Issue with drilling holes for scales.

[Fletch Helical]

I'm just finishing up a handle on a knife a made a while back that sat for a while. Now I had this happen on another knife I did before I stopped making knives for a while. I drill out the holes for the scales, put in these instances Corby's in and when everything is fitted up everything looks great. But once I start shaping the handles I notice a gap around the rivet and the scale and there's no rhyme or reason to it. In one knife it was only one rivet on one side you can see. The one I'm doing now I have two rivets in and it's noticeable one on one side of the one but the opposite side of the other . In the one I just did the Corby's were super snug in the holes and the threads lined up so it's not like it's drilling crooked or anything. Can drill press speed possibly have any effect on this in any way? The only reason I ask is I have it set pretty slow for drilling steel and never really changed it for doing scales. Also two different materials between the knives so I eliminated that variable. One was wood the other is paper micarta.

 

[HallHandmade]

Is it only on the side you start from? If so, I might recommend spot drilling (basically a chamfered starter hole) before you drill to size. Sometimes drill bits can go slightly over for the first section of the hole, and usually will start to tighten up as you go down. Spot or pre-drilling can sometimes help this. Center punching will start the drill on center, but once you start the cut, there's nothing but the cutting edge angle and taper of the hole bottom holding it on center, at least until it stabilizes and gets in a bit deeper. Using split point drills can help sometimes, but spot drilling is the ultimate.

In machine shops, some non-critical work is done using just a center punch and drill press, where slightly more critical work is done by doing a direct plunge with a short length bit on a mill. The most critical work is generally done by spot drilling first, then drilling.

 

[SBuzek]

Dimensional change in your handle material, if you drill the holes and then let the material sit for a couple of months the inside of the hole will dry faster than the rest of the piece. Sometimes the hole gets bigger, sometimes smaller. Happens a lot more in non stabilized woods and in your method of storing them.

Stan

 

[Fletch Helical]

It's not just the side I start it on. The scales on the one I just did were both drilled from the same side and the one pin has the problem by the exit while the other is on the entry. Stan, would your thoughts apply to stabilized wood and something like micarta too?

 

[SBuzek]

Stabilized wood should not be as bad but will move alittle,not sure on micarta.

Stan

 

[cainethirdmortal]

I'm by no mean a professional but would have to agree with vintagefan. I would think it to be bit drift or the table on your drill press isn't exactly perpendicular to your drill bit. Either way the holes in the scales aren't perfectly parallel. This could be the same for the tang. When you tighten the corby bolts they will become a strait rod in a crooked hole and deform the edges of the hole. You can use a center bit for the bit drift problem. I've came across this with my lathe a few times before I invested in center bits. You can get a set fairly cheep at HF.

 

[HallHandmade]

There is one other possibility I thought of... I used to have this problem with plexiglass. If the drill you are using has a rather thick web (the thinnest point between where the spirals are ground into the drill blank), it can sometimes cause chip evacuation issues. The chips will bunch up in the web, and then erode the hole walls through friction until the hole enlarges enough for them to break free and move upward, or until the drill is withdrawn.

The easiest way to get around this is to withdraw the drill bit full several times during drilling, to make sure that the chips are able to fully clear before you drill further. If any of the chips are sticking in the web and you have to pick them out, that is a good sign that this may be happening. This can often happen with stabilized woods, as they can tend to behave more like a polymer than wood from time to time. I would go no more than the diameter of the hole between withdrawls.