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Slitting and Drifting Question


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I'm planning to forge a couple karambits using slitting/drifting for the large finger ring.  The plan is to end up with a hole of 1 inch (maybe slightly larger) diameter with the material a quarter inch thick and a quarter inch wide around the circumference of the ring. 

Is there a rule of thumb for material starting thickness in terms of how much thickness will be gained or lost by drifting?  For example, if I were to drift a 1 inch hole in quarter inch thick flat stock that was an inch wide, could I expect to maintain quarter inch thickness after completing the drifting? 

I can, and will, do a couple test pieces first, but if there are some general guidelines or expectations for change in material thickness, that could help me get started in the right direction.  I'd rather not mess up a pattern welded billet if I can avoid it with some good advice.

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The drifting shouldn't change the thickness at all. It should spread the width. If you slit it, you'll have two legs roughly 1/2" wide, still 1/4" thick. As you spread the legs apart, the overall width will increase, the length will decrease, but the thickness remains the same. The legs would individually still be 1/2" wide, so you would have to thin them out which would in turn increase that 1/4" thickness. The volume stays the same, you're just moving the metal from one place to another.

Rather than starting with a slitting punch, I'd be more inclined to use a slot punch, something like a 1/4" wide X 1-1/8" long, rounded ends, positioned a little more than 1/4" from the end. As the hole expands and rounds-out, you have less adjusting to do on the width around the hole. This would have less impact on the pattern as well.

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Makes sense.   I just wasn't sure if drifting a hole the width of the starting stock might stretch, and therefore thin, the material as the hole gets bigger.   I'll probably try a slit and drift, maybe a punch and drift, and possibly even drill then drift to see what works best for me.  Thanks for the suggestions.

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The factor not mentioned is How big a hole/slit will you start with?  If you start with a 1/2" hole and drift to 1" you will see some changes to thickness. If you start with a slit with a perimeter just under 1" you can drift it without much change. 

I strongly suggest you try a few times before you solo on the pattern welded stock.  A good practice project is doing bottle openers---note that a good diameter for the hole in a bottle opener is around a US quarter, which is .955" in diameter according to the US mint...so pretty much spot on!  They are fast and easy to do and so support experimentation with punching, slitting and drifting *AND* you have something you can sell or give away or use!

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There is a "formula" and it's not as simple as JME's description, it's close but not quite.

One rule of thumb I recall was the slitter needs to have sides that equal the circumference of the finish hole. So, without whipping out the calculator the slitter for a 1" round hole needs the sides to add up to  3.1416 or 1.57" wide. remember to round the sides to prevent cold shuts.

I don't know if the above is really necessary but if you want zero material stretching it seems plausible to me. 

I usually try it a couple times before doing it for keeps. In my world the best math is a rule of thumb guesstimate.

Frosty The Lucky.

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As getting the ends to the same arc as the middle can take a bit of finessing, I'd suggest someone starting out slit a bit undersized.  A rounded end slot punch makes it easier but does remove more metal at the start.

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Hopefully I'll get a chance to try this a few times this weekend.   In the interest of making sure I understand what is being said though....

So, if I want to end up with a hole with a 1 inch diameter, then I should make the initial slit probably somewhere between 3/4 of an inch to an inch long to minimize the change in thickness when drifting?

Frosty, if I want to end up with a 1 inch diameter hole, but the slitter has sides over 1.5 inches, won't I end up with kind of a tear-shaped hole rather than a circle?  Or as the sides which are parallel to the slit get pushed out it pulls the sides perpendicular to the slit inward to end up with a circular hole? I think I may be missing something there.

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Take a piece of string and form a 1" hole.  Collapse it till the sides touch in a line.  The interior length is the starting length for your slitter; adjust up or down as needed as shown by your practice pieces.

Note as adjusting it down is easy by grinding the slitter; starting a tad bit large is easier than starting a bit too small.

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That's how it looks to me too, I've never tried doing it that way it's anecdotal and who knows how many hands anecdotal. I've always just accepted the steel was going to stretch with change cross sections. The only circumstance that method made much sense to me would be centered in a bar you don't mind losing some length from. 

I probably shouldn't have related that one, I didn't mean to sow confusion and hadn't recalled how dubious I was in the first place. If you disregard it you aren't going to hurt my feelings in the slightest. 

And if it turns out to work, Thomas's trick is far easier and more accurate then using a calculator and measuring.

Frosty The Lucky.

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TP, that clicks in my head better.  Thanks for the explanation.   I think I have my mind around it well enough to have an idea of what to expect now.

Thanks for staying with me until something sank into my skull.

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I have not done much work with this myself, but have been a bit confused. I have read the slot punch width being 1/2 the drifted circumference and that makes very good sense to me, but reading Mark Aspry’s(sp?) book there was something about the slot punch shouldn’t be more the 1/8” larger than the diameter. Something about the drift can only push. What am I missing aside from shop time?

David

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It sounds to me like a 5/8" slot punch for a 1" dia finished hole is anticipating the sides being stretched to make the circumference. 

I haven't read Mark's book so I don't know the context or have the explanation in front of me but. What else CAN a drift do except push metal out of it's way? It's literally a specially shaped wedge isn't it? 

Frosty The Lucky.

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The formula for the length of a slit chisel for a specific size hole is pretty simple.

Half the circumference of the final hole or Pi x diameter/2.

Then the slit is NOT drifted round. This will draw out your material around the hole. You upset the hole to it's proper diameter then use a drift to tune it up round.

The idea is that there are two things we minimize. Upsetting the material around the hole and drawing out the material around the hole. 

That's why we upset the "hole", not the stock around the slit, no upset or drawn out material around the hole to mess with the simple formula above.

The formula is simple: circumference/2, or any other you choose to use such as Pi x diameter/2

Lol, as usual, the explanation is complex.

In Francis Whitaker's book " the blacksmiths cookbook" is a good simple description and a chart for slit chisel lengths for hole sizes.

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Doesn't that come out to 1/2 the circumference? Sure it's a lot easier way to figure it but the same answer. No?

I'm into simpler, thanks Anvil and Mr. Whitaker's, "Blacksmith's Cookbook" is where I saw it. I THINK. <sigh>

Frosty The Lucky.

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Second paragraph above I gave both formulas, just in case.  :)  I had a moment of panic and thought I said radius instead of circumference.

I've mentioned it here a few times, but I always do my best to name my source. in my mind it attempts to maintain a continuity with our heritage.

I have a set of six slitters I use for common size holes

I rarely punch and drift holes a yore. The extra material you get for the frogs eye is dynamic as is the ability to slit and drift a 1" hole pass thru in a half square bar and not have it look flimsy and weak!

Slit and drifted Square holes take a bit more figuring,  but they work about the same, just another few steps to square the round so to speak.

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  • 1 month later...

I was only mildly successful in this endeavor.  Some of the best results were using a slot punch about 1/8 inch thick and about 1 and 1/8" long.  Since it was fairly thin stock I had some problems with material being pulled downward as I drifted the hole.   It's hard to keep the hot stock supported close to the drift all the way around as the hole gets larger.  With some more practice upsetting first and refining the technique I think I could end up with a round hole and the right thickness.  However, time is not on my side here, so it was easier to employ a carbide-tipped hole saw in the drill press to get the desired results for now.

This is something I'll have to work on when I have more time to refine that skill.  Thanks for all the input though.

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After you punch the slot, start with a small, tapered drift, think of something like a center punch, to gradually open the slot into a hole shape, start in the middle of the slot. Continue using a tapered drift to gradually expand the slot into a fully formed hole, flipping the part over frequently and working over the pritchel or hardy holes will help support the material as you are drifting the hole.

Go back and re-read my initial response to this thread, with your experience now it might make a little more sense what I was trying to explain on slot size and placement for the initial punch.

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That's pretty much what I did on my more successful attempts.  However, I still get a minor amount of material distortion where the punch/drift makes contact with the material. I start on the pritchel hole until the drift is too big and then move to the hardy hole.  I usually flip the piece after 3 strikes to the drift, but I still end up with a small section where it is just a bit thinner than the surrounding material.

Your advice was helpful in getting me close to what I'm after, but I think I'll need to upset the material a little before and/or expand the hole a bit by driving the stock inwards towards the ends of the slots before using a drift.  One thing I'm fairly sure of is that I need more practice to get it right.

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A bolster plate with a series of spaced out increasing size holes in it can help prevent drag down as well if your pritchel or hardy holes are larger than the wanted drifted hole. 

I've seen a fancy one made using a circular piece of plate---1/2"? thick.  it had a rod the size of the pritchel hole attached to the center and then the series of increasing sized holes drilled along a circle that would fall over the hardy hole of that anvil. To use: drop the rod in the pritchel and "dial" the needed sized hole in the bolster plate to lie over the hardy. Drift and dial it larger as needed.  (I plan to make one like this as soon as I get power to the shop and can drill a lot of holes!)

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I'd think that starting with a 1/8" wide slot punch, your sidewall legs would be around 3/8" - 7/16" wide each, so you could use this mass to your advantage when you go to thin the wall down to that 1/4" target. Sounds like you're on the right path and a few more tries will get you closer to your desired end result.

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6 hours ago, anvil said:

we need to know the width of your tang.

Asking about somebody's tang in public, Anvil?! :o A little bit nosy don't you think?

Frosty The Lucky.

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21 hours ago, anvil said:

To know the width of your punch, we need to know the width of your tang.

 

I can make that dimension more or less whatever I want it to be for the final product since it will be made from a pattern welded billet.  I'd prefer to waste as little material as is reasonable of course, but I'm not stuck with a particular size of starting stock.  Unless/until I get more proficient at slitting (or punching) and drifting large holes in thin stock with relatively thin side walls afterwords I'll probably stick with the hole saw.

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