Strength Question

Making own Carbide Tipped woodturning chisels. I have a box of Square Tungstan Carbide square tips (german made ones) 14x14x2mm and these need to be fitted with a countersunk M6 machine screw. (or at least the hole in the centre is 6.5mm and ones fitted to the planer/thicknesser are fitted with M6

I have plenty 10mm square bright steel bar. is that just too thin to take an M6 ?

No problem in tapping m6 in a 10mm bar but i would try 12mm square at least or even14mm reduced for the last inch..to avoid bending on long overhangs

What is the a/f (across flats) dimension of a standard M6 nut?

10mm

that is a very good point, well made.,

is bright mild steel as strong as whatever nuts is made out of? and is home tapping just as strong as however commercial nuts is ? (i'm think rolled threads are stronger than cut)

Hi Justin, I've tapped hundreds of holes of all sizes into mild steel in my lifetime at work and at home, which probably have held much more forces than your tips are likely to take, and to my knowledge none of the threads failed.

Regards Nick.

Think the O/P is questioning the CSA of the material left after bunging a 6mm thread through it - in cross section the two bits either side. Simple numbers, 10 - 6 = 4 divided by 2 = 2

Regards Ian.

Hi, well if the O/P has any doubts, just get a wider piece of steel.

Regards Nick.

Edited By Nicholas Farr on 21/04/2023 09:55:53

.

A point adequately addressed by the reference to M6 nuts

MichaelG.

A point adequately addressed by the reference to M6 nuts

Different shear plane.

Regrds Ian.

I think that something else to consider will these be used for shallow bowls or deep hollowing. This will also effect the cross section size of the bar used if deep hollowing then go a little larger which will then allow the size of hole you will need.

Personally I would use round bar which will allow you to turn the tip without the corners of a square bar from digging into the tool rest if you should get s catch

Edited By Dalboy on 21/04/2023 10:31:04

Thanks for all the replies

Have decided to go with an M5, seems to be a nice compromise.

Out of luck I managed to get another look at the carbide tipped tool that I was so impressed with for roughing out wet timber (recently felled). This was fitted with the exact same 14mm square tungstan carbide bit with the same 6.4mm centre hole that I am using - it was held in place with an M4 machine screw, the max dia of the head was 7.1mm. Clearly this is enough as it is a well used tool (it was a bought tool £40 ish.) scew head just seems a bit on the small side with only a 0.35mm lip holding it all together!

Anyways, made the handle tonight out of a bit of oak I salvaged out of a skip (old bed frame)

Hopefully get my 10mm sq bar milled drilled and tapped sometime this weekend - I'm needing a lesson for that, should be interesting.  I'm very new to turning metal and now have full access to what I believe is a decent lathe with attached mill.

Edited By Justin Thyme on 22/04/2023 00:05:00

There may be more to strength in this application than the screw.

My example is a lathe tool where a carbide insert is held in place by a small screw, M2.5 I think. It's the biggest insert I use on my 1.5kW lathe, and it can take 2mm cuts out of mild steel:

Point is the screw doesn't take much strain because the insert and holder are shaped to take most of the forces:

The geometry and fit provides most of the strength, not the screw. The insert is firmly supported underneath against downward cutting forces and against rotational forces by the flanks. In neither case does much force go through the little screw, which would likely shear if it alone had to support the insert. The screw does little more than hold the insert in the correct position: the holder takes the force.

A wood-working chisel is likely made on the same principle, that is with the insert accurately fitted into a supportive recess. The strength comes from accurate fitting against properly designed shoulders, arranged so the screw isn't stressed much. Have a good look at a commercial chisel and copy how they support the insert.

Roughly, an M2.5 bolt will break under a load of about 200kg, M6 about 1300kg. But those are static loads when a chisel experiences dynamic shock loads when it's thumped with a mallet. I don't know how high the dynamic loads peak, but it's not that difficult to knock the exposed head of an M6 bolt off with a cold-chisel and hammer.

Dave

Dave - I think he's talking about a wood turning chisel, unlikely to get thumped by a mallet (hopefully!)

Rob

I do own a set of carbide woodturning chisels which sit in the cupboard doing nothing( given to me) they have a 9mm square shaft and the screw from a very quick measurement is about 3mm. I think looking at them that the square one pictured below is not the correct tip fitted as the hole is way too big for the screw and it is only held in place by the countersunk part.

As you can see there is as much metal either side of the screw hole so I would not want to put a 6mm screw into a 10mm square shaft

I did try them when I first was given them but just can't get on with them. These only have a short shaft I would not use them in a deep bowl or deep hollow form as I could imagine that they would not take the forces put on them.

This one which is a small tipped one is a much better quality tool the bit is about 6mm diameter

I have only used these carbide ones recently, and for roughing out wet unseasoned wood they were wonderful, certainly quicker than traditional gouges

Anyway the main reason I am making them is we have a new milling machine, and I want to use that to make something!

So getting back to the topic of what size screw I should use in a 10mm Sq bar. I agree M6 seems too big, which leaves me two choices, either M5 that is only going to leave 2.5mm steel either side of screw but the countersunk is an excellent fit in the carbide bit. OR, , or go for M4, but that will leave the head of the screw only marginally bigger than the hole in the carbide bit.

What would be best compromise?

Im measuring the one in the second picture above as 3.5mm (if bar is 9mm)

It might be possible to use a 4 or 5mm high tensile screw with a special washer to centralise the insert. A shoulder for the insert about 2mm deep would keep it from moving and the hole for the thread can benifit from being offset about 0.1mm to hold the insert tight. The forces are mostly down into the seating, so the screw is not stressed too much.