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View Full Version : Made a simple sheet metal knockout punch



Jaakko Fagerlund
04-10-2013, 12:02 PM
I was doing some sheet metal work today and needed a nice looking round 20 mm hole in the galvanised 0.75 mm thick steel sheet. First tried drilling, but what a big suprise, it doesn't go well with a thin sheet (don't ask). After hiding all the evidence, I thought of going shopping to purchase a step drill or a sheet metal punch...hey, wait a minute, I'm a HSM and a tool maker! So, time to make one.

Material chosen was some mystery metal, all I know it is prehardened and possibly close to Imacro. Machines nicely, gives beatiful finish and is tough. I could have chosen some hardenable tool steel like 1.2510, but as I only wanted two holes made with it I didn't want to bother with hardening.

So, off to the lathe and first shaved out the punch part. The diameter is 20.0 mm and a 10 mm hole through the center. I first drilled to 9.5 mm and then with a 10 mm drill to make a pretty good hole for a bolt to enter.

Next part was the die. I drilled 19.5 mm diameter to a depth of 5 mm and bored it to a diameter of 20.2 mm to give clearance for the punch. And the same operation as for the punch, a 10 mm hole through the center.

Found a nice M10 x 70, 12.9 hardness grade. Took couple of washers and an M10 nut (nothing special) and now I had these:

http://i1291.photobucket.com/albums/b543/lankasahaus/sheet%20metal%20punch/th_sheet_metal_punch_1_zps3b9e45f1.jpg (http://i1291.photobucket.com/albums/b543/lankasahaus/sheet%20metal%20punch/sheet_metal_punch_1_zps3b9e45f1.jpg)

After assembling them it looks like this:

http://i1291.photobucket.com/albums/b543/lankasahaus/sheet%20metal%20punch/th_sheet_metal_punch_2_zpscac14cc8.jpg (http://i1291.photobucket.com/albums/b543/lankasahaus/sheet%20metal%20punch/sheet_metal_punch_2_zpscac14cc8.jpg)

Well, of course you assemble the thing through your pre drilled/punched sheet metal and then just tighten the bolt and nut until it makes the hole.

Once the punch goes through, you discover why the commercial punches have a reduced diameter behind the cutting face. It should be there so you can pull the punch through the plate with the screw, as the punch sits like a frozen duck in a pond in that hole it just made.

The punch I have done and shown was used to cut only two holes and required a little bit oomph when tightening the SHCS and the nut. If you want to lessen the force required, grind/mill/file the punch end so that it looks from the side like someone drilled the end. This way the punch cuts only on a small portion at a time, lessening the force required to pull it through.

Also a way to reduce force is to put a small thrust bearing under the bolt and nut. If you look closely at the first image, you can see the friction marks on the washers and nut face.

Also the clearance between the die and the punch affects the force required, but not much in this case. The usual total clearance for steel sheet is 16-20 % of the sheet thickness. For my 0.75 mm thick sheet it should have been 0.12-0.15 mm, but the 0.2 mm difference in the punch and die diameters was a good tradeoff, especially when I just wanted to see if this works :)

Mike Burdick
04-10-2013, 12:47 PM
Maybe yours is like this, I can't see for sure, but ...

I made a similar punch but "V" cut the end so it would start cutting a small section on opposite sides at at the same time. Of course this increased the travel distance needed to punch the hole but it decreased the punching force dramatically.

.

MaxHeadRoom
04-10-2013, 01:27 PM
I made a similar punch but "V" cut the end so it would start cutting a small section on opposite sides at at the same time.
.

The commercial ones are also like this, also it ends up bending the slug to a vee so it is easier to remove the slug from the female part.
Max.

Jaakko Fagerlund
04-10-2013, 03:32 PM
Maybe yours is like this, I can't see for sure, but ...

I made a similar punch but "V" cut the end so it would start cutting a small section on opposite sides at at the same time. Of course this increased the travel distance needed to punch the hole but it decreased the punching force dramatically.
That is exactly what I meant to say, thank you! :) Sometimes it is hard to find a description for something, especially when a little sleepy or hungry.

But yeah,would have made the punch very easy to operate if I had done that V shape to the punch end, but lazy tool maker was lazy and didn't like to fire up the mill or use an angle grinder nor file, not because of two holes.

Now I'm just thinking that maybe in the future I need to do more holes in sheet metal (usually for electrical work) and I'll just machine a nice set of the usual sizes (12, 16, 18, 20 and 22 mm) and harden them properly to make them last and to really get them sharp by grinding.

Or if I do the set, I'll never ever again have a need for them :D

Paul Alciatore
04-10-2013, 04:51 PM
The commercial ones are also like this, also it ends up bending the slug to a vee so it is easier to remove the slug from the female part.
Max.

This Vee shape on the cutting edge does one more very important thing. It greatly reduces the amount of force needed at any one time during the cutting procedure. With the twin Vee shape, there are only four "points" where the actual cutting action is occurring, one on each side of each Vee point. With this feature, a two inch diameter cutter only needs a very small amount of additional force over what a half inch cutter needs. If you cut the entire circle at once, you need four times the force for a two inch circle over a half inch one.

This is a more important feature than the ball bearing drives, which also reduce the torque needed to drive the bolt.

In experiments with the commercial punches I observed this relatively constant cutting force for different sizes of cutters. Square cutters need more force because they usually cut from all four corners at once.

Kind of like a straight line metal sheer with a sloped cutting edge which only cuts at one point at a time.

And yes, clearance is very desirable on the back side of the punch for easier removal. This clearance can be a simple step to a reduced diameter, a short distance behind the cutting edge.

MaxHeadRoom
04-10-2013, 05:02 PM
I just bided my time for a nice Greenlee set off ebay, now I am thinking of incorporating the hydraulic option!.
Just use my manual pump off of my hyd. press.
Max.

oxford
04-10-2013, 05:49 PM
The "v's" are nice but the slugbuster style are much better. They actually split the piece being punched out in half, they then just fall off the screw instead of getting stuck on it. Once you go hydraulic you won't want to go back, especially with big holes or stainless.

Stern
04-10-2013, 05:57 PM
Have a few sets of Slugbusters as I do a lot of Stainless hood work (making large 2"'ish holes) and don't find it that bad to do (have used hydraulic ones, but they are sometimes a pain in the ass). Part that sucks is to make the hole size I need I have to use a punch to make the drilled hole big enough for the right size punch bolt, so every hole takes 2 punches :(

oxford
04-10-2013, 07:31 PM
^^^^^^^I do the same thing when using the 3/4" conduit KO. It is easier to drill the 3/8 hole punch and punch again instead of drilling the big hole. Fwiw the KO specifically made for stainless do work a lot better than the standard ones.

wierdscience
04-10-2013, 08:32 PM
^^^^^^^I do the same thing when using the 3/4" conduit KO. It is easier to drill the 3/8 hole punch and punch again instead of drilling the big hole. Fwiw the KO specifically made for stainless do work a lot better than the standard ones.

Yes it is a PITA,couple weeks ago I had some 400amp disconnects to hang that were 12ga steel,the chewy kind.Bought one of these at the Local Home Despot-
http://www.idealindustries.com/prodDetail.do?prodId=36-300&div=3&l1=saws_drill_bits&l1b=tko

Thing worked great,better than a Uni-bit.I think I'm gonna go grab another for the 1-1/4" studs on the bigger KO punches.

J Tiers
04-10-2013, 10:25 PM
Or if I do the set, I'll never ever again have a need for them :D

Ain't THAT the truth............

I once made a double punch, to cut both holes for a standard US double electrical outlet at one time..... I had been modifying a certain kind of equipment, and had needed to cut the hole for an outlet several times.

I made it with the "v" shape and all, it worked well when I checked it out, and I was all kinds of happy at my minor success "toolmaking".... ready for the next modification project.

I used it as-intended ONE TIME, ON WHAT I DISCOVERED WAS THE LAST UNIT I HAD TO DO..............

Well, at least it worked well.....

dian
04-11-2013, 07:05 AM
i never used these punches. what alines them, just the bolt? would have to be a very close fitt.

Jaakko Fagerlund
04-11-2013, 09:14 AM
i never used these punches. what alines them, just the bolt? would have to be a very close fitt.
Yes the bolt aligns it. Doesn't have to be very close fit, as the punch and die has about 0.2 mm total clearance. A drilled or reamed hole the same nominal size than the bolt is just fine, just make sure it is at the center and straight.

Paul Alciatore
04-11-2013, 11:20 AM
Yes the bolt aligns it. Doesn't have to be very close fit, as the punch and die has about 0.2 mm total clearance. A drilled or reamed hole the same nominal size than the bolt is just fine, just make sure it is at the center and straight.

Ditto on the bolt doing the alignment. It is a close fit both in the threads and in the hole in the socket end of the punch set.

If you are talking about the hole in the socket part of the punch set, yes it does need to be a close fit for alignment. But the hole that the bolt passes through in the work can be rough and off center and it makes little or no difference.

It is also important to keep BOTH ends of the punch set sharp. It is also a very good idea to use cutting fluid (oil) on the cutting edges and some grease on the threads and under the shoulder of the draw bolt if it is not a BB type.

These punches are always available on E-bay. I have had good luck with used ones: the major problems with them is breakage or stretching of the draw bolt. Greenlee uses a bolt which is better than a grade 8 by a significant factor. You can purchase new bolts from them if you get a punch set with a bad bolt. For the LARGER sizes of the punches a grade 8 bolt will be OK. If a smaller size punch has a 1/4" draw bolt, you may need an OEM bolt as the smaller ones break more readily (see my post above on the force needed). I purchased a number of lots of punches via E-bay and managed to get an almost complete set from 9/16" to over 2" diameter for a modest cost. I sharpened and sold some duplicates to keep my cost down. They can be sharpened on the flat side of a wheel or with a cup wheel. No complicated shapes are needed.

Jaakko Fagerlund
04-11-2013, 12:06 PM
Here is link to a picture of a different kind of system. It has a speed lock (spring loaded, can be clipped in to place) and the rod is pulled with a thread or hydraulic unit. This kind of tool can have very precise holes and the shaft, basically a nice sliding fit.

http://prosource.graybar.com/sites/default/files/images/greenlee/7212SP11_7212SP-1-1-2D_7212SP-1-1-2P_SP-Lock_4_new_hr.jpg

And here is a picture of hydraulic unit, powered with a hand pump, but has those basic threaded punches.

http://www.shop.yourtoolingstore.com/images/GreenleeSlugBusterHyd7306.jpg