stripping problem when punching fiberglass

[MulticylTodd]

Hi forum users,

We are making a hand held punching machine to punch small (approx 3/16" dia) holes through 2 pieces of aluminum, one of which is .100" thick and the other is .060" thick - between which is a think layer of fiberglass material. It is a less than 2 ton punching requirement. The machine punches well and strips well if the fiberglass is not present, but once the fiberglass is introduced the punch jams up in the die. We have tried heavy duty 1.5" dia stripping springs, coated punches, and played with the cutting clearance from .008-.025," but not had any good results stripping the punch. Any ideas?

Thanks

 

[MikeHalloran]

I'd put some back taper on the punch shank.

I.e., allow a full diameter land behind the punch face to provide material for a few sharpenings, then reduce the punch diameter progressively farther from the punch face to allow clearance between the punch shank and the punched hole.

The idea is that after the punch makes the hole, it will pull some fibers from the material, which will align with the punch and jam the punch in the hole. Providing clearance, so the shank of the punch rattles around in the produced hole, provides room for the fibers projecting inward from the hole to turn around and realign so the punch can be withdrawn from the sheet.




Mike Halloran
Pembroke Pines, FL, USA

 

[Compositepro]

Mike's suggestion might help. Fiberglass does not punch well. The material will crush from the pressure causing splintering and fibrillation. Drilling with very sharp carbide drills will work better. The drill can cut through the aluminum and fiberglass together.

 

[MulticylTodd]

Thanks for the responses so far guys - we will grind a few punches and try Mike's suggestion. Compositepro, I appreciate the undoubtedly correct input, but drilling doesn't help those of us who make punching machines

Thanks again.

 

[MiketheEngineer]

Is the fiberglass sandwiched "tight" between the AL sheets??

 

[dhengr]

MulticylTodd:

MikeH’s solution for the tapered punch may well solve your problem. Maybe grind a bit of a cup in the tip of the punch so it is truly shearing the fiberglass fibers not just pushing/pulling and ripping them. They are just jamming up in the space btwn. the punch and the die. But consider these things too:

1. Have the punch hit the thin Al sheet first, so the thicker sheet supports the fiberglass w.r.t. the die. If you hit the thick Al sheet first, with the punch, you aren’t punching the fiberglass so much as ripping and crushing it, since it gets no support from the thinner sheet immediately above the die; and the thicker sheet in its deforming doesn’t produce a punching/shearing action on the fiberglass.

2. Would any sort of a partial twisting/rotating action of the punch during the punching process help. You Mech. guys will know, better than I do, if this can even be done or how.

3. Maybe your punch should (be in/should have) two cutting actions: the first to punch almost to size; and the second to really shear all three layers to final size. Again, without digging out my books on this stuff, you Mech. guys will know, better than I do, but the lead punch dia.(smaller) would clear a hole so the final punched/sheared materials had someplace to go, but with a cleaner cut.

 

[dvd]

I'd be curious to know if the aluminum sheet is dishing due to the compression of the glass layer, resulting in punching of a spherical sheet instead of a flat sheet. The material closes in on the punch as the spherical shape relaxes after punching. Are there pictures of the punched sheet?

 

[ajack1]

Assuming you use standard punches and dies from a supplier like Lane, Porter, Dayton, AWP have you tried talking to one of their technical support guys?

They are often willing to help and have a great deal of knowledge and I know for example you can buy dies with a slight nick and spiral in them to help stop slugs coming back up. I have no experience in piercing fibreglass so cannot comment but surely free advice from the people that are trying to sell the product would be a good place to start?

 

[ornerynorsk]

Fiberglass is nasty stuff to punch. You could also try heeling (stepping) the punch which not only reduces tonnage, but interupts the shearing zone, which might help.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[chicopee]

About punching a smaller hole and than finish the job by rimming.

 

[wongs9982]

add a stripping plate on your die set.
or
add plastic spring to strip out the part.