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View Full Version : Lathe, picking up length



GKman
06-01-2008, 07:03 PM
Seems simple but can't get accurate results. Trying to make several flanged bushings 3/4" ID X 1" OD. I use digital calipers clamped from the carriage to the bed to measure X travel. I face off the right end and turn the OD over 0.600", fine. Then I switch to a parting tool 0.123" thick and try to cut off the part 0.600" long and never get it . How can I consistently find the end or do something to get the length right?

http://img.photobucket.com/albums/v494/gkman/P6010046.jpg

dockrat
06-01-2008, 07:33 PM
Well knowing how much luck I have with parting off myself, I would part it off overlength then measure that length with the caliper to see how much I have to take off then turn it around in the lathe and then using the X axis caliper I would then face off the excess. It has worked for me. Good luck

Rustybolt
06-01-2008, 08:00 PM
One way I've done it in a situation like yours is to put some dykem on the end of the part while it's running. Just kiss the end of the part with the cutoff tool. Account for the thickness of the tool and zero out the indicator. Now move over .600 or whatever length you need. Should get you within a couple of thousandths.

macona
06-02-2008, 12:52 AM
What kind of parting tool are you using? Could be the tool is flexing and not cutting straight.

Best thing is to cut long flip around and face off the back.

DickDastardly40
06-02-2008, 03:22 AM
As an alternative to the above you could try this:

Set the compound parallel to the lathe bed, take up the backlash towards the job, take the smallest facing cut with the parting tool & set zero on the compound with the saddle locked.
Move the compound 0.723" to the left and part.

This method has worked well for me for any number of years.

Al

Carld
06-02-2008, 09:30 AM
GK, how far are you missing the length? Is the face where it's parted flat?

ERBenoit
06-02-2008, 09:57 AM
In addition to the other suggestions, have you: Checked for excessive axial spindle play?? Are you locking the carriage while parting?

ahidley
06-02-2008, 10:17 AM
2 things come to mind. First when you've changed to the parting tool did you run the tool up against the end of the stock. Sometimes its hard to see if its actually touching snugly. Second Is your parting tool ground SQUARE to the surface that your cutting. If not it wont cut straight in.

But for a nicer finish. cut it alittle long and flip it and face it off. All this was mentioned above.

GKman
06-03-2008, 04:38 AM
Thanks for all of the input. Looks like it's too much accuracy to expect from a parting tool. Measuring, reversing and facing to length seems to get it. Bluing the end to see when the tool finds it seems consistant within a thousanth or two.

mark61
06-03-2008, 06:38 AM
Use your dial indicator. Put it on the cross slide with the edge of the parting blade resting against half the point. Place the other hald of the point against the work. Back parting tool out and feed carrage while watching the indicator. Ussually is closer for me than trying to use a caliper or depth gage from the end of the work to the side of the tool.

mark61

GKman
06-03-2008, 12:00 PM
Use your dial indicator. Put it on the cross slide with the edge of the parting blade resting against half the point. Place the other hald of the point against the work. Back parting tool out and feed carrage while watching the indicator. Ussually is closer for me than trying to use a caliper or depth gage from the end of the work to the side of the tool.

mark61

I think I got it. Like this?
http://img.photobucket.com/albums/v494/gkman/P6030047.jpg

Pherdie
06-03-2008, 12:55 PM
I may have an answer to your problem.

Sometime back I was using cheap digital calipers and continually sustained sizable errors. Calibration checks at 1, 2, 3, etc., even inch values, against a standard, showed spot on, but construction errors still appeared. Out of desperation, I rechecked the calipers thoroughly and found they were not linear. As the reading being taken progressed away from an even inch mark, the error grew substantially.

I no longer have those calipers, I now use Mitutoyo calipers.........

Scishopguy
06-03-2008, 02:14 PM
Just a thought...How far out was the blade sticking from the holder? If you have a lot of blade sticking out it tends to flex and "push away" from the part as you cut. I make sure that I have enough blade to reach the center or to the edge of a center hole, but not much more. Hope this helps.

kf2qd
06-03-2008, 02:43 PM
Another way to part off spacers - set up a turning tool - spaced off the parting tool by the desired width - you will face the part as you part it off. Good repeatability and not a bunch of fussing around. Did this on the square post on a turret lathe, but a square post on any lathe would be up to the job.

juergenwt
06-03-2008, 03:23 PM
As said before - your tool is most likely flexing. If you are only making a small number of parts, do this if you want to use a parting tool: If you have a "good" quick change tool holder and a multiple stop set the stop for the turning operation and for the cut-off operation. Make sure you tool is not flexing. If you
put an angle on the cutting face it will most likely flex - depending on how thick your tool is. Run slower and use oil. Lock your carriage.
If you have to be precise, I would leave myself .015 stock and follow up with a facing operation.
If you have a saw and are making a small number of pieces - get a short bar - turn down for one bushing and bore on each end - cut off on saw and face to size.
You can also turn, cut off leaving stock, than drill and bore (ream) and face in a second operation (if a little run-out is OK or you have a grinding operation later. Many ways to skin a cat.