The quest for perfect concentricity...

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underdog

underdog

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Sep 11, 2006
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Watkinsville, Georgia
In an effort to get pen bodies perfectly matched up to the hardware kits, I've tried several things, with varying results.

I've always used a set of calipers to determine the blank finished diameter and the hardware diameter (nib, cap and center band). So I don't have a problem with that..

But seems like getting that blank/tube completely centered on the axis is always a hit and miss proposition. No matter what I have done, I have still been able to feel high/low spots between the nib and body.

I've tried the following:

1) using an adjustable mandrel with only enough tailstock pressure to keep it captured.
2) turning the nib end at the headstock to minimize any runout.
3) turning one body at a time to minimize runout.
4) using a better pen mill that didn't leave a burr (arizona silhouette) on the tube due to clearance between the shaft and cutter head.
5) using a machinist's dead center and 60 live center, along with Johnny CNCs bushings.

I think that's about it.

I did some measuring last night... here are some things I learned.

1) clearance between the mandrel (OD - .244") and the brass tube (ID - .248") is about .004". Plenty of slop to induce anywhere from 0-.004" runout... (I suppose that'd be .008" if actually measuring runout, eh?)

2) Although I didn't measure any runout on the precision 60° centers (from Johnny), one of my Johnny CNC's bushings still has about .002" runout, and the other is about .001". When using this setup, I still got some pretty large felt non-concentricity on my pens. I haven't tried this method with the good pen mill, though, so the jury is still out. If I can get better results I'll let you know.
 
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Option #5 is the only way to go, except I make my own tbc bushes.

Once you use these bushes all the out of round hassles disappear.

Having said that, not all your problems will disappear. I have just been measuring a kit which supposedly needs to be turned to 13.6mm for the cap (both ends), 12.3mm (both ends) for the barrel.

When I measured the components for said kit I got 13.49 and 13.69 for the cap, 12.35 and 11.86 for the barrel.

These variations can very easily be detected when you run your finger over the finished pen, in the case of the barrel they could be detected easily by eye.

I have bought kits and bushes together from various suppliers where the variations were ven larger.

This was one of the reasons I started making my own bushes and ultimately one needs to measure each end and match it to the components in the kit.
 
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BigShed said:
Option #5 is the only way to go, except I make my own tbc bushes.

When I measured the components for said kit I got 13.49 and 13.69 for the cap, 12.35 and 11.86 for the barrel.


This was one of the reasons I started making my own bushes and ultimately one needs to measure each end and match it to the components in the kit.
Click to expand...

Yep; You need to measure the actual fittings. Don't trust the manufacturer specs. JohnnyCNC's bushings are about the best. Turn them TBC.
 
Addressing only concentricity, which is out of roundness related to the bore not mating of size perfectly to components. (something I know how to address on a metal lathe, but the principle about the same on a wood lathe)

Assuming good machine precision, TBC is the only way to go, either using centers only and a great deal of care or making your own bushings to perfectly match your components. Either a dead or live center should have no significant run out if you avoid the real cheap ones, and not very much even with them when they are new.

I doubt you can achieve perfect concentricity on a (commercially made) mandrel but if one is very well made and mates up with your machine very well you should be able to get it to the point of being unnoticeable unless you are really looking for it.

In either case, technique is all important (which is where I usually fall short) and perfection is the best ideal but an unrealistic goal for most practical applications.
 
Now if we admit that there is a concentricity problem with the best of bushings and even .001 with custom bushings, just take the time to measure the components with the same ferver. It has been my experience that manufactured pen components at times show up with mismatches on one side or another not because of my mismeasurements but becasue the components are not mfg'ed to equal tolerances "across the board" of a particular line up, .001 or .002 in some cases.

Few people measure the pen components for concentricity, but I got into the habit of doing that a few years ago. Could be that this is some of the fitting problem.

I did a search a few months ago but couldn't find the thread that discussed how small of a difference that a finger could feel. It was surprisingly small. So, .001 or .002 do make a difference.
 
Would this be resolved (or minimized) by rotating the blank 180' on the bushings when you get to the final finishing cuts and sanding? Seems to make a difference on some of mine but its not totally clear to me that this is a true fix for something being non concentric. I definitely need to start turning between centers regardless!
 
Geh...:frown:

Found out that my centers aren't lining up (again). In the past, I've fiddled with (filed) my Jet 1642 tailstock until my precision centers were as close as I could make them by eye. But tonight I see a very noticable difference in height.

To top it off, I noticed a lot of fretting (and rust) between those centers and Johnny's bushings... Definitely not lined up.

Sigh. Prolly the last time I use that method, because I just can't stand messing up those nice centers Johnny sent me...:redface:
 
So.. it seems like it's back to the mandrel (or plain centers) for me.

Is there someone who makes regular bushings that fit the mandrel shaft a bit better? Or is there some way to swedge the end of the tubes to make them fit more closely?
 
Several details have already been mentioned, such as making sure the centers are aligned, the blank faces are square, the bushings and mandrel (if used) are true and have minimal slop, the tubes are deburred and free of glue; and that rotating the blank on the bushings may help. One other thing I've noticed that makes a difference is lathe speed.

When I turn at high speed, I tend to get an out-of-round (not just a non-concentric) barrel. I originally thought that my lathe had a vibration issue at higher speed, but I got a better lathe and still experience the same problem. I don't know if the OOR is caused by mandrel whip, tool chatter, or something else. I do know that if I slow the lathe down for my final cutting pass, I get better results.

I hope that helps,
Eric
 
When you put two center points together you can tell pretty close whether they are centered so you used probably the best method. what I do is put a chuck clamped on each end of a transfer punch in the tail and headstock and clamp down the head stock (my delta has a rotate-able head stock). This locks the fixed point. If I was on a quest for perfect alignment I would do this with the tail stock extended about 1/2 inch and lock the tail stock to the bed as well. I remove the pin and chucks and my tail stock are as alighned as possible.Than I would use the 60 deg drive and live centers turning between centers. Two things about the bushings, make sure they penetrate far into the tube (1/2-3/4") to reduce angular twist which will cause more runout.

These things will get you as close as I could imagine you would ever want to get!

Good Luck
Ray
 
take a piece of metal shim material and put it between centers. if they are perfectly aligned, it'll stay perpendicular to the bed. If they are off, it will move as the points push it in different directions.
 
Have you tried bringing your centers into alignment by adjusting the feet on the lathe to remove any twist that you may have in the ways? The cast iron bed of the lathe may seem really solid, but it can flex quite a bit. Enough to move your centers off center depending on how the lathe feet are adjusted. Center alignment is the final thing I check with my Powermatic 3520 when I relocate it. Small adjustments to the feet (at the tailstock end) can make big moves in center alignment at the headstock.

The 1642 is similar enough to the 3520 that adjusting your feet to solve your alignment problem might be something for you to consider trying.

Just a thought.

Ed
 
Yeah Ed... I've done that, ad nauseum. Adjusting the legs won't help when the headstock center is lower than the tailstock center.

And... Jet has sent me three tailstocks from to solve this problem. I finally took the best of the three, and filed the bottom of the tailstock where it mounts on the ways, until I got it really, really close. I couldn't really tell if it was off at that point.

But last night, I took another look, and sure enough the tailstock was higher.. Grrr...
 
Hi Jim - Sounds pretty frustrating.

You said you had it in alignment and then last night it was out of alignment. Were the headstock and tailstock in the exact same place both times you checked? If not, it could be an issue with your ways and not the tailstock. Have you tried moving the headstock and checking your aligment at different positions along the ways to check for an issue with the surface (without rotating anything or changing the extension of the quill)? If it's high in some spots, low in others and right on in some, then fiddling with the tailstock isn't going to solve your problem.

Another thought is that some crud could have built up under the tailstock as you were moving it as you turned. That could cause the tailstock to cant up (since most of the crud would get packed on the headstock side of the tailstock). Especially if your filing adjustments to the bottom of the tailstock didn't leave it perfectly flat and left a tiny gap for dust / chips to work under the tailstock as it moves.

Another thing to check is your alignment at different quill extensions. If it changes depending on how much of the quill is extended, then that's a different problem to solve.

Just some more thoughts. If you already did all this, then just ignore me.

Ed
 
On a wood lathe having the centers line up exactly aren't necessarly as important as having the centers run true (No runout). Although it is good to have them as close as possible, but the construction of the lathe will have a lot to do with how close that is.

On a metal lathe where the cutting tool is held in a tool holder and that tool holder is fixed to the compound/cross slide/carrage assembly the cutting tool travels along a fixed bed (Ways) then it is important for the centers to be lined up to both the ways and on center by height. Other wise you will end up with a tapered part, unless thats what you want, (Thats why the tail stock on a metal lathe is adjustable front to back; to make tapers)

On a wood lathe the cutting tool is held by us and no matter how good you get at turning, you will never be able to hold that cutting tool firm enough or slide it straight enough to make a perfectly straight part. Hence the centers being a little off won't make any difference; but having run out in the centers will cause out of round parts.
 
Hi Danny - a misalignment of the centers will make a difference with custom bushings designed to seat on 60 degree centers for between center turning. Both in premature wear of the bushings and in random out of round results that gets worse the more out of align things are.

Pen makers are trying to use wood lathes to achieve a level of precision and accuracy that is way beyond what the lathes are designed for. A wood lathe will never match a metal lathe, but that's not a good reason to not try and get the wood lathe as close as possible. It really does make a big difference in my experience.

Ed
 
Well now, see what happens when I start a post and then get sidetracked for a couple of hours? Still, I hope that the info is of some use to you.

underdog said:
So tell me.. besides eyeballing it, how can you ensure that the centers are lined up? And, what is the acceptable amount of being out of line?
Click to expand...

Well, depending on how anal you are, and how precise your equipment is, there's the razor blade test (see the pictures below).

You can read about it in this review of an HF 8x12 metal lathe (about half-way down the page):
http://www.annisquamgranite.com/8x12ReviewPg2.htm

Personally, I just run them up to just touching and see if they match. If not, then I start shimming things until they do. Assuming that your lathe is made correctly, you should be able to get suitable accuracy just by visually checking. As an example, my tailstock is a little bit lower than my headstock, so I've got it shimmed with about four layers of blue tape on the underside of the tailstock where it rides on the bed.

Hope this helps.
dow
 

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parklandturner said:
Hi Danny - a misalignment of the centers will make a difference with custom bushings designed to seat on 60 degree centers for between center turning. Both in premature wear of the bushings and in random out of round results that gets worse the more out of align things are.

Pen makers are trying to use wood lathes to achieve a level of precision and accuracy that is way beyond what the lathes are designed for. A wood lathe will never match a metal lathe, but that's not a good reason to not try and get the wood lathe as close as possible. It really does make a big difference in my experience.

Ed
Click to expand...
Not really, I have a cheap wood lathe I use to turn the bulk of my pens on I make my own flavor of TBC bushing and the centers have never lined up and will never line up unless I put more into the lathe then it's worth.

I've turned hundreds of pens on it and since I've started using my own flavor of TBC bushings i've lever had any of the pens turn out of round. And yes I check them.

When I had a cheap dead center that had runout no matter what I did yes I got out of round, but not with the centers running true.

If your getting out of round with your bushing, make sure your center drill is cutting a 60 degree angle, and not 58 degrees or something like that, if there is an angle mismatch then yes you will get a possible out of round condition; because the angle of the drilled center in the bushing and the angle of the lathe center dont match thus allowing the bushing to rock on the contact points instead of seating on the full area of the angle.

Think about it if you want to turn a tapered shaft; on the metal lathe you purpously move the tail stock out of alingment, and turn the part on centers, with what your saying the part will not only be tapered but also out of round.

This doesn't happen (the out of round condition), I know this for a fact I've turned tapered shafts this way.
 
When I said I was using precision centers, I meant it...

The live center is a 60° South Bend I bought from Johnny CNC. The 60° dead center I also got from Johnny. I'm not sure the brand, but there's no runout. I know, I measured it with my dial indicator..

But they don't line up when when I bring them within close proximity. And.. they are fretting something awful in my custom bushings... So I'm not totally buying your idea that it won't cause problems if I have good centers.
 
underdog said:
When I said I was using precision centers, I meant it...

The live center is a 60° South Bend I bought from Johnny CNC. The 60° dead center I also got from Johnny. I'm not sure the brand, but there's no runout. I know, I measured it with my dial indicator..

But they don't line up when when I bring them within close proximity. And.. they are fretting something awful in my custom bushings... So I'm not totally buying your idea that it won't cause problems if I have good centers.
Click to expand...
Check the angles in your bushings, and how they fit the angles on your lathe centers, if you can rock your bushing while pushing it against the lathe center then the angles are different, or there is not enough of an angle bearing surface to seat against, or a little of both.

I've seen center drills vary quite a bit on the actual angle they drill, also if your bushings are short (they dont go in the tube very far) like the mandrel bushings, that will allow for rocking, also (Less bearing surface in the tube)

In my post I mention my flavor of TBC bushings, there's a reason I started making my own. :wink:.

I could draw up some sketch's to help explain or illustrate what I'm talking about but I am swamped right now with orders for the shop (not pens) and it will be a few days.
 
Nah... I highly doubt these bushings were off. Those bushings were near perfect when I got them. And one of them even stayed on the center last night when I pulled the tailstock away. It's more likely I dinged 'em up trying to get glue on/off..
 
underdog said:
Nah... I highly doubt these bushings were off. Those bushings were near perfect when I got them. And one of them even stayed on the center last night when I pulled the tailstock away. It's more likely I dinged 'em up trying to get glue on/off..
Click to expand...


Try coloring the inside center mating surfaces of the bushing and the points of the centers with some Dykem or similar layout fluid and running the lathe with a pen barrel set up on it for TBC for a few minutes, then take the assembly apart and see if there are any shiny places on the mating surfaces of the bushings against the centers. If there are, the bushings, centers, lathe spindle/tailstock alignment or maybe something more obscure are incorrect since there should be absolutely no movement to mark the Dykem (it should look the same after taking it apart for examination as it did when it was put together.

If all is well, try it again while doing some turning of the barrel (keep in mind that slippage will cause marking if you turn it hard enough to make is slip). You might also try it under different amounts of tension.

Any shiny spots or uneven marking on the Dykemed surfaces indicate something is wiggling around or moving somewhere.
 
Made some sketches

I made some sketches
Disclaimer: This is my theory that I developed from dealing with out of round issues. It is not ment to claim or disclaim anyone elses thoughts or theorys.

Anyhow here goes. In the good angle skertch you see how a TBC bushing should match the lathe center, and the contact area.
Good angle.PNG
In the open angle you see what the contact area would look like (yellow Lines) if the angle was open.
open angle.PNG
In the closed angle you see what the contact area would look like (red Lines) if the angle was closed.
closed angle.PNG

Now if we were turning solid peices where we would drill the center holes in each end of the peice and then turn neither one of these conditions would affect the part in an out of round results. if the centers were out of line we would have a tapered part but not out of round.

But the fact that we are using bushings this brings in a few more problems with what I consider another place for movement. The fit and the length of the bushing to the tube can allow for wobble and misalingment to cause out of round conditions.

This is the reason I make my bushings as shown below, I make the part that goes in the tube to a closer tollerance then the other bushings, usally about .003" to .005" smaller the the inside diameter. Also I make this part 3/4" long to give the bushing more bearing surface inside of the tube, this gives less chance of the bushing to wobble and cause out of round. (My theory). Also I make the larger diameter (finished Pen Diameter) only 1/4" long to put the lathe center closer to the part I'm turning to help with the out of round issue (again my theory)
bushing.PNG

A bushing that is only 1/4" long and have maybe .010" or more clearance (smaller then the tube) and a center drill angle not exactly at 60 degrees would be able to wobble enough to cause an out of round condition.

I know that ever since I started making my bushings like above, I have not had any problems with out of round conditions with my pens.

I put this out just to help, if you think it hog wash thats fine.

Have a good day.
 
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