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Once in a while I need a bolt of exactly the right length and it is not one of the lengths available in stores. So I get a longer bolt and cut it to length. Here's the procedure I follow.

I screw one or several nuts onto the bolt so that they are between the bolt head and the future cut. Then I grip the bolt in a vise and cut it with a hacksaw. Then I trim the edges with a file to get them to something like 45 degrees. Finally I ungrip the bolt and unscrew the nuts and that cleans the threading of any loose metal.

What I don't like is trimming the edges requires a lot of tedious work and usually the edges won't be as clean as they were on the original bolt and so the bolt will sometimes get stuck when I start to screw a nut onto it and I'll have to remove the nut and try to screw it again. This it not a problem when the connection is to be permanent, but is definitely a problem when it has to be disconnected and connected back rather often.

Also it's quite problematic to have a cut perfectly orthogonal to the bolt axis because the threading helps the saw blade to slightly drive away. This is not a major problem, but still would be better to avoid.

Is there a more efficient procedure for cutting bolts to length that would yield cleaner edges and more precise cuts?

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nice use of 'orthogonal' in a home improvement forum :) –  dbracey Feb 24 '12 at 16:09
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5 Answers

You have the basics down pat.

The key to getting the threads working properly is:

  1. Squaring off the thread end of the bolt. Yes, the hacksaw blade will follow the threads slightly. If you have a bandsaw with a stock holding vise that can be squared to the blade, run a single nut on so the hexes will hold the bolt in place as straight to the blade as possible. Otherwise its finish work with an 10 to 12 inch mill file with proper handle and a vise. The larger file gives you control.

  2. Adding the chamfer on the thread which you use either a file and vise or gentle grinder work to produce as evenly around the bolt circumference as possible. You want it as flat and minimal as possible to maintain the thread and to follow the thread angle which is 30 degrees (half of the 60 degree included thread angle).

  3. The missing element is the thread cleanup. Threads are cut with a 60 degree angle. A standard 60 degree triangular file is perfect. You will need a rather small one that easily fits as close to the root of the thread as possible, use it to get rid of the burr that formed when you chamfered the end of the bolt off and cut a slight lead in which removes anything that departs from the proper thread angle. The thread should exit into the chamfer looking as much like a 60 degree cut thread as possible.

Improper thread cleanup is where you get the problems with starting nuts on the bolt. If it looks like half a thread coming off the end of the bolt, it will either be bent partially over, or easily deform in use so the nut cannot start properly or galls with the weak thread and jams. Either leads to cross-threading and misery.

Done more than my share during my early mechanic career working with old farm equipment where proper length bolts aren't always available. Once you get the image in your head of what a proper bolt looks like for thread lead-in, it goes rather quickly.

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I use four nuts to cut the bolt at the proper length. Two nuts on each side of the cut - one locks the other nut in place so your blade has a secure groove to cut the bolt. I also use flanged nuts on the inside facing each other to provide a better guide for the saw blade.

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The nuts will be scratched by the saw blade, won't they? –  sharptooth Sep 3 '13 at 13:23
    
@sharptooth: Yes, the nuts are expendable. –  wallyk Sep 3 '13 at 15:07
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You could try putting a nut on either side of the cutting blade to square the cut and to stop the blade from drifting along the thread.

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Welcome to the site Lin. I'm updating the answer to be in the form of an answer, we're bad at Jeopardy here. :) –  BMitch Mar 24 '12 at 0:53
    
Good idea, but I guess nuts will be damaged by the blade while the cut is being done. –  sharptooth Mar 26 '12 at 6:37
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You are doing it the optimal way for somebody that just does it occaisonally. A cut that is more square across the bolt (OK - orthogonal...) can be had by using a motorized chop saw with an abrasive blade, an a grinder will help the clean-up, like Caleb says.

If you were to do it this way, you don't need the nut threaded on to clean out the threads. BUT... it kinda sounds like you have a non-problem here. Every blue moon you have to cut a bolt and it takes a little extra time to get the nut on? That doesn't sound so bad.

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Nothing wrong with trying to improve your skills, even if it's not a frequent problem, plus you never know when a project may require 4 or 8 cut bolts, as one of mine did a while back. –  TomG Mar 25 '12 at 2:06
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After cutting the bolt to length, use a stationary grinder instead of a file to clean up your work. With a grinder, it's easy to square up the end of the bolt and apply the chamfer that you want.

Just be careful, especially if the bolt is shorter than the grinder's table. If the bolt is short, use a pair of vice grips to hold the bolt during grinding. And remember that the end of the bolt is going to be very hot after grinding!

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