Broken Bits

[glcochran]

Hope some one may have some direction for me: I am cutting with a PRS alpha , 2.2hp spindle, cutting 5/8 plywwod,@.4ips,18000rpm,full depth, using a Amana 1/4in #46170 compression spiral, which amana documentation says I should be able to cut full depth at .833ips. The strange thing is the bit is breaking about 1/4in into the collet. I have tightend the collet as tight as I can possibly get the collet, I even tried a new collet but had similar results. Any help will be appriciated. At $42 a bit this hurts!

[coach]

Gregory, I have an alpha with a 2.2 also. I cut 3/4 plywood in 2 passes at 6.0 IPS and about
12,000 spindle rpm. I could cut in 1 pass but hold down becomes an issue at times so I just use 2 passes.
Is .4 a ty-po? That is toooooo slow.
Are you familiar witht the chip load calculator?
On shopbot console click tools and go down to chip load calculator. That will help.
You will get good help on the forum.

[dlcw]

Gregory,

I cut 5/8 prefinished Baltic Birch all the time (just finished cutting 20 sheets into drawer parts) for mortise and blind tenon drawer boxes.

I use a down spiral to cut the mortises and a 1/4" compression for outline cuts. My outline cuts are done to within .0312" of final plywood thickness doing a climb-cut then reverse direction to a conventional-cut to finsh cutting the parts out. I cut at 4ips at 12K rpm like David and have not had a bit break. If you are cutting at .4ips at 18K rpm you are most likely overheating and weakening your bit to the point of breaking.

At first I was scared to run my machine that fast until Gary Campbell told me to quite whining and just try it. So, I stopped whining, sped up the feed rate, slowed down the spindle and hurry! Great cuts and no broken bits.

Don
www.diamondlakewoodworks.com (http://www.diamondlakewoodworks.com)

[glcochran]

I was using the chip load graph in the Amana catalog for the specific bit. But I will take your guidance and speed up the feed and slow down the bit. Hold down is not an issue as I am screwing to the spoil in the voids. Could that I was cutting the full depth in conventional-cut make a difference also? the edges look great and do not even need sanding, But I am open to guidance. Thanks

[dlcw]

I've found that when cutting plywood, if you want the parts exactly the right size you need to cut it in two passes. Make the first cut climb so that the flex in the machine will result in the bit being pushed away from the cut line. The second pass will be cutting very little material so the conventional cut will cause very little if any machine flex putting your cut exactly on your desired cut line. The bit should handle cutting most of the way through 5/8" plywood without breaking. Give it a try and I think you'll be happy with the results.

I'm running a 2009 PRS Alpha with 2.2 spindle and vacuum hold down with SB Control software 3.6.10.

Don

[beacon14]

Using screws instead of vacuum could (will) result in more vibration of the material especially when you are farther away from the screws. That could be part of your problem as well.

Try the Whiteside 1/4" compression bits, they are available online at $28.80 each. I've been getting very good results with them. Most recently I've ordered them here (http://www.woodworkersworld.net/solid_carbide_spirals_router_bits.shtml#compressio n), they have free shipping for orders over $29. (Was $100 last time I ordered, but I'm not complaining.) Part # is UD2100.

[gene]

I use the bits from centurion 2 flute comp 12000rpm and cut at 6 ips full .75 deep. I can push it to 7 if i need but the cut quality suffers.

[gene]

Gregory
Is the collet snapped into the nut? I know that this might be a stupid question but my helper didnt know about the snapping onto thing and i broke a few bits as well. Remember if your bit is spinning too fast and the cut speed too slow then you build considerable heat and thats carbides worst enemy.

[glcochran]

Checked and cleaned collet, now running at 12000rpm,cutting at 3ips, full 5/8inch depth, screwed with vacuum, same two flute compression bit, cut well but again broke about .25inch into the collet, after about 65 inchs. Find it hard to believe its the bit, new bits arrive on Tuesday.
Am I still cutting too slow?

[navigator7]

Gregory,
Did you pick up on what Gene said?

Does the cutter bit run perfectly true under no load?
Do you have a dial indicator?

As my nose is under the tent, it seems to me something is bending your cutter bit about .25" into the collet.

I'd sure like to see what a dial indicator said after a revolution or two on the cutter.
A reading without the nut?
Up inside the collet?

Fractured spindle??

Just kicking out ideas.

[glcochran]

On a warmed up spindle, Just measured runout inside and outside of the spindle with out nut, 0.000 runout inside and out. Inserted a bit, measured runout on the bit, again 0.000, cut 10 inches full depth 12000rpm remeasured bit runout, 0.000

[Gary Campbell]

Gregory...
Any chance that this collet and/or nut had hit the floor or something else? Most mfgr's reccommend changing collet if bit breaks.
Gary

[navigator7]

Ok super!
What happens when you push on the spindle from all different direction by hand with the dial indicator attached?

What about pushing on the gantry?

Something is moving.

[glcochran]

When new bits arrive, I am trying a new collet and nut, As I am the second owner of this PRS, eventhough the previous owner only had about 20 hours on the machine. Upon looking at the nut, i do see a spiral gouge on the tooled taper.

[ken_rychlik]

Chuck, Did you ever buy a machine?

Greg, I think David may be onto something about the sheet moving. How many screws do you use to hold your sheet in place and what are the chances of the sheet vibrating or moving?

My next question would be related to that possiblity. How much cutting do you do on a sheet before the bit breaks?

Running as slow as you were at first, you should start a fire on the table from the heat before you snap a bit.

Kenneth

[glcochran]

There may be something to the vibration in combination with a bad collet nut, however when I first broke a bit I had cut out 90% of 2 Lawn deer 4x8 sheet, as soon as it happened I gingerly grabbed the loose half of the bit and it was not even warm. Upon taking the collet off there the bit had shattered about 1/4 in up into the collet.

There is about .002in play in the x-axis when I push on the top of the plate holding the spindle, no play in the y-axis. For the last break, I had the sheet screwed in the 4 corners and the vacuum on for the hold down. So maybe it's that the collet is sliping due to the gouge on the nuts taper. Hope to try again with new bit, collet and nut when bits arrive.

I really appriciate everyones help with this! What a great forum!!!!!

[coach]

Greg, I cut the 2 lawn deer last week. If it the file that is floating around this forum,,,,,,some of the parts were hard to hold down even with a vacuum. I can't see being able to guesstimate where all the screws would need to be. As David stated,,,,lot of vibration especially with those parts.
Also, try tabs if you cut it again.

[navigator7]

@ Kenneth,
Not yet. Eager ready and willing but planet misalignment
keeps holding me away like opposing magnets.

My enthusiasm comes from working repair in a shipyard and years of troubleshooting equipment problems.

@ Gregory,
Any discoloration of the cutter bit? Any blue, browns, purples or black?

What does the break look like?
Is there any metal that looks pounded flat or worked?

Is this gouge you refer to highlighted by wear? IE is it shiny?

Is there any metal chips pressed into the collet?

[glcochran]

No discoloration on any bit or collet.

Most pieces are shine free, look like a matt finish, there are some hammered looking areas on the break of the bottom half. Looks small areas, that look like they happened after the break.

The gouge , actually 2, look like the collet slipped in the nut. Not polished , more like a spalling. width of gouge about the thickness of a finger nail. ( Collets were snapped into the nut in all cases). ( Thightened as tight as two 6" wrenches will let you.

No metal imbeded into the collet or nut, collet howerver does have some two polished rings around the lower beveled surface.

[glcochran]

More info, have 4 tabs on all parts .25in X.25in, all intact, Left .1in skin on cut to maintain vacuum on last attempt. Only 4 screws, on corners. My bot is a 48x48, so I have half of a 4x8 sheet hanging off but supported by rollers, Therefor, screws are .5in in from corners.

[keithb]

Gregory, The tools breaking up into the collet area are breaking from harmonic vibrations being generated from the tool bit. When the vibes hit a certain pitch that we can't hear the bit will break where the vibes center is, just like an opera singer may break a crystal glass. Amana likes to put a high angle clearance on their bits cutting edges. It causes them to self feed, and take a random orbit instead of a true round orbit. To aid this situation you must drop your rpm and pick up your feedrate to put more load on the bit. The bit is telling you it wants more wood to eat. You may end up around 9-10 K RPM.
Thanks, Keith B.

[glcochran]

So Keith.... With that advise, do you feel that the collet is probably OK , just really slow down the bit, and speed up the feed.

[navigator7]

Keith,
That was eloquent!
I had no idea that was possible...but I never ran a machine that could spin as fast as the Bots.

Oil platforms in high current areas such as Cook Inlet had a heck of a problem shaking the platform apart. Engineering-wise, the columns were strong enough but an unknown (At the time) phenomena similar to what you described happens on a really large scale. Its called Vortex Shedding.

8666

I wonder if a ShopBot user frustrated or in trouble might post a link to sound or video for others to help trouble shoot??

I assisted a guy the other day by asking him to hold his phone to his machine. (A concrete pump) As soon as I heard the machine running... I knew what to tell him to do.
???

[rb99]

opera singer may break a crystal glass

This never actually happens.

RIB

[gene]

Chuck,
I dont think that theres no shopbotters that own a oil rig.
Sorry i couldnt resist.

[glcochran]

All I can say is, This is a GREAT Forum, You are all great Botters for helping, and ShopBots are Wonderful.

With all of your help and suggestions I found the groove!!!!! With a new Amana 46170 .25 compression, 10500rpm, 6ips, No broken bit, a job done faster and cleaner, chips not sawdust, room temp bit, and I love it when it comes together.

Thanks to you all for the advise, this is a wonderful tool. I guess I just need to stop babying it>>>>>>

Again Thanks !!

[coach]

I know your feeling.....been there.
This forum for sure is awesome.
Congratulations !!!

[gene]

ATABOY.
Way to go. I know that when i first got my bot i broke so many bits it wasnt even funny. After you get to know the limitations and the cut and feed rates then its a good feeling when the sheet of parts come off the bot. It will get even better by fine tuning the speeds and materials you cut. Persistance pays off..