# Metric or Imperial ball screw



## j ferguson (Jan 24, 2017)

This should be obvious.  but...
(edit: it was obvious - I'm stupid)

I'm using LinuxCNC on a 6040 router and have something screwed up with the setup.  it makes everything a little small.

I'd assumed since machine was sold in US, it has imperial lead screws. ... but maybe not.

5 mm/rev not the same as 5 revs/inch.  So if i have a metric ball screw, I can't use inch machine units.

Can i simply correct the difference in my stepconf setup?

my thought would be to enter 25.4/5 or 5.08 where you enter revs/inch and then actually check the results.


----------



## jbolt (Jan 24, 2017)

Yes 5.08 is correct for turns per inch for the screw if it is a 5mm pitch but for the Mach3 settings you need to know steps per inch which is the sum of the screw pitch, stepper steps per rev, any micro stepping from the drives and any gear reduction.

Here is a link to a steps per inch calculator. http://www.machsupport.com/forum/index.php/topic,16315.0.html


----------



## j ferguson (Jan 24, 2017)

Thanks Jay.  I'm using LinuxCNC but each of these elements is accessible.  this is actually a pretty simple direct drive so I'm spared some of the uncertainty.

I also have a big pile MDF to try settings on in case of confusion.

By the way, Jay, I liked your bit about things which cannot go wrong.  I bought a new VW in 1968 to replace my 61.  Very soon the turn signal blinker module failed - not under warranty.  the replacement was $17.  i asked the parts guy why they had made a part which cost $17 to do what the previous thermal relay which cost $1.79 had done perfectly well.  "This one lasts forever" I was told.
"Then why do I need a new one."  
"Oh it's not the one you had that lasts forever, it's this new one you are paying all this money for."

They looked the same to me.



best,  John


----------



## j ferguson (Jan 26, 2017)

25.4*8000/25 = 8128 which goes in the SCALE entry in LinuxCNC  router.ini file.  It's amazing how many alternate theories I invented to explain why parts made on the router didn't quite fit.  Thanks Jay.


----------



## jbolt (Jan 26, 2017)

Considering the pitch difference from 5mm to 5TPI is about .003" I'm sure it drove you nuts. Glad to hear you got it sorted out. 

Is it a DIY router, kit or ready made? You should post some pictures.


----------



## j ferguson (Jan 26, 2017)

Gunrunner you need to order a set of these for the Z axis hand crank. Bill did you just spit coffee on your monitor? :rofl:

View attachment 253356


----------



## j ferguson (Jan 26, 2017)

Just realized that I'll need to switch back to old WRONG scale setting to finish present project because otherwise new parts won't fit.

Boy you have to keep a clear head in this stuff!!


----------



## jbolt (Jan 26, 2017)

Gunrunner you need to order a set of these for the Z axis hand crank. Bill did you just spit coffee on your monitor? :rofl:

View attachment 253356


----------



## j ferguson (Jan 28, 2017)

Hah, Mountainview I see.  FWIW all the solid modeling is done on a 1992 Sun SPARCstation10 running 1993 SunOS4.1.4  For example, I make a solid-model fuselage aft of wing using first former and last former  then cut sections at 2 inch intervals which become nested DXF arrays which then go to SheetCAM for the G-Code and then to the router.  I'm using 1/16 inch carbide diamond cut bits.  I have a little 3d printer and use the Sun to design and make stl files for parts like landing gear.

There is a fair amount of fooling around with this because I'm not following any plan.  I frequently have to do stuff over or stop model design and improve the 'manufacturing' setup.  One thing that I've found really useful is ability to make jigs on the 3d printer to make it easier to assemble wings (1/16 3 ply ribs on carbon fiber spars with carbon fiber torque tubes for the ailerons running to servos in the fuselage).  Span of first plane is 36 inch.  I have  no idea how it will fly, but it's getting close.  My plan was to make cabin section, wings, rear fuselage and tail feathers, then make the nose section as long as it took to get CG.  Thing's ended up looking like a Pilatus Porter (really long nose) so I used that as a guide to landing gear, made out of 3D printed hub and fittings, carbon fiber tubes, springs, and music wire.


----------

