Help with 3D lines

[angelosart]

See attached photo of a 3D finish I just completed today. I seem to keep getting these lines in the direction of the finish cut. I have to scrape and sand afterwards. Ugh!
Settings....
1/8" ball nose
19000 rpm
3/2 feed/plunge rate
8% step over
this particular one was done with a 45deg raster

Any reason why? This only started recently.

[steve_g]

Angelo…
A recent thread indicated that some guys are getting these marks when they lean on their machines to get a better look!
Other obvious causes are: wood moving due to stress relief when being cut, Hold down method inadequate, and “looseness” somewhere in the machine…
These types of issues can often be a challenge to track down. Good luck!
SG

[angelosart]

Steve
Thanks for the reply. I definitely don't lean on the machine when peeking at the progress. The stress relief on the wood makes a lot of sense. On this project I used screws for hold down.

When you mention "looseness" can you clarify the parts of the machine that includes? I recently check my router casing, what else could it be?

I guess I'm just wondering if this is typical for others who are cutting reliefs with ball nose bits? I've had the 'Bot for over a year and this is the worst its been.

Thanks
Angelo

[steve_g]

Angelo…
“looseness” is a very vague term… It can come from a number of sources! Step one is to give your machine the “shake” test. With the power on (but router/spindle not running!), Grab the collet and push / pull in all axis directions (X,Y and Z)… “Looseness” may be in the form of almost indiscernible “clicks”. You said you have had the machine “a year now” was it new when you got it? If so, it’s unlikely the machine is worn…
Is the material you’re using KD (kiln dried)? Is the problem worse with a particular wood? Is the heat from the router/spindle directed at the wood? (could cause expansion issues on long runs)
Obviously I’m just guessing now! You will find the issue eventually…
SG

[Brady Watson]

Angelo,
Stop machining on a 45. Raster parallel to the longest portion of the model, which should be the same direction the grain is running in your photo. It will machine faster and better than they way you are doing it now.

-B

[angelosart]

Thanks guys.
Brady can you explain why rastering at an angle is slower. I just like to understand because I had thought the opposite if I am working with anything other than a square project or component. Are there situations where rastering at an angle other than 0 and 90 is desired?

[Brady Watson]

Always machine with the grain when possible. Long straight XZ moves are preferred, opposed to short back and forth going across the grain.

Consider mowing the lawn. You have to mow a space 20x100 feet. Your natural inclination is to mow in long lines parallel with the 100' side; not along the 20' side. All that stopping and starting (going the 20' way) adds up to a substantial time penalty. Same with 3D machining. The machine has to ramp down to a stop, change direction, and ramp back up to your target feed rate. When rastering along the longest side with the grain, stop/start is minimized. Also. by machining in the same direction as the grain, you get a better surface finish with less possibility of tear out and fuzzies.

In my experience only employing 2 axes at a time results in more precise machining. Each axis you add to the mix (45 deg uses 3 axes) the sloppier things can get - (and the more commands need to get sent across USB). There can be a *marginal* reduction in machining lines when doing compound 3D curvature using a 45 deg machining angle. If you simulate a straight raster and a 45 deg one, you can see very little difference. However, if you are machining something completely round like a 1/2 sphere etc, the preferred method would be to do offset machining.

Don't take my word for it. Generate two 3D toolpaths. One at 45 deg, one @ 0 deg and save them out as SBPs. Then open up SB3 and run each in Preview mode. Observe the time it takes to machine each one.

-B

[khalid]

very nice info Brady..

[MogulTx]

I used to mow my front lawn on a 45 specifically because of the cool lines that it made in the grass. It was not a huge area, so the extra time was not an issue, but all the stopping, turning and restarting DID have a time penalty. For me, on that type of thing, it was "worth it" because I liked that I was the only person in the neighborhood who did much work on his front lawn and who paid any attention. (many thanks to my lovely wife who inspired me to work on the house and lawn to make it nicer) In fact, a number of folks in the neighborhood didn't mow on a reasonable schedule... so they saved a TON of time... (JK).... anyway, there is absolutely a direct correlation to changes of direction that cause the 'Bot to HAVE to slow down to make this type of cutting pattern. AND you can much more easily hide lines WITH the grain rather than AGAINST the grain- which results in less finish sanding.... so go with the grain! And orient your cuts along the grain as often as is possible! (That Brady is great, isn't he?)

Monty