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Most accurate way to Spot/Drill a hole?

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HellawellCustoms

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#1
Hey guys, new to machining. Just got a Precision Mathews PM25, so far I've drilled a few holes for some of my fixed blades I do. (I make knives as a hobby) Anyways I actually bought the mill so that I could start making folding knives! I've quickly realized that my old eyeball and drill methods would not work for this. I also noticed my automatic center punch just isn't accurate enough, so I bought an optical center punch, unfortunately it seems to be defective/made poorly. The optical lens has quite a bit of wobble and causes all of my punches to be off by a few thousands. So I was wondering what people's suggestions are? I was thinking of clamping the piece of stock in the vice and lining up a center point to the cross hairs on the design and then switching to a drill. I've also heard of spot drills and center drills? What one would I need? Here is a picture to show an example of what I am working with.

32625912_10155667293471314_7847167457545420800_n.jpg
 

Bob Korves

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#2
Drilling for accuracy:

Center punch as well as you can. Spot with a spotting drill having a larger included angle than the following drill. Spotting helps drills follow more accurately. Placing a center punch mark or other alternative method is not a precision operation.

Follow with a drill that is plenty enough undersized to not extend beyond the desired finished hole location. Drilling removes material. Drilling does not drill a straight hole, a hole to the desired angle, or to an accurate desired size.

Follow the drill with a boring bar. A boring bar does not start a hole. Bore the hole a small amount undersized in the desired location. Boring is not efficient for removing lots of material, and is difficult to use for making a hole to a precise finished size.

Follow the boring bar with a reamer of the size you want the finished hole to be. A reamer, used properly, opens up holes to a predictable diameter within quite close tolerance. A reamer does not remove bulk material or locate the hole. In very high accuracy hole generating, a boring bar finishes the hole. It is fussy and needs a very accurate machine such as a jig borer to achieve the desired size hole in the desired location.

Recap for accurate hobby shop work:
1. Spot and drill to remove bulk material.
2. Bore to locate the hole and bring the hole nearly to finished size.
3. Ream the hole to the finished size.
 
Last edited:

mikey

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#3
There are a number of ways to do this:
  • Use a sharp prick punch. Scribe your lines, put the point in one of the lines and run along the line until it meets your spot. You will feel it engage the second line. Tap, check to see if you hit the spot and correct if needed by angling the punch and tapping again. Tap lightly until the cone from the punch is on the spot. Then give it a tap with a center punch and drill.
  • Use the Optical punch. You have to look directly over the magnified insert or parallax will mess you up. With practice, you can get pretty accurate with it.
  • Use an edge finder and use your machine's hand wheels to get you pretty much dead on target, within a thou or two.
  • Use a wiggler. This is very quick to use and relatively accurate.
For spotters, it depends on which drills you use. If using the common 118 degree drills, use a 120 degree spotter. If you use 135 degree split points, use a 140 degree spotter. The spotter needs to be just a bit bigger than the angle at the drill tip, that's all.

Edit: Bob is faster than me!
 

whitmore

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#4
Hey guys, new to machining....r I've drilled a few holes for some of my fixed blades I do. ... I've quickly realized that my old eyeball and drill methods would not work for this... all of my punches to be off by a few thousands.]
The way I've made accurate centering (other than by dialing in coordinates on a mill) is by making a pattern and using
chemical machining (etching). A good laser-print onto clear film can be the master,
and there are transfer films that can be baked-on to a substrate (polished steel blank, in your case), exposed,
and then an etchant will make the tiny dent for you. After an etch is done, the prick punch will
find (by feel) the center rather easily. The other blank layout details can be part of the same
exposure, if your workflow allows those details to be guided by such markings.

'Thermal transfer paper' or 'photosensitive dry film' will bring up hits on eBay and such.
Etchant can be ferric chloride (messy) or dilute nitric acid, but probably salt water would work OK, with a
trickle of electric current to help out (that would be electrochemical machining).

Alternately, one can imagine a jig that would adapt to any positioned hole, and define the arcs and
other edges after the hole is completed. Drill undersize and ream for best hole quality.
 

Aukai

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#5
Would a jig, and transfer punch be too inaccurate?
 

darkzero

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#6
When I need to drill a hole spot on, I indicate (edge find or indicate a hole) a known reference when possible, then move to the hole location with the DRO, spot drill, then proceed with drilling.

If I need to locate a center punch accurately (or as accurate as I can), I use the center point attachment with my Blake CoAx to indicate the center punch.

Before I had the luxury of owning a Blake, to locate a center punch, I would use the most accurate drill bit I had (small diameter) or a thin rod with a ground point, then I would bring the quill down & touch the center point with the drill bit & watch for any deflection. I would also rotate the drill bit/spindle ever 90° or so & repeat untill I saw minimal or no deflection until I was happy. May seem or sound a bit crude but it always worked for me.

EDIT: You know, come to thinking of my drill bit center punch finding hack, I have always had an edge finder with the point for locating center punches yet I have never used it for that purpose! :confused 3:
 

Tinkertoy1941

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#7
I would imagine that you bought the Precision Mathews PM25 to do precision work!!
With the Precision of the " Precision Mathews PM25 " I would have my known dimension on a blue print and set my work piece in a vice to known dimensions. Then I would use a center drill to locate the holes precisely from the known dimensions and never have to use a center punch again.
 

P. Waller

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#8
Choose what you consider the most important feature of the part, in this case it appears to be the pivot hole, make this your zero in both X and Y.
Produce every other feature from there by moving the appropriate distance. This is known as absolute positioning.

Use the graduated dials on the machine to make the moves, this is what they are for.

You appear to be CAD savvy so dimension your drawings in ordinate dimensions from 0,0 as seen in the rough example below, be aware that the farther you move from X0, Y0 the errors increase so choose the starting point wisely.
knifeblade.jpg
 

Rooster

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#9
My-self, i carefully scribe lines and a light tap with a 1/16" Starrett center punch. I then inspect it with a 10X loupe, if all is good then a bigger center punch. As regards to drilling, center drills are for drilling lathe centers and spotting drills are for spotting.
 

Bob Korves

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My-self, i carefully scribe lines and a light tap with a 1/16" Starrett center punch. I then inspect it with a 10X loupe, if all is good then a bigger center punch. As regards to drilling, center drills are for drilling lathe centers and spotting drills are for spotting.
Yes, that it a good recipe for drilling ordinary holes reasonably accurately.
 
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