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Old 05-18-2017, 12:17 PM
cekonecky cekonecky is offline
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Join Date: Sep 2016
Location: Brooklyn, NY
Posts: 3
Default New Machine - Inaccurate Cuts

I'm cutting parts for a conference table in which the table top will have tenons resting in notches cut out in the legs. For ease of assembly I need to oversize my leg notches by 1/32". In my file I have the notches drawn at .7812" (1/32" over 3/4") but when cut they are consistently coming out at .75" exactly. To address the situation I ran some tests on a piece of scrap. I drew two rectangles in V-carve at 6" by .7812" and cut one inside the curve and one on the outside. The inside cutpath again resulted in a notch that was .75" exactly, and the outside cutpath left a piece that was .79". I tried these two cuts again, this time slowing down my ipm to 100 (thinking perhaps that the machine was moving too fast to be as precise as I wanted), but again the inside path left me a notch at .75" and the outside one at .79". Is there any explanation for why my machine isn't cutting as precisely as I need it to be? Also what could the explanation be for the machine cutting the same drawing to different dimensions based on which side of the line it's cutting on? Shouldn't my cuts be just as accurate whether they're inside cuts or outside cuts?

I'm cutting on a 5x10 cobra with a vacuum table - hold down doesn't seem to be my problem as my cuts are consistently inaccurate.
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Old 05-18-2017, 01:25 PM
Brendt Brendt is offline
Camaster Owner
Join Date: Apr 2014
Location: Northeast Wisconsin
Posts: 147

Check the diameter of the cutter with a caliper to make sure it matches the tool size that you entered in the specifications when you set up the tool. If the bit has ever been sharpened it will be undersize which could account for the discrepency you are seeing.
Brendt Bowen

Stinger II 48" x 48"
V Carve Pro, Corel Draw X8
1.7 kwh spindle
Performance Package
Hurricane Black Box Vac
Oneida 5hp Super Dust Gorrilla
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Old 05-18-2017, 03:31 PM
Xray Xray is offline
CAMaster Stinger Owner
Join Date: Feb 2011
Posts: 1,630

In toolpaths are you machining outside, inside or on vectors ?
If anything but outside, might be your problem.

Stinger 1
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Old 05-18-2017, 05:05 PM
mike.davison mike.davison is offline
CAMaster Owner
Join Date: Jul 2015
Location: Bolton, Massachusetts
Posts: 448

You might want to share the VCP and gcode files. It may be clear to others, but it's not clear to me what you're doing. Cutting on the vector? Cutting inside or outside the vector? Bit size? Feed rate? Depth of cut? If you share the 6x.7812 VCP file and the resulting g code I'm sure someone here can help.
Mike Davison
Stinger II 4x4, Recoil, spindle, FTC, Aspire, Sketchup
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Old 05-18-2017, 09:45 PM
rcrawford rcrawford is offline
Cobra Owner
Join Date: Dec 2011
Location: Alberta
Posts: 2,210

Draw a large rectangle (like 4' x 8') and cut on the line 0.02" deep with a 90 degree v-bit. Measure the size. This will tell you if the machine is cutting accurately. You might just have to make a slight adjustment in the wincnc.ini file.

If that comes out accurate, then the problem is most likely your tool isn't the exact diameter is states. If its diameter is smaller than stated, this would give you a smaller pockets on inside cuts, and large outside cuts.
Russell Crawford
Cobra 408 ATC with recoil
Alberta, Canada
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Old 05-18-2017, 10:42 PM
Gary Campbell's Avatar
Gary Campbell Gary Campbell is offline
CNC Tech & Trainer
Join Date: Dec 2012
Location: Marquette, MI USA
Posts: 2,766

Try the following:

Cut four 6"squares. 2 inside profile, 2 outside. One of each climb direction, the other conventional.

Cut a 1/8" deep slot at 100 ipm with your bit and measure the width of the slot.

Measure all and report back

Are there any divots or defects in the corners or where the bit plunges in?
Gary Campbell
CNC Technology and Training
The Ultimate Woodworking Machine

"There are 10 kinds of people in the world. Those that understand binary logic, and those who don't"
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