3/4" Holes in 3" thick rubber

[C0lson]

I am new to Eng-Tips so pardon me if this is not the correct forum. I must devise a method to drill or punch 3/4" holes through 3" diameter hard rubber spheres (ShoreA durometer ~80.) Fabricating the spheres with the whole in place is not an option. I've tried drilling with hole saws at low rpm on a milling machine with various lubricants without success. I think this is in part because to drill I a must apply side pressure from the vice to resist the rotation and in turn, this continuous pressure against the milling machine bit binds it up. Higher rpm and I burn the rubber. I've seen mention of freezing rubber before machining and I'll give this a try but my gut tells me I would need temperatures lower than the typical freezer for this to work.
It seems like an arbor punch might work but have no knowledge of the nomenclature and this in turn makes searching, or even calling machine shops, etc. very time consuming.
Can anyone here offer advice or what kind of machinery I should be searching for? Thank you kindly/C0lson

 

[johnnymat]

Hi
I think you have identified the main issues in dealing with this, Punching is impossible without getting ID curvature, drilling difficult with rubber in its natural state. Freezing is the best option and may not be so difficult depending on the rubber type. Most rubbers except silicone will freeze at -30. standard vitons at -20. Aflas and FFKM at -10
also rubbers do not go from fully flexible to suddenly solid, they progressively stiffen with lowering temps, so you may be able to get to a point with a conventional freezer ( about-20) that may be good enough
Remember rubber is a natural insulator so it may take some time to freeze through the thickness. Better if you can set up some dry ice in methanol, or play liquid nitrogen into a closed container.
Machining then at conventional speeds( no lubricant) should be OK

Johnnymat

 

[Compositepro]

Holes in rubber stoppers are made with a low speed core drill. This is a thin walled tube that is sharpened to a razor edge on the ID. It is rotated slowly while pressing into the rubber. Lubricant is helpful.

 

[C0lson]

Johnnymat and Compositepro, thank you for your advice. I have now tested -05F with no significant difference and will be using solid CO2(dry ice) allowing different Q-times in the deep freeze to ensure the material has opportunity to solidify as much as possible.

I have seen reference to core drills before and razor edge with thin walls seem like helpful characteristics. Actually used this type for custom stoppers in labs years ago. I have not found a manufacturer that produces a 3/4" diameter for soft materials. All seem to be for concrete, etc. which have thick walls and sometimes thicker 'teeth' or diamond coating. Compositepro - do you know of a core drill manufacturer that I may look into?

If these options fail I will test using small off center pilot holes perpendicular to the central axis hole and insert rods to prevent rotation and avoid compressing the rubber compound. Perhaps in the end it will be a combination of all.

Thank you again for you ideas. They keep my mind from binding up like the milling machine /COlson