Fits and tolerances

[RBX]

Here it goes..
The other day i was designing two die cast parts that are supposed to slide into another,now i get to the part were i calculate my Fits.
Now i know all about Locational clearence fits,force fits etc.. and that you have to consider the fabrication process quality ie: milling 0.0005".
I also know about the suggested fits in The handbook but there too vague

So my question is as follows all things being equal lets say you have choose to between a range of clearence fits how do you determine which one will give you just the right amount of play without play and be easily deassembled by hand?
Is there Fits gauges so i can feel what an lc6 feels like?

I know my question is very large so any input would be helpful!

 

[KENAT]

Ask one of the old hands whose been around a while and has forgotten more about fits than you learnt at uni?

Obviously not an option if you're by yourself but otherwise a good start, especially as what's appropriate will vary based on function/situation.

In many (most) cases the allowable play is driven by function and this then drives the fit.

While manufacturability is important to allow for, your first consideration is function. Work out what you think you need for function. Only if this is too tight/expensive for easy manufacturing do you need to look at a different way of doing it/reconsidering.

Give more detail on the function of your parts and someone may be able to come up with something more helpfull.

KENAT, probably the least qualified checker you'll ever meet...

 

[BobM3]

You can by go-no/go plugs and precision drill bushings to check different clearance fits. McMaster Carr or MSC (and others) sell them.

 

[drawoh]

RBX,

How accurate do you need to be?

If this were my casting, I would be very interested in a fit that relied on as-cast tolerances.

Does the fit have to be a clearance hole? Could you instead pick up a face, and edge and a point?

JHG

 

[RBX]

Thank you for all the replies

Dimensions for the part is as follows,

A square shaft .250" wide and .375 long must into blind square hole .375" deep.

I applied and LC7 fit clearence 0.004 to 0.0008


Fonction:

Must easily deassembled by hand with little play.

Die cast Tolerances are +-0.019" giving too much play between parts

So im going have to mill the parts after casting

 

[drawoh]

RBX,

How are you going to mill a blind rectangular hole? Either you chamfer your male part to provide room for milling radii, or you are using EDM. EDM is expensive.

If your female hole is through, you can broach it.

In your place, I would be determined to eliminate the secondary process on at least one of the parts. Perhaps machining the male part creates an accurate enough fit for you.

The next thing I would do would be to investigate powder metallurgy. I understand this is an accurate process. Probably, it costs more, but if it eliminates the extra machining step, you could be way ahead.

Yet another approach is to forget about the casting, and machine the whole part. If the diecasting cannot do away with machining, perhaps CNC machining can do away with the diecasting process.

JHG

 

[tripleZ]

Powder metal is an option. It just depends on the material and your functional requirements. 0.0008" - 0.004" should be attainable if the geometry is simply.

If you post a pic and spec a material, some may be able to give you some ideas.

I'll second the EDM comment about it being very expensive in a production environment.