Why did my tool dig in?

This topic has 8 replies, 5 voices, and was last updated 30 June 2017 at 23:36 by Chris Gill.

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30 June 2017 at 21:48 #305011
Chris Gill
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@chrisgill22114
Hi, I wonder if someone can help me understand what happened today.

I was boring out a 20mm hole in a bit of 12mm thick BMS. I'd drilled to 18mm and roughed out with a carbide tool and swapped to HS for finishing off but the tool chattered so badly I swapped back to carbide.

I was only running at 320 rpm because the workpiece centre of gravity was way off-axis. I was cutting on the back-stroke in the hope of getting a finer cut (yes, OK, carbide – slow – fine don't work together). And then, with 1 thou to go, it dug in and left a gouge in the middle of the hole.

Not a catastrophy in this case but scruffy and something I'd like to avoid if I could just figure out what I did wrong.

Thanks, Chris
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30 June 2017 at 21:48 #18566
Chris Gill
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@chrisgill22114
30 June 2017 at 22:16 #305012
peak4
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@peak4
Tool slightly below centre height maybe, such that any deformation (bending) of the tool forces it into the work, rather than away from it.

If it's a touch above centre height, then a minor flexing is more likely to move it away from the surface of the workpiece
30 June 2017 at 22:21 #305013
Ian P
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@ianp
The fact that the HSS tool chattered is an indication of something wrong with the cutting geometry or something flexing. Boring bars below centre will almost always dig in, its safer to have the tool slightly above centre and it be very sharp.

Its hard to be specific, not knowing the type of carbide tip but tips for steel dont usually have a very keen edge so for the final cut/s I would suggest using one of the tips made for aluminium (they are polished and very sharp carbide)

Creeping up to a finished diameter taking very small cuts only works with very sharp tools and a rigid setup.

Ian P
30 June 2017 at 22:21 #305014
Chris Gill
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@chrisgill22114
Posted by peak4 on 30/06/2017 22:16:59:

Tool slightly below centre height maybe, such that any deformation (bending) of the tool forces it into the work, rather than away from it.

If it's a touch above centre height, then a minor flexing is more likely to move it away from the surface of the workpiece

Ah, interesting. It's a while since I checked my height setting tool. Making a better one has been on my "one day" list. Maybe I should have a go.

Thanks
30 June 2017 at 22:24 #305015
Chris Evans 6
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@chrisevans6
Gosh how long is a piece of string ? Seriously though many things could influence things for the dig in. Lack of tool/lathe rigidity/gibs on the lathe not adjusted correctly/backlash in the cross slide feed screw. You say cutting on the backstroke I assume this means cutting when coming out of the workpiece ? If so you may have picked up a piece of swarf. Was the tool at correct height ? Did it have adequate clearance ? I am sure someone will be along with more reasons but so many things can prompt a dig in.
30 June 2017 at 23:15 #305019
Chris Gill
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@chrisgill22114
Posted by Chris Evans 6 on 30/06/2017 22:24:41:

Gosh how long is a piece of string ?

Ah, yes. I'd tightened gibs and backlash to the point where it was getting too tight. I'd set the tool height but there's a question mark here and clearance was good. Swarf was my original thought – may need to invest in decent coolant feed (or hose pipe)
30 June 2017 at 23:16 #305020
not done it yet
Participant
@notdoneityet
All the above, along with the question: Was the cross slide locked off? That in combination with any of the above reasons might well have made things worse.

Maybe a reamer would have been a better choice (in hindsight)?
30 June 2017 at 23:36 #305021
Chris Gill
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@chrisgill22114
Posted by not done it yet on 30/06/2017 23:16:34:

Was the cross slide locked off?

Ah, b**er, don't think I did.
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