twist drill calculations

[awjareme]

When doing feed calculations do you consider a twist drill a one flute tool? Just wondering as our old manual machines have controls to set to .00X"/rev, using a 2 flute tool you would think you would then double this to get .00X/tooth. Anyways whenever i do this, or when calculating feed for the CNC mill, it always seems way to agressive, by roughly double? Or does the normal RPM*Chip Load*#F not apply to plunging?

[RICHARD ZASTROW]

Normally, twist drill feed is given as feed =.0xx" per revolution. This is not for circular interpolation of an end mill. If you want to calculate the feed per tooth, the feed per tooth on a two flute drill would be half of the feed per revolution.

Dick Z

[awjareme]

Ok, so when a drill has a .006" chip load. It is per revolution and not per flute like an endmill would be? The charts i have just say chip load. Also when plunging with an endmill or other tool do you consider the chip load given say .002"/flute as .002"/REV for plunging, regardless of the # of flutes, or is there a better formula for plunging speeds

[wildwestpat]

Twist drills cut on the lands but push aside the material close to their centre. Is the drill being plunged into the metal or is there a pilot hole? Use tables designed for twist drill feeds dependent on size if there is no pilot hole. If there is a pilot hole then the feed rate can be higher as the rubbing on the centre of the lands is reduced by the presence of a suitably sized hole. This is why it is often necessary to work up from a small to a large drill in stages as the twist drill is much weaker then an end mill having less metal in the lands.

Yes end mills can be used to plunge holes provided they have a cutting edge at their centre - if not then you will be unable to force then through as they have a blank non cutting area in their centre that will be unable to rub the metal sideways onto the cutting edge unlike a twist drill where the point pushes / brubs the metal into the cutting edge.

Hope this helps - regards - Pat

[awjareme]

Ya that makes sense just now trying to figure out a way to calculate plunge feeds. We do lots of captured key ways and such. I've been in a machine shop forever just kinda getting into the practical end of it with a new NC. Its hard trying to find proper speeds/feeds, especially when everything else here is manual.

[RICHARD ZASTROW]

Don't confuse chip load with feed rate. In the case of plunging an end mill straight into the work, the chip load would be the feed per rev. divided by the number of teeth.

If there is a pilot hole to clear the center, you could use counter-boring feed rates.

In the case of blind key ways, you might ramp in at end milling feed rates then re-cut the slot at finish depth. Key ways are considered a precision feature so I finish them with a straight 2 flute key way end mill. But then I'm a fussy old fart. LOL

Dick Z

[awjareme]

Ya, I'm forced to use the 2 flute endmills here for keys. Sad thing is all my charts start off with feed for 4 fluters. No big deal to do the math though. Anyone got links to some good tables on the net? I know it all depends on the manufacturer as well, but general guidelines would help right now. I re-read what i first said and to make it clearer what i was doing was using a chip load given for a drill and finding the feed but then doubling it as i considered it a 2 flute tool, but i guess the chart i had failed to mention it was IPR not per flute. I found some others with the same values but it clearly states IPR. Thanks for the clarification

[wildwestpat]

Sorry to have to check on something so basic. I assume the end mills you are using to plunge cut are the type that are designed for plunge cutting and have a cutting edge that crosses the centre of the tool. There are end mills that are NOT designed to plunge cut and these have a centre recess and will defiantly resist attempts to plunge unless a pilot hole has been drilled as deep as the desired hole.

I use Machinists Tool Box (www.machinists-toolbox.com) but this is a paid for calculator - there are others - I do not know of any free ware ones. As a very rough guide a 2 to 1 change is a guide failing anything more scientific but I would use MTB every time and have learnt to trust the thing.

Hope this helps - Regards - Pat

[awjareme]

Ya plunging type. Just feeds and speeds giving me headaches. Everyone else here runs manuals, so theres lots of tooling knowledge here, but the guys here and myself haven't run a CNC miller before. Normally they run into the rigidness and HP restrictions in the equipment, before they can push a tool to its max capabilities

[Al_The_Man]

Are you using peck drilling G73 e.g.?
If you need a drill feed speed guide PM me an email add.
Al.

[awjareme]

Website says BobCAD took this over? wonder if its available through them now? I hate to phone there, as they seem to like to hound potential clients

[wildwestpat]

Gosh! you are right about the take over of machinists toolbox ( CNC Machinist Software) Glad I get mine as an Alibre add in. Why not ring before the take over bites as I bet the phone staff have the same apprehension. I had trouble with Win 7 and found the staff at MTB very good at sorting me out.

I have seen a hand held device a bit like a blackberry that is pre programmed with much of the information of feed rates etc. I think that device was a 'Machinist Calc Pro' as it looked similar to that advertised by http://www.calculated.com

Bob Warfield has a neat software pack on his web site. I had the trial run and thought it was good but l was not serious about replacing my existing MTB.

Perhaps some one will read this and chip in with experience of the pre programmed hand held calculator as this might be good on the shop floor.

Regards - Pat