Out of round on last cut

[Robin Hewitt]

Milling a 44mm hole with a 12mm HSS 3 flute through a 1" steel plate, cutting down hill.

Told it to 'drill' 26mm deep to make sure it went through. The software did it in 14 steps of 1.857mm so the last cut was a a bit lighter.

All went well until the last cut when it decided to cut all the way up the side of the hole. My fine chips became needles and it went oversize with a bulge at 10 o'clock.

It then did exactly the same on the second hole.

It's okay because I was going to clean it up with a boring head anyway and I can make the bushing that goes in it oversize if necessary.

My guess is that the tool was bending towards the center all the way down then flexed out when there was no more metal underneath it.

Is there a work around for this, or is it something else

[John3]

What was the cutter path followed?

Did you program it to drill the center out, then work outwards, or did you program it to drill close/at the perimeter?

I've found that the specific cutter path you follow has a large effect on the quality and time of milled holes.

John

[Robin Hewitt]

The software decided to do it with a centre plunge and then two cuts. 27.7mm diam, then 43.4mm leaving me 0.6mm to clean out with the boring head.

I tried the next plate cutting up hill and it went egg shaped with a lip at the bottom where the last cut went undersized. It has to be tool drag and flex somehere in the system.

Nobody mentioned drag from the cutting action bending the mill out of shape, time to get out the DTI and then lean on it a bit

[MIKE JEFFERS]

going from 27 to 43 is a big step, reduce your cutter overlap
take it it did this at full depth
think your strategy was all wrong
what software you using?
mike

[Robin Hewitt]

Originally Posted by MIKE JEFFERS think your strategy was all wrong what software you using?

Hi Mike

I'm using g-simple very badly, I should have included a finishing cut that would have spiraled down in to the hole with minimal tool drag and made it all perfect.

g-simple draws a series of paralel tool paths across the billet and then starts cutting them from bottom left in a series of passes at ever increasing depth. If it hits an outline it follows it with a clean up allowance if you asked for one. It then goes back to the paralel paths to remove any metal left standing outside the ouline(s).

Having cut the outline(s) it then does any pockets one after the other in a similar fashion. Round holes it cuts as 360 degree arcs.

Sounds primitive but it is a darned sight better than my home brewed software which spiralled in to size at full depth. I wrote that before I found this here site and got eddjukated, may need to revisit it, but it isn't easy to come up with reasonable tool paths for every circumstance. I have a notion to break the part into a myriad of tiny squares and remember how deep each one is and how deep it can go. That would allow for a lot of whittling before I went for the final shape.

Understanding the problem is halfway to a solution, now I know that I have to allow for the machine bending out of shape due to cutting drag I can do something about it. I don't want to spend thousands on software, I'd rather spend it on more tangible assets

best regards

Robin

[hoss2006]

The demo version of Mach 3 is free and comes with a bunch of wizards
that can be used to create your g-code for you.
Including Cut Circle and Circle Pocket that could help.
Check it out here.
Hoss

[MIKE JEFFERS]

try cambam too best of all its free
ace converter's also free

mike

[philbur]

Hi Robin,

CNC machining is a bit like computers. Went you first get involved you are paranoid about the hardware then, after a while it suddenly dawns on you that really what counts is the software. You've just reached that point, congratulations.

Good software is one of the most important asset you will buy for your CNC machine and it doesn't need to cost so much.

Regards
Phil

PS: I remember many years ago developing my own computer program that would perform calculates based on an input/output matrix table. After 3 months of effort somebody showed me a new, commercially available piece of software called a Spread Sheet.

Originally Posted by Robin Hewitt Understanding the problem is halfway to a solution, now I know that I have to allow for the machine bending out of shape due to cutting drag I can do something about it. I don't want to spend thousands on software, I'd rather spend it on more tangible assets best regards Robin

[MIKE JEFFERS]

can't argue with that
good software makes a big difference
try dolphin

[Robin Hewitt]

Originally Posted by philbur Good software is one of the most important asset you will buy for your CNC machine and it doesn't need to cost so much.

Unfortunately I blew $13k on a Baker rifle so I now have to wait until her indoors wants something equivalent before it's my turn again.

I don't need software to do the actual machine control, I just need to convert dxf to tool paths and G codes.

I have downloaded Cambam instantly plummetting to the botttom of the learning curve, doubtless I shall still be using gsimple for the forseeable future

[philbur]

The wizards in the free demo version of MACH3 will do what you need with virtually a horizontal learning curve.

If you have never used Wizards you should at least have a look. I think they will fit you needs perfectly.

Phil

Originally Posted by Robin Hewitt I have downloaded Cambam instantly plummetting to the botttom of the learning curve, doubtless I shall still be using gsimple for the forseeable future

[digits]

Originally Posted by Robin Hewitt Unfortunately I blew $13k on a Baker rifle so I now have to wait until her indoors wants something equivalent before it's my turn again. I don't need software to do the actual machine control, I just need to convert dxf to tool paths and G codes. I have downloaded Cambam instantly plummetting to the botttom of the learning curve, doubtless I shall still be using gsimple for the forseeable future

If you've got £85 to spend then I can highly recommend SheetCAM for 2.5D stuff - check out www.sheetcam.com

Cheers.