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Locating with two diamond locators.

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drir

drir

Mechanical
Aug 17, 2011
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Hi,

I want to locate two parts (with each two holes) very precise with locatings pins. Normally one uses a round locating pin in combination with a diamond shaped pin. However, this set-up only prevents rotation around the round pin. Translation between the two holes is still possible (pitch of the two holes, see fig. 3.19 on page 11 in the link below). In order to prevent also this translation, I found something were one uses two diamond locating pins as shown in this document (page 11, figure 3.20) instead of one round pin in combination with a diamond shaped pin.


Is this a common way so solve my problem?
 
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Translation between the holes is not possible with one round/one diamond if your holes/pins are sized and toleranced properly. If they are not sized properly, diamond pins will give you worse results than one round/one diamond. Diamond pins are not used to increase locating precision. They are for ease of assembly/disassembly. If you have poor control of the hole shape (out of round, etc) then two diamonds will help further with assembly/disassembly, but not locating accuracy.

-handleman, CSWP (The new, easy test)
 
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Hi Handleman,

Thanks for your reply!! Thus in words of accuracy one round/one diamond pin is the best solution? Or two round pins?

Thanks
 
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Properly sizing the pins will be the most important factor. Two very tight tolerance round pins will help account for the holes being slightly out of round. Diamond pins have no advantage in terms of accuracy, they are much more dependent on the holes being perfectly round to locate accurately. The disadvantage of round pins is that they are more difficult to place parts onto, and may bind up more readily with tight tolerances between pin and hole.

Comprehension is not understanding. Understanding is not wisdom. And it is wisdom that gives us the ability to apply what we know, to our real world situations
 
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CastMetal, thanks for your response! I can buy accurate dowel pins with an accuracy of h6. After drilling I will ream the holes with an accuracy of H7. If ones the parts are assembled they don't have to disassemble any more. The pins are only for placing the two parts very accurate with respect to each other.

Conclusion: two round accurate pins (h6) in reamed holes (H7) will be most accurate positioning the parts?
 
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Make sure you get orientation of diamond pin with respect to round pin correct. As in fig. 3.19.

I have seen jigs with diamond pin 90 deg. to proper orientation and engineers wondering why parts come out so sloppy.
 
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That will bring you within .0005" or .012mm of location if that is good enough for your purpose you have your answer. It not a question of "most" accurate rather whether it is accurate enough.

Comprehension is not understanding. Understanding is not wisdom. And it is wisdom that gives us the ability to apply what we know, to our real world situations
 
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When I was in Tool Design we used diamond pins when we could allow some float in one direction and need to be tightly controlled in the perpendicular direction. Also, we used diamond pins when there was a potential mismatch in CTE's between mating parts. I personally never had an occasion where I would use two diamond pins

Here is what Carrr-Lane has

You may want check out the other locating pin options available
 
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I'm looking for a manner to locate my two parts which gives me the highest accuracy! Still I don't have an answer on that question. Is this with 2 round (dowel) pins, one round/one diamond or two diamond locating pins?
 
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Accuracy relative to WHAT? Most systems are design to maintain repeatability, not absolute accuracy, since that's completely dictated by the machining process. If you are trying to align something within the parts to each other, then the tolerancing and specification of how and when the "somethings" are machined is extremely critical. Stackups can completely nullify anything you do at the interface.

And, you may want to look at using a kinematic mount approach, which actually uses more than two dimensional features to control repeatability:
TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize
 
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Let me try again...

DIAMOND PINS ARE NOT USED TO INCREASE LOCATING PRECISION!!!!!!!

The purpose of diamond pins is to ease assembly/disassembly. If used correctly with one round and one diamond, locating precision will not decrease. Depending on the final clearance condition between the pins and the holes, two diamonds may or may not give less precise positioning than one round and one diamond. If toleranced properly, two diamonds will not decrease accuracy.

The biggest effect on assembly accuracy is going to be the location of your holes. If your holes are 1mm off, no matter what pin you use the assembly won't go together.


-handleman, CSWP (The new, easy test)
 
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Using a pin & diamond pin with holes is an alternative to using a hole and a slot with pins. The round hole & pin locate the part to within the tolerance of the fit of the pin in the hole and the location of the holes in each part. The slot and pin or hole & diamond then control rotation. Essentially/typically you're making the hole & pin a secondary datum, and the other one tertiary.

Other alternatives can be using pins with some limited flexibility, such as roll pins, and plain holes. In some situations this may better simulate the pattern of holes being the datum.

To some extent you're going about this backwards, what 'accuracy' do you need based on function?

Posting guidelines faq731-376 (probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​
 
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drir,

Diamond pins are used to correct for mechanical fabrication tolerances. Normally, your hole clearance is a function of how accurately you can locate hole. The diamond pin can be shifted some distance off nominal and still engage the hole, in spite of an ultra accurate fit.

I do not see how figure_3.20 can work. It allows rotation about the right hand hole.

Critter.gif
JHG
 
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