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I need to grind down the threads on a 4" hex head bolt while preserving 1 inch of threads closest to the head. I need to do 5 bolts this way. I'm thinking bolt in vise and then using a belt sander. Have any other suggestions? I am using the modified bolts to create a roller bar on a trailer. The nut will be welded to a bracket, the bolt will be inserted then threaded down to lock in place. The roller bar is water pipe with a roller bearing insert, that's where the smooth part of the bolt comes into action.

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  • So grind off 3" of thread? I think the result will be poor... I would consider taking it to a shop and get them to do it on a lathe.
    – Solar Mike
    Feb 27, 2020 at 19:53
  • Maybe buy bolts that fit your needs? Not being sarcastic, but why not just buy new bolts? Feb 27, 2020 at 20:03
  • Are you saving the bolt shaft without the threads as a pin or cutting the extra off?
    – Ed Beal
    Feb 27, 2020 at 21:00
  • I'd do the job with my lathe. Probably not worth buying one for just this. if you put multiple nuts on your bolt to cover the thread you want to preserve, you can tighten those nuts into your drill chuck, and have at the extra thread with a simple file or sandpaper. The result is pretty neat. Feb 28, 2020 at 0:36
  • Thanks for all the suggestions. I think a shop and lathe ideas are going to be the solution.
    – Anne
    Feb 28, 2020 at 3:59

4 Answers 4

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If it's not important for the shank of the bolt to remain perfectly round then a belt sander with a coarse grit will do fine. The 80 grit is the most fine I'd try; 60 would be better. Grits as coarse as 36 or even 24 are routinely used for fast material removal by an angle grinder when working with steel. You could get a belt that coarse but it will leave a rougher surface compared to 60 or 40.

Assuming that your bolt is fairly small in diameter it might be slightly easier if you can fix the sander in place and manipulate the bolt instead. Ask an assistant to hold the sander upside-down on a work bench. Then hold the bolt to the belt while rotating the bolt. You'll be able to see the work as it progresses and it'll be easier to control. The bolt will get hot quickly; plan on using pliers (locking pliers if possible) to hold it.

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  • And dip the bolt in water periodically to keep it cool!
    – fred_dot_u
    Feb 27, 2020 at 21:52
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    This will work, having done it several times in the past. I recommend running a nut up along the bolt near the head - beyond where you're going to be sanding/removing the threads. Then when you back the nut off after the thread removal it cleans up the threads and makes it easier to start a new nut.
    – SteveSh
    Feb 27, 2020 at 21:53
  • Using a nut is a good suggestion.
    – HoneyDo
    Feb 27, 2020 at 21:59
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From my experience I have found that an electric bench grinder works best for this type of job. Put the hex head in a vise grip and carefully work your way up the bolt rotating the bolt as you go. I find that this allows you to be more precise with the grind.
Whatever grinding method you use it's important that you don't allow the grinding surface to mar the threads remaining. If you're not careful they can be damaged and won't allow a nut to be threaded on. If this happens you can probably reset the starting thread by using a small triangular file and reopen the thread path.

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Grinding the threads on a bolt to make it round can be done freehand and get good results, if you are patient and you go really slow. The deepest part of the thread will be visible as little grooves, that become narrower as you grind down to the minor diameter. Once they disappear, you've gone far enough.

There is a very specific art to using a belt sander / bench grinder to grind round objects without making them lopsided -- it can also be quite dangerous if you are doing it without the object firmly against the table or tool rest, respectively. If you are not highly skilled, you can lose fingers doing it the hoopty way.

Dipping the bolt in water's been mentioned, that's a good idea. Putting a nut over the threads you want to protect, which was mentioned also, is a good idea too -- if you are grinding for a while, chances are that you will slip a bit and put gouges into the thread if you don't have something physically covering it (tape will probably foul your grinding medium and not really protect it too well).

Be sure that you take the nut off every once in a while and look at how far down you've taken it -- a few times when doing this, I'd notice that the neck of my ground-down portions would sometimes get narrower right near the part where they were concealed...

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Grinding that much off the bolt will take forever, wasted wear and tear on your grinding wheel. I am not sure but it's also possible the heat could damage the temper of the steel.

Instead, put the bolt heads in a vise and saw off the excess. Use a hacksaw if that's all you have, a portable bandsaw would make life a lot easier though.

Before sawing, thread one or more nuts onto the thread. This will serve several purposes. One, if you line up the flats of the nuts with the bolt head, it gives more meat for the vise to grab. You can use it as a saw guide - put it right where you want to cut off - but you may damage the nut and have to throw away the nut. Most importantly, it will re-form the threads when you remove the nuts.

After sawing, but before removing the nuts, you may want to clean it up with a grinder but a file would probably be sufficient and safer.

And of course, it's way better to just buy bolts the right length if at all possible, but I am sure you already thought of that.

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