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Rickyg
12-22-2012, 10:43 PM
Merry Xmas everyone
Can anyone suggest a good 1/8 bit that lasts logger then 20 minutes.
Today I cut a 6"x6" square out of 17.5 mm thick plywood.
Running at 10 inches per minute feed rate and each pass is basically 1/8th of an inch. I get 4.5 passes then the bit is dulled to the point where is slows the axis down and then wanders into deeper wood and jams.
I haven't done enough of this to understand the difference in sounds to take that as a warning.
I tried a carbide bit I got from ebay. Broke 2 before I got 3 inches.
I have been using the Saber bits from Roto zip. They actually cut very cleanly, but as i said. don't last.

I am using my router to rebuild my z axis to use a different router that supports a 1/4 inch bit. I have more faith in that size of bit. But until then, I need bits that last long enough to cut the parts I need to build the unit in the first place.

Any thoughts would be appreciated.

ger21
12-23-2012, 12:21 AM
You're cutting much too slow, which causes excessive heat which accelerates the dulling. Add to that the fact that plywood can be hard on bits as it is, and you're in a no win situation.

You didn't mention the rpm you're using, but at 18,000, you should be cutting at 200-300ipm or more with a 2 flute spiral bit.

Rickyg
12-23-2012, 09:58 AM
Thanks Ger21.
I suspect I am in a no win situation at least for the time being.
I can't get the machine to run any faster then 15 ipm. Folks on the forum have explained its my choice of stepper causing the issue. So I figure I will redesign so parts and move to new steppers as part of the redesign. That Said, I am surprised I am to slow, but I will defer to your experience. You have never steered me wrong.

Also you are keeping in mind this is a 1.8 inch bit.

also I believe I am running at around 20k RPM, its the highest setting for the router itself.

Ok I have a part to cut today I will jack up the speed.

thanks

ger21
12-23-2012, 10:27 AM
If you have a variable speed router, then run it at it's lowest setting.

And yes, a 1/8" 2 flute bit is easily capable of cutting at 200 ipm

Rickyg
12-23-2012, 11:31 AM
Not sure I understand why running it slower helps.
But I will dial it down.
I am going to assume that the slower RPM combined with the low feed rate I have will result in lower temperatures on the bit and thus greater life span.

Will let you know how I make out.

Have a super christmas

Rickyg
12-23-2012, 12:10 PM
Depth of cut
I get the impression from reading elsewhere that I should be able to cut at 3/8 depth with a 1/8 bit. I think I would be happy at .25 of an inch depth per pass. right now I am using .1875 per pass.

ger21
12-23-2012, 12:54 PM
Cutting plywood, with a 1/8" bit, I would not cut more than 1/8" deep. At the proper feedrate, you could possibly get away with a 3/8" cut. But, cutting deeper at such a slow feedrate will probably generate even more heat. Also note that cutting deeper increases the require cutting force, which can lead to bit deflection or machine deflection. The rigidity of your machine will have an effect on cut quality, and how deep you can actually cut and still achieve good results.


Not sure I understand why running it slower helps.

To achieve maximum tool life, you need to run at the highest chip load that you can. Chip load is the amount of material each flute of the tool is removing.
Decreasing the rpm has the same effect as increasing feedrate. Both will increase the chip load.

Rickyg
12-23-2012, 07:55 PM
thanks for the info.

I will dial down the RPM and max out the IPM and see what happens.
As for Chip Load, I will go and read about that some more.. But I think you mean, that as the blade hits the wood, it cuts into it causing chips. IF the speed is high, the volume of chips goes down because the blade is contacting then leaving the wood so fast that there is little build up.
Reducing the speed means it contacts the wood longer and therefore the chip load increases. With an increased chip load, the tool then gives up heat easier and before it becomes a problem. With the load being low there is no place for the heat to go and so it dulls the bit.

Sorry if I am stating the obvious, but I needed to write it down to make sense of it.

Now if all I stated is correct, then a plunge of the bit into the plywood would result in a spike in heat. There would be a very low chip load as the bit isn't moving, result high heat. This I don't understand how to solve.

Off to read more.

thanks again

ger21
12-23-2012, 08:39 PM
Yes, plunging is bad. You want to gradually ramp into the material

bushwakka
12-26-2012, 04:09 AM
Ricky,
I find that when cutting ply, that the kerf from an 1/8" bit tends to hold the chips. I think that the bit actually packs the chips into the kerf and these can grab the bit on the backside (as well as cutting on the front side).
On occasion, I have cut double passes so that the kerf is wider allowing the chips room to move.

I also need to keep the 1/8" bit as short as possible, and tend to use bits with a short flute length (LOC) as they have less machined out of them, and don't flex as much. I try to keep the LOC just a couple of mm longer than the material thickness (so I don't have to change the bit, but minimise bit flexing).
If I use a 22mm LOC (7/8") in 12mm (1/2")ply, it will last around 600mm (2ft) and then break at the top of the grinding (which i believe is indicative of excessive flex).

Failing that:
1.slow your router rotation speed down as per ger21's suggestion.
2.travel speed as fast as your stepper motors will smoothly allow. It is important that movement remains smooth otherwise the instantaneous numbers for cutting speed will vary dramatically.
3.take a very shallow cut
4.keep your bit stickout as short as possible.
5.use a spiral upcut bit (to clear chips out), preferably a single flute (to maintain your chipload).
6. change to a sharp bit after each (or a couple) of profile operations, before they break.

Just of note, the same 1/8" bit that will cut hours worth of 18mm mdf at 2.5m/min will last only a couple of meters (at reduced depths) in plywood, and I would normally cut with 1/4" for plywood.

Cheers
Paul

bushwakka
12-26-2012, 04:27 AM
Ricky,

With regard to chip load, here is my layman's understanding of it:

* slowing the router speed while travelling the same speed means that the bit is cutting a bigger piece of timber out each rotation. The heat is generated from the metal cutting through the timber, so if you heat it up and cut it out (a big chip), the heat goes with the chip. If you heat it up and do not cut it out (a small chip), the heat stays in the timber and the bit. This will cause the bit to dull quickly.
* 1 flute instead of 2 flute effectively doubles the chip load given the same speed/rotation. The chip from a 1 flute bit will be twice as big as chips from a 2 flute. Also note, that a 1flute bit only makes half as many cuts as a 2 flute bit at the same rotational speed, so will tend to generate less heat in the timber and the bit. I also think that there is more metal in a 1 flute bit, so it is more rigid than a 2 flute bit.
* high speed rotation with slow travel speed means that your bit is cutting many more chips than low speed rotation with high travel speed. Over a given distance, you are probably making orders of magnitudes more cuts than optimum, and hence the bit will dull over a short distance of cutting.

Paul

Rickyg
12-26-2012, 10:12 AM
Morning Paul
thanks for the information.
I have moved to a single flute bit a few weeks back and I am cutting only 1/8 deep.
I have now learned to ramp into the wood and will add that to my bag of tricks on my next cut.
I have upped my speed. sadly i have some poor performing steppers so 15 ipm is the best I can get. I am trying to cut parts such that I can improve the performance of the device. First stop is a 1/4 router.
Then replacing the steppers with dual shaft higher current devices so that I can use a damper and just in general get better speed.
If all goes well I will try my first cut with all that I have learned from you and Ger21 today.
What I am not sure about is how to do a tool change in the middle of the cut without spoiling the positioning and depth. But thats another opportunity to read.
What I am confused about is that I get the impression cutting mdf is easier then plywood, I thought it was the other way around.

ger21
12-26-2012, 10:16 AM
No, MDF is definitely easier to cut than plywood.

Rickyg
12-26-2012, 08:57 PM
So today, while the boos was out chasing boxing day sales.
I gave it a whirl.
Dialed the router speed down. On a scale of 1 to 6 wtih 6 being the highest I dialed it down to 3, not a good idea. The router dragged to a halt.
so dialed it up to 5 and no issues there.
Kept the single flute bit.
Ramped into the work.
Ran as fast as I could. 15 ipm.
Vacuumed as much as I could with out getting in the way.
With fingers crossed I successfully cut to reasonable size parts successfully.
Even with a lot of smoke when I tried to drill some holes.
I think I will skip hole drilling and just use the router to mark the wood and use the drill press to drill it.
However I am sitting here happy.
Thank you. Now I am more confident that I can build the new Z axis components so that I can move to the 1/4 router.
I am convinced that the new router is the way to go.
Also my current design exhibits a number of issues with tool flex.
My new design will deal with that.

Anyway. Thanks. I am not sure whats more fun, the design and build or the possibilities that the tool presents.

Merry Christmas and Happy New Year.:cheers: