Tolerance on a conical cylinder

[magnusrm]

Hi. Are the tolerances different when making a conical cylinder in comparison to making a right (Straight) cylinder on a CNC lathe?

thank you.

 

[MintJulep]

Since you have asked in a drafting fora I'll assume that you are responsible for design of the part.

Tolerances are dictated by function.

You, as the designer, are the only one who knows what tolerance is required for your part. It's your job to specify the necessary tolerances on the drawing.

It becomes manufacturing's responsibility to produce parts within those tolerances.

 

[magnusrm]

yes, but i need to know if it is possible to do. I cant ask them to make something that is impossible... this is early state, so im asking for tips here rather than bothering the operators (dont know who i have to use yet). Should i post in another forum instead?

 

[MintJulep]

Alright, so you are interested in process capability, not tolerance.

If you want to know if "something" is possible you need to define what "something" is first.

Comparing an unknown conical something to an unknown cylindrical something isn't a useful exercise.

 

[powerhound]

Magnus,

There are some good examples of how to tolerance a cone in the ASME Y14.5 standards. Both the 1994 and the 2009 versions have them. To answer your question as I understand it: No, they are not toleranced the same. Cylinders require a single size tolerance on the diameter and maybe a form control. Cones need at least two dimensions but they be a combination of two diameters and a distance between them, or one diameter, an angle, and a length.

Powerhound, GDTP S-0731
Engineering Technician
Inventor 2010
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[CheckerHater]

Are the tolerances different when making a conical cylinder in comparison to making a right (Straight) cylinder on a CNC lathe?

Generally, no.
In fact sometimes you have to program a cone to produce cylinder.

The difference will show up if the angle of the cone is so big, it looks more like a flat face than a cylinder. Also variations in RPMs and feed will affect the accuracy, not to mention using different materials.

And don’t forget: measuring cone is different from measuring cylinder, so even if they are same accuracy it may be difficult to prove.

 

[magnusrm]

thank you powerhound and CheckerHater.

The dimensions is about 8mm to 18mm in diameter over 250mm. I need about 0,01mm tolerances, this is most important at the end with the largest diameter. The material isnt defined yet, but it'll be some kind of hight strength stainless steel. Is this doable?

 

[powerhound]

Do you have access to an ASME Y14.5 sstandard?

Powerhound, GDTP S-0731
Engineering Technician
Inventor 2010
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[MintJulep]

Nope, not possible. Not on a lathe.

 

[MintJulep]

Might be possible on a Swiss-type lathe.

 

[powerhound]

MintJulep,

Why are you saying it's not possible on a lathe? You then say it might be possible on a Swiss. Are you saying that this is just an impossible tolerance to hold unless you find the right Swiss machine to do it on?

Powerhound, GDTP S-0731
Engineering Technician
Inventor 2010
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[MintJulep]

Aspect ratio. 250mm long and 18mm diameter at the thickest, tapering to 8 mm.

No way that stays straight on a conventional lathe during machining.

 

[powerhound]

Clearly the straightness won't be perfect but you really don't think it could hold 0.01mm? Are you even thinking about using a center?

So what is the smallest tolerance you think a conventional lathe could hold?

Powerhound, GDTP S-0731
Engineering Technician
Inventor 2010
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[juergenwt]

Not possible by boring in a lathe. If you have someone who can make a specials reamer - may be, and that is very optimistic. Need to know more details on this project.

 

[magnusrm]

Im just a student, so i clearly dont have the experience, but i cant see why it can be pretty good if you use a center guide and take very small cuts? Making a conic in comperison with a straight cylinder you can have deviations in the Z axis to add to the X-deviations. So you are saying that it will be bended by the cutting force or the angle wont be constant because of the deviations in the Z-axis?

 

[MintJulep]

The cutting force will almost certainly be enough to cause deflection greater than 0.01mm.

 

[magnusrm]

but the cutting force is relative to the cut, so if i just take very small cuts?
What if i up the dimensions to 25mm - 20mm at the same length?

 

[MintJulep]

Try it and see.

 

[MechNorth]

I had people hard-turning conical tapers on hardened steels using manual & CNC machines to within (0.0025mm surface profile). Max length that I used would have been in the 250mm area. That does not mean that just ANY shop can achieve these tolerances; it was a specialty of our business (mold making). Nevertheless, it is achievable.

Jim Sykes, P.Eng, GDTP-S
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[MintJulep]

MechNorth,

On 18 mm diameter stock?