Holding a Square piece in a 3 jaw chuck

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Rips
Posts: 14
Joined: Tue Aug 03, 2004 11:47 am

Holding a Square piece in a 3 jaw chuck

Post by Rips »

Is there a way to hold a square piece of stock centered in a 3 jaw chuck? There has to be some trick to do this. I know it is quick work in a 4 jaw, but that's a future purchase.
Andre

Re: Holding a Square piece in a 3 jaw chuck

Post by Andre »

Soft jaws milled out to hold the part.

Andre' B.
Michael Az

Re: Holding a Square piece in a 3 jaw chuck

Post by Michael Az »

I guess it depends how bad you need to hold it. You could take a round solid piece of stock that is bigger than what you need to hold and saw it down the middle length ways, Put in in your mill and mill out a cavity to fit the stock you want to hold. Sandwich the part between the halfs and put in your chuck.
Michael
Doug_C
Posts: 1254
Joined: Sat Jan 04, 2003 6:48 pm

Re: Holding a Square piece in a 3 jaw chuck

Post by Doug_C »

Do you really need to hold it in the three jaw?

If you will be turning the OD then you might think about turning it between centers with a lathe dog driving it. Once round, the rest can be done in a chuck if need be. If the piece of square stock was extra long. Just turn enough to grip safely, then finish it off in the chuck.

Last option: if this is for internal features only, without any external work. If you had a boring head in your chuck, you could mount the material to the carriage and mill in the lathe.

DC
Al_Messer
Posts: 2664
Joined: Sat Jan 04, 2003 7:12 pm
Location: Mid Tenn.

Re: Holding a Square piece in a 3 jaw chuck

Post by Al_Messer »

Take a piece of round stock about one inch longer than your chuck jaws and about 1/2" bigger in diameter than the square stock is diagonally across the corners. Drill and tap a hole for a set screw on one side. Chuck it up in the three jaw and drill and bore until the square stock is a STIFF push fit. Drop a bit of Soft Solder in the tapped hole, tighten the set screw on the square stock and machine away.
Al Messer

"One nation, under God"
Patenteux47

Re: Holding a Square piece in a 3 jaw chuck

Post by Patenteux47 »

A piece of round or hex stock bored to fit over the corners of the square stock to be turned. Cut it through one side with a hacksaw and you have a collet that will hold your square stock in a three jaw chuck. Below is a photo of a similair collet used to turn a washer thinner or to bore the hole larger.

Image
gmacoffline

Re: Holding a Square piece in a 3 jaw chuck

Post by gmacoffline »

wow - a quiz!


if there is nothing more about the part that you can tell us - then these will give oyu a few ideas. there ane very many ways to depart the integument from the feline when holding items for turning work. lots depend on what machining forces and surfaces you need.

this gets a little detailed but may reveal some simpler stuff as we go along. the easiest ones are at the end so go there first if you dont need the entertainment of the longer story. each method has its uses and its limits. you have to know how to use them safely at all times. ymmv but always use caution and check everything three times and more when you are jury-rigging something to get it done without the proper tooling.

we are presuming you want to hold the square section in the 3-jaw, for brevity sake, and not a short square bar that you want to hold flat by its length in the chuck. ( doh!) I guess that would be better described as holding a rectangle in the 3-jaw but the concepts fit both ways. we are further presuming that this is a 3-jaw scroll chuck - self centering type and that you cannot reposition any part of the jaws to assist in positioning the square.

also presuming that the lathe work needed means that you either want to bore it concentrically or turn the outside to a round shape, otherwise why would one be using the lathe. this simplifies things by requiring that we hold the piece by the outside surfaces and hold it concentric with the lathe spindle axis.most methods permit this - a few cause some need for workaround methods

1- method the first is to hold it in a surrounding circular piece specially made to fit or clamp the item securely - that was already described above. go immediately to sixth method below if you want to do this method fast quick and cheap.

2. method the second is to make up certain pieces that will take up the odd space and cause the square to be held on-center correctly. this is approximated by drawing a circle around the part whilst on a piece of paper, then make shaped shims to fit the gaps. making the shims as items that have cutouts around the square edges will help to keep them in place and hold the part from slipping while tightening the chuck. these can be made of very hard wood or a suitable metal. once made, tape of glue them to the square and hold the part in the chuck, tighten as normal. check for concentricity by checking the height of the square corners passing a gauge.

method the third - easiest way yet. get some lead or other castable metal ( they make casting metal for fixturing things like this- it melts in boiling water) , cast the square into the end of a cylinder of this metal with the square positioned concentric in the bar. now chuck this bar in the 3-jaw. shim with metal or paper strips to align minor off-centricity. turn with very low tooling forces so as not to overstress the casting bond. when done machining simply melt the casting mandrel off - you can use plastic casting or epoxy for this also but its more difficult to get it to release. this is actually quite easy if you get a metal tube of some sort ( wrap some foil or aluminum flashing around a piece or pipe for a start, a scrap piece of copper pipe is wonderful - use lead filler ) line it with tinfoil if you dont want the lead to stick inthe pipe, put it over the square, and fill it up with molten lead or whatever casting stuff you like - the foil will let the hardened cylinder come free of the form and you neednt worry about it - it is easily cleaned out of the remelted material when you want to recover the casting media and the 3-jaw will bite right through it to grip the cylinder

method the fourth - also fairly easy. get a short cylinder that is larger in diameter than the square dimension across the face diagonal, and machine a flat end on it - affix the square to this with solder, epoxy, superglue ( dont laugh - this stuff works great for forces within its ability) or some other suitable adhesive or fixing compound. machine the part with low tool forces and low speeds so that if it escapes by chance it will not depart the mandrel at hazardous speeds. be sure if you use an adhesive and you are drilling the part all the way through - that you can live with the adhesive bond letting go as soon as the drill has fully pierced the glued side - becasuse it probably will let go - because you have just reduced the available holding area by a substantial amount. you can actually part the piece off of this if this is helpful and you can work the part operations out that way.

method the fifth - cheaters way - drill a hole on-center in the square and tap the hole. screw a cut off bolt into this with a washer. grip the bolt in the three jaw with the washer butt up against the jaws ends so the machining forces dont screw the bolt into the part farther as you go along. machine the part, remove the bolt, drill thru the center of part on a drill press if it needs a thru-hole. you had selected this threaded hole to be smaller than the final hole so that its vestiges are removed by the final drilling.

method the sixth - also a cheaters way. find a short piece of pipe ( plastic pipe short pieces often work fine and scraps can be gotten cheap, even buying a PVC pipe coupling just for this use is very cheap) whose ID is somewhere from just larger than the square to just smaller. cut off and slot a short section and place in the 3-jaw, insert the square and tighten. you may need to shim a minor amount to get the part centered, use hard paper same as above. using PVC pipe or most other rigid plastic types you can get a pretty firm grip because the metal part wil bite into the plastic well. make sure thay you have not used the structure or the tensile strength of the pipe to hold the part becasue the thing can let go if you have tightened it so fay that it is trying to squeeze out in some way.

there are other methods but these cover the basics. everything else takes too much work and time. I favor the most direct way that gets it done safely. I would suggest a piece of PVC pipe and method six if the part is metal - this can usually be squeezed down to hold the part without dinging the square edges too much.

btw, if you have any kind of a fitting that gives you a large flat surface, like a pipe stanchion foot or a chuck backing plate, this can be used as a quickie backplate - secure the item to it with glue, clamps, screws, solder, whatever. hold the plate by a short length of pipe threaded into the fitting, that is then held in the 3-jaw. this gets more risky as it's harder to hold the item firmly but the concept works OK within its abilities.
gmacoffline

Re: Holding a Square piece in a 3 jaw chuck SAFELY

Post by gmacoffline »

BTW and I hope you would already know this. Never- never- never- never- never, and I mean NEVER, try to hold a part like this with less than 75% of it held within the length of the chuck jaws. when you are approaching 50% of the item sticking out beyond the depth of the chuck jaws you are asking for disaster. the part will escape because the tooling forces are causing the grip to loosen with every revolution and the centrifugal forces on the part cause it to loosen more with each added degree of imbalance. it can cause serious damage and there is no stopping it in time when it starts to escape the grip of the chuck. this is an avalanche failure mode - once it gets itself started it accelerates into a very bad state very quickly until the part finally escapes.

you can hold some longer work like this if you are using the tailstock center to provide pressure against the chuck, or if you have a fixed spider in a captive bearing clamped onto it, or if you are holding it in some other kind of a centering clamp that holds pressure against the chuck. anything else is danger waiting to happen.
Rips
Posts: 14
Joined: Tue Aug 03, 2004 11:47 am

Re: Holding a Square piece in a 3 jaw chuck

Post by Rips »

These are all great ideas. Just a bit more information: the piece I am holding is a 1/8" square by 12" aluminum rod. I need to round one-half (6") and I need to make about 10 of them. I tried Patent's idea and it seemed to work fine. I have think if I had to do it again. I may try a set screw or GMAs soft solder fill method.

I am slowly learning the quirks of this hobby (the hard way!) and I do appreciate all the suggestions. The first lesson I learned was having way more patience then is humanly possible. If I rush something, if is plain dangerous and scary.

I'll have more questions soon. Thanks.
cdehoedt
Posts: 1
Joined: Sat Sep 04, 2021 3:05 pm

Re: Holding a Square piece in a 3 jaw chuck

Post by cdehoedt »

Looking for this solution myself.
I was thinking if the square section is not too big then perhaps a 4 jaw adjustable size jaws, tap holder might work?
The type that is a ratchet operated reversible action with a round handle which can be gripped with the 3 jaw chuck.
Just an idea.
Russ Hanscom
Posts: 1955
Joined: Wed Mar 15, 2006 11:10 pm
Location: Farmington, NM

Re: Holding a Square piece in a 3 jaw chuck

Post by Russ Hanscom »

If you have the option to use collets, there are collets for square stock.
pete
Posts: 2518
Joined: Tue Feb 10, 2009 6:04 am

Re: Holding a Square piece in a 3 jaw chuck

Post by pete »

The original thread was from 2004. But any answer depends on a lot more information than what his first post contained. How large/long is the work piece that he added later, level of accuracy, what other machines or tooling are available that would make building any fixture easier or a whole lot tougher without. A 4 jaw tap holder could work, but without doubt you'd still have to shim it within the 3 jaw to get the work properly centered. Making what's called a cat head chuck might be faster and easier, visualize a heavy wall pipe with 4 drilled and tapped holes equally spaced around it's circumference. Then use cap screws with locking nuts for safety. Hold that cat head chuck in the 3 jaw and adjust the cap screws and indicate the work as you would any independent 4 jaw chuck to move the work concentric to the spindle C/L. A cat head chuck can be used at each end of the lathes spindle the same way a lot of gun smiths still do it for barrel chambering & threading, although I'd guess most would always choose to use a 4 jaw on the working side. These types of shop made chucks can even be used to support longer square or even rectangular work in a steady rest. With the OP's part dimensions that's how I would have done it. But it should be noted that because of the small contact patch those screw tips have compared to any jaw type chuck when driving the work, your cutting feeds and depths of cut really need to be reduced. I have seen a few of these types of chucks made with at least two rows of screws to add more contact area.

If the work is fairly short or really oddly shaped where even a 4 jaw might not work? Another method is to use and what would be most commonly referred to in the U.K as a Keats angle plate that's meant to be bolted to the lathes face plate. https://www.ebay.com/itm/372359733970 An older invention, but almost universal piece of tooling used for model engineering work while boring or machining eccentrics and other odd shaped work.The shorter answer would be to just buy a 4 jaw independent and face plate since there's a whole lot that can't be done without those basic items and using far more effort than it might be worth.
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