Author Topic: Tapered Cylinder  (Read 4900 times)

Offline Nerdz

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Tapered Cylinder
« on: January 25, 2016, 12:11:08 AM »
Hello everyone,

Its been a while since Ive been in the shop (for various reasons, unemployment being the major factor). I decided to practice on getting a piston/cylinder to fit correctly. I initially set out to make a 0.625 barrel and cylinder. The cylinder holder would be brass (because that's what I have on hand) and the actual cylinder is Cast Iron. I also bought a boring bar with some inserts from shars, because that's what I can afford at the present moment, I did however find out I could use a 3/8 end mill in my boring bar holder. Through experimentation I found out that a HSS end mill gave a really great finish compared to the inserts I bought, so I used that. Anyway. Bored out the cylinder to the correct dimensions, and both sides matched. I did not try and fit a piston inside.

Next I bored out my piece of brass to match the OD of my cylinder liner. The cylinder liner was press fitted into the piece of brass. It was very hard to get in there. I had to use a combination of my vice and a hammer, but it fit. I measured the cylinder and found out it was now slightly smaller (0.615), so I remachined it to the correct dimensions. I then lapped it using a piece of wood dowel and some brasso. The HSS endmill left a awesome finish.

This is where things get confusing. I dont know what Im doing wrong, but I made a piston to size and come to find out, it will enter one way but not the other. I am trying to get a gas perfect fit, just for practice. I checked with a mic to see that the piston is the same size from one end to the other, but it seems that my cylinder is slightly tapered.  When I bored it out, I let my boring bar go in and out at the same setting using the same feed until it didnt cut any more.

Is there anything I could do? Also, this was the first time buying inserts, are there specific ones for Cast Iron? I used these:
http://www.shars.com/ccmt-21-51-hf-ybc152-1
http://www.shars.com/ccmt-21-52-hr-ybc251
http://www.shars.com/3-8-sclcr-indexable-boring-bar

(both chattered like hell,  then I switched back to my HSS endmill set up)

ADD: I am also using a 3 jaw chuck to do work. Should I switch to a 4 Jaw? Piston was graphite at first, but then I changed it to cast iron because of the dust.
« Last Edit: January 25, 2016, 12:29:14 AM by Nerdz »
-Chris

Offline b.lindsey

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Re: Tapered Cylinder
« Reply #1 on: January 25, 2016, 12:30:43 AM »
Chris, it sounds to me like your lathe headstock may be angled just a bit. If that is the case then even taking multiple passes at the same setting can still result in a taper. Not sure how long your cylinder is but it wouldn't take much to misalignment to give you a tapered bore though .010" sounds like a lot if the cylinder is pretty short.

Put a good piece of truly round stock in the lathe and use a dial test indicator. As you move along the round bar, check to see if you are getting any indicator movement. My suspicion is that you will get some, meaning the headstock is out of alignment even if by only a fraction of a degree.

Bill

Offline Nerdz

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Re: Tapered Cylinder
« Reply #2 on: January 25, 2016, 12:35:14 AM »
Chris, it sounds to me like your lathe headstock may be angled just a bit. If that is the case then even taking multiple passes at the same setting can still result in a taper. Not sure how long your cylinder is but it wouldn't take much to misalignment to give you a tapered bore though .010" sounds like a lot if the cylinder is pretty short.

Put a good piece of truly round stock in the lathe and use a dial test indicator. As you move along the round bar, check to see if you are getting any indicator movement. My suspicion is that you will get some, meaning the headstock is out of alignment even if by only a fraction of a degree.

Bill

Hi Bill, My stock is only 1.75'' long. I have some 0.5'' W-1 ground steel I could use as a round bar to check it.
-Chris

Offline Nerdz

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Re: Tapered Cylinder
« Reply #3 on: January 25, 2016, 11:31:10 AM »
I mounted a Dial indicator perpendicular to the chuck. Spinned the chuck around and saw that it varies from 0 to 3 thou. I moved the saddle to about how much my work piece was and noted that it varied about 1.5 thou. Is there anything I could do to fix it? I have a small 7x16 lathe, so i dont know if thats a option.
« Last Edit: January 25, 2016, 11:34:57 AM by Nerdz »
-Chris

kettrinboy

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Re: Tapered Cylinder
« Reply #4 on: January 25, 2016, 12:53:46 PM »
Hi Chris
not withstanding if there is or isn't a problem with your lathes head alignment causing the taper , you can still machine a pretty parallel bore or turn a parallel outside dia out of the chuck by measuring the taper over the length of the bore or outside dia and easing on or taking off a very small amount of cut as the tool goes up the bore or outside dia, I used to do this a lot at work when trying to turn and bore parallel using some of the older worn out lathes , you need to ease the cut on in a continuous motion over the length of the cut but with practice you can get good results , using a clock to measure how much cut your putting on makes it easier to see rather than using the crosslide dial,  hopefully if your lathes bedways are not worn it might only need carefully levelling up  with a sensitive spirit level to get it to turn more parallel.
regards Geoff

Offline b.lindsey

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Re: Tapered Cylinder
« Reply #5 on: January 25, 2016, 01:03:22 PM »
Chris, hopefully others with similar lathes can comment on headstock adjustability. Another simple thing to check would be to make sure there is no swart between the chuck and chuck mount which could angle the chuck slightly. Not sure how your chuck attaches though.

Bill

Offline AOG

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Re: Tapered Cylinder
« Reply #6 on: January 25, 2016, 01:20:59 PM »
With a 7 inch lathe it could also be warping of the bed casting. We could of more help if we knew what model of lathe you have. The good news is that there are usually fixes for a misaligned spindle and warped beds.

Tony

Offline joncarruthers

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Re: Tapered Cylinder
« Reply #7 on: January 25, 2016, 03:39:23 PM »
Does the spindle have a taper? if so lose the chuck and test the spindle holding a known good bar in a collet.

Then try it in the chuck.

Offline Hugh Currin

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Re: Tapered Cylinder
« Reply #8 on: January 25, 2016, 04:27:35 PM »
I mounted a Dial indicator perpendicular to the chuck. Spinned the chuck around and saw that it varies from 0 to 3 thou. I moved the saddle to about how much my work piece was and noted that it varied about 1.5 thou.

I can't tell how you are taking measurements for this test. I do the following though it could be wrong.

Put a length of straight bar in a 3 jaw or collet. With a test indicator measure the run-out near the spindle. Slowly rotating the spindle noting the max and min values. The center of the bar will then be the average of the max and min values. Then move away from the spindle (without adjusting the indicator) and measure the run-out again finding the max and min. Average the max and min at this new location. The difference between the two averages will be the taper.

i.e. Measure 0.000" and 0.003" near the spindle. The average will be 0.0015". Move away from the spindle say 6" and repeat the measurement getting 0.001" and 0.004" for an average of 0.0025". The difference between the two averages is then 0.0010". So the taper is 0.001" over that 6".

Taking the average of the run-out should negate miss-alignment in the chuck. As long as the test bar is straight, this should give an indication of spindle alignment. The wobble, difference between max and min, will give information on the chuck itself or how it is mounted.

I hope this makes some sense, and hope it is of use.

Thanks.

Hugh
Hugh

Offline PStechPaul

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Re: Tapered Cylinder
« Reply #9 on: January 25, 2016, 09:26:09 PM »
Some ideas:
 
1. If you have a 4-jaw independent chuck, you should be able to adjust it for minimal radial runout and get the piece on center. With a 3-jaw scroll chuck you might be able to shim one or two of the jaws to center it.
 
2. If the work is wobbling you might be able to hold it loosely and gently tap it with a soft hammer to reduce the wobble, then tighten and repeat until it's solidly held.
 
3. Since you are using an end mill as a boring bar, you might be able to compensate for the taper by alternately cutting on the front and rear of the bore.

Offline Nerdz

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Re: Tapered Cylinder
« Reply #10 on: January 26, 2016, 12:25:04 AM »
Hi Tony,

I have a micromark 7x16 lathe. Had it for quite some time, but Ive never had to do precise work. I dont have a collet, but I do have a 4 jaw. Next lathe I get (Grizzly G4000) I'll get collets for it, but for now this lathe will do. The chuck is attached via set screws on the back, and nuts that hold it on.

Hugh, that was a great help. I chucked in piece of 0.5'' W1 bar. Measured run out with the Indicator perpendicular to the bar, mounted in the boring bar holder. Run out near the chuck varied from 0 to 3 thou. Moved over 6''. Varied from 0 to 4 thou. I set my min value to be 0. So the taper from 0 to 6 inches is 0.0005 (half a thou). Ive included a pic of my setup towards the end at max swing.

Well, since Ive eliminated that as a issue, I guess its back to the drawing board.
-Chris

Offline Nerdz

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Re: Tapered Cylinder
« Reply #11 on: January 27, 2016, 02:22:45 AM »
Small update: Chucked in my 4 jaw. Bored the piece out a little bit more. The end nearest to the chuck is about a thou larger than the end furthest from the chuck (again, only 1.75 inches). I noticed that each time I ran the boring bar in and out it took some off. Sometimes it would take a little and produce some swarf, other times it would take nothing at all. I dont get what gives here  :shrug:, Maybe my work piece wasn't straight when I faced it off? I think I might just try again and chuck this piece into the scrap pile.

-Chris

Offline Roger B

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Re: Tapered Cylinder
« Reply #12 on: January 27, 2016, 07:47:40 AM »
What may be important here is to have a very sharp tool so it always cuts and does not just get pushed away when taking removing very small amounts. There will always be a bit of spring in the boring bar.
Best regards

Roger

Offline zeeprogrammer

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Re: Tapered Cylinder
« Reply #13 on: January 27, 2016, 11:30:39 AM »
Following closely as I'm having similar/same issues.

I don't think it matters if the workpiece is straight or not. The boring bar doesn't know. The hole just won't be square to the part (which will likely be a problem in assembly).

I also believe there's nearly always spring in the boring bar but it doesn't seem like that should cause taper.

But I'm still a newbie and continue to have problems boring.

Am I reading Nerdz post right? .0005 over 6 inches? I also have a mini-lathe...wouldn't I feel ecstatic? I really don't know.
Carl (aka Zee) Will sometimes respond to 'hey' but never 'hey you'.
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Offline Ian S C

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Re: Tapered Cylinder
« Reply #14 on: January 27, 2016, 12:16:26 PM »
Nerdz, was the bore of the brass bit parallel, and was the out side of the CI liner parallel before you assembled them? If so my next thought is that as you pressed them together some of the metal (probably the brass) got scraped as  they slid together so that one end of the bore in the brass piece was larger than the other.
I would make up a lap (you can go quite fancy), I make them from wood, with a slit down it's length, and a wood screw in the slit to adjust it, I use grinding paste, mine's quite fine, so it  just gets that. If you have a coarse and fine valve paste, make another lap for the fine, coarse grit will be embedded in the first lap.  If you are really over enthusiastic, you can then try Brasso, or even tooth paste.
Ian S C