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Male Plastic Polycarbonate M4 insert wont fit in an M4 hole made on an Aluminum plate.

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JJAV1983

JJAV1983

Mechanical
Nov 7, 2012
35
How do I make sure that both parts are going to fit properly (loosely)? The call out for the insulator thread is: M4 x 0.7 - 6g. The assembler has to re-tap every single aluminium plate to be able to fit both parts.

The call-out for the threaded hole is : M4 x 0.7 - 6H.

Is there a way to indicate in the drawing to make the threaded hole to its maximum major and minor diameters?

Thanks..

JJAV
 
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A proper fit is not a loose one. It is one where both components of the fastening system interface with each other within the limits of their respective specifications.

The Pitch Diameter of the internally threaded hole must be between 3.545mm and 3.663mm. The pitch diameter of the externally threaded component must be between 3.433mm and 3.523mm. You can see by the spec that if the PD of the threaded hole was at its smallest (3.545mm) and the PD of the threaded shaft was at its largest (3.523mm) the parts would still assemble.

Your problem is not in the specification. This is a manufacturing issue not a specification issue. Either the PD of the internal hole is out of spec or someone is chamfering the hole after it has been tapped and is rolling the first thread over, making it difficult to get a threaded part started into the hole. Both of these issues are fixed by chasing the hole with another tap so you'll have to figure out which one it is. Chamfering the hole before tapping solves the second issue I mentioned.

Powerhound, GDTP S-0731
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You are right Taji, thanks for the tip. I am sure that chamfering the hole will help.

JJAV
 
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If that's the problem, it needs to chamfered BEFORE tapping, not after.

Powerhound, GDTP S-0731
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Did somebody EVER use thread gauge to check tapped hole or insert or both?
 
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Is this part anodized? If yes, before or after the holes are tapped?
 
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CH, I have used thread gauges to check tapped holes; go/no-go gauges. Wasn't terribly impressed with them really.

MJ; good question. Generally anodized aluminum is tapped before anodizing, and the tapped holes plugged before anodizing. The holes are considered exempt from the anodizing because they aren't exposed after assembly.

Jim Sykes, P.Eng, GDTP-S
Profile Services TecEase, Inc.
 
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MN, you don’t have to be impressed with the gauges, you just use them. You have more choices when measuring external thread, but internal M4 – probably not much.

If thread was machined improperly as PH suggested, the gauge could catch it.
The dimensions/tolerances usually apply for final state of the part, e.g. after coating/plating. It is not uncommon to specify tolerances before and/or after if necessary.

OP seems to automatically put the blame on design, which unfortunately is also not uncommon. In my opinion if both tolerances (6g for the bolt, 6H for the nut) were clearly specified on the drawings, there is no need for any additional “magic” indication.
 
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MJ, the current plate that contains the tapped holes is not anodized, however I still have the same problem.

MechNorth: regarding what you were saying about plugging the tapped holes before anodizing, Is this something the machinist should know or should you put a note to make it clear?

JJAV
INVENTOR 2013

 
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Are the threads in the aluminum made with a cutting tap or a forming tap? I'm having a hard time believing a cutting tap is not delivering a good thread in aluminum UNLESS it's totally worn and used up. Even a bad machine shop should know better than that though. If a forming tap is dull then it could leave a small pitch diameter. Can you check and see what type of tap is being used?

Powerhound, GDTP S-0731
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If you definitely don't want the threaded holes anodized you should clearly say so.

Something like "THREADED HOLES SHALL BE FREE OF ANODIZE" should do it. Note - you're telling them the holes are not to be anodized but you probably don't care if they make holes first and then mask during anodize or if they anodize the part and then tap it - so don't tie their hands by saying 'MASK ALL THREADED HOLES BEFORE ANODIZE' as this isn't actually your requirement. I've had vendors come back and ask if they could make the holes after anodize because masking was time consuming or problematic etc.

What's the material of the mating part? Occasionally SST will gall in aluminum. I had some #4-40 holes in 60661-T6 that kept binding on the thumbscrews. Machine shop claimed it was from masking compound left in the holes after anodize but I was never convinced as when I ran a hex head screw through the hole as a crude tap I only ever saw aluminum chips - no 'gunk' or similar.

Posting guidelines faq731-376 (probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​
 
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CH, you're right about the limited options. Then again, it is poor practice to put an M4 thread in aluminum as it doesn't bear load or vibration well.

JJAV, a good process planning/manufacturing engineering group will include it in their process plans, but to be safe, it is best to put it on the drawings.

Powerhound, SST in alum is a notoriously bad idea, particularly at small thread sizes. Galling, tear-out and galvanic corrosion pathways.

Kenat, in my experience, tapping after anodizing frequently caused localized surface damage to the surrounding anodizing in proximity to the hole. Don't know if that's always the case, but it was on our parts which were milled, tapped, threaded, turned in alu plate, tube, bar and occasionally castings too. Strangely enough, each colour of anodizing behaved slightly differently.

Jim Sykes, P.Eng, GDTP-S
Profile Services TecEase, Inc.
 
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Yeah, I wasn't really suggesting that forming taps were the way to go, I was just considering the possibility that they may have been used. After all, I'm not the one tapping the holes in this part.

As a sidebar, I haven't had the foul luck with forming taps that you guys have had. I use them frequently in aluminum and especially with smaller sizes since they are beefier than cutting taps. I don't even remember the last time I broke one that I didn't just plain screw up and either try to tap deeper than the hole or get the feed and RPM messed up somehow.

Powerhound, GDTP S-0731
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JJAV1983,
You wrote that the tapped holes are not anodized.
What exactly is the issue, do the threaded parts not fit the tapped holes? Or are they too loose? Or ...?

Chris
SolidWorks 11
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ctopher

Exactly, the tapped holes are not anodized, the issue is that when the assembler tries to fit both the male polycarbonate M4 part and the aluminum M4 tapped hole, it is just not possible, he has to re-tap every single AL part. So I am thinking that chamfering will really help here as Powerhound suggested.
 
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Ctopher,

Long time no write. It might be that when they retap the threaded holes it gets rid of inconsistencies made during production. By the way I forgot to mention that this problem has been happening on brass material, we have a couple of brass plates that also need to be retapped.

 
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