Counterboring to Precise Depth

[SteveHGraham]

I retrospect, I think this part should have been designed for the lathe. There is so much symmetry in it, including a center bore and counterbore. I could have machined the bulk of it in forty minutes. The are only four holes that really call for a mill.

A few curves wouldn't have affected the function, and it would have looked better. I may redo it on the lathe.

[arthur.marks]

That's certainly not a great way to measure a depth...

I agree. Steve should use a stack of no less than grade AS1, as defined in US Standard B89.1.9, gage blocks recently calibrated and traceable to NIST on a calibrated, grade A as defined in Federal Specification GGG-P-463c, surface plate. A tenth-reading, lever type, recently calibrated, test indicator should be used for comparison measurement between the workpiece and gage stack. The workpiece plane height must first be confirmed through comparison with said gage block stack. Upon confirmation of plane height dimension, the gage block stack is calculated by subtracting the necessary depth of counterbore. A new stack is assembled for final comparison means. All work must be performed in an ambient air temperature of 68 degrees ( +/- 1 degree) and humidity level of 50% ( +/- 1 percent). All gages and workpiece must be acclimated by soaking for at least 24 hours in stated environment.



This procedure is, of course, required only if one is working in an antiquated facility where a functioning CMM, calibrated and traceable to NIST standards of accuracy, is unavailable. A depth micrometer is unacceptable in this instance as it introduces an allowable measurement inaccuracy of up to 0.0002" ( 2 tenths).



A caliper is a perfectly acceptable instrument for accurately measuring the counterbore depth on Steve's thing-a-ma-jiggy-CNC-conversion to +/- 0.001" at this shallow of a depth--especially if a depth attachment anvil is used.



Furthermore, "a caliper is to a micrometer what a ruler is to a tape measure," can be reduced to the following logical statement.
caliper : micrometer :: ruler : tape measure
I disagree. A caliper is more analogous to a tape measure than a micrometer. A micrometer is more analogous to a ruler than a tape measure.

[SteveHGraham]

My head hurts now.

The newest counterbores: 19/64". WHERE do I get a 19/64" counterbore? That's .297".

Enco has a 115-piece HSS Chinese drill set on sale for 50 bucks. I may get it just so I'll have some hope of creating counterbores. Right now I'm going to have to root through my jar of junk bits and see if I have 19/64".

[arthur.marks]

Steve - are these counterbores for standard socket head cap screws (SHCS)? What is the screw size? Counterbores are usually bought and sized by the screw and not the counterbore diameter. Is this per chance for a 4mm metric screw or possibly a #8 machine screw?

[SteveHGraham]

The motor mount attaches to the lathe where the x dial used to go. There are two metric cap screws that go through the mount into the lathe. The caps are around 2.270" in diameter, and the screws are M4s.

Now that I've actually measured the screws, I plan to go in there with a quarter-inch end mill and see what happens. I know there will be some wobble, but it could work, and if it doesn't, I'll have an undersized bore I can open up.

It looks like HF has a 115-piece HSS drill set which gets complaints for the way the points are ground, but not so much for the steel. That might be a nice thing to have around for future counterboring jobs. There is no way I'm going to grind the points off my US drills.

[arthur.marks]

Victor Machinery
Travers
KBC
Etc...

[SteveHGraham]

I don't want to blow twenty bucks on two holes. I am trying to figure out a better way. It's starting to look like a) buy a set of HF drills for this purpose, or b) try to turn something from tool steel.

[arthur.marks]

D-bit? Do you have any properly sized drill rod or do you have the ability to harden O1? Basically you take a tool steel rod the diameter you want the counterbore recess. Cut with the side of an end mill to slightly less than 1/2 full diameter of the rod. Put about a 6-degree clearance angle across the entire bottom edge. This is clearance to create the sharp cutting edge on the flat end. Then relieve the side which won't be the cutting flute. i.e. bottom left facet of a RH tool or bottom right edge of a LH tool. Probably easier to post a picture, but I'm on a tablet at the moment.

[EDIT:] In lieu of a photo, links.
http://www.deansphotographica.com/machi ... ipod3.html
http://www.model-engineer.co.uk/forums/ ... p?th=40248
It is brought up in second link, but you'll need to use a drill just under the final CB diameter. Drill with that first to less than full depth of the counterbore. The D-bit the follows and cleans up the hole to proper size and cuts the bottom to a flat step.

[Harold_V]

That's certainly not a great way to measure a depth...

I agree. Steve should use a stack of no less than grade AS1, as defined in US Standard B89.1.9, gage blocks recently calibrated and traceable to NIST on a calibrated, grade A as defined in Federal Specification GGG-P-463c, surface plate. A tenth-reading, lever type, recently calibrated, test indicator should be used for comparison measurement between the workpiece and gage stack. The workpiece plane height must first be confirmed through comparison with said gage block stack. Upon confirmation of plane height dimension, the gage block stack is calculated by subtracting the necessary depth of counterbore. A new stack is assembled for final comparison means. All work must be performed in an ambient air temperature of 68 degrees ( +/- 1 degree) and humidity level of 50% ( +/- 1 percent). All gages and workpiece must be acclimated by soaking for at least 24 hours in stated environment.

Snotty comments noted.

Furthermore, "a caliper is to a micrometer what a ruler is to a tape measure," can be reduced to the following logical statement.
caliper : micrometer :: ruler : tape measure

I disagree. A caliper is more analogous to a tape measure than a micrometer. A micrometer is more analogous to a ruler than a tape measure.

Also noted, but with no agreement. There is no doubt in my mind, a caliper offers greater ability to make an accurate measurement than a ruler, in particular with aging eyes. The point I tried to make is that a caliper of any description will not *reliably* yield a correct measurement, even with the attachment you suggested. They're simply not made to do that, in spite of the protestations of many. Sure, you can get a reading, and it may even be correct, but is it? It may also repeat time and again, but is it correct? Maybe it is, and maybe it isn't, and maybe it matters, and maybe it doesn't. That's something that all should know and understand, so when it matters, wise decisions can be made.

The bottom line is, you can compare a caliper to a micrometer in the same way you can compare a ruler to a tape measure. The degree of precision by using the proper instrument for the purpose at hand will always be better, with more reliable readings.

Do not discount the ability of a proper ruler. There was a time when I could measure linear dimensions (working from a shoulder) and guarantee my readings within .003". That's about the degree of *reliability* of a caliper. No need to tell me that yours can read closer. Mine can, too, but they are not *reliable*, which is the point. One does not use a ruler, tape measure or a caliper when a dimension is critical. Note that I am not suggesting that the measurement in question is critical. I have no way of knowing if it is, or it isn't.

Harold

[ctwo]

I thought it was called a scale? And I have one of those caliper attachment thingies...I tried it on once. It was good at hooking my sleeve or mismannered finger and take my calipers to the floor. I filed it away somewhere in case I need it for something. And then I got a Scherr Tumico depth mic set, which I have yet to use...

[SteveHGraham]

I ended up making a D-shaped cutter from drill rod. It works great...up to a depth of about .4 inches. After that the chips refuse to go out through the bottom of the bore, and the aluminum welds a new bottom in the hole. The cutter has no flutes, so I guess it will only work on short holes. Guess I'm headed for HF.

[SteveHGraham]

GREAT CAESAR'S GHOST! The local Home Depot has a single 19/64 bit for sale. I'm out of here.