Force Fit Pins. How to in ally please

[ImanCarrot]

I got to Force Fit two 3.5mm dia steel pins in the side of aboput 100 alluminium blocks 30.5mm X 30.5mm X 6mm thick, I've looked in the Machinery's Handbook but all they give you is stuff about steel- nothing about ally.

Stell and ally obviously have different co-efficinets of expansion. Is there any special techniques I should know about?

I was planning on drilling the holes undersize, reaming them a bit less undesize, heating the ally block, sticking the pins in and letting the assy cool... does that sound about right?

Looking for guidance! Thanks in advance!

Iain

[Edit] When it cools, will the steel pins warp the ally surface? cos I'm sticking an optical mirror on the block surface and that may be a problem.[/Edit]

[Geof]

Yes you will distort the plate by having interference fit pins in the edges; maybe not to the tolerances I work with but certainly to your tolerances.

However unless you have machined the plate optically flat the inherent surface inaccuracies will be comparable to the distortion from the pins so it may be immaterial.

Heating the plate could introduce a can of worms. Getting it hot enough to open the holes up far enough to let the pins simply fall in will probably relieve stresses and cause distortion.

Regarding the differential expansion if you are going hot enough for this to be a factor again you are probably relieving stresses in the aluminum.

If you rigidly glue your mirror onto the plate and then heat it you have created a bi-material strip, it will bend like crazy.

Why not have a size for size or even a very small clearance and use Loctite?

[ImanCarrot]

Thanks Geof, good advice as always. Didn't think about the stress releiving!

I'll have a think about it over a pint... just heading off home covered in engineer's blue lol damn stuff gets everywhere when you drop it on the floor

[wildwestpat]

Hi Iain

Suggest you use roll pins rather than solid pin. Not only are the distorting forces in the dowel holes in the alloy part limited by the spring pressure but also the expansion differentials are absorbed without any loss of register. Just use a small size roll pin with an appropriately thin gauge would IMO be a workable solution. (There are several types of roll pin from which to chose as well as the material. I keep a box of assorted roll pins for jobs such as this.)

Also the tolerance for the hole is less demanding for roll pins but the alignment is the same as for solid or taper pins.

In many cases using super glue to temporarily assemble the parts then to dill the roll pin holes all at the same time means that the assembly can be dismantled and reassembled whilst retaining register. Just use one of the super glues that comes apart either with hot water or a side impact.

Hope this helps - regards Pat

[billystein]

hi,
i use a fadal. it has m46 which is positive approach. when i make holes for press fit pins i normally center drill,drill,undersize end mill with positive approach and then reamer that is .01 mm (.0005 inch) undersize.
i have a cuttermaster so i have undersize end mills made for standard dowels.
if your cnc has little or no backlash then you don't need pos app but for the fadal it is the hot tip.
good luck

[louieatienza]

I was thinking of doing the same thing and wondering if just lacing the pins on dry ice would give enough clearance to fit them in a tght hole?

[Geof]

Dry ice is at about -80 degrees Celsius which is about 100 degrees below normal room temperature. The coefficient of thermal expansion for steel is about 1.2 x 10^-6 in/in degree Celsius (I think). The pins are going to shrink a very, very small amount.

It is feasible to use liquid nitrogen which is about 200 degrees below normal room temperature to shrink largish parts to fit into aluminum holes, valve seat inserts are one application for this but they are often well over 1" diameter.

[ImanCarrot]

The pins are going to shrink a very, very small amount

Totaly agree, I thought of using liquid nitrogen but on a 3.5mm dia pin as you say it's gonna shrink very little.

After careful deliberation (and a few pints lol) what I think I'll do is make the block oversize, thickness wise, end mill the holes slightly undersize, heat the block, stick the pins in, let the assy cool, stick a bit of superglue on the interface and let it "creep" into any gap...

Then! I'll machine the block to correct thickness, that should get rid of any distortion that the pins have caused or any distortion caused by heat releiving. Then I'll stick the mirror on.

Or... I might just redesign it for the customer and diamond turn the ally block so I don't need to stick a mirror on it then coat it with Silicon Monoxide or Silicon Dioxide.

Choices, choices lol l love it will post piccies when I makes them heh

[morehelium]

How about Loctite?

Just ream the holes a few tenths oversize, and use locktite to hold them in.

Easy to do, clean up any excess and you'll get what you want without any high precision reaming or distortion.

Red Loctite will keep them held as well as a slight press fit, blue will let you remove them easy.

Heat up the piece to remove if needed...

Chris

[juergenwt]

Iman - I would go with the same fit as recommended for steel.
Tight press fit H8/x8
Medium press fit H7/s6
Light press fit H7/r6

At 3.5mm dia you should not have a problem.

Never put "Force fit" on a print. It means different things to different people.

[ImanCarrot]

Thanks everyone for all the good advice.

Never put "Force fit" on a print. It means different things to different people.

Didn't know that! thanks, my customer's drawing states "force fit", but it was drawn in 1973... Hand drawn lol: most ancient drawing I've ever seen! I was nine years old when it was drawn heh.

It's llike looking at an ancient manuscript! I'm scared in case I put my coffee cup down on it

[Geof]

.....Never put "Force fit" on a print. It means different things to different people.

Back when I took draughting lessons at trade school as an apprentice 'force fit' meant to the specs given in drafting standards. This was a decade before Mr Carrot's 1973 drawing but probably the same implicit meaning applied then. I think the 'different things to different people' aspect has come in because of the demise of proper apprenticeships with their emphasis on learning and abiding by the standards of a Trade.

It's llike looking at an ancient manuscript!

I was 30 in 1973 and I am not sure I am ancient yet!!!