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Counterboring tools?

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Bo'sun01/04/2020 18:49:20
79 forum posts

Clearly, proprietary (with a pilot) counterbore cutters are the way to go, and flat bottomed drills are an alternative to finishing the base of the pocket. But are they're any disadvantages to using a conventional 180deg. drill to finish the pocket? Other than needing a deeper pocket to get the cap screw head below the surface?

Mick B101/04/2020 19:01:14
1545 forum posts
83 photos

An ordinary drill sharpened to a flat bottom is a 180deg drill. Is that what you meant?

The disadvantage is that a standard-length drill is long enough to flex, so you'd best drill most of the c/bore with a standard 118 deg point first and use the hole to guide the flat bottomed one - means keeping 2 drills in that size or regrinding between ops. I prefer to use a slot drill if I've got one in the size I want for the counterbore.

If you mean not flat bottoming the hole, the disadvantage is poorer friction-locking of the underside of the head, and poor support for a washer if you use one.

Edited By Mick B1 on 01/04/2020 19:01:39

Martin Connelly01/04/2020 19:09:12
1222 forum posts
147 photos

Mick, I think 180 is finger trouble and Bo'sun meant 118 degrees..

Martin C

pgk pgk01/04/2020 19:24:22
1722 forum posts
287 photos

Conical washers?


Vic01/04/2020 19:35:29
2489 forum posts
14 photos

I can’t sharpen conventional twist drills by hand but making flat bottom ones is easy. I’ve ground several in the past and they’ve worked well. It obviously helps to drill and then bore at the same setting. I do this on a milling machine.

not done it yet01/04/2020 19:48:10
4488 forum posts
16 photos

For the number of counter-bores I need to do, I centre with the drill size (if counter-bores are done separate to the hole drilling) and change to an end mill. Most often simply done immediately after drilling thus on centre already. So much like Mick B1 except that any end mill of the right size will do the job.

I have half a dozen counter-bore drills but have never used them.

Journeyman01/04/2020 19:55:08
776 forum posts
131 photos

Most of the counter-bores I've done have been put in with an end mill. The few counter-bore tools I have seem always to have the wrong size pin. I cannot imagine that using a cap-screw in a counter-bore with an 118deg bottom is a very good idea , it must surely stress the bolt/screw in an undesigned way.


Edit: Typos (can't make the tablet work)

Edited By Journeyman on 01/04/2020 20:05:50

Bo'sun01/04/2020 20:03:24
79 forum posts
Posted by Martin Connelly on 01/04/2020 19:09:12:

Mick, I think 180 is finger trouble and Bo'sun meant 118 degrees..

Martin C

Thank you Martin,

Yes, my poor tryping. I meant 118deg,


Mick B101/04/2020 20:49:04
1545 forum posts
83 photos
Posted by Bo'sun on 01/04/2020 20:03:24:
Posted by Martin Connelly on 01/04/2020 19:09:12:

Mick, I think 180 is finger trouble and Bo'sun meant 118 degrees..

Martin C

Thank you Martin,

Yes, my poor tryping. I meant 118deg,


Well, I thought you probably did - that's what I tried to cover. I think Journeyman's right - all the force acts on the periphery of the cap-head if it's in a 118 deg c/bore.

ega01/04/2020 21:49:21
1615 forum posts
135 photos

The screws retaining the Myford saddle to the apron are an example of counterbores which definitely need a relatively flat-bottomed hole; the counterbores are over size to allow for adjustment of the position of saddle to apron.

I use counterbores fairly frequently - some shop-made, some commercial. The Granlund counterbores are the best I know of as pilot and bore can be sized to suit the job.

John Olsen01/04/2020 22:42:36
1028 forum posts
86 photos
1 articles

It is not hard to make your own counterbores from silver steel rod. The advantage of this is that you get to choose the outside diameter and the pin size. I do it like this:

Turn the outside of the silver steel down to the size you need. Face the end and drill for a pin. With a triangular file, file teeth in the end, making sure that they are facing the correct way to cut. Also make sure that the cutting edge remains at the level of the faced end. The teeth do not need to be especially even so dividing by eye is good enough.

Then harden, eg heat up the business end to the colour of a boiled carrot and plunge into cold water. Then temper to a light straw colour. Make a pin up to fit the hole. You can make a pin with a shoulder if you want to use the counterbore on a larger pilot hole later so they are more versatile than commercial ones.

Since they are carbon steel, you don't want to try to use them at high speed, but for the kind of use they get for spot facing and counterboring they will last fine.


JasonB02/04/2020 06:56:20
17828 forum posts
1949 photos
1 articles

The pilot for counterbores often seems large but it is intended to ensure that the internal fillet where the head of the cap screw meets it's shank does not make contact with the edge of the hole therefore ensuring all the load goes to the underside of the head. If using other methods and small clearance holes use a small CSK bit etc to ease the edge.

Bo'sun02/04/2020 08:19:55
79 forum posts

Good morning all,

The reason for this post was that many of the commercially available counterbore sets have larger holes than I would like. A point that some of the posts has eluded to.

Many thanks for all the comments and suggestions.


Speedy Builder502/04/2020 08:41:42
1981 forum posts
139 photos

Small counterbores - use a "D" bit

Bo'sun02/04/2020 09:21:51
79 forum posts

It's not the "size" of the counterbore, but the (what I consider to be) oversized holes. Yes, if you're making two mating parts in different locations, the extra clearance could be of benefit. But when you have the opportunity to "spot through" to locate the mating holes, quite accurately, the oversized holes can be a nuisance.

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