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Slitting problem


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I have been trying to slit and drift for a morticed and tenoned piece as part of a spit project I am working on. 

However, I am finding that when I try to slit the piece, a chunk of 1/2" square, the slit is always very off-center and not very straight. 

What small piece of advice am I lacking? 

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hard to recommend a precise fix without being able to see where it went wrong in person, but there are a few things you can do to mitigate the problem.  These are just things that I learned from Mark, instructors at the school, and Mark's books (IIRC MTF Vol 3 has more and more detailed cross sections and information regarding the subject than vol 1, and I haven't had the time to get into vol 2 yet)


when starting the slit give your chisel (or slot punch) a half blow at first and eyeball the placement, if you start off center it becomes that much more difficult to end in the center, realign your tool and give it another thump, rinse and repeat until you are satisfied.


if your chisel bevel is not symmetric the tip will drift towards the short side as it penetrates, causing your hole to end up non perpendicular to the face you are slitting.  counteract this by rotating your chisel 180 degrees every time you take it out to quench.


it may feel like you are holding the tool vertically, this may or may not actually be true! a friend, mirror, or videocamera will all reveal this.  your hammer strike might not be coming down totally vertically (especially if your working height is less than ideal for the thickness of stock and length of your chisel to the struck end) which can cause the force transmitted to be at an angle instead of vertical.  mitigate this by making sure your tooling has a loosely crowned top so the impact occurs more towards the center of the tool rather than catching an edge first.  mitigate also by rotating the piece you are slitting (if possible) 180 degrees periodically so if there is any drift you can oscillate it and end up roughly centered overall.


last time I did a big slit/drift (hammer eye in 1 1/2" square) I had to realign my starting slit 3 times because I wasn't centered between the opposite faces and used a lopsided vice to 'correct' which made it worse, and I was frequently rotating everything.


bang bang bang, quench chisel and turn it 180 deg, bang bang bang, quench chisel and rotate workpiece, bang bang bang, quench and turn chisel, bang bang bang, quench and rotate workpiece.  etc etc


that's about all I have hands on for, im not very good at straightening out crooked or asymmetric punching yet so you are on your own there :)


you have demonstrated a level of skill far above and beyond anything I could hope to match so I hope this isn't too entry level to be of assistance!


good luck :)

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using a combination square and a sharp scriber
lay out you line exactly where you want it.
Then use a cold chisel and carefully make a good deep impression to run your hot cut in.
Something deep enough that you can easily hold the hot cut in it.

Or, get a flypress and set up a fence. (I use both methods).

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Practice. Do 20 of them, put 19 in the scrap bin.  


CAREFUL layout also helps like Arfist said above.  You can mark both ends of your scribed line on both sides of the stock to help align your punch/slitter.


On bigger stock I find it's helpful to rotate the punch and stock ever few blows to compensate for slight but repeated misalignment.  

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I think that the problem might have been hit upon: my slitting chisel has a slight asymetry to it, which makes it slide to the side. I will endeavor to correct that by rotating the punch each blow and see how it works for me. I am also going to try flattening out the 1/2" square to see if the wider and thinner material gives me more what I am looking for. 

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