Depth of Cut 2

We can be really lily-livered about depths of cut.

I just read a post where Jason recommended that someone takes a full 1/2" depth of cut with a slitting saw - sound advice.

I recently sawed two cuts through a tough steel on my bandsaw - a good 75mm of cut with no trouble at all with a nice wide kerf. Why do we blanche at much smaller cuts with a slitting saw when the mill is probably more powerful than the bandsaw?

Another example - how big a drill will you use on your lathe, or what step will you use between drill sizes?

I will happily drill 10mm diameter in steel with no pilot hole (split point drill). So why not use 10mm DOC, assuming a matching feed/speed can be achieved...? I doubt any of us achieve/attempt metal removal rates with a lathe tool that remotely approach what we take when drilling. Is it because drills are 'balanced'?

Or is it just that our typical tools aren't really ground/shaped for such heavy cuts?

Neil

Neil, you do also need to consider the diameter of the work in your lathe, turning a 10mm dia rod to nothing in one pass requires less grunt from the machine that taking the same 5mm depth of cut (10mm off dia) from say a 25mm dia bar.

" I doubt any of us achieve/attempt metal removal rates with a lathe tool that remotely approach what we take when drilling."

Can't speak for you but I can do it. 1" bar down to 1/2" in one pass

A bit more than I would usually take off but happy to drill from 3/4" to 1" which is not far off the width of a parting cut or 3mm DOC pass. One thing to watch when doing this on out variable speed machines is you need to run fairly slowly if you want to avoid chatter which then starts to mean overheating and lack of torque. So I often find it is better to run at a higher speed  quite possibly double but to take off half teh DOC per pass, really takes no longer but motor is in it's power band. This becomes more of an issue as diameter of work goes up which is why you often see me turning 9-10" cast iron flywheels at 3-400 rpm

Edited By JasonB on 02/03/2018 15:18:41

Edited By JasonB on 02/03/2018 15:25:18

I detect a bias here. Jason mentions a decent depth of cut and we get genuflection. I mention it and we get a lecture about how I'm using industrial machine tools and it's not relevant to hobby machines. Mumble, mumble, mumble. Where's the cauldron stirring emoticon when you need it?

I've stalled my lathe by being over ambitious on cuts with lathe tools. Never done so, this far, when drilling. The largest drill I use is 1-7/8". Most of my larger MT drills are odd sizes, bought cheap on Ebay, and used for ripping metal out prior to boring to size. I don't worry too much about increments. I'll start somewhere around 8-10mm and then pick a drill to give around 1/4" to 3/8" width of cut per edge. I run slow, around 180rpm for larger drills, but feed as fast as I can turn the tailstock handle. I've measured the steel swarf as having a thickness between 15 and 20 thou. That's way more than many people, including me, use when turning. Of course I've got a geared headstock and no VFD, so full power available at all speeds.

Andrew

Andrew, is not a cut through a piece of 1/2" steel with say a thin 1.0mm slitting saw about the same amount of metal removal as a 1/2" cutter taking a full width cut of 1mm deep?

Different cattle of fish to a 6mm deep full width cut as Murrey suggested which would be like cutting 1/2" steel with a 1/4" side and face cutter, Your horizontal would romp through it but a small benchtop would stall.

Will upload a couple of videos later cutting a bit of 1/2" square, was only going to load the second one as the first one failed when the 1.0mm wide cutter just stalled at 100rpm, had to do it at 200rpm to get enough power behind the cut.

Edited By JasonB on 02/03/2018 18:42:56

I'm going to post the slitting saw video in Eric's thread rather than this one which is more to do with drilling and DOC on the lathe.

I have a Centec 2, it can take a 1/4" wide cut through 1/4" steel with a new 4 way slot drill, but I've had the cutter stop and belt slip with a 2" slab cutter taking a full width cut at only 5 thou depth also on steel and at the same speed.

However I wouldn't be about to push a slitting saw too hard due to the lack of set on the blades. I don't fancy blade shrapnel flying if it jams.

Re your speed issue. It's one of the reasons I wouldn't use modern electronic speed controllers, you loose power going slower and I would be afraid of letting the smoke out.

Drills seem to cut much happier when they are taking out the whole diameter at once, rather than going up in steps. With smaller machines this is not always possible, but withing the capacity of the machine it does seem to give a better job.

John

When it comes to parting, I used to use the slowest speed possible and manual feed, but now I increasingly think that power feed (under CNC) is better and using a much higher speed so the tooth load is much smaller. Recently cut through 3/4 brass using a Sandvik parting tool at 800 rpm, no sign of distress at all.

I also agree about not drilling in steps. If the smallest drill wanders off centre you'll never get back. I have a set of Black and Decker "pilot point" drills - very hard to get now - up to 13 mm, they are excellent for hacking out lots of metal in one go preparatory for boring to size.

John,

What sort of drill point do these B & D pilot drills have? I have read elsewhere that short stub drills with regular points on them will do as excellent first drills; the secret is obviously the use of short stiff drills as the starter for 10 so to speak

Regards

Brian

ideally a pilot drill should be about the diameter of the thickness of the final size drill, pilot drill then straight to the full size, if need be drop both speed and feed, but don't allow the drill to rub, as many metals will work harden, and the edge on the drill will be blunted.

Ian S C