Rule of Thumb on cut speeds?

[johnm]

Folks -

Okay, we just got back from the Northern California Camp ShopBot in St. Helena, and I had a question I forgot to ask...

We're cutting 3/4 PVC plastic (AZEK is the brand name) and we are going to be cutting a welter of fence pickets from the material. I am seeing some milling marks from the cutter, a 1/4" Onsrud series 65, on the curves of the picket heads. It doesn't matter if it's an inside or outside curve, and it isn't too bad, but I am wondering what the matrix is between feed speed and depth of cut. We did three test picket heads, one with 3 step downs and one final pass, one with 3 step downs and NO final pass and one with 2 stepdowns and no final pass, all at 1.75ips. The quality of the cuts is very similar. I didn't have any plastic melting, tho' a fair amount of the swarf was not ejected from the cut and remained in the channel during subsequent passes.

The cutting rpm for the first cut was 12k, and for the second and third cuts it was 16k - the chips were larger at higher speeds which seems counterintuitive.

Should we speed up? Slow down? Put a hat on our heads? Jump around?

The straight cuts on the side of the pickets are of better quality, but we are getting "fretting" for lack of a better word on the curved areas of the cut. Not bad, but certainly different than the straight areas. I just want to know what is normal for the machine and how I can improve the work, if possible.

TIA

John Moorhead and Rose Davidson
Lakeport, CA

[elcruisr]

A big improvement might be moving up in tool diameter. I try to use a 3/8 polished o-flute cutter whenever possible on Azek or it's like. You can do a single pass and get better edge quality thanks to less tool flex and a deeper gullet on the tool. If I was cutting at 1.75" (I'm assuming you run a PRT not an Alpha) I'd be turning 14 to 16000 and get a very clean cut.

Eric

[jhicks]

My experience with Color core and other plastics has been to consult directly with Onsrud 1st. They have specific recommended bits and geometries for just about anything and are also very helpful with speed and feeds at their tech support desk.
Generally we leave a thin Onion skin layer on the entire profile to reduce torque on final cut outs of somewhere between .080 and .100 and accomplish successfully often without any hold downs or vacuum where it makes sense.
I too like the results of the o flutes but they aren't always the right answer.
If you can run a final pass offset it helps by overcutting prior tool path marks and profiles the entire surface on the final pass.
Speeds usually in the area of 1.7" to 2.0" on our PRT with spindle at between 13,000 and 15,000 rpm has been our general range with a 1/4" bit.
Final answer is always test and dial in to your liking or required finish based on your hold down technique, bit sharpness, material, and cutting strategy.

[propellers]

I need to make a protractor that has an OD of 20" and an ID of 12". I need to cut marks every 1/2 degree for only 30 degrees or 60 lines. Starting at 0,0 with the degree markings fanning out to 20" radius. I tried to accomplish this in PW but its apparent I need remedial training. Can anyone give me a hand with this.

[paco]

From where are you stop? What do you need to know?

Are you looking for someone to draw this (contact me) or want to learn how (need tips and hints)?

[gerald_d]

Lonnie, your spec is rather vague - this is the best I could do to meet your info. (0,0 in center):


4990

[harold_weber]

Following Gerald's picture, for each line you can specify the point 0,0 for one end and the other end can be located at X=R*sin(angle) and Y=R*sin(angle). You can calculate all these values in EXCEL. Finally you can use the little scissors tool in PW to trim off what you dont want. (You probably want to trim from 0,0 to the ID.)

If you want to wait for the Jamboree, I'm sure Dirk's presentation on using EXCEL will be of interest.

[propellers]

Paco, I would like to have the knowledge of doing this. Gerald's post is what I'm looking for but I was looking for the proceedure of drawing it in PW.

Gerald, did you draw this in PW?

Harold,I appreciate your input, I can't wait until the Jamboree, I'm building a timing ring to time the magnetos on my plane so I can fly down to the Jamboree this year.

[paco]

Do yo have a draft of your project so far?

[propellers]

Thanks all, Harold Weber was kind enough to call me a few minutes ago and walked me through the process in PW. The forum is a great asset to purchasing a SB. I once considered a Thermwood machine and feel my SB purchase was one of those smart things I have done in life.