Rule of thumb for 3D bit selection

[dubliner]

OK, Ive been asked to do a 3D topo out of HDU, havent seen the file yet, but I'm wondering how I can select a bit diameter and stepover in advance as the customer would like a ball park figure for a 4x4 ft x 3" piece. I'm actually trying to figure out a quick price and a machining strategy so that I dont waste too much time on the estimate. So based on the little information I gave you, how would you determine the largest ball nose & stepover you could use to minimize time, or would you use a differrent estimate process based on the volume of foam to cut or would you do something I havent thought of? TIA Neville

[billp]

Neville,
It's really not easy, smart to do the estimate until you have seen the file. Once you have it in hand you can at least run a projected time to cut with a few different bits. None of these will be 100% correct, but they'll be a lot closer than a blind guess. There's a huge time difference between carving a 3D Topo of Kansas, or Colorado....

[dlcw]

I would probably rough this with a 1/2" down spiral at 4ips leaving 1/16" for finishing. Then a 1/4", 1/8" or 1/16" ballnose depending on the level of detail wanted.

The roughing would not take too long. The finishing would be determined by which bit, step over and feed speed. For really detailed work I will use a 1/8" or 1/16" ballnose with an 8% stepover and feed at 4ips. For a large project like yours I would lean towards a 1/4" ballnose for finishing with a 10% stepover and feed speed of 4 ips.

Another determining factor for feed speeds and spindle speeds is the type of wood you will be using. In cedar I can cut fairly quick, but in hard maple I slow down to keep from breaking the bit.

Hope this helps a little.

[dubliner]

HI Bill, yes these questions were going around in my head too. Back to the bit selection, I suppose until I know the resolution required I cant select a bit either.

[dubliner]

I was responding just as you were Don. Yes I had only thought of using one bit, but roughing and final might be the right choice. I suspect his budget will be the decider. I'll flush that out first. Its funny some people are reluctant to express their budget, but I may have already exceeded it bringing in you two other experts :-). I have limited time to work on CNC, so I've become a little more mercenary in these matters. I dont want to waste it on estimates of jobs that could never eventuate.

[dubliner]

Just thought of something - He can purchase reducing degrees of stepover starting at 49%. The finished part will match his budget :-)

[garyb]

That must be something Irish.........price by the stepover
good one Nev

[dubliner]

No problem, Cobber, I learnt it from this Aussie Bloke see :-)