educating me - boring

I;ve bored oen hole on the mill but this wa smy first on the lathe. An ally sq block 47mm and 90mm long aiming for a through hole of 29mm.

Drilled 5mm and 10mm split points like butter then blacksmiths drills 18mm, 22mm and 25,,. I found I had to set them up for a hard start push to avoid rubbing chatter until they engaged and then they cut well.

The onto the boring bar. The best i had was a 16mm indexed cutter with (if memory serves) a 0.4mm radius point. I set it 100mm overhang. I opened up in 0.5mm passes. the first at 900rpm and i got some singing halfway down the hole so cut the RPM to around 500 next pass when i could only just hear a little singing. I used a fine power feed for the passes and was gettig a lovely finish.

The last 0.2mm I dropped the rpm to around 350 cos the singing had increased previous pass. that got me to size at the opening and was a nice finish but I thought I ought to run a spring pass since i had no way to measure at depth (calipers only) - The spring pass chattered faintly and left a slight knurl through the whole bore.

Apart from the obvious of a thicker bar I'm wondering if the chatter was actually due to making too fine a pass such that the tool was rubbing rather than cutting that then set up the oscillation? At that point it did pull the cross slide in according to the dro by 0.07mm and distracted rather than cut the metal. The bore actually measured sl narrower after the pass than before.

Since I'm pressing bushings in each end it won't be seen but i'd like to understand and avoid this in future..

Oh, and I'd had the bar set parallel to the hole.. would there have been any advantage or disadvantage to having angled the tool at the bore wall?

Thanks

pgk

Been there, had the same problem!

Angling the boring bar will make no difference. The problem is one of DOC and feedrate versus the insert radius. If one thinks about the insert geometry a DOC of cut less than the insert radius will have a tendency to push the insert away from the cut. By it's nature the boring bar is less stiff than an external toolholder and so will deflect a little bit. If the DOC and/or feedrate are small then the deflection will be a significant proportion of same. The forces on the boring bar decrease and it moves back into position, and the cycle repeats - equals chatter.

The trick is to ensure that when the boring bar does deflect it doesn't result in oscillations. I'd be using a DOC of nearer 1mm and feedrates of at least 0.1mm/rev for roughing, and up to 0.2mm/rev for spring passes.

Andrew

Best to finish with a sharp hss boring bar.

Edited By David Clark 1 on 31/07/2015 12:55:10

pgk

General rule with carbide indexed cutters is to run them at the speeds, feeds and DOC given by the maker. If no book settings are provided,whether on the box or via online reference, the inserts are rubbish and should be binned even if they occasionally appear to work OK as behaviour is unpredictable. Inserts from reputable suppliers are highly developed cutting systems designed to work as per their book settings. Normally they need to be run hard enough to put significant heat into the chips if they are to behave themselves. Cutting action under such conditions is somewhat different to that at the lower speeds used with HSS so the tip edge and chipbreaker shape is likely to be less than optimal at HSS speeds & feeds. Life at book settings can be quite short tho' so picking a home shop guy machine and budget friendly insert can be problem. Aluminium geometry positive rake inserts frequently work well at HSS style speeds as do so called "super-sharps".

#2 to Andrews point about using more DOC. Worth spending a bit of time on the drawing board or CAD program to see how a small depth of cut makes the tip act as a ramp pushing the bar away from the bore. If you consider a flat ended insert it should be clear how the end helps stabilise things once its gets into cut. Angling the bar puts a little more cutting edge to work and reduces the effective deflection forces which may stabilise things enough to kill the song.

Finishing cuts are not appropriate for a correctly employed insert. They give their specified performance over the book operating range. If you need a better finish you use the appropriate insert. Which doesn't mean you can't make the things work well outside their book range given sufficient futszing about but such expedients are likely to be unreliable. You are paying towards all the enignnering effort the supplier put into making something that works like it says on the tin so you might as well exploit it.

Clive.

That does tend to confirm what i thought (and why i mentioned the tip radius in my Q )..

<<#2 to Andrews point about using more DOC>

I was reading about tip radius effects a few days ago on a cnc mill site

The reason i ran the lathe at the 900rpm to begin with was cowardice with my first reasonably sized block of stuff hanging out of the the 4-jaw with alloy shims to stop them marking. Because of the fine nose radius i put a fine feed on but did wonder if that also lead to the rubbing and should have been coarser.

HSS I have no problem with using but didn't have thicker bar to use it with and didn't want to make one (yet) for one hole. I don't have an issue with grinding HSS and the surface finish on the sides was milled with an HSS flycutter to avodi a stripey finish after hogging off the bulk with a 25mm endmill.

thanks

Since i dolt have bore guages (yet) I turned plugs to a good fit and measured. One side was 29.40mm bore and the starting through hole 29.45mm - I had been shooting for 29mm until I hit the chatter. I'm guessing that next time in ally I'll leave the last 0.25mm for a final pass and a stiffer bar.

Try putting the back gear in.

John

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Excellent advice in a perfect world but as a 'home shop machinist' I cannot get anywhere near the quoted figures, that being said I get some excellent results.

Tony

Tony

Konw what you mean about speeds. Some time back I bought some high end Sandvick finishing inserts for £ very, very reasonable after mis-reading the recommended cutting speed as 450 ft/min. Which I figured was fast but useable. When they arrived speed turned out to be 450 m/min, over 3 times greater with corresponding increases in feed rate. Ooops!

Experimentation at more home shop friendly speeds gave very variable results. Sometimes excellent finish sometimes pretty rough with no obvious reasons beyond total unsuitability for aluminium where a huge built up edge appeared after only a few turns of the spindle.

Finally I plucked up courage to run at the proper speed. With the 1024 wizzing round flat out the carriage did a fair imitation of an F1 car off the grid when I engaged the feed cluch. The finish was awesome but chips coming off like a blue hot Manchester rainstorm need something more than the usual manual lathe guards to keep them under control. Hit the stop button about half inch before the end of cut with nerves jangling like George Formbys ukulele and took a break. Probably a bit less scary with a lathe having a conventional lever clutch instead of the 1024 on/off set-up.

Bottom line is that there are probably thousands of reasons and combinations of reasons for poor finish. When struggling with that hydra it really doesn't help if the particular insert you've got doesn't like the material and speeds you have. Especially if you have fairly narrow out of spec window where it does work. Found that out the hard way once with a different insert which worked fine with excellent finish for all the roughing cuts then did the ploughed field thing on the last cut as it dropped out of the "out of spec but still works OK" window. Naturally I'd used that insert on other, larger, jobs with more than decent results. As I recall matters some experimentation pretty much proved that anything under 1" diameter was verboten on the SouthBend Heavy 10 I was running at the time.

All far far worse when boring as you can't see whats going on and there are more variables to deal with. An easily overlooked nasty with some boring inserts is that the angles provided mean the smallest bore can be rather larger than one might expect. I have an inset & boring bar combination which looks as if it should handle 3/4" bore fine but in practice 1 1/4" or maybe a touch under is about the minimum. The bar really seems too slender for its task.

Clive

Edited By Clive Foster on 02/08/2015 14:11:10

If your boring aluminium better results are likely if 11mm triangular raked inserts are used. Better still micro polished ones specifically for aluminium as then the coating doesn't wear off - such as some pciked out from these

**LINK**

The similarly styled tips intended for finishing stainless steel are also good on other materials.

Manufacturers speeds and feed often crop up as if they are gospel. In real terms they are aimed at metal removal rates and tool life what ever tool material they are quoted for. The numbers that are needed really depends on the nature of the work and in any case in terms of feed relate to cut depth size of work and even the condition of the machine used.

John

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When I am machining aluminium I always use paraffin to stop the build up on the tool tip, works for me.

If I'm using HSS I polish the tips with a slip stone and sticking is extremely rare - sometimes happens parting off when I should have sharpened the blade - with the same stone.

John

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