Non-perpendicular Cross Slide?

Well now I have learned something. It makes sense too. But it begs a question, .0001 in 12" would be .000025 in 3" Right? Well I would have a hard time picking that up with any of my tools. so this brings me to the question would a novice even find this? (Not that I am a pro, I'm not even up to intermediate)
I think we need to know how concaved or convexed is it, over what dia.Like .0005 on a 2" dia. for what I do I would be fine there. He's on a 9" lathe thats under .0025.
One thing he should also look at would be is the crossslide out or could it be the head? A 4" test bar would answer this quick enough.(also a good project and learning exercise). Just some thoughts.
Mark
 
randyc said:
Well, here for one:

"I do notice that on the Prentice Brothers inspection chart they say the the lathe should face .001 “ low or concave in the center."

The above quote is from this thread in "Practical Machinist": http://www.practicalmachinist.com/v...ory/accuracy-alignments-machine-tools-247127/

So far as I know, this is common knowledge among old-timers (at seventy, I guess I qualify :) and has been discussed a few times in the above forum.
Click to expand...


One thousandth of an inch would be within most part tolerances. I never heard of it being built in before but does make sense. Thanks
 
Mark, you have just pointed out the reason why a slight concavity is desirable. Two parts will always mate properly, a single faced shaft will always lie flat on a planar surface and the amount of taper is so slight as to cause no problems in normal applications. BTW, the figure that was mentioned is .0002 in 12 inches, not .0001.

Since the OP has a Chinese lathe (I presume) the quality standards are unknown. Rather than .0002 in 12 inches it could very well be .008 in 12 inches, who knows ?

He has no practical means to change the concavity or convexity of his machine - it is what it is and any effort expended to correct it requires some very specialized skills, specialized tools and LOTS of time. Take a look at Ulmadoctor's post here: http://www.hobby-machinist.com/threads/biax-scraping-a-3x3-milling-knee.32957/#post-278433

Because most of us have not the skills to correct any cross-slide angular errors, measurement is a moot point. BUT if one is anal enough to want to quantify the concavity (me, for example), a "tenths" indicator will probably easily detect the concavity in inexpensive Asian machinery.

Here's the process, first noting that the cross slide must have a lot of travel to make this measurement.

First secure the largest workpiece in the headstock that can swing across the ways.

Take a light facing cut (using power feed if possible) and then use a DTI to sweep across the Far face of the workpiece, from OD to center !

Let's understand this: if we sweep from the near side of the OD to the center of the work that we've just faced, we'll find zero error because the cross-slide (to which the DTI is secured) will follow exactly the same path used to face the workpiece.

But sweeping the DTI across the FAR SIDE of the OD to the face will reveal the angular error because on the far side of the work, past the center, the cross slide does NOT track the face and will reflect the true error.

edited: Mark, good point - if the headstock was not aligned with the ways the same problem would be observed and a test bar or a two collar test will reveal that.
 
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Randy, I agree unless the cross slide was way out I would run with it. Ops you are right I got that wrong, it was .0002. still would be very small on the 9" max swing.
I brought up the head because I believe his 9x20 is very similar to my 10x22. my lathe was good when I first set it up but it moved from a small crash. when I went to loosen the bolts to make an adjustment, I found them only snugged down. there is room there for adjustment and a good chance that if he has the amount of error I think he has it is comming from the head. Or at least I would look at that first as that is the easiest to check.
Mark
 
i would like to ask a couple questions of the author of the thread,
was your carriage locked when you started the facing operation?
is there a lot of slop in your compound rest or the cross slide dials or handwheels?
is your tool ground correctly and centered to the work?

if you answered yes to the last 2- start there first.
i'd be happy to help you, if you need some instruction as to how to accomplish your goals.
 
randyc said:
Well, here for one:

"I do notice that on the Prentice Brothers inspection chart they say the the lathe should face .001 “ low or concave in the center."

The above quote is from this thread in "Practical Machinist": http://www.practicalmachinist.com/v...ory/accuracy-alignments-machine-tools-247127/

So far as I know, this is common knowledge among old-timers (at seventy, I guess I qualify :) and has been discussed a few times in the above forum.
Click to expand...

That is very interesting. But I would not put it in concrete. Especially when the source is from 115 years ago. I just checked my Clausing lathe inspection cert sheet and it shows the cross slide to be set at 90° the datum center line of the spindle. And not some obtuse angle…Good Luck, Dave.
 
The obtuse angle to which you refer would be in the neighborhood of .003 degrees, in accordance with Dr. Schlesinger's recommendations, so I think that it would be fair to call it ninety degrees since it would be extremely difficult to detect.

Mister King however recommends even LESS concavity than that, on the order of .001 degrees. Why not make your own measurement in the method described above ? I, for one, would be extremely interested in the results now that this topic has seemingly picked up some momentum :)

I'd also not set it in concrete. Who knows how lathes are set up in the rest of the world, especially China ? I made the stipulation in my original post that I referred to American lathes. Of course your Clausing is as American as apple pie which hints that it is set up to face slightly concave as are all other American-made lathes to the best of my recollection.

Insofar as the reference being 115 year old, I have two responses: the first is that manual machine tools (ignoring any onboard electronics) are still designed exactly the same now as they were in 1899 perhaps with the exception of some old shops still using overhead belt drive. I think that I saw some old photos of Thomas A. Edison's large workshop in Menlo Park, in which any of us would feel right at home with his machine tools.

The second is that the age of the reference has no bearing on what Dr. Schlesinger was attempting to convey. His point was HOW to measure machinery deviation and to suggest permissible limits to those deviations.

In any event, I'm personally happy to accept the opinion of the aforementioned expert who has been rebuilding lathes for over 45 years. He is respected, not only in the U.S., but all over the world. (I'm also a member of Practical Machinist and whenever Mr. King expresses an opinion, the rest of the professionals sit up and pay attention.)
 
Just my thoughts, I could be all wet.

On the occasion that my parting tool does not cut a relatively square plunge I have found it is usually one (or more) of three reasons:
1. The tool isn't aligned perpendicular to the work. IE: parallel to the square face.
2. The parting tool cutting lip and/or clearance was not ground square.
3. The tool has a dulled corner, usually on the convex side of the cut.

If you're using/testing the parting tool by facing the end of the work you're likely getting some side flex in the tool. With the exception of a very sharp stiff parting tool, with some relief behind the the cutting edge, it is likely you'll always have some drift leaving a convex end on the work. I have found that even thin cutoffs may not supply enough load on the parting tool to prevent this drift.

Assuming you've taken due diligence in setting the tool square to the work, if the tool isn't following the desired track when pushed into the work I would I consider the forces on the tooling before questioning the tool feed.
 
I'm going to add my 10 cents here.
I purchased a JET 13x40 as a total novice. There is a thread here describing the fiasco. Not only was it lost in shipment but when it finally got to me the steady rest and xslide rest were for another machine. furthermore, the headstock misalignment was more than 40 seconds of arc. Not trivial for a novice to contend with. However, by the time it was all done, 2 yrs later, yep 2 yrs, Richard King and Tony got me "square". The JET q/c indicated the lathe had been tested well w/i guaranteed tolerances. NOT, the tests, in my humble opinion, were all fabricated. One single test WILL NOT assure the headstock, chuck and the ways are all in alignment. Richard got my miscreant to w/i 0.0002 in 6" for a 2" OD solid bar chucked only (Phase I). So we must disagree, it doesn't take a Master to apply the principles, only one to direct the work. Even I can perform the work.

Imagine: chuck a 1" bar 16" long in a 4 jaw chuck, then slide the tailstock up to a centerdrilled hole opposite end. Do this 100 times. cut the bar to 750 thou. Soon you will not have a 4 jaw chuck, or 3 jaw for that matter. Forget the tailstock. Manf do not care. This community does. Novice read, the lateral force is tremendous.
.'. QED
 
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