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What Do You Do When Bit Extender Introduces Wobble?

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Rick M

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Mar 11, 2009
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Typical scenario: You've got your blank all clamped in perfect orientation on the drill press, gotten the hole going just the way you want it, and discover the stem on the Forstner bit is too short to reach the full travel required 🙁.

First reaction: Get out that high quality bit extender you bought and stored for precisely this situation, then assemble and re-chuck everything. Resume drilling as before.

Second reaction: Observe with consternation that the O.D. of the Forstner bit stem is noticeably less than the I.D. of the extender, therefore the bit now rotates slightly off-center.

First resloution: Grabbed a Jorgensen wooden clamp to hold the blank and just let the blank follow the wobbly rotation. Got the job done, but I suspect there might be better alternatives, especially since this would NOT work where a longer blank was being drilled on-lathe.

Pondering how to remedy this problem in the future, I considered a few ideas:
1) re-chucking the bit closer to the end of the stem (without using the extender--obviously a limited solution),
2) the possibility of finding somewhere to buy metal shim stock to do a 3/4 wrap around the bit's stem,
3) inquire here to ask others what they do in this situation.
 
Without knowing the rest of the objective, my first resolution would be to use the Forstner bit only to establish the aiming of the hole. Then use a long bit (e.g. lamp maker's) to drill the rest, guided by the first drilling. Long bits can be up to about 18" long, and small diameters too.

Tell us more about the design. There might be better ways.
 
It appears that one could be metric and the other English, even if not labeled that way. My inclination is to guess that the Forstner may not be the best quality since you did not mention the word expensive. Since your question was general and did not describe a specific application, the most obvious answer is to get a Forstner bit that matches the extender.

I have an 18 inch extender, but it is intended for a specific purpose and will only work with already long twist drills with a screw tip that are used in hand drills. These rigs are typically used by electricians for such things as drilling through a fire break in existing construction. The mating shank of the drill stem has three flats to match the extension.

Another point is that long extensions are not something normally used on a drill press because run-out is going to be a problem with any extension, but especially one that uses set screws. The hole obviously has to be oversized so if you are looking for zero runout then you are out of luck. Also, consider that each time that the DP table is cranked up, some additional misalignment is being added to complicate things.

If this were a specific application question then I might be able to come up with some other workarounds. For instance, the hole through a peppermill does not need to be drilled except for a smaller diameter starter hole and then can be turned to the necessary diameter.
 
I was using a PSI 13/16ths Forstner to make a thru-hole in a 2 3/16ths length of dry mahogany or Spanish cedar I had laying around (1st attempt at a kaleidoscope, so this is a prototype). The ~5-inch extender (more accurate image than original source) has 4 set screws--2 paired axially, pairs 90 degrees apart.

I forgot to include in the OP that I'd also considered switching to a small drill bit so I could establish where the Forstner would exit, then reverse the blank and use the Forstner from the opposite end. Certainly an option to ensure a clean hole at each end, but small bits are also short, so I'd probably still have the issue of the drill press chuck contacting the blank before a thru hole was established.

The DP table has been adjusted for perpendicularity (is that a word?) to the DP chuck (though not re-leveled with each subsequent up/down adjustment). Both the bit and the extension run true by themselves, so the wobble seems strictly a function of differences in O.D. and I.D. when paired.
 
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kaleidoscopes

I have found it is better to drill from both ends and meet in the middle. This way both ends are clean. If it is not perfect in the middle, who cares. The section with the mirror is little bit smaller and it produces no problem. Just make sure you have matching centers on both ends such as starting with a square blank. This will allow you to get accurate centers. Plus I drill them on the lathe.

Vernon
 
Rick M said:
I was using a PSI 13/16ths Forstner to make a thru-hole in a 2 3/16ths length of dry mahogany or Spanish cedar I had laying around (1st attempt at a kaleidoscope, so this is a prototype). The ~5-inch extender (more accurate image than original source) has 4 set screws--2 paired axially, pairs 90 degrees apart.

I forgot to include in the OP that I'd also considered switching to a small drill bit so I could establish where the Forstner would exit, then reverse the blank and use the Forstner from the opposite end. Certainly an option to ensure a clean hole at each end, but small bits are also short, so I'd probably still have the issue of the drill press chuck contacting the blank before a thru hole was established.

The DP table has been adjusted for perpendicularity (is that a word?) to the DP chuck (though not re-leveled with each subsequent up/down adjustment). Both the bit and the extension run true by themselves, so the wobble seems strictly a function of differences in O.D. and I.D. when paired.
Click to expand...

It seems to me that a 2 3/16 length through hole should not be a challenge and I do't see why an extender is needed for something that small. The throw of full size drill presses is over three inches and if you happen to have a very short shanked Forstner bit, get another one with a longer shank or don't bottom the shank of the current on in the chuck. If you do not get aggressive, I think that you could grip as little as 3/4 inch of the shank.

I think that perpendicularity is a real word, but that is not the issue when the table is cranked up or down. The DP table is likely to make minute lateral shifts which will result in side loading of the Forstner bit. If the hole in the extension results in an off-center fit to the bit, it does not really matter how many set screws are being used because it won't improve runout.

As far as a small pilot hole is concerned, that does not seem like a good idea if better precision is desired. The Forstner bit uses the small center tip to help reduce drift and without it, there is less to restrain the effects of side loading. Most drill presses have adjustment stops for bit travel and I think that setting the maximum limit to the point where only the tip of the Forstner breaks through will accomplish what you want so that there can be a clean hole made from both sides. Even easier is to have a backer board tightly clamped to the block that you are drilling -- that way, you can go all the way through without tearout on the exit side.
 
A hex-shank Forstner bit with a compatible extender would be a better choice of tools. Unfortunately, such an extender may be rare. My Irwin hex extender has a 5/8" OD barrel and doesn't fit my hex-shank Grizzly bits, but for about $6 (for a new Grizzly 13/16"), you could grind the bit shank to properly mate. The Irwin extender is available at Lowe's and probably Home Depot too.

Bill's suggestion of a longer bit is also good. Forstner bits work best at low speeds, and you could probably grip as little as 1/2 inch, especially with a hex shank.
 
Joe Greiner said:
Here's discussion of a brand with compatible extenders, but it might not be available in 1/16" increments: http://www.woodworkforums.com/f171/colt-maxicut-forstner-bits-112285/

Uncanny timing.🙂
Click to expand...

From that, it looks like Colt makes some extensions that use a system they call RotaStop rather than utilizing grub screws. Colt's MaxiCut Forstners were also favorably mentioned, so I did a quick check and found at least one place with bits in 1/16th-inch increments and 3 lengths of extensions. Might be worth picking up a couple of essential items, then budgeting for more later.
 
Meanwhile, after pondering some of Bill's points, another idea I might explore struck me yesterday. Next time I empty a disposable water or soda bottle, I'll experiment by cutting a narrow strip from it to see how that works as a shim. I might have to experiment with different bottles (cheaper/crackly sounding vs. sturdier) to determine whether anything fits, or if the idea has merit.
 
The DP's got plenty of travel, as long as there's enough stem on the bit. I'll see what happens when there's less of it in the chuck, but it'll still be close with that particular PSI bit.
 
Same bit

I looked at home and I have that same bit from PSI. The biggest problem is that they are not the best bits and dull rather quickly. To solve the problem get the same size Boremax bit from Woodcraft. While somewhat more expensive it will still be dirlling through several of the others.

Vernon
 
Convalco Forstner bits

In my experience the only forstner bits worth owning are the original ones, manufactured in this country, not a chinese job, Connecticut Valley Manufacturing Co.
They are not cheap, but they are worth every penny and can be had in several lengths. A sound investment, I bought some in 1977 and A few of that original set, are still going strong.

They do not have serrations, and do a very smooth cut.
My 2 cents
jorge

http://www.convalco.com/
 
Jorge Castaneda said:
In my experience the only forstner bits worth owning are the original ones, manufactured in this country, not a chinese job, Connecticut Valley Manufacturing Co.
They are not cheap, but they are worth every penny and can be had in several lengths. A sound investment, I bought some in 1977 and A few of that original set, are still going strong.

They do not have serrations, and do a very smooth cut.
My 2 cents
jorge

http://www.convalco.com/
Click to expand...

I agree. Good quality tools are actually cheaper in the long run (or at least result in less aggravation).
 
Jorge Castaneda said:
In my experience the only forstner bits worth owning are the original ones, manufactured in this country, not a chinese job, Connecticut Valley Manufacturing Co.
They are not cheap, but they are worth every penny and can be had in several lengths. A sound investment, I bought some in 1977 and A few of that original set, are still going strong.

They do not have serrations, and do a very smooth cut.
My 2 cents
jorge

http://www.convalco.com/
Click to expand...
I'm no fan of the Chinese tools, and totally agree that good long-lasting tools usually justify the cost. OTOH, when you are just getting started and need "a few things" to get started without taking out a second mortgage, you opt to bite the bullet and buy the bargain stuff--at least until you can budget for better hardware without waiting to piecemeal a set of bits (or whatever) one or two (w/ shipping, in terms of $$ and lead time) at a whack.

Those bits you referenced are certainly priced as would be expected for high quality and your recommendation is noted, as is the change in ownership and manufacturing described at the link given.
 
Bill Boehme said:
My guess is that the bit will get loose if you make a plastic sleeve. Plastic deforms too easily under load. . .
Click to expand...
Given that I had nothing to lose by trying, I trimmed a piece of disposable water bottle to make a 3/4 wrap around the bit stem and it made a very snug fit upon shimming it into the extender. After tightening all the grub screws, I waited overnight and rechecked the screws.

Taking a non scientific approach, I didn't get out the torque wrench to double check my findings, but the screws all seemed just as snug as they had been the evening before. Best of all, the bit now rotated on-center--well enough that it could be raised clear of the blank and still re-enter the hole without altering the opening. It remained that way after drilling a couple more blanks and the grub screws maintained their torque.

When I'm finished, I'll pull the assembly apart and look for any discoloration that might signal deformation in the plastic, but for now, I'd say this type plastic is a workable, budget solution to the original problem.

Nonetheless, long term, I'm in the same camp with those recommending better quality bits.
 
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