CNC Not Cutting Round

[supertom44]

Came to try making some coasters and what not and ran into an issue where my cnc isn't cutting round.
There are two straight sections on either side of the circle.
To me this suggests that there's an issue with the X axis, I've double checked the coupler between the motor and the rail and all seems fine, checked the Y axis too and all good.
This image was done at 120 inches/min but tried slowing it right down to 40 inches/min and problem still persists.



Machine is a SainSmart 4030 Proverxl running a dewalt router as a spindle.

Not really sure what else to check/adjust.

 

[Yojevol]

I've got a similar problem with my old Denford which has had quite a hard life. In making my clock gears it's important to get reasonable circularity. I think my problem is due to play in the z-axis guides which is allowing the router to be a bit floppy thus causing undersized y-axis cuts. I've got replacement guides but not yet fitted.
So it's worth checking if there is any play in the router supports.
Brian

 

[supertom44]

I hadn't considered play in the z-axis but I can see how that may be the cause, will check it over later, thanks.

 

[Sachakins]

Might be dumb question, but is the file ok?
Drawing not been stretched.

I would try doing a board with several holes across the whole 400 x 300 cutting area.
It may show if the error is in one area or consistent across all the cuts.

 

[supertom44]

File is good, saw issue with logo file so I created a new file in vcarve using the circle tool, path and preview all looked perfect.

Interesting idea about the holes, will see if I got some scrap lying around.

 

[worn thumbs]

One of these? PROVerXL 4030 CNC Router with Carveco Maker Subscription

If so,notice the lack of bracing for the gantry beam across the machine.If identical steppers are used on each axis then surely the need for a similar amount of bracing is fairly obvious-well except to the designers.I'm guessing that the gantry beam is flexing the side plates as a reaction to the stepper drive forces.I suggest you find a piece of scrap and try cutting two circles with the same cutting parameters,one clockwise and one anti-clockwise and measure them carefully.

Alternatively,it might be possible to mount a DTI to the side plate with some temporary bracketry to look for any movement.

 

[supertom44]

One of these? PROVerXL 4030 CNC Router with Carveco Maker Subscription

If so,notice the lack of bracing for the gantry beam across the machine.If identical steppers are used on each axis then surely the need for a similar amount of bracing is fairly obvious-well except to the designers.I'm guessing that the gantry beam is flexing the side plates as a reaction to the stepper drive forces.I suggest you find a piece of scrap and try cutting two circles with the same cutting parameters,one clockwise and one anti-clockwise and measure them carefully.

Alternatively,it might be possible to mount a DTI to the side plate with some temporary bracketry to look for any movement.

Yeah one of those, except I've extended it to 600x800 so I imagine this would amplify any flex. Even so I would have thought it would be fine when taking light passes through something soft such as MDF.

 

[supertom44]

I've got a similar problem with my old Denford which has had quite a hard life. In making my clock gears it's important to get reasonable circularity. I think my problem is due to play in the z-axis guides which is allowing the router to be a bit floppy thus causing undersized y-axis cuts. I've got replacement guides but not yet fitted.
So it's worth checking if there is any play in the router supports.
Brian

This was the issue!!

There are two bolts that attach the spindle carridge to the X rail, these were loose causing play.

Also noticed that the top bolt for the spindle holder has been stripped, unfortunately sainsmart don't have any replacements in stock, guessing cos they've moved onto a newer model, but doesn't seem to be causing any issues at the moment.

 

[Ollie78]

Something is loose somewhere. When it happened to me it was the coupling on the y axis ballscrew.
Just go through every connection, it will turn up.

Ollie