More on bit geometry

[khaos]

So I have purchased a few centurion ball nose bits. Interestingly they are normal overall bit lenth but very short cut length and the transition from 1/4 to 1/8 is abrupt. Not a nice taper like the onsrud bit. Its also about 1/3 the cost of the onsrud so there is that. My question is how do I add this tool to the database in a meaningful way. I mean it cant reach down next to a vertical drop more than about 5/8 after that depth it needs to be farther away (shank/2). How does the software know this?

Under cutting parameters I assume pass depth is how deep per pass. Not final depth.

My thanks in advance,

[Brady Watson]

Joe,
Add the tool as a regular 1/8" ball end mill. Take care when going deeper than the 1/4 to 1/8" transition.

If you have a spindle you can get an 1/8" collet that will allow you to use 1/8" extra long ball end mill tools.

If you have a router with no 1/8" collet available, you can get a stubby collet chuck that fits into your 1/2" collet and allows you to run an 1/8" collet while maintaining precise run-out specs. More details on this via 'BradysTricks' under the web columns off of the ShopBot main website.

-B

[khaos]

Thanks for the response.
I think I did not ask the question very well. The overall length 'B' of the bit is great. The issue is the length of the cutting edge 'A'. The centurion bits look like the image here. If the cutter diameter is 1/8 and 'A' or the cut length is 5/8s and I have a 3d file how do I make sure the tool path takes into account the transition from 1/8 cutter to 1/4 non cutter shank?

Do I simply have to get a new bit as long as the work is thick? This raises another question, how does aspire know where the collet is?

4378

[Brady Watson]

Joe,
As far as I know, Aspire does not have collet collision checking. It is up to you, the operator, to think things through and realize where you need to use caution.

Personally, I do not use stepped or necked down ball end mills. I will occassionally use a tapered ball end mill in cases where roughing is minimal and where I feel the bit can cut in one pass the entire depth of the relief. I find tapered or stepped bits a waste of money, for general 3D cutting. They need to start with a full 1/4" blank of carbide before all of that material is ground away to get down to 1/8". I prefer a straight 1/8" ball end mill and the best 1/8" collet that I can afford. This allows me to select the whole gammut of 1/8" shank tools and saves me money in the long run. My cost on a good microcarbide tapered ball end mill with 1/4" shank is around $35 (MSC wants like $53 for one!). My cost on a super extra long 1/8" carbide TiAN coated ball end mill is $12. Do the math.

So...If I was pigeon-holed into using the bit you describe, I would make sure that the bit NEVER 'fell off of' the side of the relief. I would offset my boundary vector to the inside by about .04" - and then check to make sure that the bit will never go down the sides of the relief on a sharp edge more than 5/8". If you really want to eliminate confusion & poor results, invest in the correct tool for the job. Either an 1/8" straight sided ball end mill or a tapered 1/4" to 1/8" ball will work fine. The stepped bits are really only intended for dished relief cutting where the depth of cut will not exceed the length of cut on the tool.

-B

[khaos]

Thats what I felt the deal was. Sadly I am out the cash for the bits. Is the source you use for your 3D bits available to the hobbyist like myself? If so, I have an itchy CC trigger finger and the need to replace about 6 bits. I will just stroke this up to experience.

Typically I deal with 2-3 inch work and in the past my taper bit has served very well because it could make the full pass. I did not even try to run the cut file with these bits. I am sure it would have ended in a ruined piece of hickory, broken bit, or worse yet a spilled beer.
4379 just kidding. You get the point though.

For you lurkers: I called Centurion and they do not make the tapered ball mills or the straight with a longer cutter edge length.
On the website the bits looked exactly as delivered. So its not centurions fault, its total buyer ignorance.

BTW:
I have 'Precision Collets' 1/2, 1/4, 1/8 for a while now and I have seen noise reduction (or pitch change) and some day to day ease of use. I am very happy and would order again in a minute.

[myxpykalix]

Joe,
I had the same basic issue where i was cutting molds from a supplied file created on a linux with some odd old cad pgm and could not isolate the .25 hold together holes and it would carve the holes with the rest of the part and when the 1/8th" bit went down into the .75 deep hole it would flare it out because of the taper. I too called Fred to see if they had a longer cutting shank 1/8th straight bit but no luck.

[Brady Watson]

Joe,
Send me an email. I'd be happy to recommend bits to fit your needs & budget.

-B

[andyb]

Joe,
What bits did you purchase from Centurion? I may be willing to purchase some or all of them from you. I use their ballnose bits and love them.

Contact me offline at andy(-at-)brookscnc(-dot-)com

Andy B.

[khaos]

Andy,
Email sent.

[curtiss]

As you cut with a bit that is longer and longer, (say 1/8" in diameter) I assume the bit is easier to break.

As the cutting tip moves farther out from the collet, say from 1/2 inch to an inch... is it twice as easy to break ??? 4 times easier ???

thanx

[khaos]

certainly. The cutting strategy has to take into account the bit characteristics. We need to make the speed & feed decisions with quality of cut, accuracy of cut, flex and propensity for breakage. Personally I feel if your pushing the cut close to breakage you are flexing the bit and accuracy is out the proverbial window. Your milage may vary.

Basically, same ol same ol.