Turning an Accurate Diameter

[Antman]

Hi guys and Happy Father’s Day to you Dads,
Thanx for your patience and answers. Again I haven’t been getting much workshop time, we had some refugees from Babylon here again for the weekend. I think it must be centifrugal force that throws people our way, I think that’s how we got here. First thing, I planted some fruit trees, gives me something to hold on to when my World starts slippping towards Babylon. Only my pear trees are doing really well. What do they want me to do, cruise the bars with them?
This seems like such a novice question but I’ve completed a couple of stages of my rotab axle, with difficulty and I don’t want to mess it up on the next stage. Our local Small Town Harware Shop and NOS stockist closed down and at the halfprice sale I bought some random size taper roller bearings cheap. I know it’s probably overkill for a rotab but I wanted to use one for the lessons learned. A 32005 so I know its internal diameter is 25mm, is that 25.000mm better than any standard I could ever measure to? So now I have a piece of 50mm stock in the chuck with a face and an OD that’s a good fit for the table with a small taper in the centre opening out to a small 60deg area for the tailstock behind which I must turn down to fit the inner race. I can’t use trial and error for a fit which is how I got the fit for the outer race (just a shade too big). I’ve decided to recess on both sides of the shaft where I must turn to dimension to minimize taper. What must that diameter be, 24.99mm? My best measuring tools are a Chee Po 25mm mike and a Mitutoyo 1” mike with a 4th place on the barell but no clutch thimble. I also have a 25mm micrometer setting ring, if that could be any use.
The rest of the crew has gone out for Sunday shopping so snatching some thread time … no chance I’ll be machining until tomorrow TGIM if you ‘re retired.
Cheers,
Ant

 

[gbritnell]

Hi Ant,
I'd really like to help you sir but I'm not quite sure of your explanation. A quick sketch would certainly help out unless someone else has a better idea of what you want.
gbritnell

 

[Antman]

Thanks George, I get a huge kick when one of the masters deigns to reply to my naive questions. Unfortunately a sketch is out of the question, a daughter hijacked the scanner on her way to university. Anyway I've got a 50mm diameter piece of stock in the chuck, the tailstock end of which is finished 49mm to fit in a recess in the rotab table, so I can't do a trial fit before I part off, I must hit that diameter in the next turning process. The inner race has a nominal ID of 25mm. The taper roller bearing I am using is off the shelf with a part number of "32005" and I assume the dimensions are coded into that number. So if the nominal ID is 25mm does that mean 25.000mm and what diameter must I aim for in tuning the shaft to fit? Would that be like 24.99mm or bigger or smaller? Is it acceptable practice to set a micrometer, lock it and use as a Go/No Go gauge?
Thanx again,
Ant

 

[steamer]

+.01 mm / -.000

or

+.0003" / -0.00000

is the size tolerance on the bore for a 32005 bearing.

Dave

 

[Antman]

Thanks Dave, love your longboat. When I was a kid, one of my friends dissappeared, then turned up for school every day. When he surfaced best part of a year later he had a speedboat with a 2tone plank deck he got from PM plans, advanced for its day, hull design.

I've suddenly lost all my confidence. If I eff it up now I'll probably be so disgusted I'll give up. The first machining phase, I found really difficult. I think I need a complete recap of everything I need to know to finish this part of the axle to 24.99mm . Tool? Speed? Feed? How to measure the thing using my best mike and times 25.4 ?

 

[BillC]

Ant, You just need to get yourself psyched up to do your job and just do it...Begin by getting accustomed to making your measurements.

The old tried and true way to bring a shaft to size is to leave about .002 over and use emery cloth to put it on size....

BillC

 

[Antman]

Billy, Until I know how to strip the saddle I will never use abrasives in my lathe, CI maybe, reluctantly.

 

[BillC]

Hey Antie,

A good grade of emery cloth will not do your lathe any harm. You're correct that some of the cheap stuff will allow the abrasive to strip from the cloth.

Since you are reluctant to us emery cloth, use carborundum with some light oil like WD40 - cuts fine too.

Sounds like you know plenty about this sizing operation already! ;D

Billy?

 

[Antman]

BillC? Or Anyone, Help! Whew I'm glad that's off my chest. I guess the nub of my problem is getting a decent finish last time.

 

[Admiral_dk]

Put a piece of alufoil over the lathe beds, to catch the abrasives.

 

[gbritnell]

Hi Ant,
The micrometer shouldn't be used as a go-no go gauge. The reason being if the cut is too small then when you put the mike over it, it's too late to do anything about it. Use the mike as it was intended. Sneak up on your dimension. I don't know how well and accurately your lathe cuts in a single finish pass. If for example you need to take off .10 mm (.0039 inches) and you adjust your crossfeed to that amount and it cuts to what you wanted then you know what you are working with but if it doesn't than you will know by your reading how to 'adjust' to get what you want. Ideally you want to cut the stock to what you need but sometimes trying to get .0127mm (.0005 in) is hard to do. The preferred method is to cut it as close as you can (large) and then using a good smooth mill file, file the last little bit off. The only time I use emery is if I want to polish the surface, not to try and 'hit' size.
As far as what to cut to it depends on the type of fit you are trying to achieve, slip, light press or full press. In working with bearings they are ground to size with some extremely accurate grinding machines and if the dimension says 25mm then that is what it is, so make your part accordingly.
I would guess that you are trying for a good snug slip fit so for a dimension of 25mm I would try for -.0127mm (-.0005)
I hope this helps. Oh and by the way, confidence is just knowing what your tools will do and trust yourself.
gbritnell

 

[Antman]

Sorry for not accepting that advice but until I think I know what I'm doing I won't be using abrasives in the lathe. For this piece at least I will get it done or totally stuff it up using HSS and coolant. I have a lot of practice cuts before I get to final dimension.
Ant

 

[steamer]

Ant,

Start by trying to hit a dimension LARGER than the target size. Say .050" bigger.... EXACTLY.

Keep track of how you do this, how the lathe behaves, ect.

Use it as a practice run.

Now when you get down to the "nitty gritty" you will know how your lathe behaves, how you mikes read, and how you behave.

I have done this a thousand times...it works and builds confidence.

Dave

 

[Antman]

George, your erudite replies are much appreciated. I shall have to read and reread that. In my experience (maybe lucky) the most talented, most knowledgable and most experienced people are the nicest most helpful of all of us.

 

[Antman]

OK Dave as long as I don't try to fit the setting disc into the race I'll be fine.

 

[Ken I]

Ant, as George & Steamer say - sneak up on it - make your approach cuts of a depth that gives a good finish and make each one count - ie try to hit the next step spot on - this will clue you in to the vargaries of your machine / material / cutter as you approach final size.
Also calibrate your micrometer against the 25mm (or 1") setting piece that came with it - even if you don't adjust it you will at least know how much high or low it reads as well as your "feel" for closing the mic.

Regards,
Ken

 

[Antman]

Ken, You know, I think you have given me exactly what I need now ... how to use a micrometer. Thanks man.