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Ridges in final pass of bow inside

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Forum topic by JoshNZ posted 08-28-2021 10:49 AM 807 views 0 times favorited 22 replies Add to Favorites Watch
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JoshNZ

141 posts in 2314 days


08-28-2021 10:49 AM

Topic tags/keywords: bowl ridges surface finish

Hi all,

I’m having a heck of a time mastering the final pass of the inside of a bowl, the one continuous cut from the rim to the centre. Actually I’m not even making it one inch down from the rim without seeing these corrugations. I wondered if anyone might recognise them and know what I’m doing wrong? See YouTube link and photo.

https://youtu.be/kgyswB2jx_I

The bowl in the image above is screwed to a faceplate to eliminate my chuck as the problem.
I’ve tried different woods different rpms different gouges different grinds, that one is a 40/40, I usually prefer a ~50 Irish grind but can’t do any better with either. Tried removing the heel from the bevel, tried different angles of the tool rest tried different orientations of the flute, tried a feather light grip tried white knuckling it. I’ve just rebuilt my lathe spindle/new bearings.

I’m sure it is my technique but I’m not sure what else to try at this point. The first corrugation appears only half inch after beginning the cut, before I’ve really began to swing my body or initiate any movement (so I’m quite comfortable and steady). Yet, I watch the surface behind the bevel and think all is going well and then out comes a pretty significant ridge when seemingly nothing has happened. Once in a blue moon I do get it right but don’t know why hah! Not seeing it at all on outside of bowls.

Will leave it at that anyway hopefully it is obvious to someone already!
Thanks in advance


22 replies so far

View Aj2's profile

Aj2

4072 posts in 3043 days


#1 posted 08-28-2021 01:46 PM

How about your tool rest is it with out ridges or snags. I keep a wax candle near by to make my tool rest slick for my final passes.
I also us scraper with a negative grind it leaves a acceptance surface for me.
Good Luck

-- Aj

View drsurfrat's profile

drsurfrat

1029 posts in 431 days


#2 posted 08-28-2021 03:27 PM

I can think of two things that cause that for me.

1. The wood is resonating, and causes a flex that leaves lines that you don’t intend. Sometimes it works to change speeds if you can, or at least make a very slow cut advance.

2. if your final pass is with a scraper, you might need to sharpen, and have less coffee. I never use a scraper for final pass, I’ve never had success.

2b. If using a fingernail grind/(Irish?) gouge. I make sure that I AM riding the bevel on smooth start, and only cause the curve by pivoting the handle of the gouge – not pulling around with my tool tip – so that the bevel holds the cutting edge stable. I gently lean into the cut and let the vibrations of the system allow the movement forward. Hard to explain in words, and I don’t have a video. Let me know if you need more, maybeI can draw a picture.

-- Mike (near Boston) ... Laziness is the mother of invention, necessity is the mother of exhaustion - me

View OSU55's profile

OSU55

2830 posts in 3234 days


#3 posted 08-28-2021 06:59 PM

Yes I have seen the same. Too much bevel pressure. Don’t “ride the bevel”, “float the bevel”. Also a heavier cut will add to the problem, as will a thinner wall. I suspect you are pushing the tool through the cut and pushing the bevel against the wood. Set this bowl aside, get another blank. Turn the OD, then turn the ID to ~1” wall. Now, start practicing the lightest finish cut you can take (can also practice on the OD). Just a sliver. Practice with up to ~1/16” cut depth. I usually use Ellsworth’s ID finish cut, using his grind. It touches the bevel on the left side of the tip, with the flute opened to the top. I make better cuts with it vs a push cut. Float, dont ride, the bevel on these cuts. Very relaxed arms and hands, and let the tool cut, dont push it through the cut.

AND, sharpen the tool before finish cuts.

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drsurfrat

1029 posts in 431 days


#4 posted 08-28-2021 07:04 PM

Sorry if this is obvious, but I’m trying to help

-- Mike (near Boston) ... Laziness is the mother of invention, necessity is the mother of exhaustion - me

View JoshNZ's profile

JoshNZ

141 posts in 2314 days


#5 posted 08-28-2021 10:19 PM

AJ that was one of my first thoughts, I ran a file across my rest then dressed it with a diamond stone then waxed it, it’s super slick.

Mike – all understood, i thought about the resonating thing too, and that is a suspicion I’ve been wrestling. This is a union graduate I rebuilt when I was younger, back when I didn’t know much about machining. I thought I did have a harmonic problem as I remember one of the journals on my shaft being slightly undersized. So dragged the whole machine apart and turned a new one this week (we are in lockdown at the moment). Both bearing journals are .015mm interference press fit, shaft ends are running true to within ~.02mm. and as I said I screwed that bowl to a face plate to eliminate chuck (hence the no-go black ring in photo). Also torqued up my outboard bed. I’ve got one more idea to improve this machine and I’m headed out to the sawmill today to cut a ~150mm thick slab to stick under it. The spindle sits about 6” under my elbow which is a little bit uncomfortable to work on (I’m 6ft2 + tall in the back), I’m hoping that will improve my posture etc. Nothing is too obvious to me, appreciate your time!

I’ll get the machine raised up and spend some time practicing floating the bevel, this seems like the most likely culprit as the few times I did have success were with a really gentle slow feather light grip. But I couldn’t repeat them right after, which is frustrating.

It has been quite some time since I have finished a bowl I must admit. I’ve got stacks of rough outs drying, I quite enjoy blasting some wet shavings out when I have some free time but seems I never got around to finishing any, so I thought it’d be a good skill to hone up this week with being stuck in lockdown, one I need to relearn apparently. I really hate sanding and I’m not even going to look at a bowl wall like that with sandpaper, so I need to master a decent finish pass or the pile is never going to get done.

Appreciate everyone’s responses, will update soon.. thanks!

View Woodmaster1's profile

Woodmaster1

1853 posts in 3832 days


#6 posted 08-28-2021 10:49 PM

my woodworking clubs best turner said sharpen my bowl gouge before my final pass. After practice and more practice you should do a good job.

View drsurfrat's profile

drsurfrat

1029 posts in 431 days


#7 posted 08-28-2021 11:42 PM

That is really annoying when you can’t pinpoint the problem. In your video, you seem to have a smooth pass. The only other technique I have is that I push into the piece with my back hand and only use my front to hold down onto the toolrest. While pushing, as I want to curve, I pivot my back hand like I scribbled in the picture above. It seems like you are doing that as well, but I can’t see your back hand in the video. Don’t know if the bevel has anything to do with it, maybe your’s is too steep or shallow? Here’s mine:

I don’t even stone it, just go from the grinder to the piece and I don’t get that corrugation problem. (I know, not what you want hear…)

-- Mike (near Boston) ... Laziness is the mother of invention, necessity is the mother of exhaustion - me

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JoshNZ

141 posts in 2314 days


#8 posted 08-29-2021 12:25 AM

That was the first thing I tried and I think it helped, bringing the bevel angle way down shallower so the pushing force has a larger vector component in the direction of cut rather than direction of bevel (pushing bevel into wood), if that overcomplicated explanation makes sense…

I’ve gotta get the machine raised anyway, I lose focus of how my body is positioned during cuts like these but catch myself at the end in some really awkward squatted/hunched positions trying to keep the bevel moving smoothly with the handle at right height.

View Aj2's profile

Aj2

4072 posts in 3043 days


#9 posted 08-29-2021 02:00 AM

I would like to suggest for your final finishing passes a negative rake scraper. Unlike a round nose scraper the negative rake cut just like bryd head in a planer.
The tool is fool proof it doesn’t catch wood at all. Your sanding time will go way down

I just looked at the video.I think your technique is fine have you checked to see if the piece is loose in your chuck
Good Luck

-- Aj

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JoshNZ

141 posts in 2314 days


#10 posted 08-29-2021 03:13 AM

The piece isn’t in a chuck AJ it’s screwed to a faceplate to eliminate that.

Call me stubborn but I’d rather learn to cut it properly, rather than scrape. Just for kicks I have sharpened an old skew chisel I no longer wanted into a round nose scraper, but I’m not a fan of it. Too many wood species tear or chatter with it for my liking.

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JoshNZ

141 posts in 2314 days


#11 posted 08-29-2021 09:49 AM

Here’s a few more videos of attempts tonight. I’ve got the machine sitting up on 7” blocks which has made a big difference comfort wise. Still no joy on the smooth cut.

I know the rims got a bit thin here, and it’s a bit chattery. Though maybe that’s a hint to my problem, because it’s not that thin =/.

Attmept 1 and 2 I tried different speeds with the 3/8” 40/40 grind, I realise that tool is on its last legs. Unfortunately the phone decided it was exposed wrong in first vid.
Attempt 3 is with a 1/2” swept back grind ~50 degree.

https://youtu.be/zRxY_vN52JQ

https://youtu.be/khCOpYlQ_gg

https://youtu.be/eUul_xYslms

View Aj2's profile

Aj2

4072 posts in 3043 days


#12 posted 08-29-2021 06:48 PM

I looked at the videos and don’t see anything wrong with your technique. I might have the tool rest a bit higher it hard to tell from a video it might be fine. As far as the different grind go I don’t think it matters much on a soft wood like maple.
Do you know the last time bearings were replaced . It looks like old iron powermatic?
Good Luck

-- Aj

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JoshNZ

141 posts in 2314 days


#13 posted 08-29-2021 09:05 PM

Replaced a couple of years ago, hasn’t done much work since then. Reinstalled only days ago, with the new shaft I machined. Bearings still looked brand new, still snug and stiff from new rubber seal. Journals are all press fit interference I had to bolt into headstock from both sides to fit it.

It’s a union graduate too for what it’s worth. I picked it up cheap years ago, fitted a 3p motor and VFD then turned a new shaft with RH thread on outboard end and put a control panel on the backside so it could be used right handed on the outboard end for large bowls.
Might all scream with alarm bells, and I’m not an expert engineer but I’m pretty certain ive got it right, there’s no play between the journals and shaft, or in the bearings, that much I can assure you.

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Aj2

4072 posts in 3043 days


#14 posted 08-29-2021 09:38 PM

That’s a cool lathe. I can see why you are expecting a better cut .
Good Luck

-- Aj

View drsurfrat's profile

drsurfrat

1029 posts in 431 days


#15 posted 08-30-2021 12:02 AM

Wow, it seems you are chasing ghosts. You’ve done way past what I would do mechanically. The only stretch I can make is if you have a friend with a lathe, ask them to try your blank on it and see if it is the wood or your technique – though I-we are pretty sure it is not your technique. If you get ridges, it is something about your wood, if not, it is a gremlin in your lathe in spite of all your diligence. Being in NZ, can’t just stop over to see…

-- Mike (near Boston) ... Laziness is the mother of invention, necessity is the mother of exhaustion - me

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