3mm drill hole too tight for 3mm bar

Hi Guys,

I thought I would try to make a small precision graver for making very fine undercuts on my Sherline lathe. So I cafeully set up my 4 jaw chuck for the first time and drilled a 1/4 inch mild steel bar with a 3mm bit after using a centre drill.

i am sure to many of you more experienced it would come as no surprise that the 3mm tungsten carbide bar did not fit in its hole (too tight).

The idea was to put a flat on the bar and use a small grub screw. This is to make a tiny very fine knife like cutter.

Now obviously one way is to sacrfice the drill bit and use this to make the graver but is there another method such as using slightly oversized drill bits or emery papering the tungsten carbide??

Chris

Well red faced having taken the square stock out of the lathe yes the bar fitted the hole. Obviously I was not holding the bar dead straight whilst trying it on the lathe. However rather than just editing out the post the question still is valid in terms of can you expect a 3mm hole and bar to fit and what tolerances must you have? for an application like this a good sliding fit is required as I may well have to replace the fragile engraver as it inevitably breaks during use.

Chris

Chris

Edited By Chris TickTock on 31/12/2019 15:47:00

You could try heating the 1/4" bar to expand it and then let it shrink onto the carbide.

Robert G8RPI.

Chris,

There are drill sets that increase in increments of 0.1 mm diameter that with care could open the female hole up to just snugly fit the cutter tip.

Regards

Brian

1 redrill leaning on the drill or try another make of drill (they are not all the same)

if fails measure accurately carbide bar

then -

2 use a 3.1mm (or larger drill to suit carbide tool)

put a flat on the bar (as in grind with carbide (green grit) wheel I would suggest)

You wont get very far using emery paper on carbide.

Thanks Guys, I think the drill increments are worth noting. how many thou becomes too tight a press fit for small objects?

Chris

Look up shrink fitting tolerances. That will provide some useful information, particularly if there are press-fit tolerances, too. If too slack, use loctite!

A drilled hole usually comes out over size rather than undersize, what make of drill bits are you using?

What does the carbide measure, you won't be the first to have bought 1/8" stock sold as 3mm

One option is to grind the end of your rod at an oblique angle and then use that as a home made reamer to open the hole up to the required size. Then loctite the carbide in place, you won't get much of a thread in the 1.6mm wall left in your 1/4" bar for a grub screw.

Try this link for table BS shaft tolerances - scroll to bottom to get descriptive chose the one you want and look up on clearance/diameter tables

**LINK**

You can sharpen one drill flute manually with a stone to give the tip slightly asymmetric geometry, this will then drill a slightly larger hole than a symmetrical grind.

Q. Can you expect a drilled hole to fit? A. Not very well.

Twist drills tend to produce slightly oversize holes, which conveniently is usually what's wanted for a sliding fit, perhaps fixed with Loctite. How much too big a twist drill cuts depends on several factors: how sharp it is, how closely the edges are equally ground, depth of hole, type of drill, the material and how much pressure the operator applies. All rather variable and out-of-control.

If a hole must be exactly to size, it's drilled slightly small and then opened out with a reamer or bored. It's reaming / boring that provides tight control of hole sizes, not twist drills.

There are various classes of fit, and I recall Machinery's Handbook taking 20 or 30 pages describing them all. That's for repeatable manufacturing. For ordinary workshop fitting purposes, by varying hole/shaft sizes slightly it's easy enough to arrange a:

• Loose sliding fit. (Shaft moves easily in hole)
• Tight sliding fit (Shaft resists moving in hole)
• Push fit. (Firm Pressure or hammer taps)
• Press fit. (Hammering or Hydraulic Press needed.)
• Shrink fit. (Heat expands the hole, shaft is hammered in, then hole shrinks to grip shaft.)

Perhaps a clockmaker will comment please; my experience is with larger objects than clock parts and I wouldn't want to confuse Chris!

Dave

Like that idea Jason (reaming) Take your point on thin wall but might just try using a 2mm grub screw if they are available and your bang on the walls are 1.6mm and min thickness is usually at least 1 times the dia but its light use so possibly 2mm set screw.

Chris

Edited By Chris TickTock on 31/12/2019 16:52:55

Jason's suggestion is the 'tried and true' method and for 'how-to' words and music look up D bits.

To stay 'in-house', try

http://www.stubmandrel.co.uk/workshop/29-d-bits

rgds

Bill

I was not actually thinking of a D bit, more like this where you use it like a reamer, the top one is a bit too steep an angle

Edited By JasonB on 31/12/2019 17:18:42

You could hold the carbide tip in a chuck or collet and use a diamond file or lap to reduce the diameter slightly. I would go for a sliding fit and use Loctite or epoxy glue to secure it. A small flat would stop a hydraulic lock while assembling.

Just to complete the picture. As Jason mentioned drills are most notorious for drilling oversize especially when they are not sharpened symmetrically which always happens when done manually by those 'I don't need no stoopid drill sharpening jig' experts.
However also an old drill will get worn on the cutting tip but also on the sides at the end leaving it a bit undersize for the first few mm which might have an effect on short holes, and also might lead to jamming/breakage.

Wood of course is another matter. There is a post on here somewhere from the late John Stephenson talking about how when he worked for a piano manufacturer he had to spend hours selecting drills to give the precise end result required, not even getting the same result from two drills of the 'same' size.

Not quite always Bazyle, there are a very few of us who were actually taught how to do this at the start of our apprenticeships. Having spent more than a solid week (45 hour week then) grinding drills from about inch and a half down to 1/16" one does sort of get a bit of a feel for it.

Happily it is a bit like playing a musical instrument that once acquired the skill never leaves one, and sixty-seven years later I still have it. I can still remember the feeling when the instructor put a fresh-ground inch drill into a lathe and drilled a hole with it and to see the two identical spirals of swarf coming off the tip. We had made drill angle gauges from gauge plate and calibrated them with a vernier height gauge - I still have mine.

I am set to wonder about some (most) of the so called apprenticeships today, and if such instruction and knowledge is passed on. Yes, I know that there are machines to do this particular task, but these are not always available at the time and place that a drill needs sorting. I doubt whether many are taught to file these days either, and again it is an extremely useful skill on occasion when other resources are not to hand. Perhaps there are not too many instructors any more???

So dear Baz I would prefer it if you did not call me 'stoopid' - - not too often anyway .

Compliments of the season to you.

Mike

Edited By Watford on 31/12/2019 21:34:50