Really Thin Taper Turning

[NSDesign]

I need to machine several tapered parts from 316 SS. The parts are about 8" to 10" long and taper evenly from 5/16" OD to 1/16" OD. I do not have a taper attachment for my lathe. I do have a follow rest, but I don't think you can use that for tapers.

Any ideas? The parts don't need to be super accurate - just uniform and with a decent finish.

Also, I have access to a tool post grinder, but have never used one - would that be an option?

Thanks,
Alan

[tattoomike68]

my advice, turn them an inch or less at a shot & charge $600 each.

[nheng]

Never done anything like this but scratched my head for a few minutes at least thinking of a reasonable way.

With the small OD and SS, you're probably going to experience lots of deflection rendering any sort of cutting over a 10" span useless.

I recalled a trick I learned a while back for cutting very tiny diameters WITHOUT experiencing an undesirable taper. You cut the full depth (final diameter) in one pass.

In your case, hold the stock in a 5/16" collet. Allow only an inch or so (less on the initial passes where depth of cut is greatest) to protrude at a time. Cut your taper to the full depth in one pass (may not be pretty depending on your machine). Back the cross slide out a bit, feed out another inch or so, move the cross slide back in and find the spot where you left off. Continue until done

When you're all done, you might want to take a belt sander to it but not a tpg. Den

[Spin Doctor]

As far as turining the taper this is an instance where you could use the boring head in the tailstock trick to get the taper. There wherea couple of threads on this a while back. But one thing you do have to do is use a ball center on each end

[Jesse168]

First things first. You can do it by turnig your compound to the correct degree. Second you need to put a small center drill hole in one end like with a # 1 center drill. Make sure you have some extra length on your material. Extra to chuck on and extra for tail stock end. You will need to practice on some scrap piece. It doesn't matter what it is. Take .312(5/16) substract .063(1/16) which leaves you .250 to be divided by your length. If it's 10 inches in length that means .025 per inch. Practice cuts on your practice piece until you remove .025 in a one inch length. You can't go to the very end of the piece towards the tail stock because of the center drill hole...keep this in mind. Your compound should only be a couple of degrees off ZERO parallel with the piece your turning. Biggest problem will be clearance between your compound handle and the piece you are going to turn and tail stock. Also if you keep your cut going toward the tail stock your tail stock will keep your material from moving. When ever you cut material going towards the chuck the material can slip in the chuck because of the force of your cut. It can even do this in a collet. Also a must is using a live center in your tail stock. Hope this helps.

[This message has been edited by Jesse168 (edited 01-02-2005).]

[coles-webb]

It can be done, but not fun and time consuming. If I ever get around to it I am going to write an article for HSM on this subject. I would use the offset tailstock method which allows you to turn a taper the length of your apron travel. The piece must be done between centers but feel free to turn a 60 degree center to use in your three jaw chuck and a homemade lathe dog (fig 1) that will drive against one of the tree jaws. Leave extra material at the tailstock so you will have a place to put a center drill (about 3/8 inch). Don't use to small of a center drill as the piece needs to be supported and centers can be broken off if you don't have enough material against the (shouldn't be an issue in this case though, the cuts wil be light). I will assume you are using 316 SS as you require the corrosion resistance for something. Ensure that it has to be 316 SS and 303 or 304 will not work just the same as the difference in machining is great. 316 SS is tough to machine as opposed to 303 or 304 SS (and the later are usually easier to obtain). The centers will not ride perfectly in the center drilled holes but that is only a small taper so it is not an issue or requirement for ball centers to be used. A taper attachment cannot normally be used but a modified one can be used (fig 2, 3 & 4). Remove the guts to the follower rest and make the following new pieces. The first is a brass or bronze (I used oillite bronze) that will be the piece resting against the piece. The next is a spring, bought or made to fit behind the piece of brass which will provide spring pressure against the piece. The last piece was a piece of round stock drilled and tapped for a socket head set screw (any bolt will do, fine threads is preferred and it must be fully threaded). The bolt must be long enough to push the piece of brass fully forward and push against the spring. Attached to the bolt is a foot (can be threaded on or welded) that pushes against the spring. A thin jam nut (or knurled jam nut) can be added to stop it from possibly moving. You can now provide spring pressure against the piece you are turning without bending it. The pressure from the top usually stays the same as you are only holding it in place. The pressure from behind you may have to adjust to lighter pressure as the work is smaller to prevent the material from flexing. I turned a taper on a hammer forged rifle barrel years ago for someone this way. Once I got the pressure set I left it alone because I found the at the small end near the tailstock the piece was well supported and as it moved toward the head stock in the middle of the piece the spring automatically provided more support. As the cutter moved closer to the headstock the material was thicker and was not influenced by the spring pressure anymore. Take light cuts and rough out all your pieces to about .025 within size (make more than you need to take into account learning curves). Then set up and learn how to use a tool post grinder for this project. If you do all this you will gain more confidence on the machine, learn new skills/equipment and have an addition set of jigs to use with your follower rest.









Any questions on what I've explained, let me know.

Mike

[This message has been edited by coles-webb (edited 01-02-2005).]

[kap pullen]

Thrud told me about some kind of skiving tool that scrapes the taper.
Is the big guy out there somewhere?

Calling thrud.

kap

[Forrest Addy]

A small machine shop will lose their butts on a long skinny taper like this. You'll spend a fortune tooling for it unless you're equipped with a large centerless grinder that can run 10" wide wheels that can grind the taper is a single plunge or a production rod lathe.

A production lathe (I may have my terminology worg here) is designed to work from small rod. The rod is fed through a snugly fitted bushing in the headstock and as it comes through the tool immediately in front of the bushing moves in and out turning the part to size in a single pass. There is no modign saddle. The movement of the rod is the feed rate. Once the part gets to length, a parting tool cuts it off.

These machines are very quick and efficient at making long skinny parts.

I suggest you look through the Thomas Register for someone to farm it out to.

[Dr. Rob]

I've done this, quick and easy, with you'd-be-surprised-how-good results.

Chuck it up and cut the approximate taper by eyeball in steps or actual angle. Just rough it out. Make some sort of reminder of how long it was supposed to be.

...and angle grind the rest freehand, while under rotation. Flap disc, crosslaid, compare occasionally with straightedge. Easy does it.

It was so easy, so fast and so good there is just no comparison. My taper was a foot long, from 12 mm to about 6 mm OD, in CRS. I've also done one 3 ft long in 50 mm aluminum, before I knew this technique. It was a nightmare.

And before someone freaks out about the technique, bear in mind that "...The parts don't need to be super accurate - just uniform and with a decent finish."

[This message has been edited by Dr. Rob (edited 01-16-2005).]

[NSDesign]

Thanks to everyone for responding. I really like the spring loaded followrest idea, and I may try a few others as well.

Dr. Rob, I frequently use a similar method on complex and freeform curves, so I will fall back on your plan if the others fail.

Once again, Thanks to all.

Alan

[DR]

Farm the job out. It's nonsense to think this can be done on a conventional lathe.

Occasionally, we do jobs of this type in the CNC lathe. The method we use to to start with something like 1" diameter stock and take it to size in a single pass using tailstock support. Given the material and the size of the small end I doubt that'd work for this piece.

This job is a piece of cake on a Swiss screw machine. Find a shop with a CNC Swiss and they'll do the job perfectly in about the time it takes you to describe what you want. CNC Swiss machines are mega bucks so it won't be cheap, but if you value your time it won't be cheap doing (or I should say, trying to do it) it yourself after all the time you'll waste.

[coles-webb]

I believe it can be done and alot learned in the process. People have accomplished many amazing things when they were ignorant to the fact that others already tried and failed or stated it couldn't be done. Home machinists or people starting out in business have limited resources and money and sometimes jobs must be done at a loss to ensure future jobs. This will be a time consuming process but it will get faster with each one you do and give you added skills that will used on other jobs. Progresivly lighter cuts will have to be made as the piece gets smaller to avoid any excess flexing. A grinder used to get it to size will get it "uniform and with a decent finish". Let us know what you decide and how it works out. I'm tempted to try it myself to see how it works.

Mike

[DR]

"People have accomplished many amazing things when they were ignorant to the fact that others already tried and failed or stated it couldn't be done. Home machinists or people starting out in business have limited resources and money and sometimes jobs must be done at a loss to ensure future jobs."

Mike,

Since the OP didn't give us much info I assumed he's doing this for profit considering he posted under what I think is a business name.

Of the responses, Forrest and myself both said it couldn't reasonably be done. The total of our machining experience well exceeds 50 years. We've both seen these kind of jobs and wouldn't touch them with a ten foot pole, hence the advice to farm it out to a shop with the appropriate machines.

This is a trivial part to do on the correct machinery. Why not do it right and make a profit?

About doing the job at a loss to ensure future work.....yeah, I've made that mistake too. The customer assumes you can continue doing them for the money losing cost, then where are you? Lose a little (or alot) on each part and make up for it in volume?????

Now, if you think you could actually do these parts on a conventional lathe with a "uniform and decent finish" more power to you. I'd love to see your method.