Accuracy Of Hole Layout , Center Punch, Dividing Head

[visenfile]

If a careful layout is done for hole positions and accurate center punches performed, followed with center drills, can the results be as accurate as using a dividing head? If a template is available can drill bushings match the dividing head accuracy? Should a center punch point roughly match the drill tip angle? Thanks

 

[Wreck™Wreck]

If a careful layout is done for hole positions and accurate center punches performed, followed with center drills, can the results be as accurate as using a dividing head? If a template is available can drill bushings match the dividing head accuracy? Should a center punch point roughly match the drill tip angle? Thanks

Yes, this is what jig bore machines are for. Accuratley bore holes in the jig, insert drill bushings and have at it.

 

[brino]

one thing I found that helped the accuracy of my centre punch marks is a tool like this:
http://www.leevalley.com/en/wood/page.aspx?p=45502&cat=1,180,42311
I was surprised by how well you can see the scratched marks thru the optical post, it magnifies and supplies enough room light to align very well.
It's a very useful tool.

-brino

 

[RJSakowski]

Any hole position located by marking, aligning a center punch is going to be limited by one's visual acuity. When I had better vision, I liked to think that I could reliably locate holes to +/- .02". anything better than that and I was fooling myself. There will be an error in scribing the layout lines, an error in centering the punch on crossed layout lines, a possible drift of the punch as it is struck, and possible wander of the center drill or drill bit. In a former business endeavor, I drilled or punched many thousands of holes in 16 ga. aluminum using that method.

A well laid out fixture with drill guides should get you within a few thousandths but is only as good as the fixture, as Wreck Wreck said. A dividing head will get you excellent angular accuracy if you are laying out a bolt circle but the can be an error in the circle radius or the location of the circle center.

For accurate layout, I would calculate the x and y coordinates of each hole and use the mill to make precise moves. Or, if you have a DRO with a bolt circle routine, use that.

If center punching the old fashioned way, I use a custom center punch with a sharp tip and about a 30º included angle. This gives me the best possible visual indication of being on the cross scribe marks. After marking the holes with that punch, I follow with a conventional center punch to deepen the punch mark. If I am using a Roper Whitney hole punch , I would just use the first punch as it is sufficient to locate the hole punch

 

[derf]

That's actually called a "prick punch" It is used to establish the initial dimple accurately, then followed by a "center punch" to guide the drill tip.

 

[TOOLMASTER]

x2

That's actually called a "prick punch" It is used to establish the initial dimple accurately, then followed by a "center punch" to guide the drill tip.

 

[stupoty]

I use a prick punch first, i can get quite good accuracy with a bit of time spent doing the layout.

I had to make a scribe for my height gauge, i think the angle I chose for the tool steel scriber is a bit wrong as i somtimes have some issues with one side being more angles than the other which can push you off a few 0.001 if your not super careful.

Stuart

 

[David S]

If you follow the "building prototypes.." series by Dan Gelbart, he doesn't find the centre of scribed line optically but rather by feel. He demonstrates scribing two lines at right angles and then sliding the fine punch along one of the scribed lines until it locks into the intersection. Then tap the punch.

David

 

[T Bredehoft]

He demonstrates scribing two lines at right angles and then sliding the fine punch along one of the scribed lines until it locks into the intersection.

That's how I was taught, back in the 20th century. It was suggested that with care, .005 accuracy was possible, but not to count on it. That instruction was followed by suggesting that a Bridgeport with DRO was far more accurate.

 

[planeflyer21]

That's how I was taught, back in the 20th century. It was suggested that with care, .005 accuracy was possible, but not to count on it. That instruction was followed by suggesting that a Bridgeport with DRO was far more accurate.

I suppose that sounds better than "Back in the 1900s..."