I would make your pegs first and experiment with drills in your material to get your required fit

make your pegs out of 3mm silver steel perhaps

rather than as you said making your pegs fit the holes…make the holes fit the pegs

Hi Chris,

For jobs like this, I use stub length split point drills from cncpoorboy. They are very good quality. Being stub length, they don't flex much and eliminate the need for spotting.

25mm is quite deep for a 3mm drill. Do you need to go all the way through or will a blind hole do?

Steve

If you are only needing a few thou accuracy then if blind I would go straight at then with a stub length split point drill, something like Dormer A022 series. If they are though holes then you will need a jobber length drill which is a bit more flexible so I would spot first, if you don't have a DRO then spot & drill at each position before moving to the next.

Lubricate drill with paraffin or WD 40 and withdraw often to clear swarf particularly if through holes.

Put some drag onto your rotary table as the large PCD of your peg holes combined with a small table will accentuate any backlash and have far more bearing on accuracy of position than what you are actually drilling with.

Edited By JasonB on 07/07/2020 13:11:37

Chris

I recently bought several different tapping size split point drills and found they cut oversize in all materials, especially aluminium, so best check your drills for drilled hole size in a piece of scrap before letting an unknown drill spoil your work.

I will add that the store agreed to a full refund including postage when I return the drills.

Emgee

Don't reverse with your rotary table, just complete one circle and then go round again so you are always feeding in the same direction which will help to reduce backlash.

Yes Keep the WD40 away from the actual clocks as it can go sticky over time but no problem using it as a cutting fluid on aluminium

DRO = Digital Read Out. This gives exact position of the mill's table so you can be just about certain to return to the same spot which would allow you to do several hole at one angular setting and then get the ones on the next angular setting spaced out on the same radii. As you don't there will be some play (backlash) in the handwheels and leadscrews then do all at one radius winding the rotary table in a positive direction spotting and then drilling each one then move out to the next radius and go round again.

Edited By JasonB on 07/07/2020 16:25:28

Hi Chris, the reason I ask is if you have the CNC table there is no problem with repeatability when indexing so you can go round and spot, change tools, go round and drill and then again to ream for each circle of holes.

Steve

Chris

As others have said only rotate the table in 1 direction, you don't then have to worry about backlash. Same with the cnc table, only rotate in 1 direction.

If when using your manual rotary table you go past where you you should have stopped, rotate backwards way past the desired stop point and then rotate slowly to the desired position, that way backlash is always taken up in the same direction.

If your table is very free to move and you have any backlash always apply at least 1 table lock before drilling. (edit)

Emgee

Edited By Emgee on 08/07/2020 18:22:12

The 'single point' terminology has me confused, but twist drills are a confusing subject. I believe all twist drills are single point drills, as opposed to augers, saw toothed hole drills, and other configurations. Could be wrong.

Twist drills wander for several reasons, many of which are down to the operator. Allowing the flutes to choke with swarf, cutting too hard with a sharp drill, forcing a blunt drill, rpm too slow, or a poorly shaped drill can all cause trouble. Deep holes, more than 4 to 6x diameter, are more likely to wander than shallow ones. Hand drills always wander in my experience, pillar drills much less, and mills better again. A lathe is best of all because the work is rotating, hence prefer a lathe when straightness is important.

Drills tips are shaped to optimise performance for particular purposes. Rounded tips are preferred when breaking through sheet metal. Square tips for flat bottomed holes. Angles to suit Brass, fast metal removal, or whatever. As the flutes can also be optimised for different purposes there's a bewildering variety of drills to choose from.

As life is too short, I use general purpose 118° drills, sometimes started with a centre punch, more often with a centre drill. I'm happy to believe spot drills are better, just not tried them yet. What centre and spot drills have in common is being short and stiff, just what's needed to get a general purpose drill going in the right direction. General purpose drills are cheap to make, easy to resharpen, OK with hard and soft materials, fairly stiff, with decently wide flutes for ejecting swarf, and although inclined to grab and tear on exit, they aren't vicious. A good compromise drill, but in some circumstances it's worth sending for a specialist.

I think Chris is describing the type of drill where the chisel end is ground in either a sort of S shape or 'split point' to extend the drill's cutting edge closer to the centre, which improves the drill's self centring properties. They're more expensive and wear faster than general purpose drills.

135°drills are preferred for harder materials.

Most of time I find all that's needed is a decent accurately placed hole sufficient to allow an ordinary 118° drill to make a good start. Once going correctly it will do the job. This is provided I frequently clear swarf, lubricate, and take care to cut at a sensible speed and pressure. Overdoing it always causes a wander.

The 'decent accurately placed hole' can be made with a centre punch or a centre drill. I do most of my drilling with a mill and prefer a centre drill accurately aimed with a DRO. However, if the hole is non-critical, I often don't bother starting it – I just go straight at it with the drill. I think it works because a milling machine puts the drill accurately at a right angle to the firmly clamped work and is able to make its own all important first start. Curiously drilling in a lathe always seems to need a starting hole because the drill can be seen skidding about on its chisel before. engaging. I may be doing it wrong…

Dave

Edited By SillyOldDuffer on 10/07/2020 15:30:56