Dimensional Accuracy

[jamesmcbennett]

(New Shopbot owner and new to CNC. Background in design.)

I am trying to get my CNC to make an accurate profile cut, but can't figure out what is wrong whether it is geometry or software settings or machine setup.

I took the images below which tell some of the story.
http://imgur.com/a/IQpIg

Basically the problem is....
18.0 mm in rhino = 16.14 mm when CNC'd
18.5 mm in rhino = 16.32 mm when CNC'd
19.0 mm in rhino = 16.56 mm when CNC'd
20.0 mm in rhino = 17.35 mm when CNC'd

Also I need to cut 20mm down to get guarantee cutting through the 18mm plywood of which the cut into spoil board is less than 1mm deep.

[jerry_stanek]

Have you checked your bit to see if it is under sized.

[khaos]

Have you checked your bit to see if it is under sized.

X2! bit size seems like a great idea more than an actual thing for some bit manufacturers. :(

[adrianm]

What are the dimensions of the pieces when they are in VCarve? Same as Rhino?

Moving from one piece of software to another there is always a risk of one or other not converting properly. Best to stick to one wherever possible.

Easiest thing to do would be to draw some simple shapes in VCarve and then cut those. That will eliminate the Rhino step and let you see if the issue is with your ShopBot setup or Rhino exporting.

Another basic test worth doing is to tell the machine to move 6" and then verify that it has actually moved 6".

The quality of the cuts look very bad so my gut feeling would be something is loose on the machine.

[steve_g]

James...
Is this a new machine? Something’s not right... is the part being held securely while being cut?
SG

[gc3]

looks like material moved while cutting, i would use a 3mm end mill lhf on something this small cutting slow

[MogulTx]

Something else to consider: What size are the lines? In other words, when the part was drawn in Rhino, what width were the lines being drawn and were they interpreted with a dimensional width when brought into the Partworks/Aspire?

Whenever I draw something, I do it in another program ( similar to Rhino but much less costly, and one with which I am familiar), then bring it into Aspire. That program ( any CAD program) can allow you to create lines of a specific width. If you have lines with width and which you are telling the program to cut inside of, maybe the program is doing exactly what you are asking.

My problem with that theory is that your first example is rather mis-shapen. The cuts are very irregular. Whatever the machine does, it should do with "purpose" and precision... and that photo does not look like precise cuts

To check out this possibility: Do a dimensional check of the inside of the lines in your Part Works/ Aspire program, to see what it is that it is "seeing".

Next: the first cuts are not looing precise....So could you have loose pinions? (check by: Turn on ShopBot and then push and pull on the y axis carriage and see if you get any slop in the X or y. If so, you need to address that slop. ANY movement needs to be removed by pressing the pinions back into its rack and securing. (assuming PRS or PRS Alpha) Pinions may also be worn out, depending on age/use.

Those are my two guesses and potential ways to check out. I hope you sold the mystery quickly. Good luck.

Monty

[jerry_stanek]

What Shopbot do you have size and model

[jamesmcbennett]

Wow. lots of great responses! Thanks for the support.

Jerry/Khaos: I'm not sure how bit sizes are meant to be. It is certainly 0.25 inch at top of bit on the smooth part, but the flute part is at its widest 0.22 inch tapering slightly as the flute appears. Not sure if that right or wrong? Shopbot is brand new PRS Alpha 96 x 48, 4HP Spindle. Also I am using it in millimetres.

Steve_G: Brand new machine. Just got it. Bit is very securely held.

GC3: Don't yet have a 3mm bit, but maybe should get one. Just have the Shopbot Starter kit at moment. Also. I don't have suction, but the kink on each of the edges was done on every corner, not just the last one to be cut. Given it did 4 passes which the wood wasn't loose on the first three passes, but the kink was in all of that passes rules out looseness being the main contributing factor.

AdrianM: I checked that dimensions in VCarvePro are the same as in Rhino as 18mm,18.5mm, 19mm and 20mm. I will try draw something in VCarve and cut that to see if it behaves differently now as well as doing a 6" test.

MogulTx: Not sure if I am understanding question correctly on line-width, but rhino has 0 thickness to lines unlike Autocad that can specify line-thickness. Will check the pinion test, but this is a brand new machine, machine is PRS Alpha 96 x 48, 4HP Spindle.

Will be back in 2hrs or so having done tests above and provide more info.

[jamesmcbennett]

Posted on Reddit too. Someone noticed the dimensions cut are all between 87%-89% of the specified dimension. Not sure what this means, but likely something there....

[jamesmcbennett]

Was recommended there to copy and paste Gcode/Shopbot file of a 10x10 square with bottom corner at 0,0,0.



'----------------------------------------------------------------
'SHOPBOT ROUTER FILE IN MM
'GENERATED BY PARTWorks
'Minimum extent in X = 0.000 Minimum extent in Y = 0.000 Minimum extent in Z = -18.000
'Maximum extent in X = 10.000 Maximum extent in Y = 10.000 Maximum extent in Z = 0.000
'Length of material in X = 10.000
'Length of material in Y = 10.000
'Depth of material in Z = 18.000
'Home Position Information = Bottom Left Corner, Material Surface
'Home X = 0.000000 Home Y = 0.000000 Home Z = 20.000000
'Rapid clearance gap or Safe Z = 6.000
'UNITS:MM
'
IF %(25)=0 THEN GOTO UNIT_ERROR 'check to see software is set to standard
SA 'Set program to absolute coordinate mode
CN, 90
'New Path
'Toolpath Name = Profile 10
'Tool Name = 1/4" Straight (48-005)
'&ToolName = "1/4" Straight (48-005)"
&Tool =2 'Jog Z axis to safe height
C9
TR,14000
C6 'Return tool to home in x and y
PAUSE 2
'
MS,75.9,25.3
JZ,20.000000
J2,0.000000,0.000000
J3,-3.175000,0.000000,6.000000
M3,-3.175000,0.000000,-5.250000
M3,-3.175000,10.000000,-5.250000
CG, ,0.000000,13.175000,3.175000,0.000000,T,1
M3,10.000000,13.175000,-5.250000
CG, ,13.175000,10.000000,0.000000,-3.175000,T,1
M3,13.175000,0.000000,-5.250000
CG, ,10.000000,-3.175000,-3.175000,0.000000,T,1
M3,0.000000,-3.175000,-5.250000
CG, ,-3.175000,0.000000,0.000000,3.175000,T,1
M3,-3.175000,0.000000,-10.500000
M3,-3.175000,10.000000,-10.500000
CG, ,0.000000,13.175000,3.175000,0.000000,T,1
M3,10.000000,13.175000,-10.500000
CG, ,13.175000,10.000000,0.000000,-3.175000,T,1
M3,13.175000,0.000000,-10.500000
CG, ,10.000000,-3.175000,-3.175000,0.000000,T,1
M3,0.000000,-3.175000,-10.500000
CG, ,-3.175000,0.000000,0.000000,3.175000,T,1
M3,-3.175000,0.000000,-15.750000
M3,-3.175000,10.000000,-15.750000
CG, ,0.000000,13.175000,3.175000,0.000000,T,1
M3,10.000000,13.175000,-15.750000
CG, ,13.175000,10.000000,0.000000,-3.175000,T,1
M3,13.175000,0.000000,-15.750000
CG, ,10.000000,-3.175000,-3.175000,0.000000,T,1
M3,0.000000,-3.175000,-15.750000
CG, ,-3.175000,0.000000,0.000000,3.175000,T,1
M3,-3.175000,0.000000,-21.000000
M3,-3.175000,10.000000,-21.000000
CG, ,0.000000,13.175000,3.175000,0.000000,T,1
M3,10.000000,13.175000,-21.000000
CG, ,13.175000,10.000000,0.000000,-3.175000,T,1
M3,13.175000,0.000000,-21.000000
CG, ,10.000000,-3.175000,-3.175000,0.000000,T,1
M3,0.000000,-3.175000,-21.000000
CG, ,-3.175000,0.000000,0.000000,3.175000,T,1
J3,-3.175000,0.000000,6.000000
JZ,20.000000
J2,0.000000,0.000000
'
'Turning router OFF
C7
END
'
'
UNIT_ERROR:
CN, 91 'Run file explaining unit error
END

[adrianm]

Did you try telling the machine to move a certain distance and measuring it? That will ensure you've got the right units set in the SB3 software.

When tracking down a problem like this it's best to eliminate as many steps as possible so I'd just use the SB3 control software at the moment and not use Rhino or VCarve.

Bit confused with the file you've posted as it's in metric that will be a 10mm x 10mm square which is very small. I expect the others were saying a 10" square. If it's meant to be a 1cm square then, as others have said, I'd be using smaller cutters at that sort of size.

[jamesmcbennett]

Yes, I did the distance test. Tested 2000mm and it is perfect.

I also made a drill hole and measured the internal diameter of the drill hole as 6.33mm, so pretty accurately a quarter inch that proves the router bit is accurate.

The 10 x 10 square wasn't cut, only as a test of Gcode. So the measurements show 13.175 which is equal to 10mm + 3.175mm (radius of quarter inch bit in mm.)

[gc3]

MS,75.9,25.3

end mill deflection is a guess slow down x-y speed and see what happens

i run my vintage pr in mm and when cutting 18mm ply with 6mm endmill use 40mm/sec for xy and cuts are clean 3 passes

what pp u using...i use arcs/mm

tried g-code and had similar issues so went back to sbp pp with vcp

[MogulTx]

OK James. I have a 4hp spindle on a 5 x 10, and the performance should be better, so we have to figure out what it is doing. While you are checking things: Is the rack well greased? ( Not too much, not too little?) Check BOTH sides of the Y carriage. I forgot to mention that. Make sure neither is moving. I had ShopBot come out and do a tune up on mine. I had a LOT of slop that I had not thought to check. I replaced ( they replaced) pinions and retuned the machine for me. Was a great improvement in cut quality.

[jamesmcbennett]

pp?

Indeed well greased. This is a brand new machine, so I followed instructions and well greased it.

[gc3]

pp...post-processor

[evan]

Not sure it's an issue but when drawing is in metric, I use metric tooling, when generating the tool paths, and when the drawing is in imperial I use imperial tooling, when generating the tool paths.

[Simops]

Not sure it's an issue but when drawing is in metric, I use metric tooling, when generating the tool paths, and when the drawing is in imperial I use imperial tooling, when generating the tool paths.

My bot is setup in imperial and I use imperial tools only.......I design many of my requirements in metric and occasionally in imperial and always use the inch PP......never a problem as SB3 does the conversion automatically and cuts are always spot on regardless whether imperial or metric.......great feature of SB3.

Cheers

[Bob Eustace]

Working the same way as Mike with zero problems.

[jamesmcbennett]

After more tests, its seems that the cut is consistently out by 2.85mm on a range of cuts 18mm, 20mm, 22mm, 24mm, 26mm, 28mm, 30mm, 32mm, 34mm and 36mm.

I have figured out that if I convert lines to either arcs or lines in rhino, then bring a DXF into VCarve that the kinks don't appear in the VCarve drawing but still appear in the CNC'd wood. Doesn't solve dimension problem, as this is a second problem, hope I don't confuse anyone.

[supertigre]

As the difference is consistent regardless of the size of the part the problem is not the Unit Values as these would give larger differences with larger moves.

Your drawing shows that the part is being cut with a climb strategy. Any deflection using this strategy will result in oversize parts (undersized spaces) and the difference would be consistent regardless of the distance moved. If you changed to a conventional strategy you may find that the gaps become oversize (parts undersized) because of the same deflection. A strategy to produce the most accurately sized parts (if the problem is deflection) is to cut most of the way through the material (80 -90%) using a climb cut and then finish cutting in the conventional direction.

As mentioned earlier an undersized bit would also cause an oversize part but changing cut direction will not have an effect.

[jamesmcbennett]

Climb Vs. Conventional
http://imgur.com/atOvtUd
Both are messed up, but differently.


I need to get something built, so figuring out if it needs to be +3 or -3 in different places and is working. Not an ideal way to have to make something, but a temporary solution. Hope to get this fixed as soon as possible! Thanks for all the great support so far!

[jamesmcbennett]

http://imgur.com/ywjB0II

This was intended to be 60mm long, but cut the hole 57mm (-3mm) and the leg 63mm (+3mm). So I remade a top that had the holes as 66mm and got holes as 63mm (-3mm) which fit the leg perfectly. So there is some game of three's going on.

On the right side of both top and bottom joint, the corner has been snipped off the leg creating a slight gap. I don't think this has anything to do with hardware as the corner is snipped off every similar shape in the same position every time with the same dimension and angle of the kind of bevelled snip. The same is happening with the holes of the top. The top left corner of the bottom hole being snipped and if you rotate that hole 90 degrees anticlockwise to position of top hole, same corner snipped at same angle.

[gc3]

what are you generating your toolpaths with? post the drawing

[evan]

My bot is setup in imperial and I use imperial tools only.......I design many of my requirements in metric and occasionally in imperial and always use the inch PP......never a problem as SB3 does the conversion automatically and cuts are always spot on regardless whether imperial or metric.......great feature of SB3.

Cheers

Great to know thanks.

[jamesmcbennett]

I use Rhino5, then VCarvePro that came with my ShopBot, then SB3 to play file.

Image attached that show various points of setting up VCarve. I export them to Shopbot file in MM.
http://imgur.com/a/iiN2L

---------------------------

Will try as suggested changing file to inches, SB3 to inches as bit is imperial 1/4 inch bit that came with the ShopBot.

[scottp55]

James, Newbie here, but wondering why you didn't "create fillets""dogbone" to .25" directly in VCarve? Both my Onsrud starter set .25's" are undersized. .246 and .247" respectively.

[jamesmcbennett]

Just taught myself a new trick, didn't know that feature was there......I'm very new. :)

[scottp55]

:) Did you measure your bits at the tip? Won't explain all your problems, but might help a little? Measure at least 5 times, Throw away high/lows and average middles(or keep going until you get 3 identical readings). Something is funky. Your pictures help a lot!

[jamesmcbennett]

Did inches test, still out by similar distance. Did gap of 0.7inches and got 0.644 which is about 1.5mm out.

Also did a test by putting fillets in on VCarve, again no difference.

Both tests still have inaccurate corners.

[steve_g]

James...
Refresh my memory.
Cutting speeds?
Step down?
How are you holding the part?
SG

[steve_g]

To eliminate design and tool path issues... Run this file. It’s a 1” X 3” cutout with dogbone corners. The 0,0 datum is in the center of the cutout, it’s set to cut .5” deep. move your router to the center of the spot you want cut, Z zero and also set that point as 0,0.
I’m curious as to how it will look...
SG

[jamesmcbennett]

Your Test File = Not accurate. http://imgur.com/R5jqsPn

Measurement is 1.03inch x 3.09inch


Cutting speeds? Not sure if that means just all the details in the tool database, everything is at its default value that came with the machine. So 1/4 inch straight bit (48-005) is

Diameter: 0.25 inches

Pass Depth 0.25 inches
Stepover 0.1 inches 40%
Spindle Speed 14000 RPM
Feed Rate 3.0 inches/sec
Plunge Rate 1.0 inches/sec



Step down? 4 passes at 0.2 inch

How are you holding the part? Just screwed in at corners of sheet, so last cut will be loose. I haven't got to learning tabs yet, but presume the issue doesn't lie there as most of the problems are well before the last cut.

[gc3]

those are terrible cuts my old pr is 10 times more accurate than that...go over machine and ck everything is assembled correctly looks mechanical to me

[bearcat]

Slow down the feed rate to 1.0 ips and the plunge rate to 0.5 ips and re-run the test. You also might want to change the out the single flute bit to a two flute or spiral bit to improve the cut quality.

Ed

[steve_g]

You have an issue with your machine somewhere...
The file was cutting very conservatively. It was a spiral cut so the plunge rate is meaningless, the 1X the bit diameter pass depth is something you should be able to do accurately and consistently, bit deflection should not be an issue at the 2ips speed it was cutting at because due to ramping it never got there!
First thing I’d do is physically tighten the set screws on all pinions.
SG

[jamesmcbennett]

Tightened the set screws, they were quite lose as don't think that was in assembly manual and I unfortunately didn't check how tight/lose they were. Was excited as thought this was going to improve things, but hasn't made much difference. I tried various router bits from the ShopBot Starter kit, but not much difference. http://imgur.com/85uSTqP

[gc3]

post a screenshot of nodes in the vectors

[jamesmcbennett]

Do you mean from SB3? Here is that http://imgur.com/a/GOraZ

[jerry_stanek]

When you tightened the set screws did you remove them first. On my pinions they have 2 one on top of the other. So if the bottom one is loose your pinion still may be loose.

[gc3]

Do you mean from SB3? Here is that http://imgur.com/a/GOraZ

no thats the toolpath in vcp select vector right click node edit mode select vector again

[scottp55]

No James, click on selected vector in VCP, press N,take screenshot

[jamesmcbennett]

PROGRESS!

I misunderstood what tighten pinions meant. This time I took the motors off and X1 and Y were both loose enough to fall off, the other two were fine. So did a quick cut at 9pm which I think turned out really well http://imgur.com/HMwTD2Q, but in residential neighbourhood, so will do a full test in the morning.

Thanks to everyone for so much amazing support! Very appreciative!

[jerry_stanek]

That is looking better.

[steve_g]

Sorry... likely I should have said “tighten the grub screws on the spur gears”
But it looks like we’re getting there!
SG

[scottp55]

Progress!:)

[chiloquinruss]

When peoploe ask why I bought a ShopBot, this thread is exactly the answer! Nice job gang! Russ

[scottp55]

James, Just in case you haven't watched it, TJ tapes all his classes. If you go down to machine maintenance, it might help. I usually try to attend them online because of the Q&A at the end of the class, but I think I skipped this one because it was on full size machines and we just have Desktops. I always learn stuff from TJ's classes. Sorry if you've already watched it.http://www.shopbottools.com/mSupport/tutorials.htm

[jamesmcbennett]

Reason I bought a ShopBot over anything else was also exclusively down to support and community, not just a machine. Thinking back to the last two months, not sure how I would have proceeded without it. Decision well made!

I hadn't seen that Scott, thanks for link. (Still clearly a newbie!) and will learn something new this weekend! :)

Have a great weekend everyone! Thanks for fixing my ShopBot

[scottp55]

Same here James. Would never have laid the money down on a machine I'd never seen without the forum. Shopbot support has always been wonderful to us as well, actually over and above any expectations we had. It would be a better world if every product/company was run like this. Hated seeing those initial cuts---Glad Desktop arrives assembled and ready to go. Wait until you get dialled in and better!:) Changes your thinking. Enjoy