Author Topic: Tapered Cylinder  (Read 4807 times)

Offline jadge

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Re: Tapered Cylinder
« Reply #15 on: January 27, 2016, 12:27:21 PM »
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:

It depends on the insert, depth of cut and the material. If the depth of cut is less than the radius on the insert then there is a force trying to push the insert radially away from the work. So if you take a small cut it doesn't seem to result in the size change expected, so you take another cut, same result. Then another cut and blow me, the darn work is now over, or under, size. Many types of bronze, including gunmetal, are particularly bad in this respect.

It may be counter-intuitive but bigger depths of cut when boring often lead to a more stable cutting operation. Make the hole first and don't sweat about the size. Then make the piston to suit.

Andrew

Offline Roger B

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Re: Tapered Cylinder
« Reply #16 on: January 27, 2016, 03:15:36 PM »
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.

I checked the numbers I got when I was doing some work on my Minilathe:

"The first check was the put a length of 10mm silver steel in a collet and check the run out at the far end ~200mm. Better than 0.02mm TIR so at least everything was concentric.
Next check clamp the DTI in the tool post at centre height and run it up and down. Total variation was less than + 0.02mm so the headstock is still parallel to the bed."

I make 0.0005" to be 0.013mm so 0.0005" over 6" is not far away from 0.02mm over 200mm.
Best regards

Roger

Offline Nerdz

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Re: Tapered Cylinder
« Reply #17 on: January 28, 2016, 01:29:31 AM »


It may be counter-intuitive but bigger depths of cut when boring often lead to a more stable cutting operation. Make the hole first and don't sweat about the size. Then make the piston to suit.

Andrew

I noticed this with my "real" boring bar. Cuts around 10 thou would cut beautifully, but then if I try to take a finishing cut of say 1 thou it would chatter like crazy and leave all kinds of marks. I would be happy taking cuts of 10 thou if I knew it didnt spring.

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

I am pretty sure It was. For lapping I used a 0.626 wooden rod with brasso. The wooden rod did make it all the way through with no binding unlike the piston.
-Chris

Offline zeeprogrammer

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Re: Tapered Cylinder
« Reply #18 on: January 28, 2016, 01:36:17 AM »
It may be counter-intuitive but bigger depths of cut when boring often lead to a more stable cutting operation. Make the hole first and don't sweat about the size. Then make the piston to suit.
I noticed this with my "real" boring bar. Cuts around 10 thou would cut beautifully, but then if I try to take a finishing cut of say 1 thou it would chatter like crazy and leave all kinds of marks. I would be happy taking cuts of 10 thou if I knew it didnt spring.

That seems to be exactly what I'm seeing.
I've been thinking along the lines of jadge.
Carl (aka Zee) Will sometimes respond to 'hey' but never 'hey you'.
"To work. To work."
Zee-Another Thread Trasher.

Offline PStechPaul

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Re: Tapered Cylinder
« Reply #19 on: January 28, 2016, 03:14:09 AM »
I saw that the bore is about 0.001" larger close to the chuck. The boring bar cutting edge point has more leverage away from the point of support and if there is any play in the spindle bearings, it will flex more at the mouth of the bore than at the inner point deepest in the bore and closest to the support of the chuck. Thus it may cut deeper there. It may help to use a follow rest and position the boring bar on the far side.

Offline Ian S C

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Re: Tapered Cylinder
« Reply #20 on: January 28, 2016, 12:08:22 PM »
With the metal in the chuck, a hollow piece can be distorted by the clamping forces, and if the piece is right into the chuck, are the chuck jaws parallel.
I once had a problem when I made some bronze bushes (I think they were for front forks on a motor bike), I bored them in a 3 jaw chuck, and they came out triangular by about .005".  I see that it got larger toward the chuck;  without support the metal won't cut so deep.  With the piece fully in the 4 jaw you should be OK.
Ian S C
« Last Edit: January 28, 2016, 12:14:18 PM by Ian S C »

Offline Nerdz

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Re: Tapered Cylinder
« Reply #21 on: January 31, 2016, 03:07:47 AM »
So the saga continues..I chucked in a piece of Cast iron, no reason why, just wanted to give it another shot. Before I did though, I made sure every gib was down tight. Cleaned up the chuck a bit (but I haven't taken it apart). Also made sure the back of the chuck was clean as well. Took a skim cut to get rid of the "yuck" from the casting process. Used power feed on my lathe, went slow at first but then as the cuts got shallower I turned up the speed until nothing was being taken off. Flipped the piece around and repeated the process. Using this process I used a sharpie and found out that sometimes it would only cut appr 66% of the piece. It left around a 3rd that wasn't touched..(meaning there was noticeable surface difference between the 2 surfaces). Hmmm

Put a dial indicator in my boring bar holder. Brought it up, zeroed it out and found the offending jaw. Dont know why I didnt do that in the beginning! Its out by 5-6 thou compared to the other jaws. Tomorrow I'll take apart the chuck and clean it. Maybe I have a piece of metal stuck in one of the jaws..Or Maybe I need a new chuck.

-Chris

Offline Nerdz

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Re: Tapered Cylinder
« Reply #22 on: February 07, 2016, 02:24:50 AM »
Took the whole chuck apart, cleaned it and oiled it. Tested a bar, and I get 4 tho worth of run out. Time for a new chuck! The good news is the runout is consistent all the way down my test piece, so thats good. Looks like I'll be using a 4 jaw for accurate work.
-Chris

Offline Noitoen

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Re: Tapered Cylinder
« Reply #23 on: February 07, 2016, 08:45:49 AM »

Offline Hugh Currin

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Re: Tapered Cylinder
« Reply #24 on: February 09, 2016, 05:29:20 PM »
Took the whole chuck apart, cleaned it and oiled it. Tested a bar, and I get 4 tho worth of run out. Time for a new chuck! The good news is the runout is consistent all the way down my test piece, so thats good. Looks like I'll be using a 4 jaw for accurate work.

If you are to the point of buying a new chuck, you could try grinding the jaws. Scary, at least for me, to do on a "good" chuck but if you're at wits end... Google "truing chuck jaws" or "grinding chuck jaws".

Hugh
Hugh

fcheslop

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Re: Tapered Cylinder
« Reply #25 on: February 09, 2016, 06:38:26 PM »
I would not worry unduly over a few thou run out on a three jaw self centering chuck they all do it to some degree
If the bore is as close as you say then simply either run a reamer down the bore or make a lap as described on an areo IC build Iv seen on here
If you do buy a new chuck chuck youre old one over here as its far better than my chucks
Try and machine it all in one chucking or machine it in such a way it could be fitted to a mandrel or clocked in on an independent four jaw.
If the taper is only a few tenths and is smaller at the top of the bore would it matter as if I right in my thinking its only the upper end you need to seal could well be wrong just my two penneth
cheers