Pressed dowel pins and the need to vent

[freerangequark]

I have a machine design featuring steel dowel locating pins and diamond points pressed into blind holes in :

[ul]
[li]steel castings[/li]
[li]aluminum casting[/li]
[li]steel machined parts[/li]
[li]aluminum machined parts[/li]
[/ul]

Someone suggested that we should be using vented pins to make insertion easier;

Our company's standard pins are not vented, nor do our design standards for pin use mention vented pins.

What should be dictating cases when vented pins should be used or not used?

Are low interference versus medium or high interference fits less likely to need vented pins to aid in insertion?

Where can I learn more on this subject?

Thank you,
Glenn

 

[mfgenggear]

Free-range
May be I have been out of the loop, the only holes in pins I am aware of are roll pins.
Dowels pins are normally solid.
Some time there is issues with blind holes.
Causing air to be traped. So an ol trick from an og guy. Was to make the pins slipfit os slightly press, using locktite. To prevent from moving.

 

[3DDave]

I have only seen the need for venting once. It was on a box made of separate aluminum panels, one for each side, with dowels for alignment that was sealed on the mating faces with epoxy - and then put into an oven for high temp curing.

As one might expect the epoxy around the pins made a good seal, the differential thermal expansion eliminated the interference, and the trapped air pushed the dowels part way out of the holes, where they were found on cooling, now held firm with the epoxy and the returned interference fit.

In your case, it should be easy enough to make up some sample blocks with cross-drilled holes at the bottom of the pin holes next to some holes that aren't, reamed to the expected size one after the other, and put an arbor press on a continuous reading scale and see what the effort is to push the pins in. Alternate - hydraulic press and measure the pressure of the fluid; the only variable will be the presence of the vent.

Analytically you should be able to find a contact pressure calculation that applies to all the cases and estimate the friction force from that; knowing the surface area/engagement will get you the retaining force. Again - check at the temp extremes to see what reduced interference might do an at the expected least interference expected.

Here's one calculation:

https://www.eng-tips.com/faqs.cfm?fid=1230

Compare that analytical number to the expected pressure from compression of the air.

 

[freerangequark]

Here are two types of vented pins

 

[freerangequark]

The machine I'm working on is an enclosed/sealed environment... only 1.1 atm.... but sealed nonetheless. This would affect placing a relief hole on the dowel pin hole.

The real question was about difficulty of insertion of the pin if there is no path through which the air from the pin hole can escape as it is blocked by the pin's interference fit to the hole.

Thank you,
Glenn

 

[3DDave]

Depending on the engagement, it's just compressing the air. If you want margin, drill a smaller diameter hole to leave a shoulder and have a place for the compressed air. If there is a 10:1 compression, then you have about 150 psi to push the pin out. On a 0.25 diameter pin, that is 7 pounds of force to push the pin back.

 

[TugboatEng]

I see two distinct cases where vented pins should be used. The first is in a close clearance fit hole where built up pressure pushes a hand fitted pin out. The second case is a tight clearance or interference hole with a pin that is installed by press or hammer. In this case excess pressure could damage the part the pin is being installed in.

If you don't want to run a vented pin consider using a looser clearance fit and retaining compound.

 

[dvd]

You can't bottom out a pin in a blind hole unless it is vented - that last bit of travel builds up infinte pressure (assuming that the infinite pressure cannot leak past the interference fit). My original engineering mentor at the bomb factory was a tool and die maker and he strongly advocated vented pins in blind holes. Don't understand your pressurized atmosphere, but you can't drill a small vent hole in the bottom of your pin bore? You can grind a small relief channel in the pin just to the depth of the press fit.

 

[geesaman.d]

You can't bottom out a pin in a blind hole unless it is vented - that last bit of travel builds up infinte pressure (assuming that the infinite pressure cannot leak past the interference fit). My original engineering mentor at the bomb factory was a tool and die maker and he strongly advocated vented pins in blind holes. Don't understand your pressurized atmosphere, but you can't drill a small vent hole in the bottom of your pin bore? You can grind a small relief channel in the pin just to the depth of the press fit.

I hadn't thought of it that way before but it makes perfect sense.

So if you want the dowel pins unvented and seated to a very consistent depth, then add a smaller blind hole at the bottom of the dowel hole to absorb the pressurized trapped air. Otherwise the final seating depth of the dowel pin will be determined by the pressure it can attain and the grip the pin has on the hole (which will vary).

In the machinery I work with, we use numerous straight dowel pins. They can be blind, through-bore, press fit, slip fit, line-to-line fit, and are never primary load-bearing. It's pretty forgiving in these things. Where possible I design dowel pins to have a smaller through-hole so the mechanic who needs to remove the pin can drive it back out without damaging parts.

David

 

[btrueblood]

I've only used vented pins on dowels inserted to similar hardness materials, where the hole was blind. Even then, it was overkill - if the dowel kept popping out due to trapped air, then a quick scribe line along the bore helped, or loctite.

For a lot of tooling that was stacked in layers (rubber compression molds are one example) and indexed via dowels, I would spec. the hole be bored "thru" if possible, more so that the pin (or its remains once it broke off) could be driven back out with a punch and hammer, so it could be replaced.

 

[ctopher]

I only used them once. It was a customer req't because they said it was easier for them to replace as needed.
Otherwise, needed had a need for them.

Chris, CSWP
SolidWorks
ctophers home

 

[mfgenggear]

Btrure
Yes I forgot to mention a small groove
With high speed hand grinder like dremel
Except commercial grade.

 

[tygerdawg]

search for "Flat Vent Dowel Pin", these are readily available Commercial Off The Shelf COTS items.

TygerDawg
Blue Technik LLC
Manufacturing Engineering Consulting

http://www.bluetechnik.com