I have a 3/4 steel rod in a 3 jaw chuck, sticking out about 2 inches. When I turn it down, the part farthest away from the chuck is about 5 thou bigger. At first I thought it was just the part deflecting. I added a live center to it in the tailstock, but the results were the same.
Then I decided to make a check. I have the compound moving in the same direction as the carriage. I put an indicator holder on the tool post holder. With the indicator on the cross slide I move the compound the full length of travel. I see 5 thou difference in height. I then put the indicator on the way and found 10 thou difference in height. I interpret this to be that the compound is not parallel to the cross slide and not parallel to the ways.
So my question is is this normal? I was moving the compound to turn down the stock. Am I doing this wrong and should move the carriage instead?
I must say your method of turning a piece of stock straight is a bit unusual. Yes, one normally moves the carriage, not the compound. For the most part, most engine lathes are left with the compound set at 29 degrees, just like you're going to chase a thread, and cuts are generated with the cross slide and/or the carriage. The compound is normally used for generating angles. You can use your compound for straight turns, but it usually will be in the way if you're using a center, and it's clearly a bitch to get dead straight.
I'm not exactly clear where you were using your indicator, so the readings you mentioned don't make a lot of sense. If you were tracking the top of the compound as it was traversed, unless it was finish machined, the reading may not mean anything. You have to be certain to check against the right surfaces before the reading will have any value. If you feel you have a problem, please describe it in better detail, and post a pic if possible.
If your compound is just 0.5º out of parallel with the axis of the spindle (and carriage), you would be out .017" (radius) over 2" of cut.
I sometimes use the compound to advance the tool for smaller parts. When I do this, I indicate the side of the compound by moving it along the z-axis with the carriage. Assuming the lathe manufacturer did his/her homework, this should yield fairly good results over shorter distances. 2" is excessive and yes, you should be using the carriage feed.
"Harold & Susan Vordos" wrote in news:4081eb32 snipped-for-privacy@newspeer2.tds.net:
I'll have to agree with Harold on this one...using the compound for actual straight turning is a bitch. Are you sure you don't have a slight angle in the compound producing the 5 thou? Set your indicator on the machine bed, and against the machined surface on the side of the compound, move the carriage back and forth..sounds like the compound is on a slight angle.
I was under the impression that the compound was generally set to 30 degrees (hence forming a 1, 2, sqrt(3) triangle) for the purpose of making very fine adjustments to cut depth. At least that's the way that I've used mine.
It's not recommended for chasing 60 degree included angle threads. The whole idea behind feeding at a slightly reduced angle is for the threading tool to take the majority of the cut from the side of the thread that loads the cutting tool against the lead screw. That means you have to change sides with the compound depending on the nature of the thread you're machining. Opposite sides for left versus right hand threads, and opposite sides for internal versus external threads.
There are various well founded reasons to set your compound properly. It's not carved in stone, so most guys do what they feel is correct for them. I've spent almost my entire life in commercial machine shops and consider myself a rather gifted lathe man. I've always set my compound at 29 degrees. Interestingly, I worked with one man, the finest lathe man I ever encountered, heads and shoulders above the average guy, who insisted that his compound be set at 30 degrees, and the quick change tool block set exactly square with the spindle. He always did fine work, but you leave yourself open to one little problem when you thread at 30 degrees.
The markings on machines are generally pretty good, but rarely dead on, what ever that may mean. If, by chance, the marks on a given machine are off as little as a few minutes of angle, and it's the wrong way, if you set your compound at 30 degrees, you may be set beyond the half angle of the thread. What that does for you is to leave a stair step on the back side of the thread, and eliminates the possibility of cleaning up the thread unless you plunge cut a few passes. By setting the compound at 29 degrees, you're more or less guaranteed that the thread will continue to wipe clean with each pass.
There's one big fly in the ointment where chasing threads to the formula is concerned. The formula addresses a sharp V thread, but in practice they are rarely, if ever, machined. Based on that, if you grind a proper threading tool, you have no idea how the tool tip relates to the pitch diameter of the thread. For chasing a thread that you want to fit to a nut, that might be fine, but if you're trying to work to proper dimensions, you have no clue where you are with the pitch diameter. The only way to know is to measure with wires, or to use proper thread gages, either rings or snap gages.
This argument is endless. There seems to be two factions, one that thinks that threading what I like to call "properly" is not necessary, especially for the home shop guy. I maintain that it is no harder to do things properly than it is to do them in a less than acceptable manner, so I always encourage folks to learn the "right" way. If you do, when the need arises, you know how to deal with the problem at hand and it's no challenge.
Hope this let a little light in for you, Harry. If you're interested, here's a link where there was a very heated discussion about this very subject. You might get several viewpoints to either support or debunk your concepts. Hard to say.
formatting link
It's a long thread, over 100 entries. Some VERY good.
Some other reasons to set the compound along the ways (and I've done this myself on occasion) is either because of a lack of a travel indicator on the carriage (compound dial then shows infeed), or when drilling - with the drill chuck mounted on the toolpost. (here the carriage runs to a stop on the bed, and the drill feed is done with the compound. This allows rapid backing out to clear chips, and rapid feed back in to the bottom of the hole, via the carriage traverse wheel)
Reading the original post, the *real* problem he was having with the lathe was that it was turning taper when using the compound to turn the diameter. The trouble was simply that he had the compound angle incorrectly set. One cannot rely on the divisions on compounds for 'zero.' I'm suprised he got as close as five thou in a few inches. All of the other issues about should a dial gage read zeros when swept (via the compound) across the top of a part in the chuck, or across the bed, etc. are moot.
In such a case the indicator *should* read zeros, but will only do so in a brand new machine with no wear. In principle the axis of travel of compound should be in a plane parallel to the bed travel of the carriage. Wear over time will destroy that alignment.
Which is another good reason to not use the compound to turn diameters.
Jim
================================================== please reply to: JRR(zero) at yktvmv (dot) vnet (dot) ibm (dot) com ==================================================
Harry didn't say: "A 1, 2, 3 Triangle is NOT a 30 degree triangle". You did.
Harry said "(hence forming a 1, 2, sqrt(3) triangle)" meaning a triangle with an opposite side of 1 unit, an hypotenuse of 2 units that form an adjacent side or sqrt(2*2 - 1*1= 3) = 1.732 units which he just called the sqrt(3) side.
Mount your indicator in a suitable adapter (if necessary) on the tool post and use the longitudinal feed to move it along a round bar observing the change in indicator reading. NOTE! This is done with the lathe NOT running! Do this several times turning the chuck between trials. Significant variation would indicate that either the test bar is not straight and round or the chuck jaws are not true.
Now reverse the bar end for end and repeat. If the results are different, the test bar may be tapered.
I did all this on my Smithy when I got it with the result that it will turn a few inches with well under one thou taper. Beyond a few inches, you have to consider deflection.
No one has mentioned the possibility that his tailstock was offset. If his was a new lathe, it would be highly unusual for the tailstock to be correctly aligned. I am more surprized, though about the compound not being parallel to the ways. I'd call that a serious defect in the lathe. However, if you look at the geometry of the situation, a 10 thou difference in height over two inches would not result in more than a thou or difference in diameter. But that still looks like a hefty slope for the compound. A small piece of swarf on the compound's mating surface could do it, though. I'd check the tailstock allignment and the dirt before I started agonizing over a faulty lathe.
Although, in the original post, he said that the effect did occur when *not* using the tailstock, to start.
================================================== please reply to: JRR(zero) at yktvmv (dot) vnet (dot) ibm (dot) com ==================================================
Odds are that the lathe hasn't been "leveled" so that it will cut straight. Leveling consists of making the two ends of the bed "level" so that there is no twist in the bed. Note that I put level in quote marks because the simplest way to insure that there is no twist in the bed is to make the ways level all the way across the width of the bed. In truth, you don't need the ways to be level but rather just untwisted. .005" in 2" is a bit radical but it just may be twisted that much. I'll also note that if you want cuts square with the spindle, you don't move the compound bur rather the entire carriage as the compound may not be setup properly to the ways but the carriage always is.
-- Bob May Losing weight is easy! If you ever want to lose weight, eat and drink less. Works every time it is tried!
Ok, now I feel like an idiot. It is obvious that the compound had a slight angle. If it had been off more, I probably would have figured it out. I guess being 5 thou off only eyeballing it isn't too bad. I tried cutting using the carriage travel and that came out ok.
I bought the lathe new 2 weeks ago. It was kind of a mess, but I got most of it cleaned up. I did level it (at least somewhat). It isn't levelled left to right yet, but I did work on front to back. Using a Starrett 98-12 level it looks good placing it on the ways on both ends. If I put the level on the cross slide, the bubble ends up on different marks. When I move the carriage from one end to the other, the bubble moves 1 division.
I have 2 more weeks to try the lathe. If I don't like it I can return it. I still have issues that have to be sorted out with the place I bought it from. But before I deal with those I wanted to make sure that the lathe itself is ok.
So now that I know it cuts reasonably straight, is there anything else I should look at? It sounds like sweeping from the toolpost holder to the ways may not be important. I guess this just changes the center height a bit when moving the compound.
I would rather make this lathe work for me than return it. However I don't want it to be a big project either.
I have used a mill, but not a lathe. I supposed I used the compound because it felt more familiar. I wasn't quite ready to kick in the power feed. Up until now I've been doing all my lathing in the mill.
Bill, exactly. My real point in that post was that in making linear cuts, 30 degress permits a vernier (and easily calculated) adjustment to the depth of cut that is not easily achieved using the cross slide.
While I'm an engineer/physicist and only an amateur machinist, but while a Co-Op student I was taught this simple trick by a very experienced tool and die maker.
Cutting and chasing threads is another issue. Harry C.
The 98 is a good level, but it is not the master precision level that you need for leveling the lathe. The master precision level is ten times as sensitive.
Maybe you can get the seller to set it up correctly.
PolyTech Forum website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.