Shaft Tolerance

[Chris GeorgeParticipant @chrisgeorge68271]

Im new to machining parts and was wandering about the correct Dia to machine a shaft to?

I have a shaft that has a bush (20mm ID) pressed onto the end giving it a snug fit. What size do I machine the shaft to to ensure the snug fit but not to split the bush when pressing it into position?

Regards Chris

[Chris GeorgeParticipant @chrisgeorge68271]

[DanielParticipant @daniel]

Hi Chris,

I would suggest a difference between the two items of around 0.02mm.

That is, if the bush is exactly 20mm id, then the shaft should be 20.02.

Or, shaft 20.00 mm od, then the bush 19.98 mm id.

HTH & ATB,

Daniel

 

Edited By Daniel on 14/05/2020 07:05:26

[Phil PParticipant @philp]

Depending on what the application of this part is, you could always turn the shaft to a diameter that is a nice sliding fit in the bush. Then use Loctite or similar to bond the two parts together.

Phil

[David George 1Participant @davidgeorge1]

Hi Chris what material is the shaft and bush and what does it do. Different materials need diferent size tolerance especialy if any heat is involved, can you give a clue as to what use etc.

David

[not done it yetParticipant @notdoneityet]

Rather depends on the shaft and bush materials, along with the operational temperature.

The internet has plenty of info on size tolerances for sliding, push, press and shrink fit. Many materials would be deformed rather than split the bush for anything even like a shrink fitted jobbie.

A loose fit can be filled with a filler such as loctite adhesive of even knurled to increase the shaft diameter. So no real answer unless all the details are exposed for consideration.

[Clive FosterParticipant @clivefoster55965]

Chris

As you may have surmised from previous answers this sort of thing can be a total minefield. Usually not terribly critical for ordinary jobs. Somewhere between doesn't fall off if I shake it and medium size hammer needed to fit will be OK.

This link has a pretty good starter table **LINK** . It says for steel on steel but for anything we home shop types ought to be doing its close enough for any metal. Class IV and V are the ones for what you are considering. It's imperial so will need conversion if using metric units. But its the clearest presentation I found quickly.

In a practical world much to be said for just using loctite, unless the part gets very hot. Tight end of class II or loose end of class III would generally be considered good. Its what I automatically do if using stock shat material and stoc bushes.

Generally its easier to machine the shaft to match the bush.

Clive

[JasonBModerator @jasonb]

As you have given a metric 20mm dimension you may find the metric tolerance system more useful than imperial

[Clive FosterParticipant @clivefoster55965]

Jason, that is a very nice link to explain(?) the labelling system of the metric tolerance system but, like so many links, it doesn't directly give any useful numbers.

I know my way round the system and can figure it out from first principles if need be but frankly most of the time, like everyone else, I just want to know what is too big and what is too small and the difference between them.

The system can be seriously confusing for someone running into it for the first time. Having two ways of doing things Shaft basis and Hole basis really doesn't help the neophyte. I know what is meant but ordinary guy or gal in the home shop should never need to deal with this level of detail unless they become seriously advanced workers.

The calculator links are very handy but simply give a single magic figure for a specific situation which can be very bad practice. A table of values puts things into context. Generally its far more important to know where things sit in the general scheme of things than having exact to umpteen decimal places data.

Clive

[SillyOldDufferModerator @sillyoldduffer]

Posted by Chris George on 14/05/2020 06:53:11:

Im new to machining parts and was wandering about the correct Dia to machine a shaft to?

I have a shaft that has a bush (20mm ID) pressed onto the end giving it a snug fit. What size do I machine the shaft to to ensure the snug fit but not to split the bush when pressing it into position?

Regards Chris

Is this professional or amateur work?

The tolerance tables on the web are for making interchangeable parts as needed in manufacturing. They're complicated! Originally done by having a tool-maker make accurate GO/NO-GO gauges, jigs, fixtures and other gizmos so the shop-floor can get it right repeatedly. Now CNC and other fancy machines and methods, mostly beyond small workshops.

So for most amateur work, we go for the older technique, which is fitting. Rather than make stuff precisely to a specification, instead parts are made close to size and then machined fit together. In effect one part is the gauge. With a shaft and bush, its usually easier to machine the shaft to fit the bush. The disadvantage of fitting is the parts aren't fully interchangeable, which makes mass production expensive because everything needs to be fitted during assembly and again during maintenance. But the method is great for small scale operations, ad-hoc repairs, and any circumstance that doesn't demand interchangeable parts.

Older books are often more relevant in home workshops than modern tables. Tubal Cain's Model Engineer's Handbook (recommended) has a simplified table for Shrink, Force, Drive, Wheel-Keying, Push and Slide fits. For a drive fit, TC suggests adding 0.45 thou per inch of diameter plus 0.3 thou.

Don't trust my maths but for a drive fit into a 20mm hole, I make the shaft 20.01662mm

As that's difficult to measure accurately, (an ordinary micrometer has 0.01mm or 0.001" graduations), I'd lock the micrometer between the 20.01 and 20.02 mm lines and use it as a 'close enough' GO gauge. This approach assumes the bush has an accurate 20mm hole, which it probably does. Another way to do it is to turn a bit of spare rod so it just slides into the bush, then set a divider on it plus the thickness of a thin Rizla cigarette paper (about 0.02mm). The shaft is turned down until the divider just slides over it – the result should be 'close enough'.

Dave

[roy entwistleParticipant @royentwistle24699]

This is the type of thing a 5 year apprenticeship was for

[not done it yetParticipant @notdoneityet]

Posted by roy entwistle on 14/05/2020 11:50:17:

This is the type of thing a 5 year apprenticeship was for

Which bit? Being able to measure inside and outside diameters accurately enough?🙂 Making repeatable parts? Thinking through the problem? Finding the relevant information? Etc, etc.

We have to remember, too, that bush might even change dimension if it is pressed into a hole….

[Paul LousickParticipant @paullousick59116]

All press fit bushes will compress if pushed into a hole for an interference fit and is why the manufacturers specify a toleranced hole to suit.

Machining your own bushes is a different matter but if made to correct engineering standards should be made to the recommended tolerances in the table that Jason referenced. The bodge way out is to make the bore to the required diameter and the OD undersize to the hole and Loctite it into position.

" What size do I machine the shaft to to ensure the snug fit" The shafting diaqmeter is normally the controlling dimension and the bush bore made to suit.

Paul

[JasonBModerator @jasonb]

Posted by Clive Foster on 14/05/2020 10:42:44:

Jason, that is a very nice link to explain(?) the labelling system of the metric tolerance system but, like so many links, it doesn't directly give any useful numbers……………………………….

Yes it explains the different fits so armed with that it is simply a matter of picking up your Zeuz book* and then looking at the charts in that – fit class along the top and size of stock down the side and it will give you the +/- that you need in microns. Whether the OP has the equipment to measure that accurately is another matter,

So about 21 Microns which is basically Daniel's first answer of 0.02mm

J

* This assumes an upto date Zeuz book not an old imperial one.

[Stuart BridgerParticipant @stuartbridger82290]

Its nearly 40 years since I did my apprenticeship and studied ISO standard metric limits and fits. Once you get your head around the concepts, it's not that complex.

Firstly you need to decide whether you are are working on a hole or shaft basis. That determines what is your fixed dimension and what is adjusted to make the appropriate fit.

Most commonly you are looking at hole basis as the hole size is fixed by drill or reamer and you make the shaft to fit the hole.

Then you determine the class of fit you are looking for (clearance, transition or interference), the link already shared explains this well.

Limits and Fits

You just then need to go to your reference tables, e.g Zeus Book or use an online calculator, to get the tolerances you need to work to to achieve the desired fit.

This one works well, just plug in the class of fit, your nominal size and out comes the results

Online Calculator

To help understand the concepts, it is useful to take an example of each fit class, plug them into the calculator and compare the results

Edited By Stuart Bridger on 14/05/2020 13:44:10

Edited By Stuart Bridger on 14/05/2020 13:48:54

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