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Author Topic: Problems threading on the lathe  (Read 331696 times)

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Offline ART

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Re: Problems threading on the lathe
« Reply #190 on: March 29, 2009, 09:09:23 AM »
Hi Guys:

  For any of you using the PP for theading, can you try one cut only shallow and show us a pic, Id like to see the
size of the crests on it across the thread to just the consistancy of them.

Thx
Art

Offline RICH

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Re: Problems threading on the lathe
« Reply #191 on: March 29, 2009, 01:04:07 PM »
ART,
I can do one later today and make some measurements. I just want to caution you on what you may see in a picture.
Just being very picky here but reading into what you ask for. 
First, a few comments about and from my testing and will admit that most are of short lengths of threading.

If you want to adjust lead error based on program number rounding it would be important that the test piece
be done on a very accurate alligned lathe, properly prepared piece, and the result will be only as good as the lathes screw. So if the screw has lead error, say .003 over a foot you may see that error in the pic.
But what are you seeing, error out of the program or error mechnicaly created (this would include the piece setup). Same goes for measuring it ( yes i can measure very accurately but wouldn't want to lead ya into
doing some thing based on bum info ).

If it's an adjustment to the flank cutting that is another scenario.

Here is some pics of A 0-80 and A 1/4-20 threading tests i did along with info as follows:

THREAD#    REMARK
1          This shows a taper because of setup and a poor / worn  tool
2           Same as above but  a better set-up
3           Good 0-80 thread, fresh ground tool, note the consistancy in the crest, root, pitch
3           30X mag of the good 0-80 thred
4           This is a 1/4-20, you can see good thread consistancy. Note that there are "two" scribed lines".
             The wider of the two was from wizard generated code and the other was picking up the thread
             and to deliberately do an offset of the Z start to test how picking up a thread would work usng G32.
             Tracked quite well.
4           30X mag of the tracking

Just wanted to clarify my caution noted above.

RICH
             

 

Offline RICH

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Re: Problems threading on the lathe
« Reply #192 on: March 29, 2009, 01:40:37 PM »
ART,
sent you a  PM
RICH

Offline ART

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Re: Problems threading on the lathe
« Reply #193 on: March 29, 2009, 04:16:12 PM »
Rich:

  Actually, what Stev is getting on a 4 inch or so thread, when cut shallow, and can easily be seen, is the crests at the middle all look pretty much the same, but the first 4 or 5 crests shows a narrowing of the crest, then it stabilizes to a set width to just about the end of the thread. I was wondering if others are seeing the first 4 or 5 crests showing a narrowing.. As the enclosed shows..

Art

Offline RICH

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Re: Problems threading on the lathe
« Reply #194 on: March 29, 2009, 06:42:04 PM »
Hm...
Was a clean up cut done  before doing the threading?
You want a nice round stock, centered / parrallel surface to the Z axis movment,
 no spinging of stock or lathe compund, let the rpm settle out, make sure any backlash is taken out, etc.

If your going to compare a surface after a cut you would like any mechanical item
to be elimated as much as possible.

Now see the machine can track a fine  line. Do 60  .0001" with a razor sharp v tool ( no radius on the end ) and see how it tracks the first scribed line. That eliminates any motor slowdown issues.

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

My  quess would be that with no end bevel the tool shears more material in the beginning
and over some distance settles out on the amount. Some of the above could also come into play.



 

Offline ART

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Re: Problems threading on the lathe
« Reply #195 on: April 07, 2009, 06:57:56 PM »
I suspect proper prep was done, Stev and John are pretty good machinists. Im not sure why their threads dont look like some
of the ones here, but John does report his Aluminum threads are good, its the steel ones that seem to vary in pitch as it goes.
Ill run soem diags witha special driver with them to see if turnign off spindle speed correction shows anythign interesting..

Thx for your input..

art

Offline John S

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Re: Problems threading on the lathe
« Reply #196 on: April 07, 2009, 08:31:33 PM »
First off a little background for the machine.
It's brand spank new and has been designed as a CNC from day one.
Spindle is driven by a 500 watt 3 phase brushless servo with encoder feedback to the driver board, this then drives thru a 3:1 timing belt reduction so it's at max torque at 800 rpm.
True speed at a requested 800 rpm is 800 to 804 and steady.



Ballscrews fitted with no discernable backlash, no compound, everything tight.

I'm running the same tests over and over which is a 2mm pitch thread 80 mm long, short threads don't show an error like the long ones and I'm positive that you need to cut long threads and use a tap or known screw as a pitch gauge because a thread gauge fits along the thread due to the fact it only covers about 4 or 5 pitches.

This is what I'm getting for the first three cuts




Unlike Steve who was getting a narrow land at the start and end and OK in the middle I always get a good thread at the start and an increased pitch at the end causing the land to become very narrow.
At the end of 80mm I'm about 1/2 a pitch or 1mm out.

Art has picked up on aluminium threads being OK but they were shorter ones for a job.
tomorrow, well later today as it's 1:30 am here I'll cut a long alloy thread with the same driver I have on the machine now.
Then I'll swap to the driver Art has sent and redo the threads in alloy and steel, scratch cuts at first.

John s.

Offline ART

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Re: Problems threading on the lathe
« Reply #197 on: April 07, 2009, 10:10:14 PM »
John:

 Thanks, that will be interesting. If the long alloys look the same, and the new driver also looks the same it will point in one direction, if the
new driver seems fine it willpoint in another..

(Nice pictures by the way.. Im impressed..)

  The RPM reading is good, Im pleased there.. Whats it do with averaging off? Does it vary allot more?

Thx
Art

Offline ART

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Re: Problems threading on the lathe
« Reply #198 on: April 07, 2009, 10:11:00 PM »
By the way, nice looking lathe..

Offline RICH

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Re: Problems threading on the lathe
« Reply #199 on: April 08, 2009, 01:52:48 AM »
JOHN S,
Will try a some longer threading 3 to 4 inches and see what happens here. 
Nice lathe and trust me when i say i undestand your frustration.
RICH