Taper distortion Smithy Midas 1220XL

[Jerry323]

Hello group. I'm a newbie, so sorry up-front for my ignorance.

I am trying out a three year-old Smithy lathe for the first time and am getting a taper distortion cutting 0.25" diameter 360 alloy brass rod stock. The brass is chucked into a three jaw chuck and is unsupported at the tail end. I am moving the tool from left to right and am getting a taper where the widest end is to the left. The entire rod stock protrudes approximately 1.5: from the chuck and is cut approximately 0.020" inch per pass. The diameter taper is approximately 0.020" over 1.10" length. I'm attaching a diagram showing this issue. What could be wrong?

Thanks for the input, and again, I'm a novice so please be gentle.

[Torch]

I'm hoping Harold will jump in here, but I suspect it might be related to the unsupported end. The brass may be flexing away from the tool on the right side whereas it cannot flex away on the chuck end. I would try to support the tailstock end and see what happens. Also, check the tool rake angle. I know conventional wisdom states brass is cut with 0° rake, but there are different types of brass out there. I have found a little rake (7° or less) doesn't hurt and might help counteract the push with a little pull.

[Jerry323]

Torch: Thanks for your quick reply and the advice. Your idea of supporting the tail end is a good one and that should have been obvious to me. Not being supported would allow the free end to flex away from the tool. I'll try and dimple the free end and support it with a tail stock center. I'll also try a thicker piece of rod material reasoning that it will flex less at the free end just to troubleshoot this issue.

Thanks again Torch

[Torch]

As a general rule, the amount of steel sticking out from the chuck should not exceed about 5x the diameter of the material. So for a .250" diameter rod, 1.25" is the limit. Brass is much easier to bend than steel, so I think that it really needs more support.

[Harold_V]

I'm hoping Harold will jump in here, but I suspect it might be related to the unsupported end. The brass may be flexing away from the tool on the right side whereas it cannot flex away on the chuck end. I would try to support the tailstock end and see what happens. Also, check the tool rake angle. I know conventional wisdom states brass is cut with 0° rake, but there are different types of brass out there. I have found a little rake (7° or less) doesn't hurt and might help counteract the push with a little pull.

You don't need my assistance----you've pretty much covered the problems. The only additional concern is if the part is generating a cloverleaf---a function of less support between jaws. In all cases, center support should eliminate the problem.

A little more functional ratio of diameter versus length is 1:3. That's not to say a 1:5 doesn't work, but you can expect deflection with a higher ratio, whereas a 1:3 can be relied upon for straight turning------unless the material is gripped in a sprung chuck.

When material has a tendency to hog, yet positive rake can be to advantage, hogging can be limited by creating a narrow chip breaker. As you alluded, cutting pressure is diminished considerably, often eliminating taper.

Harold

[Jerry323]

Hi Guys:

Yes, I tried end support and it did reduce the taper from 0.020" per inch horizontal down to about 0.005" per inch horizontal. I'm not using a live center (don't have one), but I increase pressure against the tail end (to reduce deflection) and it did seem to reduce the taper, but never below about 0.005"/in. Reducing my cut depth also seemed to help the situation as expected since reduced tool force decreased the deflection.

Using a 0.75" diameter steel rod, I managed about a 0.003"/ inch horizontal taper. I don't know what to expect here, but I'm learning!

Thanks for the assistance again friends!

[Torch]

If the tailstock is not aligned with the chuck perfectly, you will get a taper. There is usually an adjustment on the tailstock to move it towards or away from the tool. If the tailstock is shifted off-centre slightly towards the tool holder, the piece will be tapered narrower at the tailstock. If the tailstock is shifted slightly off-centre away from the tool holder, the piece will taper narrower at the chuck.

If the bed is twisted slightly, the tool will not travel along the same axis as the headstock spindle. Really, the first thing you need to do when a lathe has been moved is to make sure it is level and square and not twisted. Ok, technically all you need to do is make sure it is not twisted, but levelling everything is the easiest way to do that! It is surprising how much that cast iron bed can twist! Ideally, you should use a precision level, like a machinist's level or a master level, but that may not be available to you. Even a carpenter's level is better than nothing, if you keep it oriented in the same direction.

Then use "Rollie's Dad's Method" to fine tune the adjustment. Read about it here.

[Jerry323]

This is really some good information Torch. I did check the bed for level in two dimensions which should have caught the twist element. I'm betting you were right in the first place about the deflection. I bet I fixed the deflection issue with the tail stock suport, but then had the center hole a bit off center, which accounts for the taper in the second case.

I wonder if you or the group could suggest a book for beginning machinists. I have a few books, but so far, I've learned more from hands-on and guys like you who give guidance at the right points. I appreciate your interest!

All the best.

[warmstrong1955]

Hi Jerry
Maybe I can hep a bit....I'll take a shot anyway....

My first machine was a Smithy 1340.
It was out of alignment when I got it, but only about .001" in 30". Torch's advice & info on checking is excellent. May take a little shimming, checking & rechecking, but all worth it. One important thing I've learned.....machining will teach you patience.
Check the alignment of your tailstock. Mine was right on vertically, but off over .005" horizontally. Easy to correct.
Lots of tweaking required for these made in China critters for sure, plus they lack the rigidity of the 'real' machines, but there are some things that need to be checked and adjusted on all of 'em before you crank 'em up, like adjusting all the gibs...mine was loose as a goose all over....
If you don't have a manual for the thing, you can download one here: http://www.home-machine-shop.com/Smithy ... manual.pdf

Get a live center....you'll need one. That's far from all you're gonna need....and far from what you're gonna want...but don't worry about that now. (You will soon learn that the machine is the inexpensive part, compared to the tooling) The more you do, and learn, the more toys you'll want....

1/4" diameter 360......cut .001" a side....with no center....you're gonna have a taper. Been there..... One of the first things i built on my ol' Smithy was a timing tool for a Perkins engine. Started with a piece 3/8" dia 660, about 3" OAL, and I got about a .001" taper in the 2" I was turning. That was after multiple passes at the same position on the crossfeed. Used a live center....it was right on....after making it worse before I adjusted it to center.... oops.

Make sure you're tool is sharp. For my HSS home ground tooling especially, I use my secret weapon....a lighted magnifier. The ol' bifocals don't quite cut it anymore.... Smaller the rad on the cutter, the less pressure it'll require to make the cut....meaning the workpiece will deflect less, and less likely to end up with a taper....I like Harold's 1:3 rule, it works for me.

Make sure the cutter is on center-line of the workpiece.. Don't eyeball it....measure it.

Check out & join this group too: http://groups.yahoo.com/group/smithy-machines/
Ned knows about all things Smithy, and has built a lot of pretty hi-tech stuff with his 1324. A lot of good info there, and a lot of other Smithy owners who are quite helpful, with various models, including the 1220XL, who also machine some really cool and intricate stuff.

Lot of really good info here...I read here a lot.
It's all fun...as long as you like learning, like me anyway.

Bill

[Torch]

This is really some good information Torch. I did check the bed for level in two dimensions which should have caught the twist element.

Again, with what? A carpenter's level is $10 and designed to get a piece of wiggly wood "close enough". My Starrett precision level cost upwards of $200 and was required when pre-fabricating sections of piping for a nuclear reactor. A Starrett master level is upwards of $500 and just about the most accurate thing with a bubble. It's like the difference between a rule, calipers and a micrometer. Rory's Dad's Method lets you use a micrometer to fine tune the leveling.

I'm betting you were right in the first place about the deflection. I bet I fixed the deflection issue with the tail stock suport, but then had the center hole a bit off center, which accounts for the taper in the second case.

If the machine was running perfectly true, then the lathe would make the piece round despite the off-centre hole. Of course, that assumes the chuck runs true. For the highest level of accuracy, turn your test piece between two centres rather than use the chuck to hold the material. You can get a piece of plain steel, say 1" in diameter x 4" long, chuck it up and turn a 60° point on the end. Do not remove it from the chuck! Any error in the chuck itself has just been compensated for, as long as that custom centre does not move.

Now, take a test piece of material, (for the purpose of setting up the lathe, maybe you could find something a little larger in diameter than your brass rod? 1" or 3/4" 12L14 round bar would be nice, yet still fairly inexpensive) drill centre holes on each end, secure it between your centres. Use a lathe dog to drive the workpiece with a chuck jaw.

I wonder if you or the group could suggest a book for beginning machinists. I have a few books, but so far, I've learned more from hands-on and guys like you who give guidance at the right points. I appreciate your interest!

Start with the freebies listed on the metalweb site. The US Navy "Machinery Repairman" and US Army "Master Machinist" books are free, online and a good starting point for general machine shop knowledge. The bible for reference tables is "Machinery's Handbook". Older copies (15th edition) are out of copyright and still contain valid data, just not quite up to date with the latest carbide tooling. Google it and you can download a copy from several sources. The latest version (28th?) is something like $100. A happy compromise is eBay -- I picked up a used copy of a recent edition (27th) for cheap.

Bear in mind that I am still pretty new at all this too! I have a little over a year under my belt now. If one of the truly knowlegeable fellows (like Harold) around here disagrees with me, they are probably right and I am probably wrong!

[stevec]

The only things I can think of to add would be to center drill your test piece BEFORE turning the centre and leaving it in the chuck.
After the "centre" is turned in the chuck one might want to bring the tailstock up toward the chuck and then advance it's centre to the one in the chuck with a steel rule between the points. If things are misaligned the steel rule will be held crooked indicating some tailstock adjustment is required. (of course, if there is twist in the bed when the tailstock is moved back far enough to accomodate the test piece the above procedure won't have corrected that).
Did we mention that the test piece between centres would have to be driven by a dog of sorts?
Keep us informed of your progress.

P.S. Jerry no need to apologize for your ignorance, with help it is a fleeting thing.

[PeteH]

Don't forget Amazon.com... I got my "Machinery's" there, and I think I paid more for shipping than for the book. OK, it has a few dirty pages, but it still beats $100+ for a new one.