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Author Topic: Trouble cutting accurate parts  (Read 15898 times)

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Re: Trouble cutting accurate parts
« Reply #10 on: December 14, 2007, 12:03:28 AM »
One other thought,
If you were to CLIMB mill around the perimeter of the rectangle, backlash could allow the part to be bigger. ie: programmed dimension + backlash.
This isn't likely though with a small cutter in plastic.

Offline DAlgie

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Re: Trouble cutting accurate parts
« Reply #11 on: December 14, 2007, 01:38:25 AM »
Two things come to mind here, an 1/8" end mill is a tiny tool to ask to not flex and walk around in 1/8" thick material, and secondly, Plexiglass has horrible moisture absorbtion rates and will expand with just a small humidity change, you didn't say how big your parts are, on a mini mill can't be very big but still the stuff is trouble to keep a size with. It can also cause semi melted chips to build up on a cut surface after the mill has passed, causing size increases. Try a larger mill bit, and make sure the swarf is sucked out or something while it's cutting.

Offline jimpinder

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Re: Trouble cutting accurate parts
« Reply #12 on: December 14, 2007, 05:38:58 AM »
Walt - I have read all these suggestions and whilst I am not confused and they are all good suggestions, lets get down to the nitty gritty.

You have the machine in front of you - what is it doing wrong. Is it cutting too small or too large - and from that, does your axis appear to miss or gains steps. That is the first step.

Having said that, and I missed it first time round, plastic is very difficult to cut. As daft as it seems, the cutters must be very sharp, and the speeds must be properly controlled. (I only say this because my local plastic supplier who uses CNC said so).

I think I would try the cut manually first, say setting the cutter at exactly the right position with the spindle running, and then jogging the cutter up the cut, using a low percentage of the jog speed to do it with. Then measure the result and see if it is accurate.

If you CAN cut it accurately, then you can progress and see why the program won't do it.

JIm
Not me driving the engine - I'm better looking.

Offline wdp67

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Re: Trouble cutting accurate parts
« Reply #13 on: December 14, 2007, 12:35:44 PM »
I am at work right now so cant play with it, but it seems like if I tell the machine to move a set amount in the mdi line it moves that amount, but once I run a parts program it seems to cut larger than what it is supposed to. It does not matter if I am cutting plastic, metal or wood. All the results are the same, it is anywhere from .010-015 larger.

Walt
Re: Trouble cutting accurate parts
« Reply #14 on: December 14, 2007, 01:28:26 PM »
Hi Walt,
Are you using lead screws or ball screws ? What is the backlash in the area where you are cutting the parts
Did you check the travel with your indicator in the same area of the screw that you are machining the parts with ?
Are you using backlash compensation ? If you are, could you possibly be OVER compensating ? This would make the parts oversized.
I think your problem may be mechanical more so than electrical.
Wish You Good Luck

Offline wdp67

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Re: Trouble cutting accurate parts
« Reply #15 on: December 14, 2007, 01:35:12 PM »
I am using the stock lead screws. I have tried with both backlash enabled and backlash off, I honestly can not remember the results of the backlash off. When I get home tonight I will try again with backlash off and post the results here tonight. Thanks for all your guys help I really appreciate it. Hopefully I will get it resolved. The parts that I am making are not critical so it really doesnt matter, but I want to get it fixed so I can cut accuratly!

Walt

Offline wdp67

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Re: Trouble cutting accurate parts
« Reply #16 on: December 14, 2007, 10:36:48 PM »
Hey all
I played around with the machine tonight a little bit and may have found some  or all of the problem I think. When the y axis switches direction, watching a dial indicator, before it actually starts to go the other direction it moves .005 further in the previous direction. Does that make sense? It even does that if I do it manually. I can see where my parts would be .010 bigger in the y direction. I am not sure what to do to try to fix this though. It is a cnc'd harbor freight mini mill. Any ideas?

Walt

Offline stirling

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Re: Trouble cutting accurate parts
« Reply #17 on: December 15, 2007, 04:55:39 AM »
just a thought but what's the (theoretical) resolution of your machine in:

steps?
and
microsteps?

Offline wdp67

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Re: Trouble cutting accurate parts
« Reply #18 on: December 15, 2007, 11:54:39 AM »
I am not sure I can tell you, It is a harbor freight mini mill using a xylotex control box I think that is shipped 1/8 step and I have not changed it. I am using the stock lead screws which move the table .0625 per turn.

Offline jimpinder

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Re: Trouble cutting accurate parts
« Reply #19 on: December 15, 2007, 12:31:03 PM »
Walt - Mach3 cannot do this. If you reverse direction, then Mach3 reverses direction immediately.  What sort of motors are on the mill, are they servos with some sort of reaout to locate them. They certainly can't be steppers doing such an odd move. If they were steppers, they might not reverse straight away, but your DRO's have no feedback from steppers and would count down straight away.

If this is the case then Mach 3 is putting out the pulses, and the reverse sugnal, but your machine is not immediately picking up the reverse pulse - there is some delay in interprting it - as you see about 5 thous worth. I cannot think what to do. Have you a breakout board that is delaying these pulses (although if it delayed one, it sould delay them all). I don't see how you can measure the timing of the pulses without some sophisticated measurement devices.

The only thing I can suggest is slow down your traverse speeds and see if this is reflected in a lower than 5 thou readout when you change direction. It is certainly odd - anybody else have a thought.

Jim.
Not me driving the engine - I'm better looking.