Pipe clamps are easy, how do I clamp pipe?
November 19, 2015 9:19 PM   Subscribe

I need to sculpt the end of a 5/8" threaded steel rod. I'll be removing a lot of material, hopefully very precisely, and it would be good if the rod stayed absolutely rock solid while it's being worked on. Is there are standard way to do this?

I've thought about using a coupling nut either superglued or with holes drilled through to get myself a flat edge I can clamp to, but space is tight and I don't want to have to cut off the end with the nut when I'm done. Still, if there's no standard way to do it I'm open to creative solutions...
posted by Tell Me No Lies to Home & Garden (11 answers total)
Hmmm...a coupling nut with a jam nut to lock the rod in place is one idea.

Another way this is done is by welding the rod to a flat/square base -- two or three small welds are usually sufficient. A few strokes from an angle grinder should be enough to free the rod afterwards.

Both solutions assume you have access to a good bench vise.
posted by mosk at 9:47 PM on November 19, 2015

Slit a nut lengthwise, with a hacksaw or Dremel tool. Screw the threaded rod into the nut, and clamp the nut in a vise.

Again, as mosk suggested, assuming you have access to a good bench vise.
posted by Bruce H. at 10:10 PM on November 19, 2015 [1 favorite]

Wrap the threaded rod with 12 ga. solid copper wire and clamp it in a vise. 11 ga. would be a better fit, but may not be commercially available. The wire will lie in the threads on the rod and stick up above the outside diameter of the rod. 12 ga. might require two layers to protect the threads from the vise jaws.

Cut a V shaped groove in two pieces of wood and clamp the rod in the grooves between the two pieces of wood in a vise. Or drill a suitably sized hole in a piece of wood, split it, and clamp the rod.

Wrap the rod with many layers of duct tape and clamp it in a vise.

Find a piece of heavy rubber hose that the rod will slip into, or can be screwed into, at least the depth of the vise jaws, and clamp it in a vise.

A pair of ViseGrip or similar locking pliers may substitute for a vise in all of these techniques, if holding the pliers in your hand meets your standard for "absolutely rock solid". Or you can use several pairs of ViseGrips as an improvised vise, one to hold the work and one or two others to clamp the first one to your bench. C-clamps may also work, but it might take more of them.
posted by Bruce H. at 10:32 PM on November 19, 2015 [1 favorite]

You're right, "absolutely rock solid" isn't very descriptive. The cutting will be done on a bench top mill (with a good quality vice) that in theory can get me .001 inch accuracy, but given my limited skills is likely to be only .01 inch accuracy. In either case having the piece move more than .001 inch (or turning that much) would be undesirable.

I'm liking the split nut approach. I'll play with that tomorrow.
posted by Tell Me No Lies at 11:52 PM on November 19, 2015

Easier than splitting the nuts might be to just take 4 nuts and mill 1/2 + epsilon off of them, turning each one into a half-annulus.

Buy or make a V-block perhaps out of brass or aluminum?

collet blocks are apparently a thing that exists as well.
posted by sebastienbailard at 12:43 AM on November 20, 2015

They make pipe vises, for holding pipes. They're called pipe vises.
posted by aubilenon at 2:01 AM on November 20, 2015

If you thread two nuts onto the rod and then use two wrenches to tighten them against each other, they will grip the rod very tightly. Do that in two places a few inches apart (approximately the width of the vise jaws) and align the flats, and grab both pairs of nuts with the mill vise.
posted by jon1270 at 3:57 AM on November 20, 2015 [1 favorite]

The absolute standard way to do this is with the 2nd response - a nut that is slit so that it can be tightened. If I was going to do more than one I would custom make a cubical "nut" of this description so that it would register in the vice properly.

Using a collet holder is a good 2nd choice, if you have one of appropriate size. I've even seen threaded collet holders - that would actually possibly be even better than the above.

A v-block might work, depending on the orientation of the piece. If the rod is going to be horizontal (parallel to the machine table) then it'll probably work OK. If it's going to be vertical it kind of depends on the size of your vblock and the necessary clearances.
posted by RustyBrooks at 6:31 AM on November 20, 2015 [1 favorite]

You also need to account for the fact that the rod sticking out of your vise will flex. You need to keep that rod to a minimum.

Another possibility is to clamp it from both sides. Have it in this nut clamp on one side of the rod, then have say 3" or what ever you need to do your work span across to another vise. Then do your cuts, and then cut off the excess that you were just using for support and do any cleanup or additional work on that cut. This is similar to how work is setup on a lathe with a center on the tail stock.
posted by bdc34 at 7:04 AM on November 20, 2015

Also, are you using cooling fluid? If you need to hold an accuracy of 0.00X, that is down to 1/3 of the width of a human hair, thermal effects are relevant as well. Crunching the numbers considering a coefficient of thermal expansion for steel = 6.7E-6in/in*Rankine; milling cutting face temperatures of up to 450degC (810Rankine); and a tube diameter of 5/8.

Change in diameter = 6.7E-6*(5/8)*(810) = 0.003392in

The is a worst case number assuming the whole thickness where cutting happens reaches 450degC, which won't happen in reality, but in your case you need dimensional instability no greater than 1/3 of that estimate; which in my engineering judgement you could be right smack in the middle of with this machining operation- especially if this is a hard grade of steel.

Also, be sure not to cantilever it too far and support near the machined end as bdc34 mentioned.
posted by incolorinred at 12:24 PM on November 20, 2015

Just to follow up here:

The slit nut sounded like the right approach here.

I started with a coupling nut but there was just too much metal to compress, even with my bench vise.

So I switched to a regular nut and the bench vise worked brilliantly.

HOWEVER... My milling vice has nowhere near the strength of the bench vise and even worse tends to angle the piece being held if it is not relatively large and centered. The regular nut was far too thin to work.

The final solution was to use a regular coupling nut and drill a hole through the nut, the rod, and the other side of the nut. I then pounded a nail into the hole and the result was impressively solid. The mill vise could handle the coupling nut competently and so a sufficiently accurate cut was made.

Thank you to everyone for ideas and suggestions.
posted by Tell Me No Lies at 12:44 PM on December 20, 2015

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