Cutter accuracy

[Neale]

Been trying out my new CNC router with a few simple jobs. Trying to cut slots in thin ply, I found that my cutter seemed to cut consistently under size. 3mm single-flute cutter gave 2.6mm slot, or equivalent result cutting opposite sides of plain block (I.e. block was oversize after cutting by same amount). Tried a couple of other single-flute cutters (4mm and 6mm) which also seem to cut about 0.4mm undersize. Difficult to directly measure diameter of these cutters, but putting the 6mm cutter along a straightedge, the tip does sit slightly below the line of the shank. Is this normal? Do I need to, in effect, calibrate the diameter of a cutter before using? Are 2-flute cutters more accurate?

[A_Camera]

Which software are you using and in which mode do you see the error? I mean constant velocity or exact stop mode or both consistently? New cutter with 0.4mm error in 3mm is a lot, but I have received cutters with that much error from the factory, brand new. Of course, cheap eBay stuff but I complained and the seller sent me a batch of new ones and those were good. I don't know if 2-flute cutters are more accurate than single flute, but I guess they are easier to make and to measure, so I prefer using 2-flute or more, but I don't cut ply or any other wood.

[Neale]

I was using Mach3 via my CSMIO IP/M. However, CV or exact stop was not relevant here because for my test pieces I was typing gcode directly into the MDI window and just taking straight cuts. For testing, I fixed a blank to the spoil board and took a straight cut down each side. I moved the cutter sideways by a standard amount (54mm in this case) between cuts. I was taking very light cuts so do not believe that cutter deflection was an issue. What I expected to measure between the cut faces was 54mm minus cutter diameter. I actually measure 54mm minus cutter diameter plus about 0.4mm.

I do not believe that it is machine calibration error as I did these tests immediately after doing a gantry squaring test. This involved drilling 4 holes at the corners of a 100mm square, putting the shanks of new drills in the holes, and measuring diagonals. However, as a cross-check I also measured the sides of the square (all four sides) and saw consistent and accurate measurements to generally better than 0.1mm.

I was using cutters from a couple of UK sources. The own-brand 3mm cutter from CNCRouterShop is quite old now but my old router had so much play in every joint that I could never work to this kind of accuracy, so can't tell if it was like it from new. The 4mm and 6mm cutters are own-brand from Regal and are brand new.

[routercnc]

Hi Neale,

Just to check, if you plunge down and just drill a hole, then drop the shank end of a 3mm drill in does it fit? If not does a 2.5mm one fit? That would tell you the diameter of the cutter. If that is off then you will just tell the CAM software the true diameter of the tool (i.e. 2.6mm) and all will be well again.

[Neale]

Thanks for the suggestion. Checked 3/4/6mm cutters and all make accurate size holes. Checked backlash with dial gauge - approx 0.05mm on X axis. Accuracy otherwise better than 0.01mm over 10mm (dial gauge range). Also used 6mm cutter to make two holes 150mm apart, push drills into holes and measure with vernier calipers - around 0.05mm errors. Tried moving gantry to include/exclude backlash just to make sure - no obvious issues. So happy with Mach3 "steps per" settings. Clamped block of MDF to bed and machined both sides and measured. Light cuts (1mm or so, DOC 4mm, 6mm cutter, so no cutter deflection expected). Comes out approx 0.3mm oversize.

So, not much backlash, spindle positioning good, cutters make true size holes. But machine does not make accurate cuts. Baffled.

[Boyan Silyavski]

Normally cutters when new are spot on. Even Chinese. Check size though before inputting in CAM.


next step is 30mm circle in say 50mm square. 1mm depth, best i use Phenolic sheet as its very precise and holds incredible detail, same time not affected by temperature or humidity. As MDf is not right even 1 min after the cut/ i live near the sea.



If problems like in your case, then i go even simpler and do pure straight lines. dig 1mm deep and tell mach3 or controller to move 20mm. Then line will be 20+d of bit, easily measured with caliper. So its easy to figure whats happening and where.