Geometry of a slot punch

[Osvaldo]

Would need help with the design and material required to punch slots in an AISI 316 stainless steel plate, .039" thick. Slots geometry is .039" width x .500" long. The parts are"scallops" (an oil refinery equipment within distillation towers). Have tried several designs but non of them work. After several hundreds slots punched, the punch broke. The slots are separated .125" sideways and .125 length way.

 

[berkeman]

What material are you using so far for the punch and the die? How many slots are punched per workpiece?

Is there an alternate construction method for this (like welding strips together with spaces instead of having to punch the holes)?

316 Specs: https://www.azom.com/properties.aspx?ArticleID=863

 

[ChemAir]

Can you show pictures of the designs you have tried and give dimensions of the punch and die?

The big red flag I see is your slot width is very similar to your material thickness, so your tool is the same thickness as the sheet. Punch and die material of construction will be important, and the punch will still be delicate.

In some cases, if there is a further forming step later, a punch and form is sometimes done in one step to allow a stronger multi-purpose tool to make the hole and bend the piece at the same time. I have seen some distillation column packing made in a "punch/form" process rather than two machine operations. If the final shape will be different, incorporating the forming step into the same one that makes the holes might be an option, but tooling/design could be an issue.

A look at some video of a sheet perforating machine might be worth an idea or two.

My best suggestion would be that this job might be better suited for laser or HD plasma cutting.

 

[JBA]

1. Please describe in what manner the punch "broke". Are all of the punch and die designs you have tried been designed by you inhouse or do they also include punch and die sets procured from established manufacturers of these sets?

2. A key element in the design of any punch and die set is the correct punch to die clearance for the thickness of the material being processed. The use of a punch and die set with an incorrect clearance for the target material and its thickness results in either material tearing or rapid punch and die wear.

 

[jrmichler]

Look for signs that the 0.039" wide sheet is being pulled towards the punch, causing the punch to bend sideways and break. If so, try making the punch a snug slip fit inside the blankholder, and increasing the blankholder force. You may need to increase blankholder force by a factor of several times. Consult a specialty lubricant supplier about lubrication for punching 316 SS. Search term specialty lubricants for sheet metal forming had some good hits.

Also, what @JBA said about clearances.

 

[Osvaldo]

Will try to answer all question posted. First, the picture above is the shape of the part, the number of slots and so on.

1. The slots are punched 15 in a row, with the plate flat. Then is curved as per requirement. However this is not important now.
2. Punch die materials is Boheler K-100, quench and tempered to 55 HRC.
3.It is not possible to an alternate process. Welding is not allowed and laser or HD plasma are not available.
4.Can not paste the images here. Can you send me an email address so I can email geometry of the punch and picture showing how they failed. They are both designed and manufactured in house.
5. While punching, mineral oil is sprayed to the puncher. Please advise if instead other lube should be applied.
6. The design clearance is .009" (total clearance)
7. We have not tried yet to have the punch snugged. But may be we should have to.

 

[berkeman]

4.Can not paste the images here.

As long as you are not using the Mobile PF App (so you are on a laptop or using a Browser on your phone), you can use the UPLOAD button in the lower right of the Reply window to Upload a PDF or JPEG file.

 

[Osvaldo]

This is a drawing of the punch.

 

[Osvaldo]

Picture of broken punches.

 

[jrmichler]

2. Punch die materials is Boheler K-100, quench and tempered to 55 HRC.
5. While punching, mineral oil is sprayed to the puncher. Please advise if instead other lube should be applied.
6. The design clearance is .009" (total clearance)
7. We have not tried yet to have the punch snugged. But may be we should have to.

2) Bohler makes several die steels that have better wear resistance, are tougher, and low distortion in heat treatment. A tougher steel would help reduce the type of breakage you are having. Talk to your steel supplier, or better yet, talk to an application engineer at Bohler. Also, review the heat treatment with the heat treater. Different heat treatments will give different toughness with the same hardness. This is a case where spending a little extra money on a better alloy and heat treatment will pay back many times over. If you need to make a business case for a better alloy AND heat treatment, be sure to include the cost of the broken punches, including the cost of the downtime.
5) Mineral oil is about the worst possible lubricant for sheet metal stamping. You need a lubricant with specific additives for stamping 316 SS.
6&7) The optimal clearance between punch and die is 2% to 10% of material thickness, or 0.0008" to 0.0039" on each side. That tolerance includes the width of the slot in the die, width of the punch, and how accurately the die set locates the punch relative to the die. Remember that the die set has clearance in its bearings, so the punch is coming down in a different location each stroke. Check the design, and do a tolerance stackup calculation. You want the punch/die clearance to be 0.00235" +/- 0.0015" at all times. This may require working with the die set manufacturer and/or better maintenance of the die set, and tighter tolerances on the punches and the die.