Internally Stepped Shaft

[TexasRotorHead]

How difficult is it to machine a shaft with internal steps?

Some info about proposed shaft:

OD: 70mm
ID (starting from bottom): 54mm, 58mm, 62mm
Length: ~1m
Material: 440C (HRC 60 necessary)

The steps all increase in diameter, so it could theoretically be bored. The internal dimensions are needed to be machined to .002-.005 inch tolerances as they will act as a bearing race.

Any estimates on how much said shaft would cost to machine/produce?

Thanks Alot

 

[GregLocock]

Is it a blind ended hole or a 54 ID end? What are the details of the shoulders that form each step?





Cheers

Greg Locock

I rarely exceed 1.79 x 10^12 furlongs per fortnight

 

[desertfox]

hi TexasRotorHead

Why not produce a drawing and send to machine shops for quotation.

desertfox

 

[TexasRotorHead]

Here is a simplified drawing. This also includes the exterior features. I am not worried about these right now.

Machine shops are notoriously slow on quotes, so before I start asking for quotes, I want to see where I am ballpark wise. Basically if I should proceed or find a new design.

Thanks for your input.

 

[MikeHalloran]

So far, it looks producible, but the drawing is incomplete.

First, .002-.005 is not tight enough for a bearing seat, let alone a race. You might be able to press in races and use a softer shaft.

To get a quote, you need tolerances, radii, and probably finishes. Be sure to specify lead-in chamfers where you need them for assembly, and larger bores with looser tolerances in the areas where there's no mating part.





Mike Halloran
Pembroke Pines, FL, USA

 

[GregLocock]

Nice of you to tell the shaft where to break.

I'm sure someone will quote to make it, personally I'd redesign it to use an off the shelf tube size and a shrink fit or loctited insert to carry the bearings on the smaller IDs.





Cheers

Greg Locock

I rarely exceed 1.79 x 10^12 furlongs per fortnight

 

[desertfox]

hi TexasRotorHead

I possibly see a potential problem with the 62 dia bore, if your shaft length is a metre long it appears that the 62 dia bore is 800mm - 900mm long which might prove difficult to machine.
You give no tolerances except 0.002"- 0.005" which I assume is not required for the bearing race but thats not how your post reads.
I don't know what foces your shaft see's in service but I would consider a two part shaft where all the important machining was contained in one part.
Locating the two parts of the shaft together would depend on the concentricity tolerances you require between the 62,58,54 diameter's ie welding which will probably cause distortion or an interference fit.

regards

desertfox

 

[williedawg]

are the bores for bushings rather than bearings?

the 60Rc would be the killer, and as others have mentioned, 31+ inches....that's a deep hole to bore regardless of tolerances

this for a helicopter main rotor shaft ?
I'd look at another design if possible

 

[TexasRotorHead]

You guys are right, I was mistaken on the bearing tolerance (units are killing me right now).

I think I have found a design around the internal steps. I appreciate your help.

 

[Cockroach]

Yeah, I think you will be challenged at the manufacturing level on this one, TexasRotorHead. I can even hear the odd machinist screaming!

You're talking a narrow bore diameter (54 - 62 mm) with significant reach (1.0 m). There is also a hardness issue, HRc 60. A few things come to mind:

1) Narrow bore means lack of chip flow due to choking. You will need to go slowly on this and provide a means for coolant flow from the ID of the piece.

2) Length of reach imparts cantilever of the tooling. To counteract this you will need to use a heavy boring bar set up, one with devibration. These are usually quite heavy, but you are restricted by ID. Big problem. So your cuts are going to be very light. Note also the issue of cantilevering will impart tolerance errors. I hope you leave a generous range of possible ID for the design to work.

3) Hardness of the material will mean tool wear, possible insert breakage on a frequent basis. Again, buddy will need to take light and long cuts in order to hit the required tolerances. This also blends into the chip flow and cantilever argument above.

I would say this would be a tough machining problem. You are going to get hit with manufacturing costs, typically $US150.00/hours for the machine, $US60.00 machine shop time plus manufacturing setup and costing for tooling required to complete the job. These add up and are in addition to material cost and tertiary manufacturing processes required as specified by the print.

In agreement with DesertFox, send this out to at least three shops and take into consideration the quality of work and capacity to do a decent job. The cheapest isn't the best, I found it handy to heavily consider the top end cost ones as where the project will reasonably land.

Good luck with it.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada