# Cutting Thin Sheet Metal - WITHOUT any buckles!



## AES (19 Feb 2018)

Following my piece "Hacksaws, how to choose 'em & how to use 'em + other stuff" posted here in late 2017, esteemed member "custard" raised an interesting question. In brief, he needs to cut pieces of quite hard saw steel stock (ca. Rockwell 50 hardness) into 2 inch x 1 inch (50 mm x 25 mm) rectangles. His material is approx 0.030 ins thick ("thirty thou" or 0.008 mm) and he does NOT allow any distortions in the finished piece.

In reply both I and CHJ briefly suggested a method which would meet his criteria. Since then I've tried myself and here are the results:

All custard's conditions were reproduced with the exception of material - I didn't have any thin sheet steel of Ro. 50 to hand, but my stock did reveal some 0.031 inch ("thirty one thou") steel sheet recovered from a busted ink jet printer. I GUESS this material is mild steel (it's hard to be sure, the spark test was inconclusive) but it has been very well plated (cadmium?) and from the single centre punch indent I used to test it (an automatic spring punch set to max pressure - I have no proper hardness testing gear), I believe that while it's certainly not as hard as Ro. 50, it's certainly "a bit harder" than "normal" MS sheet.

Custard did explain that his job involves filing "teeth" into the sheet after cutting to size, so even though the test piece I've used for this test is a bit softer than custard's material, I'm quite sure that all three methods I used will happily cut his material. The only problem then becomes "no dents and buckles allowed"!

The method involves making a simple wooden holding jig, then anchoring the sheet firmly top and bottom very close to the cut while actually cutting.

Rather than my usual "lots and lots of words" I've decided to try a mainly WIP photos style this time.

First the jig, as shown. It's designed to allow the job to be held above bench top level in a woodworking vice just in case custard hasn't got a mechanics vice available. It's very simply made from some scraps of ply and softwood, the dimensions being based on custard's required 50 mm x 25 mm, though of course these can be adjusted to suit whatever a job needs:

CTSM - Sketch 1 - Jig






Important is to choose one of the dimensions of the jig "table" to be SLIGHTLY LESS than one of the required finished dimensions. I chose a whisker under custard's required 25 mm as dimension "X". The second dimension ("Y") of the jig "table" is RATHER MORE than the other required dimension. For this I chose roughly 75 mm. This extra allows sufficient space for clamping.

The only other important dimension concerns the height of the "fence". This should be whatever the thickness chosen for the jig "table" PLUS about 1/8th inch (3 mm). In this case the overall height chosen was 18 mm ("table" material thickness is 15 mm).

The 5 mm thick ply "fence" does need to be square to the side of the "table" side AND at exactly 90 degrees in both aspects. Here's the finished jig, also showing the roughly 30 mm x 20 mm softwood holder which clamps the jig into the vice:

CTSM - Photo 2 - Jig





Before "parcelling" (see below), one edge of the material to be cut needs to be straight and true. In the case of my scrap piece from the printer it already had one square straight edge. If it hadn't it would be a simple matter to file one side straight and flat, if necessary, "parcelling" beforehand, as described below.

The "parcelling" simply wraps the job securely into a sandwich made up of a stiff but softish lower sacrificial piece, followed by the material itself, and finished with a sacrificial upper piece, also stiff but softish. The material could be scrap ply, thickish cardboard, or even aluminium sheet. I guess that provided they're both stiff, two different material scraps could be used for the upper and lower layers. In my case I chose some stiff cardboard approx 2 mm thick for both layers, but whatever is chosen, the important point is that it should be VERY stiff while not being too thick.

The required finished dimensions were then accurately marked on what will become the upper sandwich layer.

Good thick transparent adhesive parcel tape was used to TIGHTLY wrap the three layers of the sandwich together, making sure that all three prepared straight edges are accurately lined up. Here's the three parts of the sandwich:

CTSM - Photo 3 - Sandwich Parts





And the complete sandwich ready for the first cut:

CTSM - Photo 4 - Sandwich





As shown below, I made the first cut with a hacksaw fitted with a 32 TPI bi-metal blade and used a small triangular file to make a vee cut in the right place to start the cut:

CTSM - Photo 5 - 1st Cut Setup





CTSM - Photo 6 - Cutting 1st Cut





The results shown below look OK to me - NO dents or buckles anyway.

CTSM - Photo 7 -1st Cut Complete





VERY important is to re-tape that cut edge and then file a starting vee BEFORE relocating the job in the jig ready for the 2nd cut:

This time I made the cut with a Dremel "Speed-Clic" abrasive cut off disc in a Dremel drill running at about 15,000 rpm. I found it a little difficult to get a comfortable grip on the drill which also allowed me an accurate cut and I guess a little practice before attacking the real job would have been sensible - REMEMBER the disc's direction of rotation when working out the best grip for making accurate, straight cuts! And again do remember the filed vee cut for accurate starting:

CTSM - Photo 8 - Making The 2nd Cut





And here's the result:

CTSM - Photo 9 - 2nd Cut Complete





As before, replace the parcel tape after completing the 2nd cut and BEFORE starting the 3rd cut. This helps prevent distortions as I believe that it's the stiffness of the sandwich that prevents distortion during cutting. You'll notice in the picture that part of the lower tape/cardboard did actually break away a little towards the end of the second cut but fortunately that didn't cause any problems.

CTSM - Photo 10 - 3rd cut





As stressed above, re-taping the newly cut edge to maintain the stiffness of the whole sandwich is important. I then made the 3rd and last cut with my Excalibur 21 scroll saw using a LUX brand "Fine" grade metal cutting blade. I had a bit of a problem with getting enough blade tension to keep a dead straight line (I hadn't used this particular blade before and had temporarily run out of my usual Pegas metal cutting blades). But the result (above) wasn't too bad:

Of course, as I deliberately chose a piece of material with one existing straight line no 4th cut was necessary in this case.

The finished result is shown below (cont. on 2nd, related post). 
Ah ha! Forgot, sorry folks. The limit on pix is 10, so this continues on the next post.

AES


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## AES (19 Feb 2018)

CTSM - Photo 11 - BEFORE Deburring





Overall the scroll saw is probably the quickest method (WITH a better blade), the Dremel disc is MAYBE a little quicker than using the hacksaw but there's not much in it time-wise, and without a bit of practice on a test piece with the cut off disc beforehand, I found the hacksaw quite a bit easier to guide accurately. And a hacksaw with a 32 TPI blade is the most likely to be found in just about any woody shop!

I was not sure exactly what "saw tooth cuts" custard requires for his particular job, but with the right material to hand it may well have been possible to mark those "teeth" out onto the top layer of the sandwich job before cutting, thus killing two birds with (almost) one stone.

Custard, as soon as I get your address (see your PMs please) the little jig and the finished test piece will be on their way to you by post.

HTH some members.

AES


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## novocaine (20 Feb 2018)

cough cough, 0.8mm not 0.008mm, cough cough. 20 gauge. 

:wink: 

lot of effort but it'll work, could also do it with a a piercing saw (jewelers saw) and get no deflection but it's a bit harder to cut a straight line.


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## AES (20 Feb 2018)

I'll send you some cough drops novocaine (but thanks for spotting my mistake - my bloody auto conversions calculator only has the cursed centimetres on it, not mms, so I got the decimal point in the wrong place - invention of the devil, cms are)!  

Yeah, it's a bit more effort, (NOT a lot if it's a job that needs to be done regularly IMO) but it was basically written for custard (who asked the question and who said he's had problems with that job, and who, as far I know, doesn't necessarily have a piercing saw). AND, just as you say, cutting straight lines with a piercing saw isn't all that easy - takes a LOT of practice.

Thanks for the comment anyway.

AES


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## Tasky (20 Feb 2018)

AES":2rx4af94 said:


> In brief, he needs to cut pieces of quite hard saw steel stock (ca. Rockwell 50 hardness) into 2 inch x 1 inch (50 mm x 25 mm) rectangles.


Wha's he makin' wi' this , anyways... Brigandine armour?

Only asking, as that's exactly what I did.


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## custard (20 Feb 2018)

It's for scratch stock cutters. Here I am cutting a bead with a scratch stock,






And here's the finished result,






Line many woodworkers I'm constantly making specially shaped scratch stock cutters for specific jobs, and I normally get the cutter material by cutting it out of an old hand saw plate or by using an old card scraper. The problem is that when cutting out the blank the edges get slightly wavy or rippled, which then means quite a lot of work stoning them flat to achieve a mirror surface all around the actual cutting edges.

So very many thanks to AES for his advice. I'll certainly be giving this a go.

Thank you.


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## novocaine (20 Feb 2018)

I'm lazy, I use an old stanley knife blade for scratch stock material.


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