facing cut.

The built in error toward concave may be made intentionally larger than the tolerance that could be held so to prevent convex cutting even with some carriage wear. That would compensate for some degree of clockwise twist.

big job
12-13-2012, 07:56 AM
Im definatly missing something, now I think I need help. We make blocks all the
time, set up blocks for special operations usually on a SB. What I see in the
photos really is a new one on me. I can't figure how or if this method will ever work. That must be hard on the lathe. So Im missing something or he is missing
a 4 jaw chuck. Tell me im wrong, I see a cutting tool mounted on a face plate
and the work is probably feed with the crosslide?? I dont get it. sam

vpt
12-13-2012, 09:09 AM
^ that is right and yes that piece probably could be cut in a 4 jaw. However milling in a lathe is nothing new.

Personally I have not seen this concave cutting everyone mentions on my lathe. But my lathe is worn some so maybe the offset initially set up in the lathe is gone from wear. I can set up a bar in the milling attachment and run the full travel of the compound and don't notice any tapers.

Look how strait that cut is!

http://img532.imageshack.us/img532/9537/tap005.jpg

Mcgyver
12-13-2012, 09:28 AM
As I know alittle about this, Mr. Cain is correct. It is accepted practice of scraping in the lathe carriage in mfg. to "throw" the crosslide. A facing cut should be about .0002 to .0003 concave. The idea is that the facing cut stock should be able to sit flat on a smooth surface. I think the .002 error is alittle much but a good indicator and a parallel in a four jaw should illuminate the problem.

5 years ago Daryl summed this all up very succinctly, really that says it all. :)

when scraping the cross slide, here's the test. That's a tenths indicator sweeping pins in the dovetail. the DSG spec worked out to be something .0005" over the length of the crosslide, consistent with Daryl's proffered concave amounts. this test is NOT supposed to indicate zero. You might not notice the taper in many situations, .00025" over a foot isn't much

http://i785.photobucket.com/albums/yy132/michael01000/DSG/DSC_8456-large.jpg