2.75in Forstner bit cutting a 2.857in hole?

[Tom Matthews]

I'm working on a small clock project and recently purchased a bunch of clock inserts along with the recommended 2 3/4 inch saw tooth bit for drilling the insert holes.

Since the bit is so large and my workpiece fairly small, I set up in my Bridgeport to cut the holes. I drilled a pilot hole and then started on one side and flipped the workpiece and finished cutting the hole on the other side.

I went to test fit the clock insert and the hole was too large. It measures 2 7/8 in diameter.

I checked the runout on the bit in my Bridgeport and measured about 0.035" with a dial indicator. I had purchased a 2 1/2 inch bit from the same company and checked it as well - about 0.025" on that one. I'll definitely be trying that one out on some scrap before an actual workpiece.

To rule out the Bridgeport (even though I was pretty sure it wasn't the culprit), I put the indicator on the Jacobs chuck and measured 0.003 (the chuck is probably 2-2.25 inches in diameter).

Could the runout of the bit be the culprit here? Should they be considered defective? I've got about $70 in the two bits so I'd like to be able to actually use them.

[Bruce Page]

It is hard for me to believe that a new bit has .035 run-out, that is extremely excessive. I suspect the chuck is the culprit. .003 run-out on the chuck OD is not necessarily indicative of the trueness of the chuck’s jaws. My first choice would be to use a collet rather than a chuck.

[Richard Coers]

Did you mic the bit diameter? Why the pilot hole, and what diameter? If you have a Bridgeport, I would drill out the wood with a 2.5" bit and then use a boring head or flycutter. Return the bits, they are not good.

[Tom Matthews]

Bruce, my first instinct was to use a collet, except these bits are turned with a hex shank on the end and I don't have a collet this size. I may try and pick one up just for these bits.

Richard, the pilot hole was so I could line up the bit on both sides of the workpiece. With Forstner bits, I usually score one side, flip the workpiece, and bore through the other side. This prevents blow out. Your boring bar suggestion was the second time I've heard that today - when I called my Dad to ask him his thoughts he suggested the same thing and offered his boring bar to use. I guess if you think like a metalworking machinist rather than a woodworker, you'd go right to the obvious solution. Clearly I'm a woodworker and went to "buy a new tool!".

Anyway, after some lunch and pondering, I went back down there and gave it another shot. Still measuring runout on the bits, but a single cut without flipping the board resulted in the correct diameter hole. I'm thinking I must have moved the workpiece slightly when I flipped it over, so I'm guessing the bits are fine.

Now I'm off to find some more red oak to remake the part I "overdrilled".

[pat warner]

I'm sure you're done now but a cutter like the one here (upper left (http://www.patwarner.com/images/drilling_tools.jpg)), will allow the through pilot hole. From that, you can and should waste from either side to manage the break out. The pilot will track within a few mils of center, even with excessive runout of drilling tool or chuck. It ain't cheap but it works forevermore + it's carbide tipped.

[Wade Lippman]

your subject says forstner bit, but your text says hole saw? Which is it?

[Tom Matthews]

I am using a saw-tooth Forstner bit, this page explains the differences:

http://www.leevalley.com/us/wood/page.aspx?p=45533&cat=1,180,42240

[Phil Thien]

I'm sure you're done now but a cutter like the one here (upper left (http://www.patwarner.com/images/drilling_tools.jpg)), will allow the through pilot hole. From that, you can and should waste from either side to manage the break out. The pilot will track within a few mils of center, even with excessive runout of drilling tool or chuck. It ain't cheap but it works forevermore + it's carbide tipped.

How much would one of those cost in the required 2-7/8" size?

McMaster only has adjustable in that size, in the MT, for over $700.

[Wade Lippman]

I am using a saw-tooth Forstner bit, this page explains the differences:

http://www.leevalley.com/us/wood/page.aspx?p=45533&cat=1,180,42240

For through holes, holes saws are so much better, assuming the stock is not too thick.

[Lee Schierer]

Although you seem to have solved the problem. When I am concerned about blowout on the exit side of a hole with a forstner bit, I set my Drill press depth control so that the center spur of the bit just starts to penetrate the exit side. The larger you make this penetration, the less accurate the alignment of the two bores will be. Then I flip the piece over and align the center spur with the pin hole and cut the remaining wood (usually less than 1/8") from the hole.

[Bruce Page]

Tom, I bored two 4½” holes in some MDF yesterday using my mill and boring head. You may want to invest in a boring head to go with your Bridgeport. They can be picked up fairly cheap on eBay.

[pat warner]

"How much would one of those cost in the required 2-7/8" size?"
******************
I think MADEINCHINA brands are<$150 for >2".

[Chris Fournier]

I don't think that the runout that the poster has identified is out of the realm of the possible at all. I have purchased well over a dozen saw tooth bits from a supplier that we all extol the virtues of and at least 25% of them had this amount of runout. At least!. I returned every single one of these outliers which is not to say that the rest were "true". Look at the price point and tell me that precision is in the mix - not a chance. Honestly can you imagine how messed up the chuck in a Bridgeport style milling macine would have to be to create this scenario! No way.

I would never dirll a pilot hole over say 0.0625" for use a bit like this. I have never had problems with blow out on the back side of the cut when using these bits when I use a sacrificial board underneath and ensure that the object being drilled is flat and solidly clamped to my DP.

If you require a boring head in a milling machine to do woodworking as some have suggested there is something terribly wrong with your setups and/or processes. And yes I own a DP, milling machine, forstner bits, saw tooth bits, fly cutters and boring heads.

[keith micinski]

Although you seem to have solved the problem. When I am concerned about blowout on the exit side of a hole with a forstner bit, I set my Drill press depth control so that the center spur of the bit just starts to penetrate the exit side. The larger you make this penetration, the less accurate the alignment of the two bores will be. Then I flip the piece over and align the center spur with the pin hole and cut the remaining wood (usually less than 1/8") from the hole.


I do it this way also.

[Tom Matthews]

Thanks guys for the suggestions. The replacement piece (no pilot hole this time) went perfectly. I'm betting I was misaligned with my pilot hole, which forced the bit to walk and enlarge the hole.

[Jeff Duncan]

Sounds like you got it solved, I'll just add that it's much easier to bypass flipping the stock by using a backer piece of scrap as Chris mentioned.

good luck,
JeffD

[Bruce Page]

If you require a boring head in a milling machine to do woodworking as some have suggested there is something terribly wrong with your setups and/or processes. And yes I own a DP, milling machine, forstner bits, saw tooth bits, fly cutters and boring heads.

Chris, a milling machine is just another tool. It doesn’t make you any more or less a craftsman if you use it in your woodworking. In my case I machined the “almost” 4½” holes for a snug fit on two specific, pre-made plastic parts. The mill & boring head method was the easiest and quickest way to get it done. Additionally, I have used my mill countless times to cut small wooden parts that I was uncomfortable doing on the tablesaw, bandsaw, or jointer. As I said, it is just another tool.