newby ...broken bits

[antihero]

Our shop just picked up a used shopbot prstandard with the Porter cable router. Having some knowledge of autocad i have been tasked with getting the machine up and running. I have been successful with getting parts from the drawing to finished product but am having a lot of trouble breaking bits. I have been cutting 1/2 mdf and 3/4" appleply (dense/multiply plywood) witth a 1/4" spriral onsrud carbide bit. I keep breaking bits though. I have been experimenting with feed speeds between .75 and 1.25 and have the rpm maxed out on the router (14000rpm) i think. I have been cutting between .125 and .25 depth per pass. The machine does not seem to be struggling to cut. I am overheating the bit? Is there feedrate/rpm chart that i should be consulting? Different bits better for this application? Any info appreciated. Also will this machine cut a true elipse? Is there a way to pause the machine midjob to change bits manually?

thanks.

[coach]

There is a chip load calculator. On your shopbot console click TOOLS scroll down until you see CHIP LOAD CALCULATOR.

[dana_swift]

I just addressed this issue in another thread:

http://www.talkshopbot.com/forum/sho...78&postcount=2

Probably the feed rate is too low, figure to running 2-3 inches per second at nominal RPMs. Use the chip load calculator as David suggests to find the correct feed rate for your RPM.

A depth of cut of .125 doesn't sound particularly aggressive. A major clue is the sound the router motor makes while cutting, is it struggling to maintain RPM? If so, the depth of cut is too great. Also if the depth of cut is too great the side load forces on the bit will be too high causing it to flex, flexing causes heating, and heating causes broken bits.

So does hitting clamps. It does make interesting notches in the clamps first tho..

Hope that helps-

D

[rcnewcomb]

Since this is a used system I would recommend getting a new collet. They are only about $12-$15. Plan on replacing collets 2-3 time a year or whenever you crash into something . When collets become worn or out of round then you can start breaking bits.

Where are the bits breaking?

Are there scratches on the base of the bits where they are in the collet?

If the router is a Porter Cable 75182 then you speed choices are: 10000, 13000, 16000, 19000 and 21000 RPM. We usually run at 13,000. Look at a feed rate of about 2ips. Use the chipload calculator as a guide.

[jerry_stanek]

You may also have to replace the bearings in the router. I bought a used shopbot and after cutting some samples I change the bearings and the cuts were a lot better and the router ran quiter.

[dana_swift]

Another comment on your question: "can the SB cut a true ellipse".

Absolutely YES. And oriented in any direction, including off of the XY plane. The software you use to generate toolpaths may not be able to tell the SB HOW to do that, but the SB is fully capable of it.

Any conic section (sorry to remind you of the hated high school algebra) is easily generated using mathematical formulas if you are inclined in that direction. I have cut perfect parabolas with the bot, and covered them with reflective mylar making a great solar furnace. About the only trick is to remember the path of the SB must be offset from the edge you are cutting by the current bit radius. So the SB usually does not move in a "pure" figure, however in the case of all ellipses, if you inset the ellipse exactly one bit radius, you get another ellipse.

An ellipse has the form:

X^2 Y^2
---- + ---- = 1
a^2 b^2

Where a is the radius in the X axis, and b is the radius in the Y axis. This is for an ellipse oriented along the X, Y axes. Use rotation formulas to get any other orientation, including ones that involve Z. The shopbot will cut those correctly also, if you have toolpathed it correctly.

So changing both a, and b to some other value is also an ellipse. So the SB can cut any elipse correctly within the mechanical tolerances of the machine. If you are really interested in doing a precision setup, you can get 0.002" from your bot. How precise do you need to be?

Aspire and other toolpathing software can draw an ellipse, but its not easy to specify a, and b for any specific ellipse. (A real oversight. Hint hint..)

Hope that helps-

D

[ken_rychlik]

Dana, we don't need all that algebra.

If he has a prs, he should have part works. Click ellipse and tell it what size you want. Then make a toolpath and run it.

Kenneth

[dlcw]

Chad,

It sounds like router spinning to fast, speed rate to slow, overheating the bit making it more brittle and dull thus more effort needed to push the bit through the material, repeat.

If the collets are fairly new then they should be ok unless, like Mr. Newcomb says, the router ran into the something before you got it (and I'm sure the seller will admit to that ). In that case get a couple of new collets - one for spare.

[dana_swift]

Kenneth, my answer was because Chad asked specifically about a "true ellipse". Not an oval shape that is often referred to as an ellipse. I have not tested the toolpath generator in PW or Aspire for correctness, for certain it can create shapes originally tied to their X and Y un-rotated dimensions. However as soon as the figure is rotated it cannot be adjusted parametrically. Most likely it is correct, as Vectric does such a great job.

The answer regarding the bot is "yes", whether or not PW creates the figure correctly or not. I explained why and how to guarantee it. Sometimes it really matters that it is correct. I certainly understand most botters don't care about such details. There are a few that will and even if not needing the info, its one of those things someone may remember "I saw that on the forum once" and be able to go back and figure it out.



D