Turned diameter is wrong.

[jimbies]

I have just completed the CNC retrofit to my Sherline lathe and was trying to turn some round stock to the required diameter. The finished diameter always end up smaller. I think this is a setup problem and would appreciate some help as I have been trying to get it right for the pass 3 days. Here is what I did :-

The configuration is in "Diameter Mode"
1. Chuck up a round stock, take a skim cut measure the diameter and enter the diameter in the X DRO. Did the same for the Z axis and zero the Z DRO.
2. Fire up the OD turning Wizard and enter the required parameter, X start = 23.43mm, X End = 23mm.
3. Back to Auto Prep screen, enter the stock diameter = 23.43 into the Part Zero DRO and click Part Zero button and finally click cycle.

Problem.
Measure the part and the diameter is 22.57mm not 23mm as I envisage it to be.

Is this correct or are there some setting which I did wrong ?

I was thinking to use the dimension as per the drawing, feed them in and have the part turned out to the correct dimension without dividing the diference of the dimeter by half.

Thanks in advance for your help.

[Crevice Reamer]

Check that your Steps per inch/mm are set correctly. Make your axes move a set distance and measure that it is correct. If not, you may need to tweak steps up or down slightly until axes move to exact distance.

CR.

[jimbies]

Hi Crevice Reamer, thanks for the reply.

I have checked the Steps per inch/mm and they are correct.
After the setup, I do a G0X0, the cutter moved to the other end of the circumference.

Should the cutter move to the center of the stock ?

[LeeWay]

If you have any backlash in there, it will show in this manner too.
I use Mach mill for my lathe because I have the tailstock motorized for drilling.
I have just bought Dolphin Turn and will be using it on my new lathe build.
What I do for my code on the 7-12 that I cnc'ed is to do most of it by hand. I use a standard peck drill cycle from Sheetcam for the tailstock. Always the same one. Then I just use the teach Wizard. This got me my first code and I was then able to just write the code myself after a short time. All very simple parts though.

I use the backlash in every move. I had to program that in. Move positive direction before I move into the part.

My parts are more accurate now. This is not coded for the actual distance it travels now though. It is all done by trial and error and editing the code in Mach until I have exactly what size parts I need.

This is why I bought Dolphin for the new lathe. I expect to have near 0 backlash on it.

[HuFlungDung]

The error appears to be due to confusion between radius and diameter. It would be easy to tell with the difference in position between two larger movements. So do a test with an X 25.0 command movement and see what you get. If the slide moves 25 mm, then you are in radius mode.

I would imagine the way around this could be as simple as halving the number of steps per inch on the X axis.

[jhowelb]

X0 should be the center of the part regardless of diam. or rad. mode.
In diam. mode when the tool is at center and you do "G0X25" the tool should move 12.5 increments because you are referencing the diameter of the part.

In rad. mode you must do your own math after measuring the diameter of the part.

I can't even guess why a tool would go to the rear radius for X zero.

[jimbies]

It's nice to know that this community is very helpful. Thank you all for the replies.

If I understand the problem correctly, the cause is:-

1) Based on the figures in my first post -
Stock dia. = 23.43mm
Desired dia = 23mm
Actual dia. = 22.57mm (Diameter of stock after machining)
Amount of stock to remove = 23.43 - 23 = 0.43mm
Actual dia. = 23.43 - (0.43 x 2) = 22.57mm (which is what I get.)

Which means the X axis has moved 0.43mm instead of only moving 0.215mm.
So, halving the X axis travel will give me the correct diameter but will this affect referencing and setting up ?

OR

2) G0X0 should put the cutter at the center of the stock and this would solve the problem.
Now, what is wrong in the configuration or setup ? I have tried enabling / disabling, setting/ resetting and do test cuts and still not getting it correct.

Oh, I forgot to mention that I am using Mach3 R2.63 Lockdown version.
Sorry, if the questions are stupid, I have only just converted my lathe to cnc and learning how to use it.

[jhowelb]

Trying to pick out specific controls explanations from the Mach Turn manual is a little like trying to untangle limp spaghetti, however I do believe your problem has it's roots in the "Part Zero" button. I can't find an explanation for this button but I can describe the procedure that works for me when setting up a turn job.

First I jog to a position close to where the cutting will be done then click Zero World X and Zero World Z. (This prevents some crazy initial moves)

I then take a light facing cut an click Z Zero.

The take enough of a turning cut to get a clean surface all round the part and shut the spindle off without moving the tool.

Measure the diameter of the part, enter that figure into the X DRO and press enter.

Load and run the program.

Try it and see if it works, you have nothing to lose.

P.S. Sorry if I sound as if I'm "talking down" to you. I don't intend to, I just had to reduce the subject to it's simplest form to be able to understand and use it.

[jimbies]

No problem jhowelb, this is exactly the type of instructions I needed.

Will give it a try tonight and see how it goes.

In your current setup, does a G0X0 brings the cutter to the center of the stock ?
I could only achieve this by halving the X axis travel as suggested by HuFlungDung.

[jhowelb]

G0X0 brings the cutting edge of my tool to center of the part.

On the Config menu, ports and pins>turn options>X mode is checked. Software COULD reverse that with a g-code.

Test: set jog mode for incremental jog distance 1.0000 then jog X positive. The tool should move away from the center by .5 !

[jimbies]

G0X0 brings the cutting edge of my tool to center of the part.

I have tried all the various setting and still unable to bring the tool to center of part.
I even tried reinstalling the program.
I double checked the stepping count and it's correct.
What could be wrong with my setup ???

[jhowelb]

I wonder,
if you took a facing cut from the outside in and at the point where the little tit disappears off the middle.....set X Zero.

Now jog out and use jog buttons to take a clean cut from the o d, mic the piece and compare the reading to the dro?