Drilling Accuracy (or Dealing with Tailstock Wobble)

[monophoto]

A concern that has been discussed a lot on this forum is the issue of tailstock wobble on midi-lathes, and the impact on drilling accuracy. I recently had to deal with this issue while making some small boxes, and thought that it might be helpful to post an illustrated discussion of what this problem is all about.

The tailstock casting on most lathes includes a rectangular protrusion on the bottom that aligns the tailstock to the bedways. As shown in the sketch, that protrusion (depicted as the dashed blue rectangle in the tailstock) ideally should be exactly as wide as the spacing between the bedways. However, many lathe owners find that their tailstock can wobble a bit - it rotates very slightly about a vertical axis that roughly passes through the center of the tailstock. This is because that protrusion is very slightly more narrow than the spacing between the bedways. The sketch of the 'less ideal lathe' shows that if the tailstock can rotate slightly, the axis of the tailstock may not align with the axis of rotation of the headstock. This is purely a consequence of less precise machining, and based on the comments on this forum, is not uncommon on midi-lathes produced in the Far East.

The amount of error that this wobble causes is typically quite small, and usually isn't a problem for most turning tasks. Where it is more likely to be a problem is when drilling - that small inaccuracy causes the drill bit to try to enter the turning slightly off-center. The photo shows the result of a test in which a brad-point bit that was not aligned exactly to the center of the turning carved a path 5mm in diameter around that center. This can cause the hole to be larger than the diameter of the bit. And because the bit will try to center itself in the turning, the bit to flex while drilling - and the resulting metal fatigue can cause small bits to break. While this is usually not a major problem for those who make pens from kits, the folks who make kitless pens need to achieve a higher degree of accuracy when drilling, and subsequently threading, concentric holes. That's why most kitless penmakers use machinists' lathes rather than wood lathes. However, there are a few members of the IAP community who have the skills and patience to do kitless pens with wood lathes in spite of this handicap.

The degree of angular misalignment between the tailstock and the axis of the headstock is a function of the degree to which the protrusion on the bottom of the headstock is smaller than the spacing of the bedways. However, the amount of centering error that can occur at the face of a turning depends on the ram extension, and on the size of accessories between the end of the ram and the turning - which in the case of drilling includes the jacobs chuck and the drill bit. As the ram is extended, and as the bit becomes longer, the centering error increases. I did some measurements on my 12" Turncrafter lathe and found the following:

• The protrusion on the bottom of the bedways is about 0.15mm more narrow that the spacing between the bedways.
• That amount of error is enough to cause as much as 0.48 degrees of misalignment between the tailstock and the spindle axis. Note that aligning the drill bit to the turning can zero out this error, and this maximum occurs if no particular care is taken to center the bit on the turning.
• When drilling using a standard 7mm brad-point pen bit in a jacobs chuck and with the tailstock ram extended to its maximum, the centering error can be as much as 2.5mm as shown in the photograph. Again, this is a worst-case number, and more accurate centering is possible.
• I also found that the spacing between the bedways on my lathe varies across the 18'" length of the bedways - the spacing is wider at the headstock end. This means that the amount of backlash that is possible also depends on where on the bedways the tailstock has been placed.

So this begs the question of how to deal with this problem. Some possibilities include:

• Purchase a lathe produced from a more precise machining process. I suspect that a Robust or OneWay lathe would perform much better than a less-expensive midi-lathe produced in a factory in Asia.
• It may be possible to shim out the protrusion on a specific lathe, but that's beyond my skill set. As my measurements indicate, it would not require a lot of shimming to prevent the tailstock from rotating, but achieving that amount of correction permanently would take the skills of an experienced machinist.
• The simplest approach is to mount the turning on the headstock spindle using whatever approach is preferred, and using the toe of a skew chisel, cut a small dimple in the center. Then, align the tip of the drill bit to that dimple before locking the tailstock down to the bedways. Aligning the bit to the exact center of the turning aligns the tailstock exactly to the rotational axis of the headstock, and essentially eliminates the error.
• The spur on a brad-point or Forstner bit can easily be aligned to a small dimple, but aligning a standard twist drill is more fiddley. My experience is that a twist-drill bit is more easily aligned to a starter hole created by a center bit aligned to that dimple. Also, if the desired diameter of the hole is large, it is easier to align successive bits in steps rather than to try to go directly to a large diameter bit from that initial starter hole.

 

[leehljp]

Thanks Louie for this write up, discussion and picts. I have a Grizzly 16" and a Rikon 12" and have not had a problem with either. I do not consider the Grizzly to be anywhere near the top of the line, and my Rikon is about 15 years old. I am mechanically inclined and have looked for ways to make mine go out of "sync", "round" or "off center" when I locked down - just to see if it "might", but have not. Mine seem to lock into center everytimge. I do not leave mine loose, I tighten everything snug at least. Mine have drilled true at all times, but again, I watch for anything out of the ordinary and back up if I see something before it goes wrong, which usually doesn't happen. I am meticulous and somewhat obsessive and have not run into that problem.

Even with a Jacobs chuck and extending the bit, I have not generally had a problem with runout.

That said, I have wondered what is going on with others who have the problem, and what they are doing. I haven't been able to figure it out, and don't usually answer in those situations except occasionally to say to not use brad points.

I am glad that you are able to track down the problems (at least some of the problem) and offer a solution. And maybe there are a couple of other repetitive problems that can be identified by others.

I once had a saw that had no problems with alignments what-so-ever - perfect cuts and perfect alignments with registration devices. On another forum, once in a while someone would have a considerable problem. I purchased another identical saw to my perfect saw - in order to use one overseas and one back here in the US. The newer one had (what I found by measuring) a couple of screw holes drilled a millimeter off from normal, and created considerable sliding TS miter table alignment problems. A few others had that problem but had not figured it out. Everyone without the problem wonder what the others with the problem were doing wrong. They were not doing anything wrong, they had a defective sliding miter table on their saw.

For you to track down .15 mm off is amazing. THANKS for reporting this, Hope it helps others!

 

[Mortalis]

While the tailstock "wobble" can cause some issues the issues are actually that the tail stock is tightened down and as a result will 'force' the drill to resist drilling centered all the way through the stock. If the tail stock is not tightened down to the ways of the lathe and is pushed from behind to advance the bit into and through the stock the drill will naturally find its center as long and the chuck is not above or below the center. That is why many 'old timers' make handles for their chucks and will not use the tail stock to drill long distances.

Another factor for a drill bit to drill off center is if the flutes of the point are not ground evenly. This will cause the drill to want to veer to the side of whichever flute is longest.

Just adding my 2 cents

 

[egnald]

The standard tailstock clamp that fits under and between the ways on my lathe, a Jet 1221vs, was one of those big round stepped washers. A few months ago, I replaced it with a custom stepped square shaped tailstock clamp that was made by Rick Herrill (rherrell). It not only provided significantly more surface contact with the ways by being square instead of round, it also was machined to a tighter fit with my ways. Needless to say, it was a big improvement in keeping my tailstock in alignment with the headstock even when it is un-clamped down.

Kudos to Rick!

Dave

 

[TDahl]

Louie,

Thanks for taking the time looking into the problem, figuring out a solution, and then creating such a detailed write up for the rest of the members. I have been looking for a solution to the tail stock wobble for some time now.

 

[leehljp]

This thread today reminded me to post a recent experience:

Drilling on the Lathe

It seems with drilling with a 12.5 mm bit the hole doesn't maintain the same size, or the hole shifts because as you can see the brass binds when inserted into the blank. I'm using a 12.5 mm jobber bit. I'm curious what bits you use and if you have any idea why it's binding like this? I do go...

www.penturners.org

I'm bringing this back to the forefront because I did experience this on my Grizzly 16" lathe this past week. It was not an ordinary pen drilling use though. I needed a very specific washer with a very specific thickness and a very specific inside diameter and outside diameter to use as spacer on a dado saw blade modification.

Although I do not have a metal lathe for machining, I do have some end mills, which I tried to use (and did use successfully) on my Grizzly wood lathe. I needed a 5/8" hole in the approx. 1 1/4" washer. I put the washer in one chuck and the end mill in the tail stock chuck. I did not have an MT2 drill Chuck that would handle 5/8" so I had to make do with what I had. Initially, it went well when touching, but as I moved almost imperceptibly it began to wobble. I backed off and checked and tried to wobble the chucks then the end mill. I couldn't. Then I tried really forcing the end mill and chuck to wobble by using a wood handle as a lever. I did move it. I don't know scientifically how much more pressure I put on the chuck with a wooden leverage - with one end in between the ways and me pushing on it against the end mill, but I did move it with a bare deflection - some seemed to be in the chuck and some in the MT2 housing, although it was very minor in the deflection, but enough. I could not tell if it was the whole tail stock unit moving between the ways, but it seemed to be a combination of the individual parts plus the whole thing. I was careful (in my mind) not to bend or break anything but I did not force it any more than it did during its wobble. Back to my hands, I couldn't move it.

Knowing it was tight and secure, and the tail stock Lock was secure, (I also knew this was not a metal lathe machine) I continued to "machine" the washer to a 5/8" hole. It took me about 10 minutes to "drill" less than 1/`16", with a drop or two of cutting oil. (680 rpm, slowest it would go.)

If at any point, and there were several points, when I applied too much pressure it would wobble. I did do what I have read from some others, I put my hand on the tail stock that was locked in place and gripped real tight to absorb the vibrations. It came out well and worked fine.

MY CONCLUSION for me: too much pressure and that too fast in the feed, even if it is minuscule, can cause alignment issues that result in wobble. This is not most people's problems and is not scientific, but I did see that under some circumstances I could introduce wobble that I could not do with my bare hands.

 

[Mike Debany]

square shaped tailstock clamp that was made by Rick Herrill

how to contact him?

 

[rixstix]

Rick's Custom Tools

Rick Herrell's custom turning tools

www.penturners.org

Should get you there. Or send him a PM

 

[randyrls]

I just saw this thread. Louie has id'd the situation well. If a solution is needed, a spring(s) could be installed on one side of the bedway on the tail stock to press the tail stock against one side of the opening. This would add some drag to the tail stock, but when locked down, it would hold that angle.

The easiest method is just to press the tailstock firmly against one side or the other. but you need good clamping pressure to keep the tail stock from moving.

The small lathes with a round tail stock clamp are notorious for moving under pressure. The square clamp is a good fix.

 

[Woodchipper]

FWIW, I drill on a drill press. Much quicker than setting up the lathe for one blank. The only way I would drill on the lathe would be drilling several blanks at a time and changing the bits.

 

[farmer]

Buy a gun drill ...
Gun drills (through coolant drill) are straight fluted drills which allow cutting fluid (either compressed air or a suitable liquid) to be injected through the drill's hollow body to the cutting face. They are used for deep hole drilling—a depth-to-diameter ratio of 300:1 or more is possible. Gun barrels are the obvious example; hence the name. Other uses include moldmaking, diemaking, and the manufacture of combustion engine parts such as crankcase, cylinder head, and woodwind musical instruments, such as uilleann pipes, as gun drills can drill long straight holes in metal, wood, and some plastics. The coolant provides lubrication and cooling to the cutting edges and removes the swarf or chips from the hole. Modern gun drills use carbide tips to prolong life and reduce total cost when compared with steel tips. Speed of drilling depends on the material being drilled, rotational speed, and the drill diameter; a high speed drill can cut a hole in P20 steel at 30 inches per minute.

Gun drilling can be done on several kinds of machine tools. On lathes, it is generally practical with hole depths of less than 50 diameters. There are also purpose-built gun drilling machines, where longer aspect ratios can be drilled.

 

[KenB259]

Buy a gun drill ...
Gun drills (through coolant drill) are straight fluted drills which allow cutting fluid (either compressed air or a suitable liquid) to be injected through the drill's hollow body to the cutting face. They are used for deep hole drilling—a depth-to-diameter ratio of 300:1 or more is possible. Gun barrels are the obvious example; hence the name. Other uses include moldmaking, diemaking, and the manufacture of combustion engine parts such as crankcase, cylinder head, and woodwind musical instruments, such as uilleann pipes, as gun drills can drill long straight holes in metal, wood, and some plastics. The coolant provides lubrication and cooling to the cutting edges and removes the swarf or chips from the hole. Modern gun drills use carbide tips to prolong life and reduce total cost when compared with steel tips. Speed of drilling depends on the material being drilled, rotational speed, and the drill diameter; a high speed drill can cut a hole in P20 steel at 30 inches per minute.

Gun drilling can be done on several kinds of machine tools. On lathes, it is generally practical with hole depths of less than 50 diameters. There are also purpose-built gun drilling machines, where longer aspect ratios can be drilled.

You should write up a tutorial on how to use a gun drill on a Turncrafter 12 inch wood lathe or for that matter any midi wood lathe. This isn't t the first time you've recommended using a gun drill for penmaking. I'd love to see it.

 

[farmer]

You should write up a tutorial on how to use a gun drill on a Turncrafter 12 inch wood lathe or for that matter any midi wood lathe. This isn't t the first time you've recommended using a gun drill for penmaking. I'd love to see it.

LOL out of the 7 lathes I own ,, I do not own one wood lathe,, Never really even I ever wanted one or used one .
Someone must not of told me you needed a wood lathe to make a pen.
Buy me one of the lathes you mentioned with what ever size of bit and no issues .

 

[KenB259]

LOL out of the 7 lathes I own ,, I do not own one wood lathe,, Never really even I ever wanted one or used one .
Someone must not of told me you needed a wood lathe to make a pen.
Buy me one of the lathes you mentioned with what ever size of bit and no issues .

My point is simply a gun drill is not practical for pen making.

 

[farmer]

My point is simply a gun drill is not practical for pen making.

Have you ever used a gun drill on wood ?

 

[KenB259]

Have you ever used a gun drill on wood ?

I make pens, most pens are 3 inches or less, most drill bits are around 4 or 5 inches long. Those drill bits are around 10 bucks, give or take. Again since the point of my response seems to elude you, now read this slowly and look up any words you don't understand….. here it comes…. My point is again.. GUN DRILLS ARE NOT PRACTICAL for making pens on a midi wood lathe. Sorry this is snarky, hasn't been the best day and I'm tired of being told I'm wrong when someone knows what they are saying is idiotic but insists on pushing the issue. I'm not responding any further on this. Buy all the gun drills you want , you do you and I'll do me.

 

[farmer]

I make pens, most pens are 3 inches or less, most drill bits are around 4 or 5 inches long. Those drill bits are around 10 bucks, give or take. Again since the point of my response seems to elude you, now read this slowly and look up any words you don't understand….. here it comes…. My point is again.. GUN DRILLS ARE NOT PRACTICAL for making pens on a midi wood lathe. Sorry this is snarky, hasn't been the best day and I'm tired of being told I'm wrong when someone knows what they are saying is idiotic but insists on pushing the issue. I'm not responding any further on this. Buy all the gun drills you want , you do you and I'll do me.

 

[farmer]

How do you know again have you ever used a gun drill ? Clearly you have not or you wouldn't be dancing around the question .
Is there a drill bit that drills faster then a gun drill the answer is No.
Is there a drill bit that drill wood cooler , the answer is No
Your out to pick a fight over something you have never used ... I don't care how you drill your holes.
If it works for you great, why are you so upset about me suggesting some that does work ?
A gun drill works give me one reason I can suggest it ?
Because no one else in here has one ?

 

[KenB259]

How do you know again have you ever used a gun drill ? Clearly you have not or you wouldn't be dancing around the question .
Is there a drill bit that drills faster then a gun drill the answer is No.
Is there a drill bit that drill wood cooler , the answer is No
Your out to pick a fight over something you have never used ... I don't care how you drill your holes.
If it works for you great, why are you so upset about me suggesting some that does work ?
A gun drill works give me one reason I can suggest it ?
Because no one else in here has one ?

No one here is stupid enough to spend hundreds of dollars on a single drill bit and then hundreds more reworking a wood lathe, which includes a whole new tailstock assembly to allow the extra long drill bit to be driven behind and through the tailstock through a sleeve and then into a pen blank. Let alone the air compressor you also need. Are kidding me? You still are going to argue that this setup is practical for penmaking. Looking for even one picture of a pen you've made, I found none. Let's see your gun drill pen making setup.