End Mill For Enlarging wrist pins

[Dave33]

I'm looking for a recommendation for and end mill for enlarging the hole in a wrist pin. The reason for doing this is to lighten the pin by about 22 grams. The pins now have a 9/16" bore and I want to enlarge to 5/8". I'm looking for the correct end mill for doing this. I know I need a carbide mill because the pins are extremely hard and probably a ball nose. Do I use a 2 flute, 3 flute, or maybe more? Thanks in advance.

 

[cvairwerks]

You don't want to do this with an end mill. You want to use a boring bar in a lathe instead. The amount of time to get the pin set exactly vertical and centered under the head is going to drive you nuts. Any misalignment makes it possible that the mill will grab and toss the pin out of you holding fixture.

 

[cannuck]

Wrist pins are usually lightened with a tapered cut from a boring bar or reamer BEFORE they are heat treated. AFTER hardening, they could be HRC50++ - not something easy to machine (except grinding). You best bet is to order a properly made set of tapered bore pins.

 

[Dave33]

I'll try to find a place to have them made. To bad as I have a number of new stock pins that I can't use.

 

[cannuck]

There are quite a number of suppliers. Here is one of the best (think they do street engine stuff as well) http://trendperform.com/c-1149648-wrist-pins.html

 

[MushCreek]

If you have a lathe, chuck one up and try it with a carbide boring bar. If you only have a mill, you could try a boring bar. It's gonna sound horrible, but it should cut it.

 

[RoninB4]

You should also consider what the wall thickness would be after enlarging the inner hole. Less reciprocating mass may come at the expense of catastrophic failure. Machining can/will induce stress into the workpiece. Leaving grooves from boring can provide a location for a stress crack to develop and propagate. Not saying it will, just things to consider when seeking improvements. Agree with others that doing this in a milling machine with a carbide end mill isn't advised. Try the lathe (indicate it in the chuck for run-out) with as beefy a carbide tipped boring bar as will fit in the 9/16 hole. Keep the length of the boring bar as short as possible and take small (.010) cuts per pass too. Like MushCreek said, it's likely to sound horrible. Good luck and report back.

 

[cannuck]

You can't just chuck up the wrist pin, you will damage the finish that is needed to have a working bearing surface. You could do it with a fairly soft collet, but why not just buy one from someone who does this all day every day?

 

[Monza Harry]

Dave I agree with the other posters about this being a bad idea, I would consider a carbide tipped reamer [would require at least two reamers more likely 3-4, so cost compared to replace is quickly closing in on a wash] or ID grinding, [a better choice by far] to do the machining but that pin is now compromised strength wise, a material upgrade would be in order, a higher alloy likely to a lesser heat treat [additional temper to reduce brittleness of the stronger alloy from the existing 50-55+Rc to a less brittle 46-48Rc(?) Wrist pins are already a quality tool steel so the next step will "Leave A Mark" on your wallet! An even better choice, I feel is to look at new rods with a lighter small end, lots of rally good choices for those. Harry

 

[RoninB4]

You can't just chuck up the wrist pin, you will damage the finish that is needed to have a working bearing surface. You could do it with a fairly soft collet, but why not just buy one from someone who does this all day every day?

-You're right, I neglected to mention that SOP for critical surfaces is to wrap a sheet of brass or 3-4 layers of paper around the work to protect it from the chuck jaws. If you don't have either a single strand of solid copper wire wound around the work can be used. Indicate the work in within .001-.002 TIR. I sometimes forget to mention things to others I do automatically. Good catch.

 

[Monza Harry]

-You're right, I neglected to mention that SOP for critical surfaces is to wrap a sheet of brass or 3-4 layers of paper around the work to protect it from the chuck jaws. If you don't have either a single strand of solid copper wire wound around the work can be used. Indicate the work in within .001-.002 TIR. I sometimes forget to mention things to others I do automatically. Good catch.

Masking/vinyl & duct tape is used alot as well. I recently used three layers of masking tape on an aluminum knurled finish, 3/4" counter bore in a 1" dia. with no damage. Pretty much SOP for professionals and yes we often take that for granted. [Embarassed emoji here]. Many also use emery cloth [PSA, and plain backed] for less delicate surfaces with "traction" issues, certainly not for this application. Harry

 

[cannuck]

The stress introduced into a low mass part such as a wrist pin from trying or "succeeding" in cutting a taper into each end in the final heat treat condition would, once said stresses relieved themselves by deforming the part would likely leave concentricity and even overall diameter well outside of the tolerances it was originally made to. Part of that would be due to the uneven clamping pressure of a 3 or 4 jaw chuck (thus why I mentioned using a soft collet). Even then, still not a good idea. Just buy a proper set from a specialist and be done with it.

 

[Dave33]

Hi all and thank you for all the advise. Given the chance of damaging or weakening the pins I've decided to have a set made. It won't be cheap but they will be correct and strong.

Thanks again for the help.

 

[metlmunchr]

Precision Products Performance is a good source for light and strong wrist pins from either H13 tool steel or C350 maraging steel. Website is http://pppcenter.com