pos tol and counter bores / sinks?

[ozzkoz]

Hi,
I read a thread on this forum regarding holding the pos. to. of a counter sink relative to the through hole and have checked out the ASME standard for how to apply the pos. tol but I still have a question or two which I m hoping someone can clarify.

I believe a counterbore should have a countersink such that a cap screw with a max fillet radius bellow the head will sit flush with the bottom of the counterbore. I've set up a note which looks something like this:

ctrbore_symb diam down depth
ctrsnk_symb diam 60deg
pos_tol @MMC relative to thru hole @LMC

my question is this:
does the FCF apply to both the counter bore and counter sink?

Does it make sense that I use LMC on the datum? The ASME standard shows the datum at MMC but the counter bore has to be large enough so that when the screw is line to line with the thru hole there is no interference between the head and counterbore. The min clearance would then be when the thru hole is at LMC, and bonus tolerance can be allowed for as the hole gets smaller.

Thanks for helping me clarify this.

 

[ctopher]

One problem I have seen with this is which surface does the c'sink lay on? The 60 degree angle is from which side?
If you make one FCF and point to or indicate both c'sink and c'bore, it refers to both. If you want separate, separate the FCF for both. Personally I only use the FCF for the hole and c'bore together, not the c'cink.

This was discussed here before: thread1103-197198

Chris
SolidWorks 09 SP4.1
ctopher's home
SolidWorks Legion

 

[ozzkoz]

The call out should indicate a 60 degree included angle. to a specified diameter. I think that since the ctrsnk dia is smaller then the ctrbr diam it should be understood that the ctrsink is at the bottom of the cntrbr.

The way I have seen it in the past is all in one leader, one line specifies counterbore, next gives the countersink, and the last gives the pos tol. I think this means the pos tol applies to both sink and bore, but I am not certain.

 

[ctopher]

Nothing should be assumed that everything is understood on a drawing. Be specific of what you want.
One leader with multiple FCFs indicate the tol applied to the dims it references, no order of how the features are machined with the order of FCFs.

Chris
SolidWorks 09 SP4.1
ctopher's home
SolidWorks Legion

 

[CoryPad]

Our standard practice is to the format you proposed minus the LMC callout.

 

[dgallup]

LMC is when the hole is at it's biggest value. I don't think that is what you want.

 

[MechNorth]

The 60-deg is an included angle. The countersink must appear on the final machined feature, and because it has a reduced diameter, it must be at the bottom of the c'bore. As Ctopher pointed out, one FCF at the end of the entire callout means it applies to all features within the callout; if you want separate, then you need to put a separate FCF under each of the sub-feature (c'bore, c'sink) callouts.

In general though, it bothers me to see people control the c'bore & c'sink to the local thru-hole as an INDIVIDUAL datum feature. It is more costly for inspection as it now requires a new datum simulation at EVERY hole. Unless there is some EXTREMELY important relationship between the c'bore, c'sink & thru-hole, don't do it that way. Plus, don't forget that the majority of c'bore installations are fabricted using a single tool now, so the thru hole, c'bore & c'sink are all made using the same tool in a single pass.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[CoryPad]

Jim,

I don't think I follow your "individual datum feature" comment - could you comment further, maybe suggest a drawing note. Thanks.

 

[MechNorth]

Fig. 5-37 ('94) shows the same tolerance for the pilot & c'bore.

Fig. 5-38 ('94) shows different position tolerances for the pilot & c'bore.

Fig. 5-39 ('94) shows the situation where each c'bore is located wrt that local pilot hole. That local pilot hole is labeled as an 8X INDIVIDUAL datum features C. The c'bore at each location is located wrt the local individual datum C.

The extension for the countersink would follow the same principal.

I am curious, though, why a countersink would be specified rather than a general chamfer note.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[NormCrawford]

Thanks Jim. I was reading along and wondering; "Why not just specifiy a good ole fashioned chamfer?"

Norm Crawford
GDTP-S
Applied Geometrics, Inc.

http://www.GDandT.com

 

[desertfox]

hi ozzkoz

I was under the impression that the hole clearence for the shank of the screw was large enough to miss the fillet on the underside of the head or I am wrong I have never seen any csk's at the bottom of a c/bore.

desertfox

 

[NormCrawford]

The hole clearance could be large enough to clear the radiu of the bolt or screw, but then again it depends on the design.
The other common alternative is to use a shoulder bolt. You know, undercut shank at the head of the bolt to avoid this problem and reduce the cost of having to machine the chamfer in the part in the first place.

Norm Crawford
GDTP-S
Applied Geometrics, Inc.

http://www.GDandT.com

 

[ozzkoz]

wow, lot's of response. Thanks all,

- Yes I could use a regular chamfer callout, but I think they are the same thing so I believe it is just style, or just specify to break the edge to a radius greater then the fillet.

- I could make the thru hole large enough to clear the fillet, this is simple, but also adds to slop in the assembly,

- I could just use a washer, but it would be easy for an assembler not to install the washer and being at the bottom of the bore easy for an inspector to miss.

- as far as controlling position to the each hole individually, I think this would be needed when the thru hole is positioned with a primary and secondary datum, but no 3rd datum for clocking, else I don't see what prevents the counterbore from being in the 1 o'clock position and hole at noon,

-lastly I think LMC is optimal because when you insert the fastener into the thru hole the closest the head can be to the ctrbr is at LMC so this is where you need to control the pos of the ctrtbr closest. As the thru hole tightens up the faster can not float as much and therefor the ctrbr can wander a little more - aka bonus tol.

 

[fsincox]

"as far as controlling position to the each hole individually...."
ozzkoz,
I may be misunderstanding your point. In fig 5.39 these are two diameters shown to be concentric, one is a datum and the other toleranced directly to it, how does clocking come in?
In fig 5-38 here clocking doesn't come in because of the simultaneous requirement 4.5.12 (1994) they are to be treated as a single pattern of features all good at one time. If your part is not that simple (they rarely are) you may have a tertiary datum on both callouts they still must act as a single pattern do to this requirement.
I like the c'sink to a diameter if you are going to do it, might as well go all the way.
I think LMC/MMC scheme you propose is good, again if you are going to do it,. The actual numbers needed should determine. We are talking generalities here you have give no numbers.
I think what most people here are saying, in essence is, "is it worth it". I have the same type of problem in my philosophy thread. Some seem to think it is a waste of my time to call out a small length pilot perpendicular to the face. “Can’t measure it”, “It will be OK anyway”, “the pilot will bend to give way as required”. Where do I stop? What is that magic number/proportion that says OK now it is important?
This is why we come to this forum.

 

[ozzkoz]

I'll check out the section you refer to about simultaneous features, that might clear up my confusion about the clocking of the counterbore to the thru hole.

I'll also read through the philosophy thread, but my thinking is that you should GD&T wherever it will allow you to guarantee assembly / functionality. After all, if a lot of parts meet drawings, but won't assemble then all that scraped material (or re-work time) is on the engineer for designing a bad part. I'd rather spend a little more money on inspection then have a VP wanting to know why I can't design parts to fit together and just scraped a large order.

 

[fsincox]

I do not disagree and am probabally more extreme. I believe the old methods of engineering drawings are flawed and were shown to be so in the war. This is part of a modern engineering system.