Need to Delcare Chamfer on Screw Clearance Hole? 3

[grailo]

Quick Question:
When working with NX 6 I noticed that when creating a screw clearance hole it put a small chamfer on both sides of the hole. For a #10 hole the chamfer was 0.015in by 45 deg. When I create the drawings to get this part fabricated will I have to declare this small chamfer or is it assumed by just the deceleration of the hole on the drawing?

Thanks!

 

[KENAT]

I can't recal if you get the lead in by default if the thread is called up properly. Check out ASME B1.1, or if you don't have it I'll try and make time to look.

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

http://eng-tips.com/market.cfm?

What is Engineering anyway: faq1088-1484

 

[KENAT]

I didnt' see anything in B1.1.

Y14.6 shows how to dimension it but not explicitly how big it should be.

http://www.pmpa.org/technology/design/threadrelief.htm

I believe the chamfer will generally be put in to help with the thread forming process, at least if tapping.

We don't explicitly call it out on our drawings, if you really need to control it though then it should be on the drawing.

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

http://eng-tips.com/market.cfm?

What is Engineering anyway: faq1088-1484

 

[CoryPad]

grailo,

You will need to specify any chamfers that you want.

KENAT,

grailo is talking about clearance holes, not a threaded hole. Clearance holes get chamfers to provide clearance for the underhead fillet on screws. Tapped holes get chamfers to avoid having a sharp first thread.

 

[KENAT]

Doh, yeah my bad, should have re-read the question.

Cory's right, you'll need to specify it if you want it. ASME Y14.5M-1994 1.8.11 & 1.8.12 detail how to spec it. Essentially, in the order hole, c'bore, csk. My machineriy's handbook has table 5 from ANSI/ASME B18.3-1998 which gives the sizes of CSK needed for common Socket Head Cap Screw sizes.

Not sure how I got a star when not answering the question asked.

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

http://eng-tips.com/market.cfm?

What is Engineering anyway: faq1088-1484

 

[PeterStock]

If you have a note calling for a break edge unless otherwise specified, the chamfer on a clearance hole may not need to be called out, if the default break edge is large enough for the screw head fillet.

Peter Stockhausen
Senior Design Analyst (Checker)
Infotech Aerospace Services

 

[SDETERS]

I do not like that Break Edge Note. How do you inspect the break edges? Do both sides really need to have a chamfer for the clearance hole? This makes the machinist setup up on that hole twice. Once to drill the hole and chamfer. Then unclamp the part, then clamp it again, find the holes center and add the other chamfer. I would only add the chamfer on the side that mates with the screw head. Unless the part can be installed either way I would only include the chamfer on one side.

 

[fcsuper]

I detect thread hijacking. Break edge is a whole other topic, so to discuss that, I suggest starting a new thread.

anyway, from my experience, unless you are using very very tight clearance holes, there should be no need for the edge of the hole to be affected unless you are using a particular type of bolt, machine screw, or screw that requires it.

I would also say that it is not automatically added. There might be a setting in NX where you can disable its inclusion.

Matt Lorono
CAD Engineer/ECN Analyst
Silicon Valley, CA
Lorono's SolidWorks Resources
Co-moderator of Solidworks Yahoo! Group
and Mechnical.Engineering Yahoo! Group

 

[SDETERS]

SOrry Bout the possible thread Hijacking My bad. If you do not think the chamfers are needed take them out.

 

[ewh]

There is an option for this in NX6 which can be turned off, which is what I have done. On non-critical features, it is as annoying as someone modeling all of the edge breaks.

"Good to know you got shoes to wear when you find the floor." - [small]Robert Hunter[/small]

 

[KENAT]

Another star? I don't understand you people sometimes;-).

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

http://eng-tips.com/market.cfm?

What is Engineering anyway: faq1088-1484

 

[CoryPad]

fcsuper,

Using a chamfer is helpful for any size clearance hole. For example, large clearance holes usually are there because the mating parts can have variable positioning, which means the screw may be on the outer edge of the hole rather than the center. The chamfer prevents the screw underhead fillet from being damaged by tightening against an unchamfered hole.

 

[EdMcCabe]

per asme b18.3

"it is considered good practice to countersink or break the edges of holes which are smaller than (D Max + 2F Max)in parts having a hardness which approaches, equals or exceeds the screw hardness. If such holes are not countersunk, the heads of the screw may not seat properly or the sharp edges on holes may deform the fillets on screws thereby making them susceptible to fatigue in applications involving dynamic loading." where D=Bolt MMC (i.e.: .164 for #8) and F= the maximum fillet of the screw head (i.e. .012 for #8). Therefore the countersink dia. for a #8 screw is .188 minimum. PS not sure what OP means by deceleration.

 

[MikeHalloran]

One of the valuable design lessons that I learned in my first encounter with a union shop, also applies to ISO-xxxx shops:

"If it's not on the print, it had damn well better not be on the part."

Okay, I paraphrased a little, but you get the point.



Mike Halloran
Pembroke Pines, FL, USA

 

[fcsuper]

Ed, that sounds like a hella specific reference to a general question. However, it one is using 304SST and "18-8" screws in that scenario, it would apply. Usually, I'm just bolting 6061 stuff together with SST screws.

Matt Lorono
CAD Engineer/ECN Analyst
Silicon Valley, CA
Lorono's SolidWorks Resources
Co-moderator of Solidworks Yahoo! Group
and Mechnical.Engineering Yahoo! Group

 

[ctopher]

I have seen drawings without chamfers shown, but the part has it because the machine shop added them based on their tools. The part is rejected.
If you need the chamfers, or are added because of a machining process, call them out on the drawing.

Chris
SolidWorks 09, CATIA V5
ctopher's home (updated Aug 5, 2008)
SolidWorks Legion

 

[EdMcCabe]

Matt that's true it goes a little beyond the OP's post. For what it's worth, I agree that one should only call out the chamfer if/when it is needed, and even then rarely on both sides of the hole. Just trying to provide a little extra info. to those that might not know this rule and/or realize why it is used. Also, at our company we design rather intricately machined aluminum housing and stainless steel fittings, and sometimes the chamfer on a hole or a tapped hole isn't for clearance with a mating screw or pin, but rather to provide a flat surface to the mating part, as drilling or tapping can sometimes raise the material around the perimeter of a hole.