Taper and tool deflection.

Scott_ · Feb 4, 2018

I have taper problem that is really got me all stressed out! I am machining small diameter samples that require a tolerance of +-.001 on the OD. The diameter is .250" and I am using a 35 deg neutral handed insert and holder because of clearance issues. I am consistently seeing a .003 taper, small on the tail stock side. I am using a .016 nose radius and taking .010 for a finish pass on mild steel. I have been able to program a taper to counter the unwanted taper but it's not pretty. It's all over the tolerance in both directions. I'm assuming that I am dealing with tool deflection and maybe I need to find a better insert geometry?

I should have stated that the machine is level and not twisted.

gregormarwick · Feb 4, 2018

Deflection in a lathe invariably results in tapering larger towards the tailstock, not smaller.

If your part is running on a centre in the tailstock, then tailstock alignment is the probable issue.

Conrad Hoffman · Feb 4, 2018

How far does the part stick out? Are you using anything to support the other end? I don't do CNC but it's common to have the unsupported end come out larger because it gets forced away from the tool. Tools requiring more pressure make the problem worse. Sharper insert, smaller nose radius and positive rake will help, but may degrade finish.

Finegrain · Feb 4, 2018

How long is long?
Using a center in the tailstock or unsupported?
How is the surface finish?
Tool on-center?

.01" is a bit much for a finish pass on a .25" diameter.

Might try a sharp Al-style insert for the finish, and only take .005" final cut.

Regards.

Mike

Scott_ · Feb 4, 2018

The part is hanging out 2-1/2" and is supported by the live center. The surface finish looks great so no problems there.

Finegrain · Feb 4, 2018

Scott_ said:
The part is hanging out 2-1/2" and is supported by the live center. The surface finish looks great so no problems there.

Oh

Good chance your tailstock is bowing the part.

Minimally, the part is pushing away from the cutter in the middle. I would expect the middle to run big, and the ends to be on-size. If one end is bigger than the other, you have machine alignment issues.

Regards.

Mike

Nerdlinger · Feb 4, 2018

I'm with Mike. It sounds like you get to align your tailstock...don't worry...there's plenty of youtube videos and tutorials available if you haven't done it before. Can you put an indicator on your turret and go along the length of your part and see what it says?? Dealign with tapers is kind of part of life with a fixed headstock machine. Fun fact - what you are dealing with is probably the number one advantage to sliding headstock (i.e. "Swiss") machines.

Scott_ · Feb 9, 2018

This is a brand new machine only had it a month.

guest · Feb 9, 2018

I think Finegrain has it right- start by turning your tailstock pressure all the way down until it has just enough power to move the quill. You're pushing on a piece of spaghetti, be gentle.

Scott_ · Feb 9, 2018

I will give it a try, thanks.

guest · Feb 9, 2018

Wouldn't hurt to put an indicator on the chuck and sweep in the tailstock to verify it's on center.

You didn't say what your starting diameter is, but it may also help to break it down into sections. 2-1/2" of .250 dia. is 10:1, pretty flimsy.

Say you're starting with 3/8" material. Starting at the tailstock end, rough 1/2" long down to .255" dia. Move down, rough another 1/2", etc until you get your 2-1/2" length. That way you have the full diameter of the material to support the roughing passes. If you turn the full length in one G71 cycle, your final roughing passes are pushing on a very flexible piece of material.

Bobw · Feb 9, 2018

Sounds like a swiss part to me..

Are you sure you aren't in NM and working for the guy I used to work for?

I hope you don't have to do a lot of them.. If its more than 10 or 20, and
not a NOW emergency, I'd say fuck it and send them out to be ground, or
sub the whole damn job out.

Are there other features? What is the start diameter?

There are tricks. On some parts they work, on some they don't..

One part I do on occasion has 2.5 or so inches of 1/4", threaded for 1.5"..

I start with 1", turn about 3/4 back and thread, turn another 3/4 back and thread.
Then finish it, which is no big deal at that point.

sometimes you just need to start with bigger stock. 10:1 becomes 3:1. Then act like
you are running a swiss machine and take it all in one shot.. Starting with 3/4" that
would only be a 1/4" doc.. really light feed and done, single pass, no tailstock.

StinkyDan · Feb 10, 2018

If its in tolerance why mess w it? Could always just get out some sandpaper and smooth it out when ur done and its still spinning in the machine

Dubbie · Feb 10, 2018

guest said:
Wouldn't hurt to put an indicator on the chuck and sweep in the tailstock to verify it's on centre.

Beware of indicator stand/holder sag when doing this. You can easily get 5-6 thou of false reading.

michiganbuck · Feb 10, 2018

I am consistently seeing a .003 taper, Constant seems you can program it out.. tweak the tail(if using tail)..
Varying .003 is a different story..does the part change length, the center at the same depth. the part long and deflection more as the cutting tool dulls.
Guess more information needed.

Holding at the head stock only, mild steel would a HSS bit be sharper? would a positive side and back rake help?
Is the chuck or collet good.

Oh [hanging out 2-1/2" and is supported by the live center.]any hour glass or all straight?
Pulling your tail handle to the same draw position every time?..
wobble center washing out the center?
getting hot and so getting lose for finish?

re finegrain QT Might try a sharp Al-style insert for the finish, and only take .005" final cut.
if all is same and tight.

Search

Largest Manufacturing Technology Community on the Web

Search

Taper and tool deflection.

Scott_

Plastic

gregormarwick

Diamond

Conrad Hoffman

Diamond

Finegrain

Diamond

Scott_

Plastic

Finegrain

Diamond

Nerdlinger

Stainless

Scott_

Plastic

guest

Guest

Scott_

Plastic

guest

Guest

Bobw

Diamond

StinkyDan

Aluminum

Dubbie

Plastic

michiganbuck

Diamond

Similar threads