8" Mini Shear Break

This is a combined 8" sheet metal shear and an 8" bending break.  It's sold by Harbor Freight as their SKU number 90757 and by Northern Tool + Equipment as Item # 143361.  The Harbor Freight version is painted red and the Northern Tool version is painted blue, but otherwise they seem to be the same.  The Micro-Mark 83213 is painted White and includes the cutoff guide (Missing Accessory on the other two versions).  The Micro-Mark Manual has some different information from the Harbor Freight manual.

This may also be called a pan and box break because it can bend metal that already has prior bends.  Also called a press break, which is different than the folding type of break.  The Harbor Freight 18" break 39103 is similar to a flat table top with a hinge.  To use it you need to trap the metal using the supplied bar with C clamps then pull up on the handles.  But this type of break can not make a box where there has already been a prior bend.

This is made by Sieg, the same company that makes the Mini Machine Tools.

There is a Profiform version that comes in two widths called the 200 (8") or 320 (12").  It's manual goes into some detail about setting the cutting tools.  When properly set you can cut typing paper.  This version has many accessories available that would also work on the Harbor Freight version.

The shear is at the top and consists of a horizontal top cutting bar and a sloped lower cutting bar.  There is a gap in the top of the casting that's less than 2 mm  0.079" deep that limits the thickness of material that you can cut (1.5 mm spec = 0.059").  1/16" printed circuit board material (0.0625) just fits and is easily cut.  It would be better if the casting was sanded to knock off the high spots (mainly paint but also casting bumps).  This was my application.

4 Jun 2007 - The shear will cut the ExpressPCB Mini boards (2.5" x 3.8") where typically the first cut is about 1.25" with no problem.  But if you're cutting a board that has 3 or more inches under the blade then it's very difficult.  I've found using a scribe to weaken the board on the top and bottom prior attempting to shear it helps.  For really large boards I still use the scroll saw that bolted under a table in the garage.

27 Oct 2007 - The Harbor Freight 38413 Throatless Shear may be a better way to cut printed circuit boards?  If you have one let me know.

A 3 foot floor mounted sheet metal shear that's foot operated will cut a 12" PCB like it was butter, but they are big and cost more than what I want to spend for this.

I think the problem is the design of the mechanism that drives the blade.  There's not enough leverage and with more leverage the drive pins will snap.  But I haven't done any calculations to see what the stresses are in the shear and frame.  If they are low for say a 5" wide 1/16" double sided PCB then it would be worth looking at a better drive system, like a hydraulic ram.

Missing Accessory - Cutoff Length Guide

The Lower Blade Support Bar (Item 8) has a couple of 8 mm holes in each end and there are a couple of pinch bolts that are intended to hold something.  As of 16 Feb. 2006 I think the missing accessory is a right angle cutting guide.  When installed the guide would allow you to align material on the guide and then make a cut 90 degrees from the guide.  This would allow squaring up material that started out with only one straight side.  But I have not seen anyone offering this useful accessory.  As it is now if you cut out a piece of metal that's small enough to fit into this mini shear using tin snips or a power sheet metal cutter (Harbor Freight 92148 electric sheet metal shear) it will not be square.  So without the missing guide you need to do layout and marking of the work piece just to get it square.  With the missing guide it would be a snap to square it up.
The main purpose is to both allow cutting the same length each time.  If care is used to set the stop bar parallel to the cutting tool then it also allows the cut edge to be parallel to the free end of the material.  It would make a nice accessory to have a fence at right angles to the cutting tool to allow squaring up a sheet of metal.

Guide (Item # 16)

The Guide (16) has a couple of thumb screws that pinch on the two 8 mm rods coming out of the lower "V" groove die.  The purpose of the guide is to both set the length of metal to be bent and to keep it parallel to the bending line.  The Guide has a lip that must be positioned on the top side and facing the bending dies.  It would be wrong to feed the metal below the lip and above the rods because doing so would not allow the metal to move upwards as it's bent.  So the tip of the lip must be the intended stop for the metal.  But the amount of lip protrusion is only 4.83 mm (0.190").  This is about the length of the flat on top of the lower die, but it is not long enough to be over the bending line.  If it was half (0.311") of  the lower die width (15.71 mm or 0.622") then when closed the length of the material to be bent would be zero.  If the guide was moved 0.25 inches away from the lower die the material that was bent would be 0.25" , etc. making setting the positon of the guide easy.   BUT that's not the case.  So was the guide made improperly, or is there some reason for the strange lip protrusion distance?

Also, how to determine which "V" goove in the lower die should be used?

Shear Blades (9 & 10)

The blades are rectangular in cross section so that they can be installed 4 different ways, each of which presents a new cutting edge.

• Cutting Printed Circuit Boards - I received an email saying that the 8MSB they received would not cut a 1/16" PCB out of the box.  I'm guessing this particular unit got mis-aligned during shipment and/or has a number of surfaces that are painted instead of smooth and greased.  TBD.
  
• Making Zero Gauss Chambers (mini boxes made from Mu-Metal)
• Prototype sheet metal work
• Learning what can easily be done and what's impossible
  

Bending Metal

In the above photo the top 8 inch die has been repositioned to the left and a couple of the small top dies have been installed on the right.  The hope was to have just one setup, but for a mini box that's 1 x 1 x 3 3/4" it did not work.  After some bends  you need to remove the top dies to get the part out of the break.

I'm making drawings and printing them at 1:1.  Taping the drawing onto the metal and cutting the metal to the drawing.

Do not use scissors or shears to cut the notches.  It will cause the metal to get bent (not flat).  Do use a nibbler which does not bend the metal then with a vise and file clean up the edges.  I tried using the 18 Gauge Electric Metal Shear 92148 but the thin metal was very distorted after cutting.
Klein makes a nibbler very similar to the one I have.

Maybe after a lot of practice my precision will improve, but for now some tolerance needs to be part of the design.  Even better is to make one half of the box, measure it then make the other drawing to fit the existing part (selective assembly was the norm before tolerances were developed.)

8" Digital Calipers (Harbor Freight 47260) are very handy for design and layout of parts for this shear break.

Cutting PCBs

Each time you use the 8MSB for cutting anything, cycle the blade up and down to be sure that it moves freely and that the blades do not collide.

The stock 8MSB has paint runs on the top surface of the frame casting and these will keep a 1/16" from being positioned in some spots and may keep it out all together.  Sand paper will get rid of the high spots.  Still working on a good way to do this without taking apart the top cutting blade.

Drive Pins

29 April 2006 - Yesterday I received a replacement 8MSB and cycled the cutting blade up and down with nothing in there and it worked fine.  Then tried to cut a 3 3/4" wide PCB, but it would not fit, so used sandpaper to knock off the paint runs.  Then cut the board up with no problem (this done without bolting it down).  This means that the first 8MSB was put out of alignment by my wife using it as an anvil.  The hammering on the top blade clamp did something to the blade alignment.

When I get the new drive pins for the old one I'll learn how to do the blade alignment.  This also means that you should not use washers or other spacers to widen the gap since they may not be stable enough to maintain the blade alignment.

20 April 2006 - Tried to cut a panelized printed circuit board but it would not cut.  I don't have the shear break bolted down and in the past was able to easily cut printed circuit boards that were narrower.  So tried using more force, but you can't use much when the shear is not bolted down.  Then without hearing or feeling anything unusual the lower blade stopped moving.  After taking it apart is was clear that both drive pins had sheared.  Harbor freight offered "used" parts, i.e. taken off another mini shear break that was not sellable, but I opted to get new pins which will take 6 to 8 weeks. 

After more inspection the following problems are apparent:

• Many surfaces where metal parts are rubbing against other metal parts are painted on one or both surfaces.  The paint needs to be removed and replaced by grease.
• Where metal parts rub against each other there are burs that need to be filed down.
• The Drive pins (Item # 19) are a poor design.  The tapped hole extends to the narrow neck area making the neck very weak.  A 0.1365 inch diameter hole through a metal part that's only 0.280 O.D. leaves only a 0.071 inch thick wall for an area of  0.047 square inches.  If the tapped hole was a little less deep the area at the neck would be 0.062 square inches or 130 percent more.  In addition to the difference in area, the hole also acts as a stress riser further weakening the part.  The tip of the drive pin should be a sphere, but these look more like a couple of chamfers were used to approximate a sphere.  There are shiny metal spots on the edges of the flat part.
  

I expect that between the paint and burs the sliding lower blade assembly jammed on one side causing it to rotate and jam tighter.  Rotation would also tend to break off the drive pins.  My wife was using this as an anvil, pounding on the top blade clamp with a small hammer and may have knocked the blade out of alignment.

A new mini shear break has arrived and is not being taken apart. 

Harbor Freight will take orders for spars parts, but it takes many many months to get them (so for nothing has arrived).  Micro Mark stocks cutting blades but will not order any other spare parts.

Shear

On a few occasions I've been able to make a nice cut, but the next time not.  You would think it would work on the 10 mil Phosphor Bronze (see below or brass).
Maybe there needs to be another adjustment or fix.  So examining the parts:

The Lower Blade (9) sits on top of the Lower Blade Body (6) and they both ride in an open channel on the front of the base.  Any slop between the moving parts and the base will result in a repositioning of the lower blade. Lathes have gibs to adjust this slop but here there are none.  Maybe offsetting each end of the blade relative to the lower blade body is the adjustment method?

The base (1) has the Upper Blade Holder (14) and the gear box parts: 30 Driving gear 27 Gear Shaft (with hole for Handle (31) in front of the gear shaft you can see the end of 22 transfer gear shaft and the 23 transfer gear is out of sight. The two gears have the same number of teeth and serve to change the direction of rotation. The far end of the main shaft (20) rides directly on the casting in the open hole at the left of this photo.  In a similar manner the near end rides on the casting. The left open channel for the lower blade body (6) and lower blade (9) can be seen at the left of the photo at the front.

These are pending more work on the old unit.  Coming about June 2006.

• removing paint that's between metal to metal moving surfaces and replacing it with grease.
• polishing the drive pin ends into shiny spheres and greasing them.
• Confirming that the slide assembly moves smoothly up and down without binding and greasing it.
• Remove the top cutting blade and it's support bar and removing all the paint on the top.  This is the bottom of the slot where the printed circuit board needs to fit and if the paint is too thick or has a run a 1/16" PCB will not fit.  So this is mainly to be sure the board fits cleanly.
• Clean off the old waxy grease from the drive gear teeth and replace with fresh grease.
  

I expect that the new replacement parts will also have the tapped hole too deep.

Brooke's web pages

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Length & Weight Measurements -  tools
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Mini Cut-off Saw
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