[sixpence]

Eureka!

I have finally made my first cuts. I have cut a square 100mmx120mm but it is exactly 1mm out all round.
The lead screw is 10 tpi, guide rails from igus. Hobbycnc board + motors.
Running acad, sheetcam, and mach2. The angles are perfect 90deg. the bit diameter is set correctly with outside offset.
Where could the problem be?
What should I check first and what sort of accuracy are other people getting?

[ger21]

If you're cutting wood, I like to make a single pass into the wood and measure the width with a caliper, to get an accurate tool size. Also, cut some different size squares and see if they are all out 1mm.

[sixpence]

I have now done what you have suggested. It was not a consistant error and much to my surprise it was not perfectly square! I ajusted the bit diameter to suit and reset the alignment. The error is now consistant 1mm too small all over the machine on the x/y axis. any further suggestions would be appreciated since your last one was so helpful.

[blue_luke]

Hello all, newbie here from Montreal...

Two things.....

Once a machine is constructed, what would be a suggested method of insuring or verifying squareness, precision, repeatability?
In other words, how can I spec a machine just built?

Also for sixpence and any other, how the Igus bearings fare?
I am considering the Igus "Drylin W " guide system.
Any comments?
Thanks, Luc

[sixpence]

Hi Luc
I have encountered no major problems with the igus rails so far exept that they were a bit over kill on specs for my machine. The smaller round rails would have been sufficient

[blue_luke]

Sixpence...
What rail guide systeme did you use and what kind of price?

[ger21]

Are you cutting different sized squares? Cut 50mm squares, 100mm squares, 150mm...
Then, if they are all off by 1mm, I'd guess that possibly your machine is flexing. Try cutting one square conventional (CCW) and one climb cutting (CW). Are they different? If so, it could be the tool flexing.

[blue_luke]

Sixpence...
What igus rail guide system did you use and what kind of price?

[sixpence]

Are you cutting different sized squares? Cut 50mm squares, 100mm squares, 150mm...
Then, if they are all off by 1mm, I'd guess that possibly your machine is flexing. Try cutting one square conventional (CCW) and one climb cutting (CW). Are they different? If so, it could be the tool flexing.
__________________
Yes I am cutting exactly those size squares and they are all out by the same. Pardon my ignorance I am still very new at this could you pls explain those terms to me conventional and climb cutting?
Thanks

[sixpence]

What igus rail guide system did you use and what kind of price?
Hi luke
Drylin WS Q-20 -rails 1220mm x2 =R1542.66
WJS200Qm-01-20-slidesx4 =R386.28

Drylin WS Q-20 -rails 610mm x2 = R771.34
WJS200Qm-01-20-slidesx4 =R386.28

WS-10-305mm railsx2 =R256.20
WJUM-01-10 slides x4 =R201.60

The currancy convesion is at present about 6.60 rand to the dollar

[sixpence]

I looked it up silly of me!
cw- clockwise
ccw counter clockwise

Duh!

[spoiledbrat]

If you are using mach 2, it could be that your encoder steps-per-unit is slightly off.

[ger21]

If you are using mach 2, it could be that your encoder steps-per-unit is slightly off.

That's not it, or different size squares would be off different amounts.

[wizard]

How about thepotentiality tht a bit of backlash has creeped into the syste?


DAvE

That's not it, or different size squares would be off different amounts.

[sixpence]

My router mount is made of wood and is not the best mounting possible I 'll try and fix that first and then try and comp. for backlash in the software if the problem does not improve. Could the fact that the board does not have microstepping be a factor? What advantage does microstepping give you?

[Evodyne]

Sixpence,

Lance here (Evodyne). I was just signing off for the night after sending a brief reply to your PM. Lo and behold, you were back online. So I've put off some much needed sleep for another couple of minutes.

Regarding microstepping: your 200 step per revolution motor, with x10 microstepping, would require 2000 steps per revolution, giving you 0.18 degrees of resolution. If you do nothing to speed up the pulse train driving it, you'll also be ten times slower. I'm sure you can factor this in inside of Mach 2, but at some point Windows itself will be the limiting factor to the maximum pulse rate if you are going off of the parallel port. This is where separate indexing hardware comes into play.

So better resolution. What else? Smoother operation.

Steppers can have RPM ranges where resonances appear. You might experience it, or you might not. I've got some small motors that have vibration dampening flywheels attached to them to remedy this. Microstepping can also be a cure, either partially or totally, to such resonance.

Lance

[ger21]

Regarding microstepping: your 200 step per revolution motor, with x10 microstepping, would require 2000 steps per revolution, giving you 0.18 degrees of resolution. If you do nothing to speed up the pulse train driving it, you'll also be ten times slower.

A bit misleading there, imo. If you're gcode has an F word in it (which it should) and you set your steps/unit correctly, you don't have to do anything to go the same speed. The control software will output the correct amount of steps for the speed you wish to travel.

And in regards to microstepping increasing resolution, not always true. I post this link a lot. http://www.machinedesign.com/ASP/vie...strSite=MDSite

If using 10x microstepping, each microstep will only have about 15% of the motors torque, increasing with each microstep. If that's not enough torque to move the machine at that moment, it may take a few microsteps before the machine moves. So, you may not get more resolution.

[Evodyne]

Gerry,

Misleading how? Regardless of how it is accomplished, the pulse rate has to increase to compensate for the microstepping. If all it takes is properly set up G-code (the F word you mention) then all the better. I'm not a G-code know it all, for sure!

Depending on the PC hardware one has, the increased pulse rate needed (and possibly only at higher speeds) might exceed Windows capabilities.

As for the resolution, you make a good point with regards to the torque per step. Something I wasn't aware of. It's the difference between a theoretical explanation and real world experience I suppose.

Lance

[ger21]

Gerry,

Misleading how?

You said if nothing is done, then you'll go 10x slower. I say that if the "F" is already in the g-code, you'll go the same speed without doing anything.

[Evodyne]

O.K., I see where you are coming from. Thanks Ger!

Lance