PDA

View Full Version : taper turning problem



bdarin
08-21-2001, 07:23 PM
I have to turn some long, slender rods with a slight taper. The taper means I can't use the follow rest cuz it binds as the diameter increases up the taper; but without it the rod is visibly larger in the middle portion. The big Q: what's the best way to turn these rods so they come out right? Is there a way to use a follow rest on tapered work? Or do I just have to be content taking 1 million very light cuts. This could take a while as the rods are 12" long and each pass takes 15 minutes! TIA

Thrud
08-22-2001, 01:11 AM
I was at an open house a while back and Haas was demonstrating turning 36" Acme lead screw on a cnc lathe. It had a special ball bearing follower programmed to stay with the cutting tool as it performed magic before your eyes (wish I had one). It could do what you are asking easily - you could look for a Haas Factory Outlet near you , they might do a "demo" for you if you ask on a slow day. Our local HFO let kids from high school come in an use the machines to get ready for a National Skills competition here in Edmonton. It is worth a try at any rate.

SGW
08-22-2001, 07:10 AM
Hmmm...............
One thing that might help is to use a sharp-point tool with very little to no front radius, so it cuts only sideways. The idea is to have, as much as possible, only side thrust from the cutting forces so they get directed back along the axis of the work. Set up the tool so the cutting edge (the side) is perpendicular to the work, or even with the point leading a little bit.
I'm not sure that will be enough to solve the problem though, and I can't think of a nifty way to "really" solve the problem. Thrud's cnc lathe idea would certainly do it, but coming up with one of those may be difficult....

Wagg
08-23-2001, 11:02 AM
This is a prodlem i have had in the past and there is no real soluction unless you have the hi-tech equip. thats used in production.
There is not much info in your request so i'am assuming the work is less then an inch in dia. to have this prodlem.
i would set RPM about 1200 @ .012" feed (per rev) and start cutting fron the headstock end. I would take small depth of cut but depending on dia. of stock.
Slow rate of feed causes tool wear and excessive point pressure and therefore causes work to push away.
doing it this way won,t solve your prod. completely so dress the affected area with a file.

I would use a turning tool with postive rake. excessive front & side clearance and angle it away fron the direction of feed so that there is min. tool contact with work.

Good luck. let me know if it works.

Thrud
08-24-2001, 01:11 AM
I use high positive geometry small nose radius carbide inserts and you get deflection no matter what you do unless you use a follow rest. I know this from making a pin punch for a specific application - it was "only" 1" long but had a neat curve (the first try) on the punch. By the way - Allen wrenches make nice punches!

SGW might have a point though, I never tried my sharp 60* external carbide insert threading tool as he suggested, but after examining my toolholder it would hit the shaft making it impossible to proceed more than 0.25" along a shaft.

I would still talk to a local machinery dealer about a "demo" or ask if they have a customer that could turn them for you.

halfnut
08-24-2001, 05:28 AM
Hair brained idea. Half brite, half right.

If you were working on centers, offset the tailstock to twice the taper, adjust out the taper attachment to remove half the taper and the follow rest will have a straight surface to run on, on the back side. But the top is still tapered. How to kake care of the top, that is now the question.

One might make a hinged pad to bear on the top with an adjustable weight arrangement, won't take much pressure to keep shaft down if tool is sharp.

Will work I think, will be strange looking, and do you have a taper attachment, that might be next problem.

Easiest way would be to use a CNC Swiss style bar machine, where the stock feeds out of the spindle and the tool stays still, or in this case moves in one axis to generate taper.

Good luck

SGW
08-24-2001, 08:53 PM
I think you'll probably get less deflection with a *sharp* HSS toolbit than with carbide. Because of its brittle nature, cabide requires a larger included angle on the cutting edge than HSS, hence more power to drive it, hence more deflection. Or so I've heard.

Sharpen up a HSS toolbit with liberal clearance angles, on the generous side of what is recommended, to give it a real cutting edge, then stone it to a fine finish on an oilstone (use a 10X glass to check, to be sure you're not kidding yourself).

bdarin
08-24-2001, 09:49 PM
Seems a combo of all you guys' help works best. Made a razor sharp HSS bit with normal top and side rake, but just a CH of front rake for less pressure forward and most of it sideways. Toll is RCH below center. Increased speed to around 1000 rpm and feed to .070, but still toward the headstock. This seems to work. Cuts are .020/pass, less than .010 causes mild chatter. This is minor as final passes will be with a tool post grinder. Thanks for all the help.

halfnut
08-25-2001, 05:56 AM
Glad you are having success, was hoping someone would try out my hairbrained idea, was wondering if it would work.

.070 feed wow. or .007 ?

Wagg
08-25-2001, 09:49 PM
Halfnuts idea of using the follower rest won't work.
The follower rest is attached to the carraige and not the crosslide so therefore the taper attachment won't let it follow the taper.
also. if you could rig it to the crosslide it would have to be on the same side as the cutting tool and the cutting tool would have to be on the same center line has the follower rest support and that support is about 30 deg. below the centerline and the taper attachment will pull on the horozontal plane.

some hair brained idea's work but not this one

bdarin
08-25-2001, 10:08 PM
oops. yeah .0070

halfnut
08-26-2001, 04:23 AM
Wagg,

The follow rest will half follow it, just on side but not on top.

OK see if I can explain my idea, lets pretend that we have taper already turned, a sample part say, mount between centers and offset tailstock till the backside of part is parrellel to the bed. The follow rest which is mounted to the carriage will now follow on back side, but the top won't. That is the reason for tensioned top pad of some sort on the top, the cutting pressure is an approxamate constant unless the taper is large, so an adjustable down pressure on top side.

Then to follow taper with the tool the taper attachment must be used.

I thought on this for a couple of days, best I could come up with.

I am sure that somewhere sometime in the past this problem has presented itself, might be interesting to see how it has been handled with low tech solutions.

firbikrhd1
08-26-2001, 07:54 PM
Another hair brained idea: If you are turning between centers, how about putting a "cat head" and a steady rest about the middle of the work to stabilize the work turn the taper to that point, stop the operation, move the "cat head" and steady rest to the other side of the cutting tool, reset it and finish the taper cut. This would reduce the deflection of the work by artificially making the work half its actual length. Just a thought...
Steve

bdarin
08-26-2001, 11:26 PM
Steve.....I never thought of that. Might be do-able. Any other thoughts on this approach?

halfnut
08-27-2001, 02:50 AM
I thought of something more, easy as pie.

You said a slight taper, so cut stock to large size and cut the taper in one shot using a taper attachment and a follow rest just ahead of tool, riding on the straight section.

Would work like a charm if machine is heavy enough and you have taper attachment that is long enough.

This one isn't too hair brained, I love thinking up hairbrained Ideas, sometimes they even work.

Maybe this is the reason they leave the wierd stuff for me at work. First shift leaves me these jobs, I wonder if they think, wonder how he's going to do that one. I usually get them done one way or the other.

This is the reason I visit these message boards, can pick up lots of ideas here and a couple of other places.

Shucks, people pay good money to attend seminars and tuition to classes. This here is free.

Wagg
08-27-2001, 08:23 AM
halfnut
your idea of using follower rest and doing it in one cut just might work.......depending on depth of cut. might screew up the first couple try's but when you get the settings right it should repeat consistantly.??????/
your "hairbrain" won't work.
the follower rest is attached to the carriage and won't be pulled away from or towards the centerline by the taper attachment to follow the taper. In cutting a taper using the taper attachment the tool is pulled towards or pushed away from the centerline. thats how the taper gets cut.
i agree with you this is a great way to get info.


I don't understand the "cathead" idea. they only permit you to work on center or another centerline like cutting a eccentric or cam.

C. Tate
08-27-2001, 11:37 PM
Could you possibly spring load both supports on the follower rest? If you place the correct amount of force in direct oposition to the tool you should get the support needed without deflecting the work. Kind of like a box tool on screw machines. The spring pressure could be adjusted by using a screw of some sort so that the supports retract as the rest travels up the taper or vice versa. I have not put to much thought in on this one... just brainstorming.

halfnut
08-28-2001, 04:56 AM
Wagg,

OK, my idea uses tailstock offset and a taper attachment. By offsetting the tailstock and using the taper attachment together the backside of shaft is parrellel with ways. So one pad of the follow rest will work, but the top won't.

It was a wild idea that I had to throw out there, see if it got anyones attention.

One shot with a taper attachment is the answer, don't know why I didn't think of it right off.

If I had a follow rest for my Hendey I'd whip one out, just to see if it worked as well as I thought. Great thing about the Hendey, got a nice taper attachment, we have a new import machine at work, shortest taper attachment I have ever seen, just 6 inch travel.

I'll tell a story about cutting radius's with a taper attachment.

Friend of mine got a job making some large spherical swivels, the outside parts are bolted together like a connecting rod, and the center is flat with bolt holes. The sphere is about 12", he mounted them in his OLD American lathe, made a link for his taper attachment 6" c to c and cut radiis, worked well. Nobody else could figure out how to cut them, not a big enough job for the CNC shops. It paid for the old Lathe.

I've got another story about this machine, called Charlie and the Oil Can.

I'll tell it someday, one of my favorites, you really need to know Charlie to fully appreciate the story.

Lead Screw
08-28-2001, 05:52 PM
Halfnuts idea sounds good to me. Ive put the follow rest ahead of the tool many times---but only with straight passes. Youll need good and round stock to prevent copying out of roundness also you need your stock running true in the machine. What are you guys calling a "Cathead"?

bdarin
08-28-2001, 11:03 PM
I thought about the spring idea for the follow rest but dismissed it because the spring pressure would never be constant. As the rest moves up the taper, the rods move out and compress the springs more. The more the springs are compressed, the more they resist compression, so the force isn't constant, but increases up the taper. At some point they will bind and it's just not worth the hassle. Good idea, though. Pressure could be held constant if the rods on the rest were in an airtight chamber, with compressed air pushing them against the rod. Intended leakage past the rods would hold the pressure in the chamber constant, thus holding the rods at a constsnt pressure against the turning regardless of diameter.
This whole thing is getting really far fetched!!

JIM DEWOLF
08-28-2001, 11:37 PM
This is my first go at this but am having a ball reading all your comments. Re this tapper problem, first, you don't say what diameter you are working with, but if I was doing this, I would chuck it with only half out of the chuck (providing you have a big enough spindle bore, offset the compound and do a section at a time. My lath e has 5 inches of travel so I could do it in 3 passes easy. Am I nuts and don't know what you guys are talking about?

farmwrench@aol
08-28-2001, 11:50 PM
Air would be the hard way to get consistant pressure. Gravity is free and constant with little slidding resistance on seals. Try a weight to supply the pressure. Now here is an even more hair brained idea. If you did this sort of thing often why not make a second taper atachement to move the support the other way of the tool? The support could ride on the set of ways on the cross slide that are exposed (or does your tapper attach cover that up?)

rmatel
08-29-2001, 04:21 PM
Hi bdarin,
I'm new to this game but it occurs to me that a taper that was out of spec due to bowing could be "fixed" by grinding. Maybe a tool post grinder using multiple light passes???

firbikrhd1
08-29-2001, 04:22 PM
Leadscrew, if this works, it will take you to a picture and some info about a Cat Head.http://members.aol.com:/firbikrhd1/cathead.jpg
Upon giving my original idea some additional thought I came up with the following: a collar could be turned larger than the outside of the work being turned taper, and bored with the same taper internally as is being turned on the work, the size so that the collar would fit the work just to the headstock side of the middle of the work after it has been turned taper. This would be used in place of the Cat Head. The procedure would be to use the Steady Rest on the outside of the stock about midway from the ends. Begin turning and as the cutter reached the Steady Rest, the work is removed from between centers, the collar slipped on the already turned portion and the Steady rest reset on the collar. The work would then be returned to the centers and the cut finished. That would be faster than resetting the Cat Head concentric with the work every time.
Steve

bdarin
08-29-2001, 08:30 PM
In response to rmatel....this is exactly what I'm doing. The TP grinder takes out the bow in the middle first, then the cleanup passes are made. This has proven to be the easiest way to solve the problem. I do, however, appreciate the HUGE response this problem has generated. Shows you guys are thinking.

Wagg
08-29-2001, 10:25 PM
I do not think halfnuts idea of offsetting the tailstock and using the taper attachment will work.
#1 The follower rest is attached to the carriage and the taper attachment is attached to the cross slide. Therefore, it won't cause the follower rest to follow the cut taper.
#2 If the tailstock was offset towards the operator, the stock would cut small on the tailstock end. If you were successful in mounting the follower rest on the cross slide so that it would follow the cut taper using the taper attachment, as the carriage advances towards the headstock, it would cause the follower rest to follow the cut taper but, at the same time, pull the cutting tool into the work.

Lead Screw
08-29-2001, 10:42 PM
Hey Firbikdh1--Thanks--it went to the cathead photo. That looks like a good idea for running irregular shapes in steady rest. That must be from an old book---I like the part about using wooden pads for finish work.

halfnut
08-30-2001, 06:25 AM
wagg,

This is getting wild, I am having a blast.

Offset tailstock away from you, this would normally make the tail end bigger, now set the taper attachment to cut double the desired taper with the large end towards chuck. In other word cut double the amount of taper desired and take half of it out by offsetting the tailstock.

The follow rests that I have used fasten to the carriage, the position of the cross slide doesn't effect it.

This would be a bear to set up, was just a wild idea.

farmwrench@aol
08-30-2001, 08:31 AM
Would being able to put the work under tension instead of compression between centers solve the problem for this and other problems. Whow do they make the neddles for slider carbs? They are very small D. and long, with small tolerance and need to be repeated.

Wagg
08-30-2001, 04:40 PM
halfnut

yes, this is fun..

you must be missing my point. the follower rest is attached to the carriage so therefore won't follow the taper attachment setting.

bdarin
08-30-2001, 09:38 PM
I've created a monster.............

charlie coghill
08-30-2001, 10:26 PM
You sure did create a monster and I have enjoyed reading all the hairbrained ideas. Even learned a thing or two.
Charlie

Toolhawk
09-09-2001, 02:24 PM
Make a sample of your finished tapered rod, then put a new rod in the chuck or collet, with about one inch sticking out of the jaws or face of collet, then machine to .025 over finished size of small dia. Now pull rod out another inch or so and put live center in small dia. and machine down to the last cut you made, then pull rod out another inch or so and start over untill you have rough cut the entire length of the pin. If you stay within one inch of chuck you can take very heavy cuts. I hope you can understand what I am talking about, good luck.

brunneng
10-04-2001, 08:42 PM
I know this is a month old but I'm new here and this topic seemed to generate so much interest that I need to try an idea on you all I came up with while reading this.

This would only work with a taper attachment.
You would use 2 upright mounts. One behind the work on the crosslide holding a spring loaded guide against the back of the taper. stay with me.
The other upright is mounted to the carrage behind the first upright and holds a pulley. A cable attached to the follower upright runs around the pulley and is attached to the back of the follower itself...pulling it through the holder.

So as the crosslide moves the tool away from the work pulling the cable with it, the cable will pull the follower away from the back the same distance. You could also use rigid rod linkages and a swivel bar in the rear upright.

This should fix the uneven pressure problem with the spring loading.
Late but maybe helpful in the future.

The Maker
11-17-2001, 06:38 AM
Possible Problems In Your Setup???

1)If you are using a chuck,check jaws for taper if you can slide .001 feeler gage between the tip of the jaw and the part,you will have part flexing and vibration.(use 2 layers of paper to compensate)

2)If you are using centers & Dog, check center drill angle to live center angle. (use New C-drill) that will also cause part to flex on you.

3)Tail stock is sliding back Fix Lock Down Nut. (Tail stock pressure)

Depending on material you are turning,use sharp tool with chipbreaker and coolant.

"Hope That Helps"

trapper
11-19-2001, 05:17 PM
I have enjoyed reading the previous ideas and I would like to add my comments from the UK.

You don't say what diameter the bar is but if you use sharp tools as already described (Kennametal KCUX inserts would be ideal)set VERY carefully on centre, you should take just one full depth cut with half to one third normal speed for material (depending on diameter) and very slow feed with lots of coolant. Taper can be set with tailstock set over or taper attachment.Use larger diameter material than you would normally for the job and the "Solid" portion to the left of the tool should support the bar as you cut and produce a superb finish. This will actually be quicker than taking lots of small cuts. If the rods are very slender, then try the same method and use the oldest and most sensitive steady there is......your finger on the back of the bar....with suitable protection of course.I have used this method many times even on modern CNC machines as well as my old "Atlas".

MarshSt
11-21-2001, 12:44 AM
Another late reply: There was a reader's letter that I remember from a few years back from a guy set up drilling gun barrels in his shop. He mentioned a hydraulic steady rest. If you had a follow/steady(three point contact) rest attached to the carriage that would self center and adjust for diameter (hydraulics would work well for this), it would follow the tool down the part(using crowned rollers) compensating for the change in diameter caused by the taper. Probably way more trouble to build than it's worth, but it sure would be cool if it worked.

Steve

harry philips
11-21-2001, 08:22 AM
Being new on the scene to this problem, and I must say you've had quite a response. Here's my penny worth.
Have you tried loading spri9ng behind ther steady fingers, the springs compresion could quite easily be adjusted to compensate for the tool point pressure . Just a thought?.
All the best Harry.

kap pullen
12-07-2001, 12:29 PM
I'm late too.
Set steady between carriage and chuck.
Start cut as close to chuck as possible,feed tword tailstock.
You may get 1/4 length or so before it starts to chatter.
Slide steady out and set up as close to tool as you can(allow for slop in the taper attachment).
Set steady with an indicator with say .001" of pressure(experiment).
Resume your cut. With luck you will make it all the way. You may have to reset steady one more time.
A bar restrained three places is much stiffer than two places.
Once you learn how far you can go before it chatters you can set the steady before the noise starts.
You may have to reset to finish the headstock end of the bar(or finish it first).
We used this method to turn shafting
for the cardboard box industry.
We also made knurled pull rolls up to 120" long (about 4.25 dia) using pinch knurls for the knurling.
Forming the steady rest jaws to fit the shaft will reduce wear and use heavy oil or gear grease.
kapulllen