Author Topic: Fixing a tapered bore  (Read 9849 times)

Offline rleete

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Fixing a tapered bore
« on: January 02, 2009, 05:15:04 PM »
Yeah, I know the easy solution is a reamer.  Problem is, I don't have any reamers, yet.   :scratch:

It's only out by the tiniest bit, but enough that the piston will bind at the bottom.  Caused, I'm sure, by the runout of the spindle on my bottom of the line mill.  I'd bore it out a little using the lathe, but I don't have a 4 jaw chuck (bore is offset), nor a boring bar holder.  I'd use the mill, but I don't have a boring head yet, either.  Make a boring bar holder, and offset it in the 3 jaw using shims?

Turn it upside down and redrill?  Bore is through, so I can get at it from both sides.  I want to avoid any mismatch caused by not having it perfectly square when going in from the bottom.

So, not having all the tooling I need, what would you do?
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Offline cedge

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Re: Fixing a tapered bore
« Reply #1 on: January 02, 2009, 08:16:49 PM »
Sounds more like a job for a nice lapping tool to me, but I'll defer to wiser minds.

Steve

bogstandard

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Re: Fixing a tapered bore
« Reply #2 on: January 03, 2009, 06:05:47 AM »
What Steve has said is very sound advice.

The are many ways to get rid of a tapered bore, and some of the purists will have you making or buying all sorts of wierd and wonderful contraptions to get this simple job done. But seeing that you are very limited in resources, I will try to explain the way I personally would go.

You don't say what the cylinder is made of, as that does have a bearing on what the lap should be made of.

You basically use a lap of the the next softest material in the pecking order downwards, so a steel cylinder would use say brass (or bronze), a brass or bronze cylinder, you would use ali, for ali, I have used a nylon rod before now and had great results.

So I have done a sketch at the bottom to show the method I am going to describe.

What you do, is turn a rod about twice the length of your bore, so that it only just fits into the larger end of the tapered bore, and make sure it is the same diameter all the way along.

Next, you file or shape a very slight taper on about 1/4 of the length so that it fits a little way into the smaller end of the bore.

Get yourself a bit of hardwood, plus some grinding medium. For starters, I would use some very fine valve grinding paste, or if you haven't any of that, some liquid metal polish.

Dab a bit of the grinding agent onto the rod, and working around the lap, rub all over with the stick. What you are doing is making a very fine but accurate file to dress out the hole, by embedding the cutting agent into the face of the lap.

Now what you do is turn on your lathe at the slowest speed possible with this lap in the chuck, and gradually feed the small bore end over the lap, and in a slightly twisting motion in and out to 'file' away very gently at the excess metal. Be very careful as the two parts can easily 'gall' together, so you have to be ready all the time to let go of the cylinder if it starts to spin with the lap.
Once you have got all the tapered portion into the smaller end, turn the cylinder around and come in from the large end and repeat the 'filing' process. Whilst doing all this, keep recharging the lap with cutting agent, as it breaks down under use.

Eventually, you will get to a stage where all the straight bit is thru the bore. What I personally would do then is make a new lap, but with no tapered end on it, and using a very fine cutting agent, such as a very fine metal or paint cutting polish (or even toothpaste), reintroduce it into the bore for final lapping and polishing.

You might need to make a new piston after you have staightened out the bore. It all depends on how accurate you want everything to be.

I have most probably forgotten a few things, so if anyone can fill in the gaps, please do so.

Bogs

Offline rleete

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Re: Fixing a tapered bore
« Reply #3 on: January 03, 2009, 12:26:20 PM »
Thanks, guys.

I guess I needed to provide more info.  Cylinder is 6061 aluminum, bore was supposed to be .438" diameter.  Length of bore is about .88" long.  Piston is brass.  Taper was because I hadn't locked down the ways of the mill, and the backlash allowed the cutter to walk around in a circle.  The narrow end was right on, but the upper end was about .006 larger.  Hard to measure holes with calipers.

What I ended up doing was sorta what you described.  I took a piece of sandpaper, and wrapped it around a "mandrel" (actually a fine point Sharpie pen), which had a taper almost custom made for the bore.  Sanded and checked the fit, sanded some more, etc.  Ended up getting it to fit, but slightly out of round.  Oops #2, story of this entire project.

So, I set it back in the mill, being very careful to line up the chuck to the bore.  Then, I used the next larger drill to round out the hole, taking very slow cuts with plenty of oil.  I also locked down the ways, to avoid the taper problem again.  See?  I do learn!  Luckily, I hadn't made the 2nd piston, so no more wasted material.  It's not as nice a fit as I'd like, but I'm getting to the point of saying F-it, just finish the damn thing.

I really need to get some more tooling to avoid all this rework and frustration.
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Offline Bernd

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Re: Fixing a tapered bore
« Reply #4 on: January 03, 2009, 01:14:57 PM »
rleete,

Do you have a faceplate for your lathe? If so you can mount the cyclinder on the faceplate and bore out the cylinder.

Ya, I know. I'm a bit late with that info. :wack:

Bernd
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bogstandard

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Re: Fixing a tapered bore
« Reply #5 on: January 03, 2009, 01:29:09 PM »
Six thou is a little large for what I suggested, two thou would be pushing it.

Lapping is a slow process, with only minute amounts removed at any one time, you would have been there for hours.

John

Offline rleete

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Re: Fixing a tapered bore
« Reply #6 on: January 03, 2009, 01:37:31 PM »
Do you have a faceplate for your lathe?

No.  It's one of the things on the list.  I got the lathe and mill, and some cutting tools, but not much else. 

First thing is a QCTP.  I was given a whole slew of nicely ground HSS bits by an old timer at work, but they're all 3/8.  My lathe takes 5/16, so I can't use them.  BTW, I have a handful of 1/2" HSS lathe bits (also given to me) that I can't forsee using.  Can you use them?
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Offline rleete

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Re: Fixing a tapered bore
« Reply #7 on: January 03, 2009, 01:42:12 PM »
Six thou is a little large for what I suggested, two thou would be pushing it.

Bah.  2 thou I would have lived with, and made the piston to be a sloppy fit.  Even perfectionists like me have a point where they give up and move on. 

Lapping is a slow process, with only minute amounts removed at any one time, you would have been there for hours.

I was anyway.  Sand and check, sand and check.  I have it pretty good right now.  Probably tighter than it needs to be, and I'll have to do some final lapping, but I think it'll work.  Just stopped to check the 'puter for oil groove sizes, then I'll part off the piston.  Funny, I thought I'd have plenty of time to finish this thing, and post pictures.  As it is, I'll be lucky to have it done next weekend.
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Offline Darren

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Re: Fixing a tapered bore
« Reply #8 on: January 03, 2009, 01:52:46 PM »
An adjustable reamer sounds like what you need.

I have one, the type that has an adjusting end screw, but there are other types.
I use it to make bearings for turntables, each turntable may have slightly a different shaft size (nominal manufacturing sizes come into play here).
I start by drilling the bearing, then bore it to my standard undersize and then ream it to an perfect fit to match the shaft in question.

Reamers like this do a wonderful job with a superb finish. One could add a quick lap to finalise but I found this unnecessary in my application and some oil retention was of benefit.

Regards
Darren

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bogstandard

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Re: Fixing a tapered bore
« Reply #9 on: January 03, 2009, 02:13:32 PM »
rleete,

Quote
Bah.  2 thou I would have lived with

I think you got my statement wrong, I wasn't on about tolerances, but how slowly lapping removes material. Depending on the lapping agent, it can take say 1/2 hour to remove 1 thou of material.

When you said at first it was only slightly tapered, I thought maybe a max of 2 thou taper, not not a funnel shape. :D

Bogs

Offline rleete

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Re: Fixing a tapered bore
« Reply #10 on: January 03, 2009, 04:08:49 PM »
Very funny, Bogs.   ::)

I finished the piston.  Damn near line to line fit on both ends, with a slight binding in the center.  A bit of lapping should fix it right up.  Actually, it's probably a little too tight.  It's still got a slightly rough surface on the bore, so it is gonna have to be run in anyway.  I'm pretty satisfied with it.

Thanks for everyone's suggestions. 
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bogstandard

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Re: Fixing a tapered bore
« Reply #11 on: January 03, 2009, 04:36:58 PM »
I think that once you have yourself sorted out with a few techniques, and a little more basic equipment, you will have very little trouble achieving what you want.

I think where new machinists go wrong, they stick to the plans as though they are gospel written in stone, and worry over a few thou here and there. So what, if the bore is a bit bigger, just make a bigger piston to fit, and modify the end caps to fit in the larger cylinder, job done, and only you know about it.

If you make a mistake, you can usually find a way to 'fix' it, and carry on as though nothing had happened.
No one is going to send a bolt of lightening your way, just because it isn't exactly to plans. If it runs, it is right.

Bogs

Offline Bernd

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Re: Fixing a tapered bore
« Reply #12 on: January 04, 2009, 09:41:13 AM »

No.  It's one of the things on the list.  I got the lathe and mill, and some cutting tools, but not much else. 

First thing is a QCTP.  I was given a whole slew of nicely ground HSS bits by an old timer at work, but they're all 3/8.  My lathe takes 5/16, so I can't use them.  BTW, I have a handful of 1/2" HSS lathe bits (also given to me) that I can't forsee using.  Can you use them?

Got an idea for a face plate for your machine. I forgot what size lathe you have. Anyway, if the jaws of the 3 jaw chuck are reverseable you could make yourself a faceplate to fit the 3 jaw and mount your cylinder on that. I'll have to mock up one of my chucks and take a pic. Would be easier to understand.

Those 1/2" tool bits can be used for parallels. So I'd say keep them for that.

Bernd

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