Best way to get a precise sliding fit in a home shop with just a decent lathe
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    Lightbulb Best way to get a precise sliding fit in a home shop with just a decent lathe

    Hi all. Little challenge for you today - I have a rescued and rebuilt CVA (think UK made version of 10EE, but simpler mechanical drive) which I may have praised rather too highly in front of friends. Anyway, one has kindly given me a challenge since I have such a splendid machine (!) of designing and making a quill shaft and housing for him for a machine that hasn't presently got one (think tailstock but not). I intend to fabricate the body of the housing and mount it first on my mill to skim the base flat after welding, then probably mount it onto the cross-slide of the CVA to bore the housing for a bush, take the housing off again, make the bush (with undersize bore) on the lathe, press the bush in place, then mount back onto the cross-slide for finish boring. My question is - is it better to turn the quill before final boring and then bore to suit the (finished)quill size, or bore the housing bush first and finish turn the quill to suit? Since he wants the best possible sliding fit are there any other techniques or tips to improve accuracy/reduce play, that can be used in a home shop environment? (without going to the expense of sending out for grinding / honing etc). I have heard of heli-laps (helically split long cast iron lapping tubes that are expanded with a tapered mandrel) and understand them to produce very accurate bores, and possible to use at home, but what could be done to true up an OD?
    Many thanks, Phil

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    If you're going to fabricate the housing from weldments, as your post implies, be sure to stick the whole thing in a oven and stress relieve it prior to any machining. If you are after a accurate end product this is mandatory...IMHO.

    Stuart

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    I usually turn something as close as I dare, then finish with a file and/or emery paper. You can correct taper, finish, and size issues this way. For a bore, you can lap it or hone it, but sometimes it's hard to measure accurately, especially the whole length. For me, I'd get the bore close to where I wanted it, then machine the quill to fit.

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    Thanks Stuart, with you on that - I did not want to go thro' the whole design in the post - it could have expanded into an entire treatise so easily!!

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    Thanks MushCreek, I have had some experience with honing which is only really a slightly better controlled process than emery, and tends to leave any out-of roundness errors. I'm going to wait for lots of comments and go with the concensus anyway. No need to rush. Phil

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    Honing really doesn't leave out or round errors if done right, it is the best way to get a straight, round hole, and to a specified size with a great finish to spec.

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    Thanks greggv, not sure the entire community will agree that its the best way! and I've certainly had problems in the past with o-o-R not being taken out entirely.

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    Brake hone or Sunnen hone ?

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    Sounds like the female portion would be harder to remake if it gets off size. So I would get the female portion bored ground or what ever and the male part made to fit.
    That way if you screw up the easier/cheaper to make male portion only would have to be remade.
    Bil lD

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    Thanks diggerdoug, I'm not sure of the machine manufacturer, but professional outfit claimed it could straighten and correct oval bores on alloy crankcases, but turned out it couldn't. I've used Delapina (UK honing machine company) kit and had better success with their horizontal hones, but not perfect. Automotive "cylinder bore" hones are generally junk. I also once borrowed a helilap lapping tool and used it in the Delapina machine to great effect. I liked that one. Phil

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    Good point Bill D, but I'm probably going to get the male hardened so it will probably end up more expensive than re-making the sleeve bush ...... Phil

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    Assuming you will not be able to measure perfectly one needs dead nuts repeatability on setup.

    Where we are going is one will need to use the parts for the final fit.

    So one will need to remove from machine to test fit then replace in machine.

    Between centers is the only way.

    If the line boring bar is between centers it could be removed then fit tested without moving part but the cut may be altered.

    Having a second lathe available is helpful as one needs to insure the bore is to size and straight.

    A second lathe can be used to make a plug that can be used to slide through the bore to insure no hangups and verify size and shape.

    The spindle can be made between centers to allow it to be returned to machine.

    It also allows a "test part" to be made.

    Just like the above plug a second chunk of material for spindle can be prepared and when close light cuts only and repeat same cut on both parts but only cut second after first is tested.

    That way if too much is removed the other one is good.

    Sent from my SAMSUNG-SGH-I337Z using Tapatalk

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    Quote Originally Posted by avturphil View Post
    Thanks greggv, not sure the entire community will agree that its the best way! and I've certainly had problems in the past with o-o-R not being taken out entirely.
    Not trying to start an argument, but I don't think the problems you've had in the past are inherent to the honing process. Honing can produce super accurate results, excellent roundness with great diameter control and very good finish. However great results can be difficult to achieve if you don't have the proper equipment or technique. There's definitely a learning curve, and it's easy to make a lot of junk at the beginning of that curve!

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    I don't believe professionals would be using files and emery cloth etc. The process would be to turn the quill to size. If adequate surface finish could not be obtained on a lathe then have it cylindrically ground. The bore would be made second. It would be bored on a lathe or boring mill to a few thou over size and then honed to final size by hand fitting the ground spindle if necessary.

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    +1 for using judicious amounts of sandpaper on a split wood dowel.

    The difference between a loose bearing fit and a snug fit is very little at all: I've just been boring some timing pulleys to 8mm and 18mm as well as a 26mm bearing into aluminium. The bores measured bang on to size with my primitive measuring gear but were too tight for the various shafts and bearing. Both pulley bores had a couple of spring passes of the boring bar, then a less than 30 second hit of 320 grit sandpaper and were a nice snug fit to the shaft.

    It's as if the machined surface finish was rough and the small amount removed smoothed the finish and gave a good fit.

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    Though I love my Sunnen hone, you can get gage pin and ring accuracy with nothing more than traditional honing/lapping processes, and you can do it in the comfort of your own home. The tools rely on being quite rigid, nothing like a brake hone. You can buy or make them, but IMO cast iron is best. You can get a good idea of how they work here- American Lap Company It is nothing like using emery paper.

    If I had the right tooling for the Sunnen, I'd hone something for a friend no problem. Just takes a few minutes. Note that they also make very good (but obscure and expensive) external tools. Using the traditional methods, I'd probably pass unless that friend was rich and generous.

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    dimensions of the contraption? attempted precision (how much can shaft move in bore)? material of sleeve? what kind of use will it have to withstand? consider a bearing ball for final sizing. also cold plating the shaft (nickel).

    as has been said, 1 micron makes a difference in fit. i would tend to buy the shaft, e.g. morse taper extension.

    edit: you can cast the sleeve with "moglice" that you make yourself. do you have a dro? because without, i dont see this happening.

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    Avturphil you sprang a new one on me with your "Heli-lap", and I found a company in the US that sells a variation on it.
    We make some parts that require what we consider to be very close fits, and the requirement was for roller burnishing and air gaging. Our part material is a soft non ferrous metal so it may not work as well with steel. Roller burnishing simply roll forms the peaks of the tool pathways down on a very microscopic level, you can go from just the top 1/3 of the ridge tops to almost mirror finish in the entire surface, and it simultaneously work hardens the metal.

    I mention that just for comparison, as I have no idea what helical laps cost but I can see how a helical lap would be far more accurate than just lapping compound on a brass split lap.

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    Make the barrel and the housing and leave a few thousands .003 to .005” and take it to an automotive machine shop where they can grind the shaft in a crankshaft grinder. They can then hone the barrel in a sunnen conrod hone to match the two perfectly. It will not cost you too much as for them this will be a simple job.
    Guv


    Sent from my iPad using Tapatalk

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    Are you using oilite bronze bushings? If so ignore this. Most valve lapping compound is too course. Get some diamond compound and lap the bore. You can end up with a mirror finish and hold .0001". Lap can be made from any soft steel like 12L10. On shafts I've used 1000 then 2000 grit sandpaper to get final size.


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