diy cnc cutting jagged. Does anybody know why?

[koisuruusagi]

1/4"mdf cutting 1/2" x 1/8" x 5"

Does anybody know the usual reason for this to happen?
(This is my first cut on the cnc.)

My guess is its either shabby cnc build, drill bit, spindle speed, or feed rate.

Specs:
black and decker RTX spinning at medium. (rated 24,000rpm)
feedrate 10" in mach3
bit- rotozip sabrecut zip bits

I would like to know if any of these are causing it before i remake the machine.

Any feedback is much appreciated.

[Benonymous]

Looks like the tool is really bouncing around there. There could be a heap of reasons why the tool is chattering. Machine stifness, loose router/rail/trucks, feed rate too high. I would think you'd pick up some clues from watching the machine work.

[koisuruusagi]

Thank you for the quick reply.

From what i was looking at while it was cutting, it looked like the bit couldnt cut fast enough so i think it is the feed rate.

Is it normal for a small piece like this to take 30mins total to cut?
20min on roughing and 10minutes on finishing.

[metalworkz]

Hello,
You did not mention what the pass depth was set to? Using a small diameter bit and a large pass depth might explain the tool flexing and causing some chatter? Also I am not sure what the size of the cut area is from your description, is it .5" wide x .125" deep by 5" long, or can you explain (1/4"mdf cutting 1/2" x 1/8" x 5") better?

[koisuruusagi]

Hello metalwrokz,
The stepdown is set to 50% tool diameter which would be half of 0.125.

[koisuruusagi]

Here is the dimensions.

[metalworkz]

I think if you can use a larger diameter bit or a bit with a shorter cutting length you will have better results. The saber cut bits appear to be fairly long at the fluted end and will probably allow more flex than a 1/8" bit with say a .5" cutting length, and I am not sure the tool geometry of the bit being used is the best choice for cutting mdf like you are doing. Most of the rotozip bits are designed to drill through the material and then cut with the side of the tool whereas you are only using the end of the bit to cut with. If you can not get another tool to use perhaps reducing the cut depth might yield somewhat better results and would not take much to try on a piece of scrap.

[Benonymous]

Have you got a picture of your machine?

[koisuruusagi]

I've attached the pictures.

I have been messing with it the whole night.
The Zmount needs to be reinforced since it looks it the support screw ripped off.

I just got the black and decker rtx today in the mail.
I examined it right now since it didn't cut this off before, and the rtx rotary tool seems to have a lot of play within itsself.

I will get a shorter bit for the 1/8" and a thicker bit for roughing tomorrow as metalworkz has suggested.
I knew the accuracy wouldnt be so precise due to the build of the machine, but it has cut something more accurate before this drill so i will see what i can do with that.

Thank you guys for the comments!

[JerryBurks]

I tried Roto-Zip bits a few times on my CNC and could not find any setting that gave a reasonable result (I tried Plywood). Not with the regular bits (up-cut) and especially not with the down-cut Sabre bits. They don't give a clean cut, flex because they are too long and slim and get completely dull after a few feet of cutting.

Carbide bits like the ones from Precisebits, Onsrud or Centurion are just vastly superior and hold up for a long time. They cost about $10 for an 1/8" bit with 0.5" cut depth.

[osphoto]

I started out with MDF. Used the rotozip as the spindle. However, I quickly went to the carbide bits. VAST improvement. I've also used burr's and they cut fairly well in MDF.

In looking at your cuts. It looks like your circle cuts are segmented. I could be wrong, but in comparing the other cuts, they look to be the same "poor" cuts?

In my CAM software, I can choose between segmented circles or arc circles.

If you slowed your cutting process, do the cuts improve?

As for cutting, these look like your rough cut pass. Did you follow up with a finish pass in the images?

As for getting carbide bits, I'm plugging: End Mill Discount - Carbide End Mills. Max, is the guy that runs this site. Max is blind and this is how he makes his money. He's treated me well and the prices are difficult to beat.

[koisuruusagi]

Thanks @JerryBurks I think that is what i wanted to hear with the rotozips.

@osphoto It went through about 20mins on roughing and 10mins on finishing. I will look if it is set on arc cutting. I use rhinocam an i am new to CAD and CAM so I am still unsure. Thank you for the links to the bits. Which bits would you use for roughing and finishing for cutting a part like this?

I've found a link to fix the play on the rotary tool on instructables.
How To Tighten Up A Sloppy Second Hand Dremel