Accuracy problem

[GeeksGoneBad]

Hey guys! the CNC is working pretty good - still have some flexing / racking so I'm working on plans for machine #2

any way I'm trying to cut some pieces for it and my measurements aren't coming out exactly right - I'm cutting very slowly so there's no flex/rack...

but basically I tried to cut a pocket of 3/4" (.75) and it's not ecaxt - it's too small - by like 1/8" almost!

if I lay a ruler down on my machine and tell it to (for instance) g0x10 it stops right at the 10" mark - and the same with the y axis - it seems tuned correctly...

any ideas?

[Regnar]

If your using a cam program make sure your have it set for the right tool or that you have the right tool in the machine. Might be lost steps under load but if the on all sides I think it has something to do with the way your cam software is setup.

[GeeksGoneBad]

in this example I'm using Mastercam and Mach3... I told Mastercam I was using a 1/8" flat endmill which is not exactly accurate, I'm really using a 1/8" rotozip wood cutting bit that's not exactly flat - it's close... I figured that should still give me the same offset comp - but I really don't know what I'm doing LOL

How would I know if I'm loosing steps? - should I slow things down even more? I'm only cutting at like 10ipm (or whatever the 10 stands for in the "feed" parameter)

[Gir]

How would I know if I'm loosing steps? - should I slow things down even more? I'm only cutting at like 10ipm (or whatever the 10 stands for in the "feed" parameter)

You would hear a loud clicking or vibrating noise. It's usually pretty obvious (quite loud). 10ipm should be fine on your machine, but if you're unsure, just as a test try cutting that pocket at 2ipm to make sure. Painful, I know

[GeeksGoneBad]

I remember hearing that sound when tuning the motors - but I'm not hearing it now - but I'm only off by a small small bit so if it's doing it I may be missing it...

oh well - I think I'll chaulk it up to a "first machine glitch" - there's still too much flexing and racking going on for me to be happy anyway - so I've decided to start building machine #2 already I will use this machine to make the parts for it - as if I cut slow enough it's at least repeatable in what it does

Look for a new thread soon

[georgeburns]

Are your circles diameters consistent if measured from all sides?

Have you tried cutting them in the opposite directions?

Have you tried cutting squares in opposite directions?

Would really like to see you solve the problem as opposed to move onto new machine.

[GeeksGoneBad]

Small Circles are really good - for instance I made 10mm holes for the cross dowels and they're perfect - 6 in a row

I haven't tried cutting in the other direction - yet I will

on the outside cut of my part - it was supposed to be 6" X 4" and it's probably 1/32 too small in each direction

also - to add the new machine idea isn't because of the measurements - it's because of the visible flexing... like when the bit plunges I can see the router being pushed up rather than plunge into the wood - I'm pretty sure I'll have the same measurement problems on any machine because it's probably the code that's wrong hehe - and I'd also like more cutting area now that I'm messing around with stuff - got like 15" X 12" now and that's JUST not big enough to route out a guitar body - which is something I'd like to play around with

I will have time to cut some of those ideas in the morning - so I'll update

[skipper_52]

Not saying this is the problem, but might be a place to check.
#1. If you using an 1/8" bit and the hole is off by that much, it sounds like your toolpath has been created on a centerline and you didn't offset the line by the bit radius.
#2 If there's any endplay in your drive screws, this might be the problem also. When I 1st setup my machine I had a 'flat spot' on the Y axis. Went over the machine and found the endplay. Tightened up the screws and the problem was gone.
#3. If the error grows with the size of the piece then you might have the spi
(steps per inch) calculated wrong.

[GeeksGoneBad]

I've got Mastercam set to computer compensation and it looks to be offset when I verify in Mastercam... I cut some things yesterday using the 1/4" router bit and they are damn near perfect - although I didn't pocket anything it was all 2d cuts

how would I find this "end play"? what would I look for?

thanks for the help guys

[skipper_52]

help me guys... if you can grab any carriage (x,y) and move it the slightest without the steppers on, you have end play. Let's call endplay slop for lack of better terms. This 'slop' has to be taken up before the carriage actually begins to move again. I dbl nutted all my shafts to keep them where I set them or you could use a nut with a nylon insert (lock nut). Too tight on the nuts and your shaft won't spin freely. Too loose you got endplay. Like I said in another forum, I cut to .003 because I have slight endplay. My second machine will not have this problem.

[GeeksGoneBad]

Oh - OK - endplay = Backlash? - yeah there's virtually no end play I can detect at all - even before I put the machine together I checked for that with the nut and rod when I got them and I can't feel or see any movement at all - but I DO have flex / racking - so if I grab an axis - I can move the router...

[ynnek]

Originally Posted by GeeksGoneBad in this example I'm using Mastercam and Mach3... I told Mastercam I was using a 1/8" flat endmill which is not exactly accurate, I'm really using a 1/8" rotozip wood cutting bit that's not exactly flat - it's close... I figured that should still give me the same offset comp - but I really don't know what I'm doing LOL How would I know if I'm loosing steps? - should I slow things down even more? I'm only cutting at like 10ipm (or whatever the 10 stands for in the "feed" parameter)

I have seen these rotary saw bits flex, try using a .125 end mill