Questions on threading of reinforcing bar

As you said, rebar material is pretty low quality, and rather inconsistent in alloy. I don't believe you will ever get very good results with thread cutting.

Are you machining the bar to "major diameter" before threading? I would do that, then run the thread die -- without removing the bar from the lathe between operations.

Also, get in touch with the thread die manufacturer for recommendations on feed/speed and lube

Rebar was never meant to be machined. The material properties are poor, as your finding out. It can be welded though...

I'd would not take that project on as it is destin to fail, but I offer a suggestion, use a good weldable aloy like 1018, and machine the thread onto that and weld it to the rebar.

up side is you can use your lathe with a bar feeder and have the lathe cut the bars to the right lenght. the time you save in load and unload may offset the cost for the steel bar, and the scrap your generating. not only that you can control the quality of the threads much better.

Labguy, its my understanding that rebar cannot be welded because of two things:

1. the steel alloy is pretty low quality, meaning that it is usually high carbon. (from all the carbon containing items in the scrap steel, like dead cats and car tires..) High carbon means that welding will be tricky and the whole bar may need to be heat treated to avoid cracking in the heat affected zone.

2. The mechanical properties of the rebar in part come from the deformation at relatively cool temperatures. We lose these properties if we heat the bar.

Kind of a endless loop...BUT: if we could weld to rebar, we could weld on commercially available threaded rod that some megaplant turns out by the ton!

It is my understanding that threading rebar is a common process for things like building anchors and the like. It just isn't pretty.

Jon.

I am sure rebar with high carbon content is weldable by using 312 stailess steel coated electrodes that would even weld dissimilar metals or unknown ferrous material to another. The only extra precaution is peening the weld to remove stress.

vshwn7@aol.com

Hello Milo

What else can I say.

Kind Regards....

From someone with 57 good answers to their credit, this is great feedback!

Neon letters are cool.

Thanks.

milo

Garbage in Garbage out...

$ in $ out. Plus a reliable fuzz free product at any stage of handling. Your approach should be listened too. But then he could have 50 tons of the stuff stockpiled in the yard. Using before mentioned sleeves will do the job best it seems.

It's pretty easy to give an answer to these questions that is all negative, that we are all idiots and there's only one better way to do the job--your way, using expensive materials chosen for threadability. Maybe we should be using leaded free cutting steel. I don't understand why its considered a good answer. Has no one noted that the end use is for rock bolts and therefore the deformed bar has another use--it needs to be grouted into the rock and the deformation aids in the retention?

The fact of the matter is that rebar is threaded all the time...we just haven't done it and are looking for POSITIVE suggestions on how to do it. In other words, shorten our learning curve.

There have been a few very interesting answers, such as swedging a threaded section onto the bar. It neatly gets around the issue of heat affecting the rebar's cold worked properties. Then a commercially available section of threaded rod could be attached. Some of the other suggestions about premachining, cutting speeds, tooling, cutting oils, and feedspeeds have also been helpful.

Jon.

pre machining, won't actually effect the cold worked properties nor enhance the finished thread profile.

Not knowing what length of material I will yet recommend heating area of bar to be threaded and then pack in sand and slow cool to anneal or minimize the arbitrary conditions.

Coating on the dies will not help either because the machine doesn't go fast enough to take advantage of the coating. And then a coating will be rendered useless when die is sharpened.

I have threaded re-bar and no you will never get thread you would like but you can compensate by using heavier soft nuts (heavier = thicker)

If you choose to stub a easily thread-able metal onto the re-bar, as you weld heat the re-bar to cherry red 2"-3" of length at the weld area. This will normalize the weld and minimize irregularities.

Hi sawmilleng.

I've been out of the country and just now checking my email.

Well, the detail about the rockbolts needing grouted was not obvious in the initial post, but with a more machinable material, grooves can be added econmically to create interstices to give the grout something to infiltrate and hold against. MAybe even deep knurls.

In the 1980's and early 1990's I was plant metallurgist for a little cold finish mill and we sold lots of cold finished steel bars to the guys who supplied the mining industry, some for roof bolts, some for tools, tool holders, and bits.

In my career I've seen many applications get bullied by the MBA's from what made engineering sense, to what makes MBA's cents.

The sense of exasperation in achieving the desired quality in the initial post painted a picture of a process not using an optimum material.

As this is an Engineering Forum < I took a process approach> to describe how the PROCESS Issues output was a reflection of the Quality of the KIV -MATERIAL INPUT ( Key input variables).

Had I known that REBAR is a must because of the deformations, I would have still tried to explain how the rebar properties affect the performance of your friends process.

Swaging a threaded sction onto a piece of rebar will require increased cost as now one of the pieces will need to be drilled out, the other likely reduced, unless it doesn't have to be same diameter as balance of piece, and the strength of the swage has to be validated. inorder to swage over say 10 % you will need to use a veryu low carbon bar, say 1008, in order to have sufficent % elongation to give you a significant swage. 1008 1008 will thread roll nicely, but drilling it out will be even less fun than threading rebar.

Sorry you felt no value in my response. I felt it appropriate and complete for the facts provided, especially as it was process focused.

Thanks for the feedback.

Milo

I don't know much about manufacturing, but I assume threaded items like bolts and screws are made by stamping pieces of metal, hot enough to be soft, between 2 halves of a die, or something like that.

Since he has many thousands to do, is this an option? It would only take a few seconds to do each one, and you could have say 10 in a furnace warming up, putting another one in as one taken out. Might need to run an ordinary rotary die over to dress them up, but that would only be for finishing touches.

Cheers.......Codey

Maybe grind a shallower approach angle on the chasers! It might help ease the agression on the front few teeth! But as said before, It's got bits of Lada in it!

Hello sawmilleng

Some 40 years ago, we were in this same situation, for making rock-bolts.

We first used a lathe to trim up the workpiece, and make it more "round".

Then we used a "Rigid" screw-cutting machine, with 4 automatic jaws which did two cuts, first a light cut to get the basic thread form, then the jaws auto-opened, the cutters auto-backed off, and auto-closed for the final cut.

Rebar has changed in the last 40 years, but at least there is generally no WW2 battleship armourplate pieces in amongst the melted steel.

Cutting fluid we used was an evil-smelling high-sulphur cutting oil, if you got it on your clothes or skin, it was far worse than piggery effluent, and the aroma followed one around for several weeks.

It may be worthwhile, if the contract allows for it, to use the next size up rebar, machine the end to 1

Dependent on quantity and delivery timetable, it may be cheaper to discard your rebar pieces, and get the completed articles manufactured in China - it may be worth looking at if you don't mind doing that sort of thing.

Kind Regards....

I liked Sparky's reply best. That dark sulfur smelling cutting oil is still the best in my opinion for cutting nasty steel. Your idea of getting TIN or AlTIN coated dies would probably increase the tool life but not likely improve the cut quality. I also agree that taking a cut on the lathe to make sure that the part is as round as possible and not oversize in places will certaainly help. If you can make some sort of an extension of the die holder that would have a roller guide or even a static guide to lead in front of the die you might end up with a a straighter thread. As for checking it, there is really no match for using a mating part, without getting very sophisticated and expensive with something like a laser probe from Renishaw.

Look into rolling instead of cutting. You would probably have to turn the threaded section down in size first, but that would provide a circular cross-section for the rolls to work on. Along Codey's line, if they were heated, you might be able to use a rolling process to reduce the initial diameter, and a second to roll the threads.

I am a machinst in a manf.setting we often thread rebar.

1 machine the high parts off , the two rails that run eather side

2 round or make pointed so die will start straight

3 flood with coolant, turn @10 fpm

this works for me building grizzles for rock crushers, thats what it sounds like you are building

You beat me to the punch and sometimes you've got to work with what you've got.

as poor of machining quality that rebar is it might be an avanue to try or consider and that is drop forging if you have a good diemaker he could make the threads in the 2 halves of the die

the heat would normalise the metal and you wouldn't be losing much due to flash at this point you could just run a thread chase on the rods deprnding on the tolerances that are required