Trouble holding bore size ????

rockfish · Nov 7, 2012

I am currently running a 500 piece order of gear blanks. I have a .625-.626 bore that needs to be held. I am drilling the hole with a .578 drill, then boring with a 3/8 boring bar with CMMT insert (not sure if that's right.......... it's the little 80* diamond insert, positive rake). Anyway, I've got this set up using that 3/8 boring bar to rough the bore to within .015 of size, then I come in with another 3/8 solid carbide boring bar with the same insert for finishing. Throughout the day, I am constantly changing offsets on this job, as the bore keeps getting smaller and smaller. How much changes in size would be considered normal ??? I'm shifting my offsets up to .002 to .0025 from my starting point in an 8 hour day. That seems excessive to me, and I can't figure it out. I thought I was having trouble before, because I was just using one boring bar to rough and finish, but now I'm using two and the finishing bar is taking very little stock out and should not be wearing out.

Frank

athack · Nov 7, 2012

Frank,

Not sure of material hardness and so on but if I had this job I would drill and ream. Probably do all 500 with one reamer...if you leave .015 to ream.

But to answer your question yes that seems like a big offset move for 500 parts. Then again if material is hard different story.

Athack

rockfish · Nov 7, 2012

It's 4140 material.

rockfish · Nov 7, 2012

I need to pick up a reamer holder of some kind. I don't have one. Consequently, I've never reamed a hole in this machine before and I'm not sure how to program that. I did think about it, and I'm going to have to give that more thought next time around.

Frank

Ox · Nov 7, 2012

It sounds like you are experiencing "thermal growth".
Your hole keeps gitting smaller eh?
That is b/c as you run and run your machine, the X axis screw warms up slightly and essentially gits longer. Before you know it, your tool is now .001 further away from the thrust bearings, and you are boring .002 undersized.

Sounds like you are having much better luck with solid carbide bars than I ever have...

------------------------

Think Snow Eh!
Ox

gocubs68 · Nov 7, 2012

you could reduce the amount you are leaving for your finish pass to reduce tool pressure or you could run your finish pass twice to eliminate tool flex

jdj · Nov 7, 2012

gocubs68 said:
you could reduce the amount you are leaving for your finish pass to reduce tool pressure or you could run your finish pass twice to eliminate tool flex

Running the finish pass TWICE has helped me out on some operations! Give it a try Frank! It isn't for every situation, but sometimes it's a charm!

asticx · Nov 8, 2012

.002 over 8 hours is normal.

danny

racen857 · Nov 8, 2012

I'm boring 4140 today too. I just checked a part and recorded the bore @ 7:30. I have a +- of .005 so I'll let it go for awhile and I'll post later today and let you know how much it changes.

Mazak 250-II msy

4140 @ 450sf .006feed @.830 rough size finishing @.847 5/8 boring bar with a tcmt insert. We make tons of these parts and I mean litterally tons, we make our parts from tubing rather than doing all the drilling.

strokersix · Nov 8, 2012

Sometimes it works good to draw the cutter out slowly as a spring cut. Dependent on cutter geometry and workpiece material.

racen857 · Nov 8, 2012

racen857 said:
I'm boring 4140 today too. I just checked a part and recorded the bore @ 7:30. I have a +- of .005 so I'll let it go for awhile and I'll post later today and let you know how much it changes.

Mazak 250-II msy

4140 @ 450sf .006feed @.830 rough size finishing @.847 5/8 boring bar with a tcmt insert. We make tons of these parts and I mean litterally tons, we make our parts from tubing rather than doing all the drilling.

ok results are in @ 7:30 .849,.....9:30 .848,.....11:30 .8475 Thats no adjustments no insert changes did not touch anything, just checking parts. Machine ran nonstop except for bar changes about 1 min x 6. and 1 10-15 min break around 10.

So around .0015 difference figureing in tool wear, thermal change and any other machines varibles.

Joe788 · Nov 8, 2012

Racen, if you're still running parts - try keeping track of a nice OD finished dimension, as well as the the ID. If the bore keeps getting smaller, and the OD keeps getting bigger, you'll be able to get some idea of how much of it was tool wear, and how much of it was thermal growth.

Philabuster · Nov 8, 2012

racen857 said:
ok results are in @ 7:30 .849,.....9:30 .848,.....11:30 .8475 Thats no adjustments no insert changes did not touch anything, just checking parts. Machine ran nonstop except for bar changes about 1 min x 6. and 1 10-15 min break around 10.

So around .0015 difference figureing in tool wear, thermal change and any other machines varibles.

Newer Mazaks like your machine will compensate the axis for thermal growth via several temp sensors in the machine.

My old ass 1986 10N (no thermal compensation) will move .001" on dia from cold machine to warmed up. Been very repeatable at this for the 15 years I have run it.

You can run an experiment by probing a tool you are not going to use for a given job right when machine is switched on. Write down the tool geometry value. Re-probe same unused tool now with machine all warmed up and write down tool geo. Next morning when machine is cold, re-probe same unused tool once again and write down value.

This test also measures several other factors like turret repeatability, probe repeatability, and so forth. Ideally, you want to isolate only one factor for testing, but these are real world variables when machining parts and should not be ignored.

Terry Z · Nov 9, 2012

Is this machine near a door that gets opened and lets cold air get to it?? Also do you have RR tracks near by? If you ream you need to program it like a drill with the correct S & F.

racen857 · Nov 9, 2012

Joe788 said:
Racen, if you're still running parts - try keeping track of a nice OD finished dimension, as well as the the ID. If the bore keeps getting smaller, and the OD keeps getting bigger, you'll be able to get some idea of how much of it was tool wear, and how much of it was thermal growth.

you ask and you shall receive(with in reason).

Ok here it is again,....

6:45 od 1.6287 id .8495,.....7:45 od 1.629 id .849,.... 8:45 od 1.6287 id .849,....9:45 od 1.6291 id .849,.... 11:00 od 1.6284 id .8485,......11:45 od 1.6284 id .848,....11:45 od 1.6289 id .8475

The id stayed about the same on the changes, one thing to keep in mind on the od it is a cnmg insert .016 radius cutting @ 650 sf .0112 feed per rev.

The od is less critical than the id so kinda blasting thru it so the od changes are probley due to surface finsh varitions and tool wear. it still held at .0007 thru the entire sample period witch is not bad concidering the cutting speeds and feeds.

CarbideBob · Nov 9, 2012

The first thing I would look at is the insert's nose wear.
( As an insert making guy my view is way skewed)

Put it on a comparator and see if the wear accounts for the size loss.
How much flank wear? Once this starts it pushes your tool away from the part. ID bores are more sensitive to it than ODs.
This size change will depend on stiffness of the tooling. Can you fit a bigger bar?
I'm guessing that on a little bar like this you have done all you can to reduce overhang. Small amounts here help a lot.

Your numbers do seem high to me.
This could easily be grade selection, tool geometry, tooling stiffness, thermal growth or machine servo problems.
Blame the tooling first, then check the worn tool to see if this makes sense.
In many cases inserts with a "M" in the third letter are not a great choice for finishing tight tolerances although they will generally settle down quickly in the first 50 parts.
Bob

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Trouble holding bore size ????

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