Shortening HSS drills

I have been wondering how much of a jobber drill is HSS. I am wondering if I could shorten some of my old drills to be like spotting drills or make countersinks.

Yes you can. The HSS usually goes all the way to the end of the flutes. Be aware though that the web in the middle of the flutes gets thicker towards the shank end of the drill so you may have to thin the web at the tip on the corner of the grinding wheel. Not needed on countersinks where there is already a hole in the job in that area of course.

Edited By Hopper on 14/05/2021 10:28:19

If you want to shorten the fluted section then grind it back and re-sharpen it shorter.

For the shank you only need to look at a drill that has slipped to see that the shank is soft. Try a hacksaw on one, if that doesn't work then grind a score around the shank, hold the part you want to keep in the vice and tap the piece you don't want and it should break off where the score is.

Martin C

Edited By Martin Connelly on 14/05/2021 10:30:59

There shouldn't be a problem shortening drills;if they are carbon steel drills they would make nice stub drills which will have more rigidity, HSS will make decent spotting drills but you wouldn't need many, 2,3,,4 6 mm would suffice for most needs. One thing to ensure is that you maintain the correct tip angle 118* & equal length cutting lips otherwise you will drill oversize holes.

George.

Generally the web will need thinning as Hopper says. The tendency to drill oversize may be reduced, as I think it's caused by uneven pressure on the longer of 2 unequal cutting lips flexing the drlll elastically, and the shorter drill will be stiffer - nevertheless still get the lips as even as possible.

Shortening jobber drills is ok for the odd job but they still flex as the shank is soft where the true stub drill is all hard

Could you please provide an explanation of how stiffness (which is a function of Young's modulus and geometry) correlates to hardness. Thanks.

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Not a direct answer, but : You may find this of interest: **LINK**

https://core.ac.uk/download/pdf/96764953.pdf

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Keywords: Mechanical properties, High strength steel, Elevated temperatures, Post-fire residual strength

MichaelG.

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Edit: Young’s Modulus is only really valid for isotropic materials ... 

Edited By Michael Gilligan on 15/05/2021 08:50:19

Possibly because the soft shank is drill rod, aka silver steel, or the like whereas the hardened fluted part is HSS.

Could you please provide an explanation of how stiffness (which is a function of Young's modulus and geometry) correlates to hardness. Thanks.

Lifted from the net - a bit vague but copvers the basics

The hardness (H) of a material tends to increase with an increase in the elastic modulus (E), yield strength (σ_y) and the strain-hardening exponent (n). Several materials with different combinations of elastic and plastic properties can exhibit identical true hardness (for a particular indenter geometry/apex angle). In general, combinations of materials that exhibit relatively low elastic modulus and high yield strength or strain-hardening exponents and those that exhibit relatively high elastic modulus and low yield strength or strain-hardening exponents exhibit similar hardness properties. Depending on the strain-hardening characteristics of the indented material, (i.e. n = 0 or ), the ratio H/σ_y ranges, respectively, from 2.2 to 2.6 or 2 to 20 (for indentations with a cone angle of 70.3°. The materials that have lower σ_y/E and higher n exhibit higher H/σ_y ratios. The commonly invoked relationship between hardness and the yield strength, i.e. H ≈ 3σ_y, is not generally valid or applicable for all power-law hardening materials. The indentation hardness of a power law hardening material can be taken as following the relationship H ≈ (2.1–2.8)σ_r where σ_r is the representative stress based on Tabor’s representative strain for a wide range of materials.

Seems like a overly complicated discussion.

Yes, drills can be shortened. Yes, they will be stiffer (less deflection) than a longer drill ( how many use extra long drills for ‘normal’ duty?). No, it won’t make a jot of difference re hole size for simple spotting use. Drilling is an axial cut, so deflection (stiffness) only becomes important if the drill is forced in and will likely then a) cut an oversized bell-mouthed hole and b) not drill straight. Very small diameter drills are more susceptible to bending.

Yes, you can shorten them and use as countersinks, but why do you think countersinks are manufactured and sold as a distinct product? There are good reasons why drills are not always good for countersinking.

Short or shortened jobber drills are just that - short jobber drills.

I have quite a lot, collected because my mill is head-space restricted. It’s why I have them and how I use them - for drilling holes.

Spotting drills are specially produced for spotting - stiff and drills at the point. A specialist item retained for a specific use. Most ordinary twist drills require a spot to locate it (to prevent skidding on the surface).

Short cuts are not always the best way to go - unless just to get the odd job done quickly or without the best tools.

I admit to being surprised by this - how would the manufacturer vary the composition of the steel as between the soft shank and the hard flutes?

DC31k has it right, the Young's modulus of HSS (M2 grade) is 200 GN/m^2 HSS whereas carbon steel is 207 GN/m^2 (Data and Formulae for Students, Anderson, Hum, Neal, Whitelaw). It is a common misconception that increase strength or hardness necessarily give increased stiffness. If you harden silver steel it also gets stronger, but its Young's Modulus doesn't change at all as that is largely down to chemistry

Hijacking my own thread, the last time I dealt with Young's modulus etc was 1982 at uni.

I am thinking of making a QCTP and holders. I understand hardening would reduce the wear and I could live with a little wear, but would unhardened be strong/rigid enough?

Unhardened steel will be plenty good enough for your home workshop tool post, I thought we had established that 'hardened' steel is no more rigid than 'unhardened' steel?

Tony

My rear toolpost was made from a length of ground steel of unknown provenance, but was not particularly difficult to machine, so probably Mild Steel. (It consists of a number of laminations, each machined differently from the others )

It has survived well over a decade without showing any obvious signs of wear.

A toolpost is not normally subjected to high rubbing speeds whilst loaded. If heavily loaded whilst static, wear is unlikely, although there may be some deformation if the loads were great enough.

If the loads imposed on a toolpost were so great as to produce permanent deformation, the rest of the machine would suffer also.

Given the likely load carrying areas in a toolpost, you can probably survive without resorting to anything too exotic in material or heat treatment, especially in a hobby, rather than an industrial, environment.

Howard

Well, they might induction-weld the HSS active portion onto a CS shank prior to finishing flutes, point, shank and cylindrical lands.

I don't know, but my guess is this would be more expensive than making the whole thing out of a single HSS rod, finish machining and then heat-treating to harden only the fluted part - so I think that's what they'd do..

Tend to be from HSS Rod for the full length, hardened (not the shank) quite early on in the process then ground, larger diameters may be part formed before hardening and finishing.

Drifting a little off topic, railway waggon buffer heads were flash butt welded to their shanks. Bring them into contact, pass a large current to get them very hot and then squeeze. You might use f

Riction welding nowadays.

If you want to make stub drills, you could do worse than make straight flute drills from silver steel.

They have several advantages, but don't drill deep holes.

Another easy style to grind is a triangular cross-section drill with a pyramidal tip, and a notch in each edge of the tip. Is supposed to work well on hard materials.

Neil