Cross drilling problem

[retailer]

Hello all, I need two cross drilled holes in a 0.5'' diam bar for my current project. I've done a search in this forum and also using google, but can't find an answer, there is plenty about cross drilling but not about drilling into an already centre punched mark.
The first hole has been cross drilled in the correct position, the project partly assembled and the second hole position marked with a centre punch, how do I now locate the centre punch mark at exactly top dead centre so I can cross drill the next hole.
I have the following gear:
Lathe, mill, rotary table, v blocks, edge finder and the usual array of DTI's. I also have a small good quality 3 jaw chuck mounted on a 3M holder, this fits into the 3M centre hole of my rotary table

I have tried lining it up by eye and magnifying glass. With the rotary table and small chuck mounted horizontally on the mill table, I have a short length of 0.25" rod turned to a fine point held in the drill chuck and by finely adjusting the x /y feeds and also turning the rotay table I think I can get it spot on. A trained tool maker or machinist could probably do a very good job this way but I'm only an office worker with no formal training and very little experience and think that there must be a better way than simply by eye.

 

[Bastelmike]

Hello,

not sure whether I understand the situation right.

You need two cross holes in one round bar, and the axes of the holes need to be perfectly parallel?

I guess You will have to use the mill. Maybe the punch mark for the second hole was a mistake. You can remove it with an endmill that cuts in the centre, endmill with capability to drill.

If Your problem is to get the bar clamped with the first hole really vertical, put a tight fitting straight round bar in it. Then You can check it with a machinist level.
Warning: this only works if Your mill table is LEVEL.

If I misunderstood Your problem, maybe You can add a small sketch?

Mike

 

[Jasonb]

I assume you are having problems making sure the punch mark is exactly at the top of the bar so the hole is not off to one side slightly.

I would clamp the bar randomly in the mill vice and then using an edge finder on the fixed and moveable jaws set the mill central to the bar, lock that axis.

Next put your pointed rod in the chuck and with the vice jaws fractionally loosened rotate the bar until the punch mark lines up with the pointer and tighten the vice. Check again with your pointer or preferably a centre finder and adjust as needed.

Drill hole

J

 

[lensman57]

Hi,

I am not clear as wether the holes are prallel or perpendicular to each other?

In any case you do not need a punch mark, grab the part between two indentical v blocks in the vise ( using the mill of course ) and using an edge finder find the edge of the bar using the inside edge of the blocks if you preffer. Once the edge has been found knowing the diameter of the rod you can locate any point on the bar using the X and Y travel. Use a center drill first and then drill to size using peck drilling. This is how precise holes are located and drilled, a center punch is never accurate enough unless you are working to a tolerance of 0.003" or worse.

Regards,

A.G

 

[Jasonb]

I suspect the punch mark was to locate where the hole should be placed by spotting from the assembled parts.

J

 

[retailer]

Hello all and thank you for the prompt replies.

The holes are about 45deg apart radially, I didn't know the exact position of the second hole until the first (smaller) hole had been drilled and I could do a trial assembly and then mark the position of the second hole. Now that it is marked with a centre punch mark I need to set it up to cross drill with the centre punch mark set at exactly top dead centre, I din't plan to use the punch mark as a hole starter, like lensman57 says I was planning on using a centre drill, but I do need to be sure that the punch mark is exactly at the top, if it isn't then even though the hole may be exactly cross drilled the radial displacement between the holes will wrong.

I've attached a photo of the first one I made, it didn't turn out as I wanted and I could see as soon as I started to drill it was in the wrong position, I rotated the part a few degrees and as I hadn't drilled too deep I was able to restart the hole with a larger centre drill, this time it looked ok so I rotated the part 90deg and redrilled from the other side, it turned out right except that now the hole is oval where I had originally started it, I am going to remake the part again but don't want to make the same mistake. Hope the photo makes it clear what needs to be done.

 

[kvom]

If you can and want to redo the part, I would use the rotab and chuck mounted vertically on the mill. Use an edge finder to center the bar, then use a small endmill to make a flat for the first hole, then spot drill and drill. Then just rotate the bar 45 degrees and repeat for the second hole. No punch mark needed.

Otherwise Jason's method is the way to go (use the v-block in the vise to ensure it's chucked horizontally). I'd question how accurately the punch mark is anyway, but once you've positioned the spindle I'd still use a spot drill to start the hole.

 

[MachineTom]

Based on the photo both holes are in the lenght of the bar, no hole in the end of the bar, Yes? Depending on how accurate your 45° spacing needs to be. Put a piece of AL in the vise and mill it parallel, it should be about 1/2" thick, this to support the work when drilling.using the edge finder, between the jaws will give you dead center, using a .200 D edge finder, zero dial at the fixed jaw then read again at the movable jaw, your dial should be adding up to .300, nowdivide by 2, = back off .150 and the spindle ts dead center over the work D. Even though you know the D of the part, by touching off both jaws, the variance of the edgefinder x your tecnique will be offset with this process. even though you know its .500 D, the dials may show .299 or 301, just half those numbers and it will workout. Will mounted for the first hole use some parallels scribe the end of the part, then use whatever you have angle block, protractor etc to position it 45° for the second hole

 

[retailer]

Hello all and once again thanks for the replies, I've decided to redesign the project a bit. The small hole in the part is for a spring and two ball bearings to act as a detent, rotation is limited to 90deg, at each limit of the roatation the larger hole is meant to align accurately with a hole in the main body, it needs to be spot on. I hadn't decided on the final design details until I'd mostly machined the main body and stumbled on a way to limit the rotation of the part to 90deg, it's all very well to design something but at the end of the day it has to match my skill level to be feasible. The redesign will effectively combine 3 parts into one and be less of problem for those with insufficient experience to machine, it's how it should have been done to start with. I'll post a photo of the finished assembly once I'm done in a day or two