Long Z travel and router bit length

[dttworld]

I would like to make some deep wooden molds for vaccuum forming. I measured a max Z travel of 9" on my PRS Standard 96x48. I plan to lower half of my table bed down approx 9" to accomodate the longer z travel.

What is the longest router bit you'll think can be safely used in the ShopBot 2.2HP spindle with ER25 collet? I would be using a ball end router bit for the finish passes. Max diameter bit for ER25 is 5/8". Max rpm on the spindle is 18k.

thx

[Brady Watson]

6" OAL, 3 or 4" LOC, 1/2" shank is about max...which means that your material is going to max out at 3 or 4". More than this, unless foam, is going to result in major cutter deflection and vibration. You'll need 3" for material, 3" for bit length & 0.5" or more for safeZ clearance.

Your best bet for models over 3 or 4" is to use slicing in the software.

-B

[Ajcoholic]

ER25 can indeed go to 5/8" collet, I bought one and a 5/8" endmill and ball endmill to do my chair seats and cutting boards.

I dont think the cutter flex is going to be a problem, the 5/8 shank mills are pretty beefy... cant you work your way down in steps? As to avoid trying to take too much off at once?

AJC

[dttworld]

....

Your best bet for models over 3 or 4" is to use slicing in the software.

-B

Forgot about slicing - thx

[dttworld]

ER25 can indeed go to 5/8" collet, I bought one and a 5/8" endmill and ball endmill to do my chair seats and cutting boards.

I dont think the cutter flex is going to be a problem, the 5/8 shank mills are pretty beefy... cant you work your way down in steps? As to avoid trying to take too much off at once?

AJC

What's the longest 5/8" cutter you've used? I'm a newbie with machines that can go to 18k rpm. The most my Chinese X3 mill went to was 2000 rpm. Kind of worried a long bit at 18k RPM could act like a big bullet if broken.

[wowhuh.mike]

I would like to make some deep wooden molds for vaccuum forming.

thx

You might consider a material other than wood. I realize you need rigidity to hold up to force of the vacuum, but foam cuts fast and some of the worries of long Z issues go away. Consider casting your thermoform patterns into a foam mold you cut on the machine. Then do your finish sanding on the cast plug.

I'm guessing you are using a DIY vacuum form??? Maybe an industrial setup which require form to be tolerant of higher temps, but there are all kinds of two part plastics and additives that can be used.

check this page out
http://www.wow-huh.com/projects/satellite-dish-sculpture/
notice image 3
that is foam I cut on bot (covered with fiberglass and resin mold). pretty quick to machine, then took fiberglass mold (the red negative in the next images) then I cast either parts (white pieces) into that mold. You could use the white part of this type of process either cast solid in a two part plastic or cast a shell and back with rigid foam.

This isn't the easiest process if you are not pretty comfortable making molds, but it is a nice way to transition from a material that is CHEAP and EASY to cut to a material that does what you need it to in terms of strength, rigidity, and resistance to certain forces.

Not sure if this helps or confuses the issue more

[Brady Watson]

What's the longest 5/8" cutter you've used? I'm a newbie with machines that can go to 18k rpm. The most my Chinese X3 mill went to was 2000 rpm. Kind of worried a long bit at 18k RPM could act like a big bullet if broken.

Danh,
You are correct to be cautious. The long length of the cutter really increases the chances of deflection and vibration, even when you are very conservative with your stepdown distance. 5/8" or 1/2" shank...makes little difference. They make longer (8" & 12") end mills specifically for foam, but they recommend a max RPM of 6,000 to prevent whipping....which kinds puts a damper on cutting speed.

The way to do 3D in any material with any bit is to just use slicing. If you are precise with your material thickness and slicing values, it will go together perfectly. If you are attentive to the tool, you shouldn't run the risk of breaking a bit. If the tool 'tells you' it needs more or less RPM or move speed, make the adjustments and resume. If a tool does break on you, it will most likely just fall over & not shoot out at you. It's the large diameter cutters (like a 3" planer or core box bit) that you should really be careful with. Those can go flying when they break!

-B

[wowhuh.mike]

I also highly suggest slicing method along with method mentioned above. With slicing you can layout all parts in sheet stock and program as one tool path and typically reduce the amount of material you must clear away than if you had a solid block. Also makes dust collection easier than steep sidewalls that often occur when cutting at once.

[myxpykalix]

Danh,
Here are some pics of my table and my indexer bay and you can see how i accomplish at least another 3-4 inches of travel below my table height and i don't see any issues with deflection so i think you can accomplish what you want to do.

[dttworld]

.....
This isn't the easiest process if you are not pretty comfortable making molds, but it is a nice way to transition from a material that is CHEAP and EASY to cut to a material that does what you need it to in terms of strength, rigidity, and resistance to certain forces.

......

That's a neat process I'll keep in mind. Looks like you could also use the ShopBot to help trim the fiberglass pieces afterwards.

[dttworld]

..... If a tool does break on you, it will most likely just fall over & not shoot out at you. It's the large diameter cutters (like a 3" planer or core box bit) that you should really be careful with. Those can go flying when they break! .....

-B

Makes sense. The metal near the outer circumference of a large diameter bit is spinniing with more velocity than near the center.

[dttworld]

Danh,
Here are some pics of my table and my indexer bay and you can see how i accomplish at least another 3-4 inches of travel below my table height ......

Sunken indexer is a future project for me. So is a 5-axis conversion but now I'm getting way ahead of myself ;)

[myxpykalix]

Danh,
What you could do is take and lower your table, then make a "torsion box" the size of a sheet of plywood in order to raise your flat stock and have your indexer next to it, in effect creating a indexer bay.

[Ajcoholic]

5/8" or 1/2" shank...makes little difference.

A 5/8" diameter cutter has more than 50% more material in its cross section vs a 1/2" diameter. That to me would/should mean more than 50% stiffer assuming they are made from the same material. I would think, therefore a 5/8" diameter cutter would be much less prone to flex vs the same length 1/2" one.

That being said, I think my cutters are 5" long? No where near 8 or 9"... that would even fit on my Buddy :) I have only taken 1 1/2" cut in one pass in hard maple, with the 5/8" diameter end mill. No chatter what so ever. But I havent needed to try going deeper.

I am just discussing - no way I am an expert or claim to be. There is only one way to try it out... and that's to try it out :)

AJC

[Brady Watson]

Andrew,
Thanks for the feedback. Please measure your cutter's overall length, cutting length and tip to collet nut. Every bit you eek out of the collet nut (making the shaft longer) adds quite a bit to deflection and vibration. How were you holding down the material when you were hogging along at 1.5" deep? How fast were you going? I ask about these numbers because it sounds almost unbelievable - I've seen/heard cutters screaming with vibration on new machines only cutting 18# foam with 6" OAL 1/2" end mills...

-B

[wowhuh.mike]

There is no way I could cut that deep in hardwood. My z axis flexes way too much. I do have a 12" travel though. I really need to make a torsion box to raise my cutting level up to stiffen z when cutting sheet goods.

[Ajcoholic]

Andrew,
Thanks for the feedback. Please measure your cutter's overall length, cutting length and tip to collet nut. Every bit you eek out of the collet nut (making the shaft longer) adds quite a bit to deflection and vibration. How were you holding down the material when you were hogging along at 1.5" deep? How fast were you going? I ask about these numbers because it sounds almost unbelievable - I've seen/heard cutters screaming with vibration on new machines only cutting 18# foam with 6" OAL 1/2" end mills...

-B

I had the data for the cutters on my computer...

5/8" diameter , 5" OAL, 2.25" cutting length. The bit was chucked in full depth of the ER25 collet (I dont have one here to measure) but the end of the bit was flush with the end of the collet top, if that makes sense.

I was cutting at 18,000 rpm (I know now, that was too fast) and 180 inches per minute. Hold down by screwing the blank to the MDF top.

The only issue is that the two 5/8" cutters I bought here cost me $160 a piece.

AJC

PS I am the first to admit, for the amount of wood working experience I have, I am very new to the shopbot experience... if I am doing something potentially bad, I'd like to know...

[Brady Watson]

I wouldn't come from a place of 'bad'...It just seems awfully aggressive with such a long bit in tough material! I tend to err on the side of conservative rather than taking that hard of a cut...especially with a $160 cutter. I'm guessing you are running a 4HP HSD? - I don't think the 2.2 would push through that material. I have stalled them before...by mistake.

-B

[Ajcoholic]

I wouldn't come from a place of 'bad'...It just seems awfully aggressive with such a long bit in tough material! I tend to err on the side of conservative rather than taking that hard of a cut...especially with a $160 cutter. I'm guessing you are running a 4HP HSD? - I don't think the 2.2 would push through that material. I have stalled them before...by mistake.

-B

Yes I have the 4 hp HSD... would have gotten a 10hp if they had it :)

AJC

[wowhuh.mike]

I need to see more of you guys' machines! My Bot is the only one I've ever seen in person.

AJC - do you have ATC?

[Ajcoholic]

I need to see more of you guys' machines! My Bot is the only one I've ever seen in person.

AJC - do you have ATC?

Sure I do... I automatically change the tools whenever necessary ;)

I have a Buddy 48 Alpha, with a 6 foot powerstick for a 48"by 48" work area. So far its treating me very well. Next up is a decent vacuum hold down ...

Some day, maybe a larger machine with ATC. I am always dreaming of the future! :)

AJC