Drilling jig

[Dave ShedmanParticipant @daveshedman]

I am looking to make a simple drilling jig to allow me to accurately locate some 9/16 holes in steel (Hot rolled plate).

My question is can I use the jig to plunge the 9/16 drill straight into the component or should I create a pilot hole in approximately the right location first to help the drill as I am thinking that a twist drill only cuts on the flanks and not at the tip?

Any advice much appreciated.

Thanks Dave

[Craig BrownParticipant @craigbrown60096]

That will largely depend on what drilling machine you have at your disposal, but generally speaking the machines usually found in the home workshop are not going to have a particularly pleasant time ploughing a 9/16 hole straight through steel. The effort required is reduced if a split point drill is used however its still a decent sized cut. Your best bet may be to make your jig with say 5/16 holes, use that to drill the plate then remove the jig to open the holes out to 9/16. I suppose how accurate the size and location of the holes also is a consideration.

[Andrew JohnstonParticipant @andrewjohnston13878]

A twist drill does cut at the centre. For a conventional two facet ground drill the rake angle is negative so cutting forces are high and require significant torque. A 4-facet ground drill will be better. Drilling 9/16th without a pilot hole is feasible but it depends upon what type of machine is being using. It will need to have enough power, low speed torque and rigidity.

Andrew

[JasonBModerator @jasonb]

Make the jig with 9/16″ holes and then make a reducing bush so you can run say a 6mm or 1/4″ bit through first unless you have a fairly substantial drill press.

Other option would be 9/16″ jig and 9/16″ annular cutter (Rotabroach) which whould do it in one on a lighter machine.

[Dave ShedmanParticipant @daveshedman]

Many thanks for the quick replies. Your responses have confirmed my suspicion regarding the large drill plunging into virgin metal. I will make a reducing bush as suggested. For info I will be using an seig SX2.7 to make the holes.

Dave

[SillyOldDufferModerator @sillyoldduffer]

If a drill-mill is available, there’s a better alternative, more accurate though slower.

Twist-drills aren’t a good way of making accurate holes.   For various reasons they tend to start in the wrong place, wander, and cut non-circular holes.  On the plus side, they’re fast and ‘good enough’ for most purposes.

When accuracy matters, rather than speed, the answer is boring.   First step is to twist drill a hole and then bore it out.  Doesn’t matter if the drilled hole is wonky, because a boring bar spins in a perfect circle around the spindle access, creating excellent accurate straight round holes.   Boring is also adjustable to produce any diameter hole, and because it’s a single-point cutter, the stress on the machine is much reduced.  Though it will take longer, a mill will happily bore much larger holes than it can drill.

Though a jig helps improve the accuracy of a twist drill, the combination is probably inferior to positioning the work XY relative to an edge found reference with a milling table, and then boring.

The big advantage of a jig/twist drill combo is for repetition work, when it’s important to quickly do the same thing over and over again.   Is this  a repetition job?

Dave

 

[BazyleParticipant @bazyle]

Back in the day when machines were less powerful it was normal to centre pop a mark, which gave a big enough dent to accurately pick up with the 3/32 or at most 1/8 drill and so in stages so that the poor cutting centre of the drill was always in clear air. Centre drills sometimes used for starting as short and rigid (no, not spotting drills as they are newcomers to the amateur scene).

People expecting to go straight in with a >1/2″ normally have come from an industrial shop with a big radial arm drill.

[JasonBModerator @jasonb]

If it is a SX2.7 then I would go 6mm, 10mm, 9/16

[larry phelan 1Participant @larryphelan1]

I have done this type of job exactly as Jason says, by using a jig with different bushes, and it works well.   Might be a bit slow, but it works !

[HopperParticipant @hopper]

Being averse to work in general and unnecessary work in particular, I would skip the bushes and just spot through the jig with the 9/16″ drill to make a nice conical indent in the job, which can then be used to start the smaller pilot drill nice and central in the hole.

It does rather depend on just how accurate you want the hole location and size as to how exactly you might proceed. Some idea of what it is you are making and why you want to use a jig would be helpful.

Usually a jig is used only for production work of many parts that you want to come out all the same. Or at home for several parts that you want to fit together. But often you can simply clamp the two parts together and drill as one operation, or at least spot through from one part to the other instead of using a jig.

There may be better ways of doing whatever it is you are planning.

[noel shelleyParticipant @noelshelley55608]

You start by mentioning accuracy ? The work will only be as good as the jig. How accurate will the jig be ? Unless your making several items, a ruler, caliper or dividers and a dot punch to mark out each ? For 9/16″ pilot hole at 5 or 6mm and use a split point drill, common now. Like Hopper I don’t like work more than needed ! Noel.

[Dave ShedmanParticipant @daveshedman]

For info I am planning to make a simple jig to the Martin Evans method for manufacture of my coupling rods, i.e you make steel drill bushes that are first located into the axle-boxes as set-up in the loco frames (Axle boxes set to correct ride-height). These bushes have a threaded portion that protrudes allowing them to be clamped by a nut to a long flat plate with two holes. One of these holes is elongated to allow one bush to be clamped exactly the correct distance from the other. Hence when the jig is removed from the loco frames and clamped to the job, holes with the correct spacing should be drilled & reamed in the embryo coupling rods for both sides. (Lead-bronze bushes fitted later).

I was also considering just using the x DRO on the mill to lay-out the hole spacing but to derive this measurement from the loco-frames would be problematic as I would be using calipers onto the axles and am worried about introducing a small error that would lead to rods that do not move smoothly/fight each other).

Thanks for all the replies.

Dave

[Howard LewisParticipant @howardlewis46836]

If using a Milling machine to drill the holes, position the first hole, and centre drill, and then gradually increase the drill size up to 9/16″.  FWIW I would use a 35/64″ as the last drill before the 9/16″

In this way, the cutting forces are reduced, and with it, the tendency to swing off centre.

Then reposition the work, using co ordinates, to place for the next hole, and repeat the centre drill, pilot drills, finishing drill sequence for each hole, before moving to the subsequent holes.

Accuracy of hole size will depend on how accurately the drill has been ground.

If the lips are unequal lengths, no matter how the drill has been ground, you will get an oversize hole.

Properly ground four facet drills are superior to jobber drills, for accuracy, IMO

Howard

[Chris CrewParticipant @chriscrew66644]

Unless I am misunderstanding his response, I must disagree with Andrew Johnston when he states a two facet drill has negative rake angles. This is simply not so as the drill would not cut as those who have tried to sharpen a drill with a Picador type jig, unaware that they have to rotate the backstop thumb wheel whilst stroking the drill bit across the grinding wheel to impart the rake, will have discovered. The Reliance jig drill rest is off-set to its axis of rotation and rolling movement to automatically impart a rake to the cutting edge which is why it is superior to the Picador type.

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