hey all,

I have a 7x10 harbor freight lathe that I have cnc'd with mach3.  I have been trying to make a spring loaded engraving tool for my mill and I have a couple of questions:

1) When I enable back lash the axis movement starts to act strange.  I hit the up arrow to move my x axis in and the axis move the desired length then it adds the backlash movement, plus it moves too far.  My mill does the opposite.  it moves the backlash amount (at desired speed) then moves the entered distance.  Should I be using backlash on the lathe wizard? Does is act the same as Mach Mill? My back lash is about .005"

2) Can I turn "good" threads at 200 rpms?  I am using the acme threads that came with the lathe and simply added steppers to the ends for the x-z movement.   I have single slot encoder and the speed is accurate. It seems to do AL ok but mild steel seems to be an issue.  Would I better off using brass? Should I add ball screws so I can turn faster?

3) And when I do turn, I find that the measurement guides for turning threads do not work for me. I always need to adjust the major and minor thread size to get a bolt to go one smoothly.  I would think this has something to do with backlash but see item 1.  Or I would think this has something to do accuracy of the steps/distance.  It would be nice if I could set the steps/distance like mach mill. but I have good distance if you do not could backlash.  For example, I can zero the x by touching the diameter and set it to .3" then I can turn it down to .2" with the wizards and I get a good result.

4) Are there any videos that explain more about mach3 thread and the wizards? I see a million people show them making threads but nothing about how they actually got there, what measurements they use, why they are going a certain rpm, what tool is better.  Or is this the nature of the beast?  My expectation is to place a piece of rod in, set the parameters, cycle, walk away and come back with a perfect thread...


Sorry if the questions seem stupid... I am hack at machining and the only parts I have made are parts for the machine over a 3 year period. this is my first real project with the lathe.  The Mill works great!!  I use cambam, turbocad and mach3 and have made many parts for years.

Phill

Never used backlash so cant really help too much but it does work on the lathe the same as mill except possibly on the X depending on whether you are in Rad or Dia mode.

200 rpm may be too slow but it will depend on dia, material, cutter type and material.

Not sure what you are meaning in Q3 possibly you are using a wizard that gives you guides for major/minor? If so then it may be assuming you are using a sharp tool and you may using a tool with the coorect root radius ground into it.

How good you will manage to do threading will depend on how good your machine is, how accurate the screws are, whether your spindle has enough power, whether your axis have enough power.

Hood

Hood, 

The backlash started working after I rebooted the PC. and the x axis is half the distance since I am in diameter mode.

200 RPM is the fastest I can go with the acme screw on the z axis. I using AL 3/8" and Steel rod (from ace hardware). 

The cutters are from harbor freight that I grind to a point. No clue if they are C2 or C5??

I will keep experimenting...

One thing I have found is that machining is like art.  There is never just one option, one answer, and there is never enough information to understand why.. Maybe I should take a class.

Threading is easy once you get it. I use full profile carbide inserts and can produce a perfect thread each time no matter the pitch or dia, took me a while to find the sweet spot though. My lathe however is fairly big and doesnt move around even if I take serious cuts so things are easier for me, might not find it so easy when I finish off the wee lathe I am doing.
Once you find the amount you need to take off for a certain pitch you can just plug in the same numbers even if the dia changes.
Good thing about CNC is you can edit and re-run the thread as long as you have not removed from the chuck, so always best to err on the high side until you find where you should be.
Hood

Phil,
Have a read of Threading on The Lathe - Mach3 as it covers threading from beginning to end and then some.
I used backlash on the lathe when threading. Yes you can use it but it needs to be setup correctly, but even then, threading in terms
of meeting some tolerance will be challenging.
Threading will only be as good as your system, so you need constant spindle rpm, thread lead and and depth of thread will only be as good what is mechanicaly driving the axis.

On the rpm question....suggest,when in the wizard click the Calculation button as that will tell you what your axis velocity and acceleration is and warn you if you are exceeding them. 200 is ok , but, depends on the tpi you will cut, and when fooling around with threading on my little lathe at 120 ipm seemed like driving a car at 150 mph both of which is just too fast for me!

RICH

I've also had lots of problems threading on my CNC converted Super 7.  Much of the problem seemed to be down to accurately knowing the tool tip radius, I haven't returned to try again yet but have got a microscope set up so I can measure it now.

Also had big problems with backlash compensation on the lathe.  The documentation on it is poor, but it does say somewhere (can't remember where) that it doesn't work for programmed moves that are smaller than the backlash.  For a lathe without ballscrews that's a big problem since the BL on the main leadscrew can be quite large - for example on mine, which is relatvely unused and unworn, there is 0.25 mm on the Z axis.  So for example, if you want to zero the tool on a faced end of the stock, then back the tool towards the headstock and turn towards the faced end, you can't rely on the backlash compensation to correct it AFAICS.  You could imagine building backlash into the G Code, so wizards for example account for measured backlash when planning moves, but this sounds a lot of work and would rule out using third-party G Code.  I am biting the bullet on this and working out how to fit ballscrews on each axis!

John.

Much of the problem seemed to be down to accurately knowing the tool tip radius

Good point John and it makes a big difference. You will never have a zero radius on the end of the tool and if doing fine threads
doesn't take much to trash a thread, heck all threads. The tolerance on small fine threads is very small ie; 0-80 the thread depth is only .011" so if you don't account for it and don't set up properly the thread will be wrong before one even starts cutting it. The problem even becomes worse when the tool is just hand ground and the tool angle is not correct since even if the radius was zero the angle may be incorrect and cuts over or under relative to the pitch line.

Full profile inserts like Hood mentioned are great in that the thread is being chased, but, you also need  a lathe capable of using them and
they are not cheap.

RICH