Thread: Pre-drilling

If I have to drill a 14.5mm hole in steel, I pre drill it with a smaller size first right? What size pre drill should I use? Is there a rule on these things?

you mean pilot drill or drill a pilot hole.
there probably is some kind of rule for it but i cant remember what it is.
depends on what you are drilling with, in a drill press probably pilot with around 5 or 6mm drill then finish with 14.5mm.
if using a hand drill start with same size but maybe go again with something about 10mm then finish with your 14.5mm.
remember, if using a hand drill the bigger the drill bit you use for pilot holes (ie the last size you use before the 14.5mm) before your last drill bit the more it is likely to bite and rip at your wrists, chip the drill bit etc etc.

if you are keen, and your drill bit is nice and sharp, and you have plenty of coolant and a drill with quill feed you could do it without a pilot hole but i guess you are using a generic type of drill press.

If you are really really keen do it in the drill press with coolant in a spray bottle, a sharp drill bit and clamp your job well, then you wouldnt have to pilot either.

Yep, pilot hole. I'm drilling it in the lathe, no coolant

ok, first up get the job running true, use your 4 jaw and clock it (dial indicator) if it absolutely must be on centre. clock it on a surface in which the bore must run true with (if you have previously machined spigot or size etc that must run true with the bore).
or, if using bare stock, if you have them use some soft jaws in your 3 jaw and bore them slightly larger than the job then clamp job into them, this will have it running true to the spindle, and if bright steel bar within reasonable tolerance to the od of the bar.
if accuracy is not super important use your 3 jaw and jump straight in.

centre drill it first.
use about 6mm pilot drill
then use your finish drill (and ream if required).

if tolerance is important use smaller drill and finish with ream to 14.5. if not important just bang in the 14.5mm after the 6mm pilot. alternative bore to size you can get your smallest boring bar into and finish it of with cutting tool.

make sure you pull the bit out and clear swarf a couple of times (depending on depth of bore) during the operation. squirting up the bore may not (actually probably wont) get to the cutting edge but it helps

if you can get some coolant and use it( id be on a lathe with coolant pump so would have strong stream shooting up the bore), squirt it up the bore as the drill is working and over the drill when out clearing swarf, the jet of the coolant will help clear swarf if drill point is exposed. if no coolant, use rocol or other goo for lubricant, the coolant in this case is for lubrication rather than cooling.

this is how I would do it anyway, for what its worth

last of all, have fun.

Thanks for that

Is 14.5mm the right tapping drill for a 5/8" UNF thread? It's worked perfectly fine when threading brass, but in steel the tap is super tight

yep, that the size, according to the sutton chart I have (i fell over 3 things in the garage getting it)
it also has this disclaimer at the top saying that it is inly suggested sizes and it may vary to individual requirements.
but in any case it will do the job, for a 5/8 unf x 18tpi thread.

i take it you will bu tapping the thread not cuttin gin the lathe?

the tap will feel tighter in steel as it is harder to cut than brass.
same deal here, plenty of goo (rocol, trefolex, oil) and take your time, 1 turn in half or 1/4 turn back to break the chip then in again, back again till you are through or bottom out.
if bottoming, start with an inter or taper tap its much easier if you have these at your disposal to start the tread off.

what is it you are making that requires such a thread?

Thanks, yep tapping not chasing.
I tried some oil (just standard oil) and it worked a treat

The component is a jig for making the timber infill used in my marking gauges, it's pretty much a steel version of the brass marking gauge.

cool, glad to be of assistance.
were you doing the job as you were asking for advice on here?

on a slightly different path, jigs are good, if you are running production of any kind for your tools you will find jigs are a major time saver, the more you can make to standardise your work the easier it will be.
at one point in my life, i used to get some jobs that were semi production in as far as we may get an order for 15 of a component, in amongst orders for several hundred of standard componenets. it was my job to make the special stuff, so i was forever either marking out individuals or making my own jigs for them, depending on if there would be a repeat order down the track.

on a more odd path, I think i may have briefly spotted you at the hare and forbes sale the other day, i was looking at the metal lathes and i am sure i saw a forum shirt with your name on it wander past me, when i was finished with the lathes i had a look around but couldnt see you anywhere.
but, there was heaps of people about so I might have just missed you if you were still there.
i didnt buy anything, have you bought from hare and forbes before?
what are they like for service and after sales backup, I have read mixed opinions here.

Yep, I had the laptop down in the workshop whilst turning

Yep, I was at the H&F sale on Saturday. I bought my lathe and a heap of other bits from them and their service has been top notch.
If you see me again, feel free to introduce yourself

When I was an apprentice we were taught that the pilot drill should be larger than the dia of the point of the big drill. The reason being that is takes a bit of pressure to make the point of the drill cut and as the drill size goes up so does the amount of pressure required. The pilot drill has a lot smaller point so is easy to run through.
bollie7

Hi guys,
Can I add my 20c worth.

I am sure what Bollie means is the flat (end) point on the drill.
This advice goes along with what I have been taught by the fitter types that I have worked with.

The idea being that the contact point on the bigger diameter drill is on tits cutting edges which centers the big drill and starts a hell of a lot easier as it is initially cutting the corner of the small hole.

I can understand that on the lathe ,after facing cut, one would use the center drill which could perhaps preclude the use of the pilot drill- if big enough, but heck! many of you will only have a smaller lathe or maybe just a drill press ,so why overload it ?

Grahame

HI,
We have had a Discussion about Pilot Drill Size on this Forum before. If You searched for it You should find it. As I remember Saying in it the Rule of Thumb is 'Your Pilot Drill should not exceed 25% of what the Finished Hole will be (Might not be word for word). So for example for a 1" (25.4mm) Hole Your Pilot Drill should not exceed 1/4" (6.35mm).
Just found this on a 3mm Sutton Drill Bit Packet - on the back it Says "HELPFUL HINTS Use the fast speed on your electric drill, withdrawing regularly to clear flutes. Generally, a lower speed on your electric drill is required on sizes over 1/4" (6.0mm). For sizes over 3/8" (9.5mm), it is advisable to drill a small pilot hole first, no greater than 25% of the larger drill and keep a steady pressure while drilling."
All The Best steran50 Stewart

When I was an apprentice we were taught that the pilot drill should be larger than the dia of the point of the big drill. The reason being that is takes a bit of pressure to make the point of the drill cut and as the drill size goes up so does the amount of pressure required. The pilot drill has a lot smaller point so is easy to run through.

Agreed, the reason is if the pilot hole is smaller than the point or land on the drill. The major drill will wander off and the benefit of the pilot will be lost.