Strange boring issue, LeBlond Regal Servo Shift

moonlight machine · Dec 29, 2019

Boring a 4" diameter 4" deep, First drilled to 2 1/4 then bore with a 1" diameter solid carbide boring bar wit a new insert. Not pushing it, .125 DOC. .004 taper in 4". Back the tool out, leaves a nice coil spring type groove that fades away before the tool is all the way out. If I do another cut without changing the diameter on the cross feed the bore has no taper, as in dead straight, 0 deviation on the dial bore gage. been a while since I checked level but if it was out why would it bore true the second time around?

plastikdreams · Dec 29, 2019

moonlight machine said:
Boring a 4" diameter 4" deep, First drilled to 2 1/4 then bore with a 1" diameter solid carbide boring bar wit a new insert. Not pushing it, .125 DOC. .004 taper in 4". Back the tool out, leaves a nice coil spring type groove that fades away before the tool is all the way out. If I do another cut without changing the diameter on the cross feed the bore has no taper, as in dead straight, 0 deviation on the dial bore gage. been a while since I checked level but if it was out why would it bore true the second time around?

Even a 1.000 solid carbide boring bar is going to flex on a .125 cut...

moonlight machine · Dec 29, 2019

plastikdreams said:
Even a 1.000 solid carbide boring bar is going to flex on a .125 cut...

It's only 7075, and if it was flex why does it flex more at full depth than when the cut just starts?

specfab · Dec 29, 2019

Happens to me all the time, and I have to peg it to boring bar flex and sometimes an exacerbating factor, chip buildup (non-optimal tool relief or height setup, maybe) between the tool and the inside wall of the bore. If there is a radius on the tool nose, that will also contribute to flexing to a smaller effective diameter result. Spring pass for finish is your friend when doing close-tolerance boring.

plastikdreams · Dec 29, 2019

moonlight machine said:
It's only 7075, and if it was flex why does it flex more at full depth than when the cut just starts?

Because it's basically ramming into the material, the deeper it cuts the more friction builds and the tool starts to climb the cut deflect. .004 in 4.000 isn't really much deflection. If seen way more on much lesser cuts in steel.

Where's the problem

moonlight machine · Dec 29, 2019

specfab said:
Happens to me all the time, and I have to peg it to boring bar flex and sometimes an exacerbating factor, chip buildup (non-optimal tool relief or height setup, maybe) between the tool and the inside wall of the bore. If there is a radius on the tool nose, that will also contribute to flexing to a smaller effective diameter result. Spring pass for finish is your friend when doing close-tolerance boring.

It might be chip build up between the bar and the bore. The bar is pretty close to the surface of the bore. Edit, just did the final cut, a spring pass, .0001 over size .0001 taper. That'll work.

gustafson · Dec 29, 2019

There are a bunch of things that could be going on. could be rocking the whole crossfeed, whole carriage, pushing the thrust bearings, it is endless

That is why they invented finish cuts

I'd be pretty damn proud of myself I my finish bore was basically perfect

Thinks ya worry too much

moonlight machine · Dec 29, 2019

gustafson said:
There are a bunch of things that could be going on. could be rocking the whole crossfeed, whole carriage, pushing the thrust bearings, it is endless

That is why they invented finish cuts

I'd be pretty damn proud of myself I my finish bore was basically perfect

Thinks ya worry too much

Ya it's just a fixture to be used once for one part. It's going to hold a 16" long piece of 17-4 over the edge of the mill table so I can drill and thread mill an 8 hole bolt circle, 16x2mm holes.

gbent · Dec 29, 2019

The wear in the longitudinal ways allows the carriage to deflect more as you move into the bore. Your spring pass doesn't have enough cutting pressure to deflect the carriage. Boring often acts different than od turning because the forces are in the opposite direction, and your carriage is further from the chuck. LeBlond's angled way is more optimized for od work than id work.

guest · Dec 29, 2019

moonlight machine said:
It's only 7075, and if it was flex why does it flex more at full depth than when the cut just starts?

It drags on the way out- sure sounds like the bar is pushing off.

I assume it's not just chips wrapping around the bar- you wouldn't be asking the question if that was the case.

Are you doing anything at the bottom of the bore? Like releasing a brake or something like that?

What I am wondering is where it starts happening. Have you looked at the drilled hole? If it's a cumulative thing, you might be leaving a little more material at the bottom of the bore with each pass.

Get to the final pass, you are taking more material at the bottom than at the top.

I'd document each pass- starting with the drilled hole, just out of curiosity. Does every boring bar pass have the same .004"? Or is it .001" on the first pass, then .002", then .003", then .004"...

Limy Sami · Dec 29, 2019

Sounds pretty normal for a ''aged'' and worn lathe, and has been already been said ;- that's why we have finishing cuts.

moonlight machine · Dec 29, 2019

Limy Sami said:
Sounds pretty normal for a ''aged'' and worn lathe, and has been already been said ;- that's why we have finishing cuts.

Aged? That baby was new in '77, it's over 20 years younger than me!

moonlight machine · Dec 29, 2019

guest said:
It drags on the way out- sure sounds like the bar is pushing off.

I assume it's not just chips wrapping around the bar- you wouldn't be asking the question if that was the case.

Are you doing anything at the bottom of the bore? Like releasing a brake or something like that?

What I am wondering is where it starts happening. Have you looked at the drilled hole? If it's a cumulative thing, you might be leaving a little more material at the bottom of the bore with each pass.

Get to the final pass, you are taking more material at the bottom than at the top.

I'd document each pass- starting with the drilled hole, just out of curiosity. Does every boring bar pass have the same .004"? Or is it .001" on the first pass, then .002", then .003", then .004"...

The bar runs about 3/16 from the bore, I was breaking the chip, chips could have laid on the on the bar when it was in the bore and pushed it out a bit. I could not see with the coolant flowing. No bottom of the bore, it is a through hole. Don't know about the taper when the bore was smaller, I did not check for taper till 3.95 dia. Finish size is 4.040. I only made one as it is a fixture for another job.

Limy Sami · Dec 29, 2019

moonlight machine said:
Aged? That baby was new in '77, it's over 20 years younger than me!

May well be

……………. but at 42 it's no spring chicken either

john.k · Dec 29, 2019

This is a simple test for an allegedly scraped and ground lathe...if the saddle twists on the bed ,a boring bar forward of the rack pinion will pivot around that point as far as the fit of the saddle permits......net result ,tool cuts on reverse pass,more cut ,more wear uncorrected.....Ive found it is a good way to deflate the asking price.....in practice its easily worked around ,and not a serious fault.

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