Lathe is cutting a taper

[Hustlerkc42]

I appreciate any input anyone might have, so I have a sb 9a and about a year ago I was having some issues trying to get it to cut straight and after endless trying and making the bottom of the table adjustable I was able to get it to cut withing .001 over about 9 inches. That was about the length of my 1.5" aluminum round stock that I used for a 2 collar test and even today I redid the test with the same piece of stock and its still within .001. However, when I turn a piece of steel between centers over 4" its turning about a .007 taper. The only thing I can think of is deflection of the piece, it being pushed away from the cutter the further away from the headstock it is?? If anyone can help with this I greatly appreciate it.

 

[jim rozen]

When turning between centers you can adjust the set-over on the tailstock to miminze the taper. There are two screws on the upper part of the tailstock the bear on the lower part. Slack the clamp bolt, slack one of the adjuster screws, and snug the opposing one. If the taper gets better you chose the correct one to slack.... Otherwise, reverse the operations on the screws.

 

[michiganbuck]

You can test a machine's straightness with a dead straight bar to tell the straightness for one length (10" 18" ?).
Someplace in between (Shorter/longe) may differ.
Good to micrometer the part and note the straightness before you attempt to turn it. Then with the part between centers come in to check to see zero/zero, or the need to take more or less to the right or left.
There should be no need to turn a part to check a machine's straightness. In precision grinding between centers, you have to come in and take a tenth or two and so need to be dead straight from the first touch.

Deflection can come from poor tool/cutter geometry (shape). It is often in the middle of the part that is in error, not the difference between one end and the other.

.007 taper in 4" is ridiculous, you can feel .0005 with just touching your tool bit to a shim on the part reading the dial numbers with not even using an indicator.

Deflection? If you can push on the part at the head, any place, or at the tail with two fingers and see a .001 change with an indicator there may be something wrong, and you should expect problems.

for example a 1/4" dia part 10" long between centers, and you push the midsection with two fingers and see .002 movement, expect a problem making it straight.

Tool rake angles for the material and sharpness have a lot to do with success at turning parts.

Most carbide inserts are not sharp enough for a light-duty lathe, or a long part between centers.

Fingernail shaving is near sharp enough for a light-duty machine.

Note: The SB 9a ia a very decent/good machine.

 

[CarbideBob]

/// between centers over 4" its turning about a .007 taper. .

hmm.... between centers

 

[SLK001]

The "two collar" test only tests for your level of the lathe. If you're cutting between centers, your tailstock is probably not on the centerline. If you're not cutting between centers, you are probably taking too deep of a cut, or your tool isn't sharp enough, or it's not on the centerline of the axis, or your lathe has a lot of wear, etc.

 

[plastikdreams]

Just remember, your lathe is one step up from a toy. Push it too hard and it's going to deflect or push off.

 

[michiganbuck]

Generally, a head-end center and the tail center should line up point to point when brought together, looking down on them and horizontal at them. If the lathe bed was perfect that line-up would continue to the length of the lathe, if not the tool bit would be higher and lower on that part at different lengths parts. With a large part, perhaps 1” in diameter set between centers you should be able to one hand push and pull this part at both ends and see that nothing moves. The saddle should be tight on the V way for the whole the bed, it should not be riding on the top of the V but riding on the angled sides of the V. Often you can Sharpi blue the V way to see if the bed is tight all along the entire distance.
If the lathe is tight in the manner a near straight part should be possible even with great wear on the machine. The method is to measure the part before you do any turning and mark it zero-zero. Or + .1,2, 3 whatever. You can use a rounded nose tool bit for a touching gauge. you set the part between centers and come in with the tool bit to just touch the part ( eyeball, touch a shim,) touch the part looking at you dial you can that both ends of the part match at the place where to real tool bit will touch/cut the part. Yes, you then adjust your tail to make the part where it will be turned straight.
*This simple test should get you better than + .001 without not taking a cut.
A lathe that is an overly worn-out machine may make a varied part diameter along the part but at least the ends of the part will be the same.

*Trying to use an indicator to straighten a part will often fail because the indicator point will/may not touch the part the same as the tool bit will touch the part.

You should have the part/tail adjustment to +- .001 before you touch the pat with a tool bit and everything about the machine and the part should be on hand wiggle tight/solid.

With both ends the same diameter on a wore out machine you may need to abrasive paper the mid area of the part to make it perfect.

Oh, if a live center or a center held in a chuck has a wobble the the wobble end of the part will be smaller.

 

[jim rozen]

Generally, a head-end center and the tail center should line up point to point when brought together,....

A good start. A taper of a few thou like this, over a 4 inch workpiece, will be resolved when the TS adjusting screws are moved by about the width of one screwdriver slot on the screw head - a change not visible to the eye in the test of looking at the lineup of the center points. No test bars or dial gages needed, just start cutting the part and make the TS screw adjustment to zero out the taper.

Caveat - turning between centers means exactly that - too often an attempt is made with the left end in some kind of chuck and the right end in a center - that never ends well.

 

[Hustlerkc42]

Thank u to everyone that helped, ultimately Jim's suggestion at the end is what did it, I was over compensating with my adjustments on the tail stock when I should have just been making minimal adjustments until I got where I needed to be. Thanks Jim and to everyone who replied... much appreciated!

 

[michiganbuck]

Guess I should have given the short answer, but because you said that the machine would give straight for one part and taper for another I thought you needed more.

The short answer is that the tail adjustment is often very close to half the error you find in the part.

I never turn an unwanted taper part because I measure the part before turning and adjust the tail to +- 001 or better before I turn a chip, using my cross dial numbers like a micrometer.

Trying to adjust the taper when on the part can lead to scrap.

 

[jim rozen]

Glad this worked out for you.

 

[chipss]

Another easy way to check tailstock alighment:

Measure the tailstock quill diameter. For my little SB 9" it's 1.060"
Chuck and turn a short bit of metal to that exact diameter. '1.060 and whatever +- tenths.
Mount a dial indicator horizontally on the tool post and zero it on your machined part.
Lock the tailstock down at the far end of the lathe.
Run the dial indicator down and measure the quill. There's your runout.

 

[SLK001]

...There's your runout.

Not "runout" - it's "offset". Runout is completely different.

 

[chipss]

Oops my bad.