I'm not sure those can be CNC milled. Inside corners and the only radius spec is 0.1mm max on the key. That's a pretty small endmill.. Punching is probably the best option outside of laser, or EDM. Maybe broaching the corners, but that could be tricky too. And tedious.
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Curta Calculator Scale Replica
- Thread starter racer8412
- Start date
- Joined
- Feb 18, 2016
- Messages
- 451
I agree. In fact, I don't even know if we can punch these out to within acceptable tolerances. This whole adventure is new territory. I'm going to challenge Racer to study up on punch and die and let us know if he can build tooling that will meet the required tolerances. 
If nothing else he will have learned quite a bit about about punch and die stamping, perforating pressures, die clearances, and other important information conerning this manufacturing technique. He may even discover that he can't meet the tolerances he requires, and have to reject the punch and die method. I'm just proposing lessons.
It's up to Racer to do the "homework".
A Note on Tolerances:
I'm thinking that, although we should strive to meet the tolerances called out on these drawings, in reality, we can probably loosen up on the tolerances a bit. The reason I say this is because we are building this as a handcrafted item. Therefore we can work with looser tolerances via the assembly of a one-off handcrafted item.
Keep in mind that the tolerances called out on these drawings are for manufacturing to assure that all these parts will fit on any Curta Calculator even as replacement parts. If our parts are slightly off on some dimensions, we can probably take care of that during the handcrafted assembly. For example, my handcrafted gears may not fit your replica of the Curta Calulator, and your gears may not fit directly into mine. But as long as they work in the calculator they were handcrafted into that's all that matters for our purposes.
So we have a little more room for error in tolerances than are called out on these drawings simply because we are building a handcrafted replica,.
~~~~~
None the less, Racer is going to need to design a punch and die that is going to punch out gears as close to the drawing tolerances as possible. This is going to require that he gains a really good understanding of the punch and die processes, especially concerning clearance issues. He may need to consider shearing angles of the punch and die edges as well.
He'll no doubt need to do some preliminary trial and error work. I would (and will) suggest in future homework lessons to begin by making a simple circular punch and die. Get the brass sheeting that will be used. And punch out some circles. And see what you get. Try different clearance tolerances, etc.
Hey, I never said this was going to be EASY!
I'm not claiming to know how to build these things. All I plan on doing is teaching Racer how to go about figuring out how to build it. He'll need to overcome all the obstacles. I'm just an innocent by-stander. Honest.
If nothing else he will have learned quite a bit about about punch and die stamping, perforating pressures, die clearances, and other important information conerning this manufacturing technique. He may even discover that he can't meet the tolerances he requires, and have to reject the punch and die method. I'm just proposing lessons.
It's up to Racer to do the "homework".
A Note on Tolerances:
I'm thinking that, although we should strive to meet the tolerances called out on these drawings, in reality, we can probably loosen up on the tolerances a bit. The reason I say this is because we are building this as a handcrafted item. Therefore we can work with looser tolerances via the assembly of a one-off handcrafted item.
Keep in mind that the tolerances called out on these drawings are for manufacturing to assure that all these parts will fit on any Curta Calculator even as replacement parts. If our parts are slightly off on some dimensions, we can probably take care of that during the handcrafted assembly. For example, my handcrafted gears may not fit your replica of the Curta Calulator, and your gears may not fit directly into mine. But as long as they work in the calculator they were handcrafted into that's all that matters for our purposes.
So we have a little more room for error in tolerances than are called out on these drawings simply because we are building a handcrafted replica,.
~~~~~
None the less, Racer is going to need to design a punch and die that is going to punch out gears as close to the drawing tolerances as possible. This is going to require that he gains a really good understanding of the punch and die processes, especially concerning clearance issues. He may need to consider shearing angles of the punch and die edges as well.
He'll no doubt need to do some preliminary trial and error work. I would (and will) suggest in future homework lessons to begin by making a simple circular punch and die. Get the brass sheeting that will be used. And punch out some circles. And see what you get. Try different clearance tolerances, etc.
Hey, I never said this was going to be EASY!
I'm not claiming to know how to build these things. All I plan on doing is teaching Racer how to go about figuring out how to build it. He'll need to overcome all the obstacles. I'm just an innocent by-stander. Honest.
- Joined
- Feb 18, 2016
- Messages
- 451
I was actually planning on having you build something more like this one. Only one that runs at a higher speed and has a guard on it so you can't stick your fingers into the punch press.
The punch and die will be a lot more complicated too. Although when you first build the punch press you will want to try it out with just punching circles out first.
But that wasn't the homework assignment I gave you. You're supposed to be learning about the process of stamping metals. And learning the formula for calculating perforating pressure.
You'll need to be able to calculate how much pressure you'll need to punch out the gears you want to make. And that will change as you design the dies and choose the material you'll make the gears from.
The final punch and die you'll need to make will look a lot like the compound die on page 3 of the booklet I asked you to read.
And the final punch press I'll have you build (assuming you ever get access to a lathe), will be more like the one in the video I posted above. Except it will have a much faster punch speed and more safety guards around the actual punch.
I might be expecting too much.
If you tried to punch the gears out using a hydraulic jack like in the video you posted it would take you a week to punch out 20 gears.
You want to be able to punch out 100 gears in a few minutes. So you'll need a fast press like the one in the video I posted only even faster.
Besides a faster punching speed actually produces a far better finished product. So you don't want to be pushing the punch through slow with a hydraulic jack. You want it to just punch the part out in a split second. That will make a much cleaner punch.
The punch and die will be a lot more complicated too. Although when you first build the punch press you will want to try it out with just punching circles out first.
But that wasn't the homework assignment I gave you. You're supposed to be learning about the process of stamping metals. And learning the formula for calculating perforating pressure.
You'll need to be able to calculate how much pressure you'll need to punch out the gears you want to make. And that will change as you design the dies and choose the material you'll make the gears from.
The final punch and die you'll need to make will look a lot like the compound die on page 3 of the booklet I asked you to read.
And the final punch press I'll have you build (assuming you ever get access to a lathe), will be more like the one in the video I posted above. Except it will have a much faster punch speed and more safety guards around the actual punch.
I might be expecting too much.
If you tried to punch the gears out using a hydraulic jack like in the video you posted it would take you a week to punch out 20 gears.
You want to be able to punch out 100 gears in a few minutes. So you'll need a fast press like the one in the video I posted only even faster.
Besides a faster punching speed actually produces a far better finished product. So you don't want to be pushing the punch through slow with a hydraulic jack. You want it to just punch the part out in a split second. That will make a much cleaner punch.
- Joined
- Feb 18, 2016
- Messages
- 451
A Sneak Preview
I ran across this video animation. This is a bit advanced as I haven't addressed all the design considerations for the punch and die. But this animation is almost precisely what we need to build. It's a simplified version of our compound punch and die.
The animation is great. The narration not so hot. Kind of robotic sounding.
There are two top punches. One (in blue in the video). This will punch the outer profile of the gear. The second punch is the center punch wrapped in a spring. That punches out the center hole of the gear. The pink block (held down by the spring around the top center punch) holds the gear blank in place during the punching operation.
The bottom die is the reddish part in the center of the bottom. It has both a center profile to accept the top center punch, and the outer profile of the gear teeth to match up with the outer blue punch on the top.
The spring-loaded yellow stage is used to hold the feed material flush with the top of the bottom die.
In this animation they are just punching out round washers with a hole in the center.
In our case the finished product will be a profiled gear with a center hole that has the wedge key we need.
This is a very complex punch and die set up. Making this alone will be a masterpiece of machining. So this punch and die setup is not something we want to rush though. How well this is made will determine how nice the gears are that it punches out.
But this is the idea. I'm really glad I was able to find this animation. This animation saves a lot of explaining.
Key issues of importance will be the clearance between the punches and dies. I've done punch press work before and I can tell you that another factor of how well things go will be determined by the speed of the operation. This tends to be something we find by trial and error once it's all up and running.
There will be two perforating forces. Each one being applied at different points in the punch stroke. In this animation the Blue top punch cuts the outer profile first. Then the center punch punches through the center hole. Because of this staggered operation the punch press only needs to be rated for one of these two operations. Obviously it will need to be rated for the higher pressure operation which will be the outside profile.
In any case, there you have it. It's almost designed for us. All we need to do is calculate the clearances we need for our material and the exact dimensions of the two top punches and the two cutting edges on the bottom die.
As soon as we get this made we can start punching out gears.
Of course, we need to build the punch press itself too. But let's not get too bogged down in the details.
Let's take it one step at a time.
I ran across this video animation. This is a bit advanced as I haven't addressed all the design considerations for the punch and die. But this animation is almost precisely what we need to build. It's a simplified version of our compound punch and die.
The animation is great. The narration not so hot. Kind of robotic sounding.
There are two top punches. One (in blue in the video). This will punch the outer profile of the gear. The second punch is the center punch wrapped in a spring. That punches out the center hole of the gear. The pink block (held down by the spring around the top center punch) holds the gear blank in place during the punching operation.
The bottom die is the reddish part in the center of the bottom. It has both a center profile to accept the top center punch, and the outer profile of the gear teeth to match up with the outer blue punch on the top.
The spring-loaded yellow stage is used to hold the feed material flush with the top of the bottom die.
In this animation they are just punching out round washers with a hole in the center.
In our case the finished product will be a profiled gear with a center hole that has the wedge key we need.
This is a very complex punch and die set up. Making this alone will be a masterpiece of machining. So this punch and die setup is not something we want to rush though. How well this is made will determine how nice the gears are that it punches out.
But this is the idea. I'm really glad I was able to find this animation. This animation saves a lot of explaining.
Key issues of importance will be the clearance between the punches and dies. I've done punch press work before and I can tell you that another factor of how well things go will be determined by the speed of the operation. This tends to be something we find by trial and error once it's all up and running.
There will be two perforating forces. Each one being applied at different points in the punch stroke. In this animation the Blue top punch cuts the outer profile first. Then the center punch punches through the center hole. Because of this staggered operation the punch press only needs to be rated for one of these two operations. Obviously it will need to be rated for the higher pressure operation which will be the outside profile.
In any case, there you have it. It's almost designed for us. All we need to do is calculate the clearances we need for our material and the exact dimensions of the two top punches and the two cutting edges on the bottom die.
As soon as we get this made we can start punching out gears.
Of course, we need to build the punch press itself too. But let's not get too bogged down in the details.
Let's take it one step at a time.
- Joined
- Feb 18, 2016
- Messages
- 451
Good work.
I've been off in this thread, A Lawnmower Punch Press, preparing to design a punch press for us to build.
I even got a lawnmower to use.
I was going to take you though building the compound die one step at a time, but perhaps you can design the compound die on your own now?
From what I've learned in the Lawnmower Punch Press thread from Jim Dawson, using half-hard 260 brass should work well for this purpose.
The shear stress for half-hard 260 brass is about 52400 psi.
Have you learned enough from those videos to take a stab and calculating the clearances we'll need?
You could probably start simple with just the center hole punch and die.
See if you can figure out what size the punch and die need to be. We can deal with the more complex outer gear profile later.
The center punch will be extremely simple to make. It's just a rod with a v-groove in it.
The die will be a round hole with the triangular key sticking out of it.
Personally I would make the die in two pieces by just drilling and reaming a round hole. And then cutting a keyway slot to accept an insert we can slide into it from the bottom to stick out as the triangular groove cutter.
Don't worry about actually making the punch and die. Just calculate the clearance. And then see if you can figure out what size the punch and die should be to produce the correct size hole.
Tip: In real life engineers often cheat by doing things trial and error to some degree. You can make a round punch, a round hole die, set up a way to keep them lined up. Then put some brass material between them and whack the punch with a nice heavy hammer. This should produce the hole and the a piece that comes out of the hole.
You can then measure the hole and the piece that came out of the hole and see if you get the results you are expecting. If not, back to the drawing board. Adjust the sizes of the punch and die and try again.
You can use this same method for the larger circle on the outside diameter of the gear. You want to make sure you can punch out a nice blank that has the precise diameter you are hoping to punch out.
Note: If this proves to be too difficult to produce a perfect gear we can move forward to option #2 which I haven't yet mentioned. Option #2 still requires that we punch out round washers. So we'll still need the punch press and compound die to punch out are washer blanks. We can then machine those blanks to finial precise profile dimensions by stacking them all on a transmission shaft arbor and machining them all at once in a batch milling operation.
So we'll need to make these round brass washers either way. And we'll still need to punch out the center hole with the wedge key. So this is the path forward. Unless you have better ideas.
Others have been suggesting using EDM, but that requires sending the brass out and having someone else make the gears. That would kind of defeat our whole purpose right? Another suggestion was to have someone else make the compound punch and die tools. And then we just punch them out. But again, doesn't that defeat our purpose? That would also be extremely EXPENSIVE.
My suggestions have been within the scope of what we might actually be able to do.
If these parts were 10 times bigger this would be a lot easier to do. But they aren't. They're tiny. So that's what we're stuck with.
I've been off in this thread, A Lawnmower Punch Press, preparing to design a punch press for us to build.
I even got a lawnmower to use.
I was going to take you though building the compound die one step at a time, but perhaps you can design the compound die on your own now?
From what I've learned in the Lawnmower Punch Press thread from Jim Dawson, using half-hard 260 brass should work well for this purpose.
The shear stress for half-hard 260 brass is about 52400 psi.
Have you learned enough from those videos to take a stab and calculating the clearances we'll need?
You could probably start simple with just the center hole punch and die.
See if you can figure out what size the punch and die need to be. We can deal with the more complex outer gear profile later.
The center punch will be extremely simple to make. It's just a rod with a v-groove in it.
The die will be a round hole with the triangular key sticking out of it.
Personally I would make the die in two pieces by just drilling and reaming a round hole. And then cutting a keyway slot to accept an insert we can slide into it from the bottom to stick out as the triangular groove cutter.
Don't worry about actually making the punch and die. Just calculate the clearance. And then see if you can figure out what size the punch and die should be to produce the correct size hole.
Tip: In real life engineers often cheat by doing things trial and error to some degree. You can make a round punch, a round hole die, set up a way to keep them lined up. Then put some brass material between them and whack the punch with a nice heavy hammer. This should produce the hole and the a piece that comes out of the hole.
You can then measure the hole and the piece that came out of the hole and see if you get the results you are expecting. If not, back to the drawing board. Adjust the sizes of the punch and die and try again.
You can use this same method for the larger circle on the outside diameter of the gear. You want to make sure you can punch out a nice blank that has the precise diameter you are hoping to punch out.
Note: If this proves to be too difficult to produce a perfect gear we can move forward to option #2 which I haven't yet mentioned. Option #2 still requires that we punch out round washers. So we'll still need the punch press and compound die to punch out are washer blanks. We can then machine those blanks to finial precise profile dimensions by stacking them all on a transmission shaft arbor and machining them all at once in a batch milling operation.
So we'll need to make these round brass washers either way. And we'll still need to punch out the center hole with the wedge key. So this is the path forward. Unless you have better ideas.
Others have been suggesting using EDM, but that requires sending the brass out and having someone else make the gears. That would kind of defeat our whole purpose right? Another suggestion was to have someone else make the compound punch and die tools. And then we just punch them out. But again, doesn't that defeat our purpose? That would also be extremely EXPENSIVE.
My suggestions have been within the scope of what we might actually be able to do.
If these parts were 10 times bigger this would be a lot easier to do. But they aren't. They're tiny. So that's what we're stuck with.
- Joined
- Feb 18, 2016
- Messages
- 451
Racer,
What I'm proposing may even be physically impossible. It's up to you do find out.
The hole we want to punch in the center is only 2.3 mm in diameter.
The material we hope to punch the hole into is 0.6 mm thick.
That means our hole diameter is only about 3.8 times larger than the thickness of the material.
We must be dangerously close (if not already exceeding) what a punch and die can even do.
See if you can find out what the limits are for the smallest hole you can punch in a given thickness of material.
We may need to give up on the punch press idea entirely.
I honestly don't know the answer. I've never worked with such small parts before. This is new territory for me.
What I'm proposing may even be physically impossible. It's up to you do find out.
The hole we want to punch in the center is only 2.3 mm in diameter.
The material we hope to punch the hole into is 0.6 mm thick.
That means our hole diameter is only about 3.8 times larger than the thickness of the material.
We must be dangerously close (if not already exceeding) what a punch and die can even do.
See if you can find out what the limits are for the smallest hole you can punch in a given thickness of material.
We may need to give up on the punch press idea entirely.
I honestly don't know the answer. I've never worked with such small parts before. This is new territory for me.
- Joined
- Feb 18, 2016
- Messages
- 451
I found this on this web page: Design Tips
Minimum Hole Diameters
Holes can be punched most economically when the hole diameter is 1 1/2 times greater than the stock thickness. Minimum diameter is related to the shear strength of the material. In softer materials this can be equal to or less than stock thickness. Stainless steel, on the other hand, would normally require a hole diameter equal to 2 times stock thickness. If the hole diameter is less than material thickness (or less than .032 dia.), it must normally be drilled and the burr removed.
Based on this "Design Tip" it looks like we might be ok. We're 3.8 times larger than the stock size. That should be ok according to this shop tip.
See, I'm even doing your homework for you.
Minimum Hole Diameters
Holes can be punched most economically when the hole diameter is 1 1/2 times greater than the stock thickness. Minimum diameter is related to the shear strength of the material. In softer materials this can be equal to or less than stock thickness. Stainless steel, on the other hand, would normally require a hole diameter equal to 2 times stock thickness. If the hole diameter is less than material thickness (or less than .032 dia.), it must normally be drilled and the burr removed.
Based on this "Design Tip" it looks like we might be ok. We're 3.8 times larger than the stock size. That should be ok according to this shop tip.
See, I'm even doing your homework for you.
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