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Saddle causing .150 taper 3” off chuck no tailstock need help.

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Uguessedit

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Have a BT1337g it’s been a great lathe. Recently it’s been gaining an appreciable amount of taper to an astounding .150 thouthandths when I chuck a 5” 6061 1.5” round bar and dial it in to zero concentric. To add further clarification this isn’t a tailstock issue or a bed level issue. Bed is perfectly level front to back and lathe is fastened to the floor. Yes I reverified leveleling and it hasn’t changed in 6 months. The tailstock when used is aligned perfect and I can center a perfect hole in a part however whether I use the tailstock “or not” the part tapers. Majority of the time I don’t use a tailstock it gets in the way for the little parts I make. I’ve been talking to the manufacturer and they believe it’s the lead screw nut and cross slide gibs. I tightened the gibs to the point I could barely crank by hand to attempt ruling out issues and checked the bed for wear though were talking a 1 year old lathe with a hardened bed. Using a straight edge it is straight across showing now wear. So then I thought perhaps an issue with the gap bed and though I’ve never removed it I did have a fastener come lose recently and saw the manufacturer had short screws where the threads barely caught and the casting let loose so I removed the gap bed and cleaned it all up, paint removed, grease, etc.., cleaned to near new and reassembled with longer screws and the problem still exists. The saddle/carriage feels solid and tight physically when I grasp it but it’s obviously pulling away the closer it gets to the chuck. Spindle bearings are solid and no runout the machine is overall healthy. There is approx .015 play in the lead screw with the saddle locked down. I don’t see how that is going to give me .150 taper. The taper is back to back no matter which lathe chuck I use or how I set up the machine It duplicates itself precisely every time. When I used just the cross slide to cut instead of whole carriage I still got taper but it wasn’t as much. Does this sound like I’ve just worn out the gibs and lead nuts? I’m going to have the parts shipped 2 day air in the morning and see if they help but I’m a little Leary if it is going to fix the issue. If anyone else has ever had this occur that would be great to hear your experience. I do use the cross slide and carriage a lot and the lathe is used every day for the past year since new. It gets a good 2-3 hours a day use. Hopefully I’ve covered all questions you may have. Photos of taper attached. Thanks.

image.jpg


image.jpg
 

pdentrem

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If it is not the gib on the back of the saddle, then I have to think more. Have you put a long straight bar in the 4 jaw and used a DI to zero it and then run the DI from end to end? Not spinning the shaft. What is the runout? Run it on the top and then the side where the tool would hit it.
 

stupoty

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If you try turning something between centers you can check that the head hasn't moved out of line, a part between centers "should" turn with no taper even if your head is out of line.

only mention as you say you get the same using the compond set to zero degrees (sorry if i miss understood that)

Stu
 

Uguessedit

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I did some further testing and took the saddle down and saw the lead screw nut has the locking screws on it to remove backlash so I tightened it up and put it back together and locked my turret in place (no swing/etc), and it still tapers. I cut 4 different parts back to back all down to the exact same specs using the power feed for consistency. The 4 parts are all exactly the same identical taper. This is weird. I’m starting to conclude something happened and this lathe is junk. I did have a parr crash a few months back but I’ve used this lathe hundreds of times since. It’s the past couple weeks that it really aggressively changed where parts have become unusable. If the manufacture can’t help figure it out I’m going to ask if they will trade it in at a discount of course. Even if half it’s better to let them rebuild it and deal with it and start new. My only other thought is convert it to Cnc but I don’t really have the time and it’s nice to have a manual lathe around the shop. I do have all the components here, ballscrews and servos and a few PCs I could go ahead and do it. I feel like it’s defeating me.
 

Uguessedit

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If you try turning something between centers you can check that the head hasn't moved out of line, a part between centers "should" turn with no taper even if your head is out of line.

only mention as you say you get the same using the compond set to zero degrees (sorry if i miss understood that)

Stu
It tapers no matter what. If I push all my body weight into the carriage while it’s power feeding I can almost eliminate the taper. If I push on the saddle or turret it does nothing. It’s a carriage issue and the bed is straight and not worn. The next thing is probably pull the carriage maybe where it rides on the bed has worn. That’s gotta be the issue I’ve tried everything else.
 

francist

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Looking at your first two pictures where you are measuring the bar, I see a difference of 0.0145", not 0.145". Or am I reading your calipers wrong?
Agreed still a taper, but an order of magnitude less.

-frank
 

Uguessedit

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Here is a link to a YT video I made that best explains the issue.


It is a private video and requires use of the link out of respect for the manufacturer until the issue is resolved. Any issues let me know.
 

Uguessedit

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Looking at your first two pictures where you are measuring the bar, I see a difference of 0.0145", not 0.145". Or am I reading your calipers wrong?
Agreed still a taper, but an order of magnitude less.

-frank
I’m sorry you’re correct I’ve been running on a couple hours sleep. Frankly I’m a bit burnt out at the moment between work and trying to alleviate this issue. I did upload a video here that clearly shows the issue occurring. It also doesn’t help I’m a metric guy and I know everyone here is probably standard units. I would normally resort to .41mm ;-)
 

JimDawson

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That almost looks like the head is out of line with the bed, rotated a bit clockwise looking down on it. Not sure how that could happen, if it was going to move I would expect it to go in the other direction.

You need to put a bar in the chuck and run a dial indicator down it as @pdentrem said above.
 

stupoty

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Now that you mentioned a crash I believe the headstock has shifted. The test I mentioned earlier will prove it.
Pierre
Thats why i sujested turn between centes as a test as i dident know if OP had a test bar for mesuring it.

Stu
 

jwmay

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It’s an interesting problem. I’d suggest getting some sleep, and coming back with fresh eyes.
 

markba633csi

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Does it cut taper when hand feeding also? Seems like it must be a shifted headstock, I would check for that
Mark
 

pacifica

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I've had a shifted headstock and it cut a .005" taper over 10 ". It takes about an hour or two of adjusting(by very small amounts) and then taper goes away.
After adjusting I'm at .0001" taper per 10".
 

Latinrascalrg1

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a part between centers "should" turn with no taper even if your head is out of line.
That is Not correct.....If either the headstock or tail stock or both are "Out of Line" with each other it will turn a taper, Unless (and this is a HUGE "UNLESS") Both the head AND tail stock somehow happen to be "off center" the exact same distance making the barstock parallel to the carriage travel which is Not Likely to happen.
 

stupoty

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That is Not correct.....If either the headstock or tail stock or both are "Out of Line" with each other it will turn a taper, Unless (and this is a HUGE "UNLESS") Both the head AND tail stock somehow happen to be "off center" the exact same distance making the barstock parallel to the carriage travel which is Not Likely to happen.
Yeah I see what your saying but a center in the head stock and slightly tweaked, lined up with a tail stock center aligned to the head stock center (OP said tail stock was dead nuts to center) will exhibit much less taper than an out of line head with a chuck holding a part in a fixed axis.

I don't see how you would have the tail stock not lined up with the head stock if you set it up to match your head end? You would generally dial in the tail stock to align to the head I would have thought.

Might be just the way I align my tail stock, I check their point to point when turning between centers and adjust the tail stock to remove taper. If you could do that it would show the saddle was tracking repeatably parallel and not some sort of gib issue.

But yeah caveats apply mileage may vary.

And I probably worded it badly.

Stu
 

Latinrascalrg1

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I don't see how you would have the tail stock not lined up with the head stock if you set it up to match your head end? You would generally dial in the tail stock to align to the head I would have thought.
Yes I agree and would think unless the person was new to the trade and uninformed that they would dial in their machines if possible. However, from what ive learned thus far, depending on how one goes about this Alignment process the tail stock may or may not align perfectly with the headstock over then entire bed length based on the conditions of the mating surfaces and how true they are on each machine. Normally most of the wear is close to the headstock so if you placed the tailstock in the well worn area of the bed to dial it in with the headstock then there would be a very very good possibility for it to be off in areas that are not as worn or vise-versa.

Now i may be WAY OFF here but right or wrong this is what ive learned on the subject but that don't mean I'm not willing to learn something new about it so please correct me if I'm wrong.
 

stupoty

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Yes I agree and would think unless the person was new to the trade and uninformed that they would dial in their machines if possible. However, from what ive learned thus far, depending on how one goes about this Alignment process the tail stock may or may not align perfectly with the headstock over then entire bed length based on the conditions of the mating surfaces and how true they are on each machine. Normally most of the wear is close to the headstock so if you placed the tailstock in the well worn area of the bed to dial it in with the headstock then there would be a very very good possibility for it to be off in areas that are not as worn or vise-versa.

Now i may be WAY OFF here but right or wrong this is what ive learned on the subject but that don't mean I'm not willing to learn something new about it so please correct me if I'm wrong.
Yeah I see that bed will cause some issues with this, I do "point to point" my centers for alignment but my sadle and tail stock travel on two different sets of ways so I guess thats another source of inaccuracy. The sadle ways are somewhat warn near my head stock and the tail stock ways are fairly immaculate all over (apart from where a previous owner ground an extra bit of swing for something that didn't fit).

You could use this factor to find the most worn area of your ways like this I guess.

I'm getting the thread off topic , sorry about that.


Either way It to me looks like a head miss alignment issue rather than a saddle issue especialy as the same result happens with the compond slide set to zero degree's, definitely worth a check.

As others have said i'f you have some known strait stock or test bar you could mount it in the chuck, maybe use the tail stock to move an indicator , this would rule out a saddle issue. OP has prism ways so it should be stable enough to slide the tail stock with it lose and still be quite stable with regard to position.

Running a test indicator up and down a test bar using the saddle, wont be very helpful if the saddle is suspected as the root of the issue perhaps ?

Stu
 

jwmay

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Any progress here?

Not sure if this is helpful at all, but my machine sits on crappy basement concrete, and it moves all the time, according to the weather and precipitation. Could that be a contributing factor in your case?

I don't know much, but this shifted head stock thing seems like the most remote of possibilities in my uneducated and inexperienced mind. But something is definitely very solidly misplaced...otherwise where would the repeatability come from? Imagine a chip embedded in the rear way. As the saddle moved across it, if the chip stayed put, the saddle would cant less and less as more of the saddle had moved past it. Or imagine the path of a flat plate with one dowel underneath it. The leading edge would climb for a certain distance, and then fall for a certain distance. Each time you rolled the plate, it would travel along the same line...Like a teeter totter with a moving hinge point. Anyways, I obviously don't know the answer, but maybe this little imagination exercise would be useful. HA!
 

stupoty

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Any progress here?

Not sure if this is helpful at all, but my machine sits on crappy basement concrete, and it moves all the time, according to the weather and precipitation. Could that be a contributing factor in your case?

I don't know much, but this shifted head stock thing seems like the most remote of possibilities in my uneducated and inexperienced mind. But something is definitely very solidly misplaced...otherwise where would the repeatability come from? Imagine a chip embedded in the rear way. As the saddle moved across it, if the chip stayed put, the saddle would cant less and less as more of the saddle had moved past it. Or imagine the path of a flat plate with one dowel underneath it. The leading edge would climb for a certain distance, and then fall for a certain distance. Each time you rolled the plate, it would travel along the same line...Like a teeter totter with a moving hinge point. Anyways, I obviously don't know the answer, but maybe this little imagination exercise would be useful. HA!
Is your lathe sitting on leveling feet , free standing or bolted down?

If its bolted down this could cause a twist in the bed which may be causing the taper, if it is free standing it may have twisted a little to allow all feet to make contact with floor. If leveling feet then check level with a reasonably acurate level.

Stu
 

stupoty

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Any progress here?
I was on my phone when I replyed , sorry I thought you were the OP. my bad :rolleyes:



Bed is perfectly level front to back and lathe is fastened to the floor. Yes I reverified leveleling and it hasn’t changed in 6 months.
I just re read the thread to see if I missed something, you say the lath is bolted down to the floor and level, you could try loosening the hold down bolts and giving it a try.

Also perhaps the pre load on the spindle bearings has changed due to wear etc. this might be making the whole spindle cock a bit when you start cutting, although it does look rock solid on your video from what I see.

I did the angle math and I came up with about 2 degree taper. Thats quite a lot.


Stu
 

Uguessedit

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Follow up: Well I’m still contending with the issue. The manufacturer has been very slow to return calls and today I finally was able to put a parts request in although I have to wait to get a quote back in a few days it’s a painstakingly sloooowww process. In the interim I removed the compound slide and installed a machined piece of 4140 that has removed .22mm or .0085 so it has lessened the taper by 50%. Rather than two sheets of paper thick of taper it’s now one. It’s still too much for some parts I need to make that slide together. The next thing to do is to replace the y axis lead screw and nut. If that doesn’t fix it then a new carriage top. If that doesn’t work then I will call a local repair shop to come out and see what they can do. I suppose the lead nut could be moving that much under load it’s really hard to tell I cannot get anything to indicate that tells me I’m having an issue. I did cut a piece of tool steel and a piece of 6061 and both tapered identically so that was a good sign I suppose that it’s not affected by material and it’s a mechanical issue. After all this I think it’s time to upgrade to a 16” lathe if after a year of use we are wearing these parts out this fast. I would hope to get 3-5 years before replacing mechanical components.
 

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Cadillac

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Reading through all this the one thing I haven’t heard mentioned is the saddle adjustment on the flat way of the bed. What could be happening is the saddle is lifting at the start of the cut and settling in getting closer to the chuck. One test you can do is put a mag base on the ways or casting and have it read the saddle then the topslide. Pushing down and pulling to see and verify if any play from saddle to ways. You can do the same with the compound. Have you checked for play in the spindle bearings? Pry bar, blocks of wood and a gauge try moving chuck up/down and side play.
I wouldn’t think your leadscrew have anything to do with it unless it’s slipping in the nut which you would see the handle moving. Finding the slop with a gauge will lead you in the direction.
 

Bob Korves

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The obvious likely problem is that the head is not aligned with the bed. I did not see in the posts above any reference to testing properly for spindle not in line with bed. Start there. It is even possible that the bolts that hold the head stock to the bed were never finish tightened after alignment at the factory, or perhaps that alignment was never done.
 

Technical Ted

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This is out of your owner's manual. Check B below. Sounds like the issue to me.

===================================================================
10.5 Making small taper when external turning
A) It is not on the same line between the spindle center and tailstock center. Please adjust
the tailstock according to operation manual instructions.
B) The moving line of carriage does not parallel to the spindle center. Please loosen the
lock screw of headstock and adjust the spindle center to required position and lock.
======================================================================

Good luck,
Ted
 

Cadillac

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I’d make sure the lathe is dead level so your not compounding the error. Step 1 is level bed then you can check spindle alignment.
 

stupoty

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if after a year of use we are wearing these parts out this fast. I would hope to get 3-5 years before replacing mechanical components.
Has the lathe had very hevy use ? As cadillac pointed out vertical movement can also cause a taper to occur, this might be caused by ware on the ways, but from the amount of taper you are describing the ware would need to me massive.


The next thing to do is to replace the y axis lead screw and nut.
This is so unlikely to be the reason you are having taper issues as is any backlash. You already said it happens when using the compound slide set at 0 degrees.

You could turn the part using manual feed on the hand wheel and see if the same thing happens if you wish to prove it to yourself.

The one thing we do know is that your taper seems to be repeatable. A lose/wobbly item is unlikely to give such repeatable errors.

You have vertical element to your ways on the back side of the lath, you could put a dial indicator on the saddle and run up and down the back side of the ways to see if the caridge is moving relative to it.
 
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