More punching on the Hydraulic press...

[JNewman]

Your press looks good Larry. Looks like it is working well for you.

I like David's idea for a punch holder but it does mean that the base of all the punches needs to be machined because they rely on a flat surface to align them. This means that forged punches either have to be done very carefully so as not to mess up the collar or the machining has to be done after forging and annealing. I suppose the collar could be forged in a die and the end ground flat but it still involves being careful not to mess up the collar.

I was thinking about it this afternoon while I was doing some milling and I thought of using a simple collet to hold punches. I like the idea of making any complication in tool holders rather than the tool, so tools can be really simple to make on the fly. How about this idea? I have left the sides off so it can be viewed and there would either have to be a way to break the taper loose or the taper could be made steep enough that it is not self holding. The plate on the bottom would be bolted to the body to tighten the collet. Different sized collets could be made fairly easily for different punch stock sizes.

[Larry L]

I dont know John...

I think the idea has merit but seems to be fairly involved... I assume The holder and the collet parts would all have to be alloy/tool steel and heat treated with very tight tolerances and the tooling would have to be clean and smooth? Also I know a collet will bite with a lot of rotational grab but do you think it hold 40 ton of pull on the punch? When the tools are pulling out of my holder how it peels off a 1/8" deep by 1/2" wide curl of H 13 steel on the set screw as it pulls out...

[david hyde]

John, yeah you're right about it means you can't do forged or extra wide punches with a machined shoulder. For wide punches I use a 2 part system with the top part having the shoulder and the punch part screwed to it with a 12.9 grade allen bolt.

If you look at one of the photos in the thread on punch geometry, it might explain it. The image is on my laptop which is at the workshop so I can't post it here. I'll try to take some more detailed shots today. The punch in my hand is one of the 2 parts (note the thread), the top part is near the edge of the bench by the welding clamp. I guess Larrys concerns about upsetting apply to the interface of the 2 parts but I've had no problems in quite a few years. My press is only 15T but the system works a treat so I'm sure it should for 40T if suitably scaled

The only difference between what I suggested to Larry and my punch is that mine uses a screw on collar to "clamp" the punch, my proposal uses bolts. I'm sure if these were big enough, with a suitable thread area they wouldn't pull out. With hindsight I think I would have gone for something like this to start with.

I agree TOTALLY with you about making the punch simple and the clamping having any comnplicated machining. With the two part sytem, its a compromise, all you have to have is a turned parallel end with a thread in it. I do this before forging for material such as H13 that are difficult to machine after forging

Having said all the above, I like Johns idea if it can stand the pull out forces

Photos will make more sense of my verbiage.

[JNewman]

While hardened and perfect fits would be nice I am not sure that it would be necessary. A wedge grips things pretty strongly. I think as long as the bolts are tight a crude collet would hold a punch more securely than a set screw. If the wedging action is not enough to hold the punch in place, there could be a combination of David's collar and the collet. The collar would not have to be as precise because the shaft would align the punch rather than the flat on the top of the punch, however changing the tools would be more difficult. It also starts to eliminate the simplicity.

As I was typing the above I had another thought. Kiss, what about a larger set screw and a flat ground on the shank like a Weldon flat on a milling cutter. The set screw on a 1" shank on my flypress was about a 3/4" bolt.

[FeWood]

I know I'm a little above my pay grade here but while reading this thread, it occurred to me that a hybrid of JNewman's system with the flat on the chisel and a bolt through the collar. What about a wedge instead of the bolt? It would mean a bigger coller to occomadate the 2 holes need for the wedge. Then tolerances could be a little looser for hand made tooling. My description is a bit vague but I could take a picture of what I mean. I have something like this on a power hacksaw.

[Larry L]

Yes.... pictures please

[Grant]

Interesting idea. I've always gone for tools with a head like the ironworker punch. In long run production, I've never found anything else to work. Your wedge is actually much easier than it first seems. You'd only need an intersecting cross hole in the holder and a flat on the tool shank. Grind a taper flat on a round bar that fits the hole and drive it in. Taper angle wouldn't matter either, the shank will rotate to match automatically. Needs to be of generous size and heat treated, you sure don't want it getting distorted or you'll never get it out!

[david hyde]

Grant am I picture something different but what you're decribing sounds like the cotter pin that grips the shaft the a bicycle on the crank pedal (don't know the proper terms)

Out of curiousity does any one have any idea of the ball park figures for the tonnage needed when pulling the punch out. (maybe a percentage of pressing forces) I've often looked at my stripper plate and thought it looks under engineered but it holds up ok over the years

[Larry L]

Yep... Cotter cranks is what I thought of too.... How big do you think the pin would need to be? 1"?

[FeWood]

Yep... Cotter cranks is what I thought of too.... How big do you think the pin would need to be? 1"?

Thats the idea "cotter cranks"

On the one I will take the picture of I think the shaft is 3/4" and the pin is 3/8".
I think it has more to do with the material around the shaft than pin size although pine size is relevent...