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inconstant speeds?
« on: February 27, 2008, 08:48:14 PM »
Hope this is an easy fix.
Mach3 and a HobbyCNC driver board on a 3 axis CNC.

The part file is set to cut at 100units/min and on the lines every thing moves at those speeds.  When I get to a curve every thing slows way down, 30ish.  I know that the G-Code only has the speed set to 100.  Is there a setting in mach3  that I need enable/disable?

Offline Graham Waterworth

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Re: inconstant speeds?
« Reply #1 on: February 28, 2008, 03:39:59 AM »
If you are doing lots of tiny moves then Mach will slow down, try using CV mode.

Graham.
Without engineers the world stops
Re: inconstant speeds?
« Reply #2 on: February 28, 2008, 10:19:30 PM »
Messed with CV and was able to speed things up a bit 40/100.  Any other ideas?

Offline jimpinder

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Re: inconstant speeds?
« Reply #3 on: February 29, 2008, 03:21:04 AM »
As Graham siad - Is this a curve - i.e. a G2 or a G3 generated by the G Code, or is it in fact very many straight lines just joined together to look like a curve.

The first is easy to produce since the two axis accelerate/decelerate at a predertermined rate together and the whole move can be computed at one time.

The second one is different, and whether you like it or not, the machine has to stop between each move, pick up the new information for the next straight line, then accelerate away again. Since the angles on the curve are different, the rates of movement of each axis, in relation to each other are different for every move. If some of your lines are millimeters long, this takes time. The CV mode cheats in that it picks up the next move and as axis slow to a stop from the previous move, they accelerates again up to speed for the next one - the overlap can be set.

I don't know what is writing your G Code - but it may be quicker if, instead of straight lines, you did away with say half of them, and used G2 or G3 to join up the curves - that, however is quick complicated.
Not me driving the engine - I'm better looking.
Re: inconstant speeds?
« Reply #4 on: February 29, 2008, 11:12:50 AM »
All of these movements are X, Y, and some Z movements, No G2, or G3.  I am using MeshCAM to gen my G-Code.  How would I convert  the following G-Code into G2, and G3 moves?   I have no problems writing some code to help with this issue.
X6.025Y1.991
X5.981Y2.005
X5.713Y2.106
X5.668Y2.125
X5.534Y2.175
X5.490Y2.195
X5.400Y2.228
X5.222Y2.307
X5.177Y2.323
X4.910Y2.440
X4.865Y2.463
X4.820Y2.482
X4.731Y2.527
X4.692Y2.544

Offline jimpinder

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Re: inconstant speeds?
« Reply #5 on: February 29, 2008, 02:30:48 PM »
As you can see they are tiny movements. You do not say, but I hope this is in inches.

I would plot them out on a graph and see if I could replace two or three with a curve. You have the start and finish points so the only thing to study is the radius of the replacement curve - and you can fill this in GCode using R instead of IJK. I would have though a suitable curve woud give a better line anyway - unless you are designing a stealth bomber.
Not me driving the engine - I'm better looking.
Re: inconstant speeds?
« Reply #6 on: February 29, 2008, 02:55:21 PM »
Inches.  Ummm, Not quite a stealth bomber.  But very close, as I am cutting out wings molds for RC airplanes.  So, no true curves, this is all constant changing curves that cant be represented by G2 and G3, I think.  So I think I am stuck with the slower speeds.  I will live.

Offline jimpinder

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Re: inconstant speeds?
« Reply #7 on: February 29, 2008, 03:58:20 PM »
Being the old **** that I am, I did a Vis Basic program to calculate the various positions in relation to a straight line, given the figures that you have quoted.

None of the figures is more than 1 percent, and most are under 1/2 percent of a stright line - so you are working to fine tolerances. I hadn't noticed your comment before I started - and so I did it.

I am a steel man, myself, and these minute changes of curve would be all but un-noticed - do they honestly make all that much difference - I know the competition is quite fierce out there.
Not me driving the engine - I'm better looking.
Re: inconstant speeds?
« Reply #8 on: February 29, 2008, 05:24:36 PM »
When you are trying to build an airfoil and it is based off molds, the quality of the molds is VERY important.  Every defect in the mold is 10X worse in the fiberglass.  So, I care allot about those 0.01% changes.  I would just like to figure out the cause of the slowness.  The CPU is only at 10~14% usage.  Why is the CNC slowing down on the curves?  Is there different driver software that is not effected by lots of little movements?

Offline Hood

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Re: inconstant speeds?
« Reply #9 on: February 29, 2008, 05:46:20 PM »
I think the problem is just as has been mentioned before, you are doing lots of tiny moves and although you are in CV mode the axis still decellerates  at the finish and accellerates at the start of each line of code. When the lines are longer the motors have time to spin up before they have to start slowing again so the machine has time to get up to the cutting speed. Maybe you can tweak your motor tuning a bit, if you can get the acceleration higher without losing steps you will see vast improvements, one way you might manage this is to lower your velocity but really messing around is the only way as each system will be different as there are so many variables.

Hood