Grade 8.8 "High Tensile" Bolts - advice on drilling through.

[Eric The Viking]

I blame Wuffles!*

I'm trying to make a new removable shaft+handwheel for the tensioner at the top of my bandsaw.

It's a sloppy 12mm i.d., socket with a notch in the ring surrounding it. It's supposed to take a 12mm shaft with two diametrically opposite keys (I'm guessing this). Presently there is a T-shaped bit of flat bar that goes into the central hole with a wooden handwheel glued on, but it's really uncomfortable to use. It was made by the last owner, and he did it well, but my hands are bad and I need something easier to turn.

My idea is to use a 12mm bolt, threaded to M5 diametrically, with a bit of studding glued or peened into place to make the two keys. An M12 bolt fits one of Axminster's replacement handwheels nicely, and the combination would do the job really well, I think.

I have a leftover box of BZP 12mm Grade 8.8 high-tensile bolts of suitable length. I found a chart on the web that says this:

Class 8.8:
Proof strength: 660 (MPa),
Yield strength: 640,
Tensile strength: 800
Medium carbon steel; quenched and tempered

Can I drill it and tap it fairly easily? I'm sure it will cut with a hacksaw, and can be filed without problems as I've done this before with bolts from the same box. I only have HSS taps and drills but I also have cutting/tapping compound and know how to hand-tap steel (or worst-case do it in the pillar drill with the belts removed).

Should I try to anneal it beforehand? Worth the effort?

It's a very low strength application, so if I do soften the steel it probably won't matter.

Thoughts appreciated.

E.

*I should explain - he's had problems with his BS tensioner, and talking to him the other day reminded me I should get my finger out to sort out my own one!

 

[AndyT]

This video popped up the other day in Google's suggestions of things they thought I'd like to watch on YouTube - maybe they knew you were going to ask!

https://youtu.be/dTGB9Od0ND0

It does seem to answer the question on how to drill. Tapping might be another matter.

 

[Rorschach]

If it is metric and marked 8.8 it is actually only grade 5 equivalent so quite soft really.

 

[Rorschach]

If you are worried about tapping drill slightly oversize, 50-75% thread depth will be more than enough for your application I am sure.

 

[Eric The Viking]

Thanks guys. The actual drilling + tapping I'm OK with, as long as the stuff is mild enough to do it with - seems like it is, from what you say.

Now need to go dig out my Vee blocks...

E.

 

[oakmitre]

Which end of the bolt are you tapping ? Is it the head or the tail, or are you cutting the head off ?

Does threaded diametrically mean a concentric thread to the existing thread. I have done this on an M10 bolt of the same spec with an M3 tapped hole.

The main problem I had was finding the centre. Also my pillar drill is small. To align I did the following.

Assuming the chuck of your pillar drill is 13mm, then the M12 bolt should chuck up. I can then be lowered into an unsecured pillar drill vise which is moved into position and gently tightened up as not to flex the bolt. once it is snug, it can be tightened up securely and the vise clamped ( or bolted in my case ) to the table. Release the bolt from the chuck, fit a small centre drill and tickle the bolt to get a punch mark.

It helps at this point if you have thought about the various lengths of drills involved to avoid having to rotate the table to fit one ! (stub drills are helpful) I drilled it out by gently pecking, I found that my drill table flexes surprisingly easily.

The pillar drill is then unplugged and a tap chucked up, establish the first few threads on the pillar drill by turning by hand - no need to take the belts off.Just do half a turn at a time.

 

[Eric The Viking]

I'm just drilling and tapping across a diameter, to make a couple of pegs stick out of the sides.

Sorry if I was confusing: I should take some pics really.

But that is a good process to get a centred hole without a lathe - bookmarked for future reference!

Many thanks,

E.

 

[oakmitre]

Sorry Eric, now I think about it diametrically does make sense.

Here is how I have done it myself in the past.You probably know this already - but just in case.

It might cut down on chewing the threads or having to file a flat spot, it also allows for fine adjustment along the length of the bolt.

1" thick piece of MDF cut 2" wide and 8" long with straight sides. Mark all the way round the MDF strip as if you were going to hand saw it in half.

Put the MDF into a pillar drill vise with the thin end pointing upwards and make sure the 2" flat surface is parallel to the drill bit.

Find a drill bit a bit smaller than 12mm, anything will do, an odd size tap drill or imperial bit and drill straight through the mdf with the drill point on the line.

You can then drive the bolt into the MDF, clamp the MDF flat on the pillar drill table. As long as you peck drill on the marked out line you should hit the threaded rod section dead on and you can do very fine adjustment by turning the bolt in the hole and measuring from the edge. A piece of thin steel glued or screwed to the top of the MDF stops sideways movement of the drill bit, sort of a poor mans bushing.

 

[Eric The Viking]

Thanks - another great idea for the armoury!

It's a bolt though, and happily I need to drill through the unthreaded bit. I spent a weird hour or so yesterday, looking at youtube videos on how to use vee blocks, and I've found my set on a (dusty) shelf, together with a cheap and nasty machine vice.

So I'll have a go that way, as I've got a pack of decent 1.5mm drills to get a good pilot hole (and the Rohm chuck is now working sweetly - see other thread). Once I've got the pilot hole taken out to tapping diameter, the tapping itself will be slow but straightforward.

And I have a box of bolts, in case I'm really hamfisted.

Cheers,

E.

 

[Inspector]

I wouldn't bother tapping the hole, I would use a roll pin, the ones with the slot down the length.

Pete

 

[Eric The Viking]

Good thought, but...

... just been experimenting. Gone as far as M6 quite successfully (although I cheated and drilled out to 5.5mm).

Trouble is that it engages with slots in what looks to be a white metal cast component on the end of the tension adjuster rod ( :roll: ). So I'm a little worried about putting a round thing in there when a flat thing would wear it a lot less. The flat plate on the handmade (wooden) knob is 6mm thick (almost), which fits the slots.

So plan B: drill/tap to M8 (nearest handy size*) then, after cutting a bit of studding or an old screw to length, file flats on that to give 6mm thickness, & wind it in with a dab of Araldite or Loctite on the threads and leave it to go off.

If it wasn't for the white metal, a roll pin or similar would've done fine I think.

E.


*I once bought some M7 bolts, as used on Rover 200-Series battery tray clamps (no, I didn't believe it either) - I'm never trying to do that again! I'll just have to file more off the M8 studding.

 

[woodpig]

I just screwed some all thread (M8 I think) into a hand crank and locked it in place with a socket set screw. So much easier to tension the blade now on my BS300 than with the little plastic wheel.

 

[DTR]

all thread

It's called studding!! [-X

Sorry, that particular Americanism really grates me, for some reason.

When I bought my Draper bandsaw (second hand), I also had to replace several fixings and knobs, levers etc. It was obviously built to a price