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

[Cadillac]

Put a indicator on the flat way of the saddle. Have to point riding the flat way while taking a cut and see if there’s movement in the cut. You shouldn’t get more than acouple TENTHS of movement on any sliding surface if not NO movement. Good luck

 

[RJSakowski]

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.

Turning a taper should not be affected by the cross feed lead screw or nut. Nor should the tailstock be the cause if not using the tailstock center.

Turned parts will tend to have a slightly larger diameter at the tailstock end, the amount dependent upon tooling geometry, condition of the cutting edge, the depth of cut, the material, and the diameter of the stock. Since the tailstock end is smaller in diameter, this isn't the cause of your prob;lem

Aside from that, there are only two conditions which will cause a turned taper. The headstock is misaligned or the lathe bed is twisted. The effect of headstock misalignment is fairly obvious. Rotating the headstock in a horizontal plane will bring the part closer or further from the cutting tool as the carriage moves away from the headstock, depending which way it is rotated. Some lathes like the Sherline actually rotate the headstock to purposely cut tapers.

The carriage rides on the vee way at the front, being prevented from lifting by small plates at the front and back of the carriage that ride on the under side of the ways. The vee way determines the distance of the cutting tool to the spindle centerline. If the bed is twisted, if corkscrews which will raise the front edge of the carriage and drop the back edge or vice versa. Since the cutting edge is several inches above the pivot point, the effect is to move the cutting edge closer to the spindle axis. The effect increases as you move away from the headstock causing a taper to be cut.

There is also a slight shift in vertical position of the cutting tool which moves the cutting edge up or down depending upon the direction of twist. This effect is usually minor compared to the lateral movement although it will increase if the cutting edge is significantly lower or higher than the horizontal plane though the spindle axis.

By the same token, if the headstock were rotated in a vertical plane it would have a similar effect, raising or lowering the toll relative to the spindle axis as you moved away from the headstock.This would be extremely unlikely unless the headstock was removed from the ;lathe or there had been a severe crash.

Considering that the lathe had been performing satisfactorily in the past and that you have not seen any serious change in level and the amount of taper that you are experiencing, I would suspect the headstock has shifted. It is possible that the mounting bolts were not tightened sufficiently and a crash could have caused a shift. In any event, I would check that first. I would position the carriage close to the headstock and lock it in place. I would mount a faceplate with a small boss about half the cross feed travel distance from the centerline. Mount an indicator on the crossfeed and rotate the faceplate so the boss is at the front and more or less horizontal with the spindle axis. Zero the indicator on the boss and mark the contact point with a Sharpie. Rotate the boss to the rear and move the indicator with the cross feed to intersect the mark on the boss. A difference on readings would mean that the headstock is not perpendicular to the cross feed travel and, by inference not parallel with the ways.

There are some caveats. The assumption is that the carriage cross feed travel is perpendicular to the carriage ways The second is that any degree of twist is not affecting the measurement. This is probably a safe assumption since the effect of the twist is least with the carriage near the head stock.

Here is my faceplate modified for this measurement. The boss is a bolt with the head slightly domed and polished.

 

[bretthl]

Just a suggestion ... mic the work piece at 1/2" intervals and plot the diameter vs distance. If it is a straight line trend then it may be, as others have suggested, a head stock alignment problem. If there is a shift in the trend (not a straight line) then perhaps the problem is in the carriage or ways.

 

[stupoty]

Hay , hows it all working out for you at the moment? Have you managed to nail down your issue yet?

Stu

 

[pacifica]

I think I would do this first:

. Along with the face plate test by rjsakowski, check runout in spindle bore, level lathe,turn a test piece of 1.5 to 2" steel that is 10" long(no tailstock) and go from there(align headstock?).

 

[petertha]

Misaligned headstock & related taper cutting problem has been discussed quite a bit. I know I have posted & included these same pics just cant find the link(s) right now. There is a very fast & accurate way to verify.
First - do your lathe levelling due diligence
Second - buy a MT3/cylindrical test bar like this inexpensive ($50) import I bought on Ebay. They are cylindrical ground & plenty accurate. If you have an MT socket like MT5/MT3, plug that in the spindle, then the test bar so it is extending cantilever. No chuck is involved. No tailstock is involved. No material cutting is involved. Mount DTI to saddle, run down the test bar along the horizontal plane. What do you read? If its pointing in or out (anything but zero) you will be cutting a taper longitudinally and a cone in cross feed. Its as simple as that. You need to correct this by loosening your HS bolts & carefully adjust the shimming/jacking bolts so the HS rotates in so the bar is parallel to bed axis. The screws are uber sensitive so just crack them & re-measure.

While you are at it, repeat test with DTI along the top of the bar. Is it pointing up or down? Trickier to solve but same basic problem.

Only when this is remedied can you introduce the tailstock into the picture.

 

[petertha]

ps - I notice several of the same vendors also offer MT5 ended cylindrical test bars if you want to go that route if your spindle is MT5. This eliminates the MT5-MT3 intermediate sleeve, but I would assume they are pretty ground accurate too. My tailstock is MT3 so it can serve double duty there. Basically its acting as a big long lever exaggerating any angular deviation.

For example (no affiliation)
https://www.ebay.ca/str/atoztoolstore/Lathe-Test-Bars/_i.html?_storecat=18372947012

 

[stupoty]

ps - I notice several of the same vendors also offer MT5 ended cylindrical test bars if you want to go that route if your spindle is MT5. This eliminates the MT5-MT3 intermediate sleeve, but I would assume they are pretty ground accurate too. My tailstock is MT3 so it can serve double duty there. Basically its acting as a big long lever exaggerating any angular deviation.

For example (no affiliation)
https://www.ebay.ca/str/atoztoolstore/Lathe-Test-Bars/_i.html?_storecat=18372947012

I'm really tempted by a test bar, I was thinking of getting it in MT3 and have a sleeve so I can put it in the MT4 head stock and my tail stock.

I wonder how much difference their is between sleeve brands for run out ?

Stu

 

[Rex Walters]

I'm really tempted by a test bar, I was thinking of getting it in MT3 and have a sleeve so I can put it in the MT4 head stock and my tail stock.

If I could only afford one, I'd buy the larger test bar for the headstock. A sleeve is probably accurate enough but will more than double potential errors (two mating surfaces for grit/burrs/whatever to cause you to pull out your hair).

The tailstock is far less critical IMHO. Drills and reamers will tend to follow their own path anyway (you'll need to use a boring bar regardless for accurate holes) and the tailstock is adjustable for cutting/not-cutting tapers when turning between centers. The spindle taper you want dead nuts aligned to the bed ways in both dimensions (horizontal and vertical, maybe ever so slightly pointing higher toward the tailstock — a few tenths over 12" at most).

A taper test bar isn't a mandatory tool unless you have reason to distrust the accuracy of your lathe (or are planning to rebuild it). It's kind of a one time use tool. (But I own both!).

 

[petertha]

The tailstock is far less critical IMHO. Drills and reamers will tend to follow their own path anyway

That's the beauty of being able to plug same MT# test bar into either head or tail stock. You can similarly check TS alignment by mounting a DTI in the lathe chuck & turn it around the OD. Yes, you can also do this with a regular dead center, but what the longer bar is providing is extended cantilever length so its going to exaggerate the angular discrepancy: up/down/left/right. Yes big drills & reamers might find their own drilling holes & such but its still better to have the TS aligned. Turning between centers depends on this. No sense having a 0.0002" live center when teh TS is in or out 0.002". Another example, after breaking 2 teeny carbide center drills in tool steel it finally dawned on me to check the TS, sure enough it had migrated off a couple thou. Actually that's not quite true. What I learned is that just tightening the TS down against the displacement grub screws while adjusting can drift it (on my particular lathe). So now I leave an indicator ball on the bar as I'm setting it up & just tweak it down while the needle is giving me constant reference.

Yes it probably goes without saying you have to clean any mating surfaces, particularly between rotating parts. Setting up the machine is the one time you should be extra diligent unless you want to repeat this whole process. HS alignment is a PITA. Maybe I got lucky but before committing to the MT3 test bar I put a tenths indicator on the MT5 ID taper of the spindle, measured nil runout. Inserted the MT5-MT3 socket adapter, exact same thing. Pulled & clocked it a few different ways, same thing. So I trust it.