Tapping

[Lonsdale73]

I have a 2.5mm sheet of stainless steel that I want to attach to an aluminium mitre bar that came pre-drilled with 6mm threaded holes. How do I best go about it? Can I simply drill a slightly smaller pilot hole and let the bolt cut it's own thread? O do I need to use a tap to cut a thread in the sheet? I've no idea what the pitch is of my bolts.

 

[Rorschach]

If the aluminium bar is already tapped you just need to drill a clearance hole in the stainless sheet. 6mm holes will do it or 6.5mm if you want a bit of wiggle room.

 

[Lonsdale73]

Thank you

 

[AES]

If it helps Lonsdale, EXACTLY what Rorschach said!

 

[Lonsdale73]

If the aluminium bar is already tapped you just need to drill a clearance hole in the stainless sheet. 6mm holes will do it or 6.5mm if you want a bit of wiggle room.

Reading that again, will it make a difference that I need to screw through the bar into the plate? The bar's already countersunk and needs to be on the underside of the plate.

 

[Lons]

If the aluminium bar is already tapped you just need to drill a clearance hole in the stainless sheet. 6mm holes will do it or 6.5mm if you want a bit of wiggle room.

Reading that again, will it make a difference that I need to screw through the bar into the plate? The bar's already countersunk and needs to be on the underside of the plate.

That's a bit different but maybe you need to post pics or a drawing to clarify.

If there's a thread in the bar and the countersunk bolt needs to screw through that first then into the plate then you will need to drill and tap the plate to 6mm to suit the bolt.
That isn't an ideal way to get a tight fix however as for best fit the bolt needs clearance through the bar hole to pull tight to the plate. 2.5 mm isn't a lot to get hold of.

Think when you screw though a bit of wood into another, unless you've pre drilled the clearance hole the screw will tend to push the 2 bits apart as it goes through similar result can happen.

 

[AES]

Still agreeing with everyone else if it makes any difference - this is 1 case where a picture (rough sketch will do) really is worth the oft-quoted 1,000 words.

 

[CHJ]

1. Drill the bolt clearance holes in the 2.5mm plate.
2. Clamp the 2.5mm plate to the mitre bar with holes aligned.
3. Countersink the plate and the threaded hole deep enough to allow a countersink machine screw to sit flush with the plate surface.

 

[Lons]

1. Drill the bolt clearance holes in the 2.5mm plate.
2. Clamp the 2.5mm plate to the mitre bar with holes aligned.
3. Countersink the plate and the threaded hole deep enough to allow a countersink machine screw to sit flush with the plate surface.

That's the way I would fit it as well

 

[AES]

Once again "me 3"!

 

[John Brown]

But the OP says "through the bar, into the plate". Seems to me that drawing shows through the plate, into the bar. But I guess it's all conjecture in the absence of a sketch from the OP.

 

[Setch]

If you already have threaded holes in the bar, drill clearance holes in the sheet and insert the screws from the other side. 2.5mm isn't much thickness to tap, but should be enough to countersink for a screw head.

**this is what the posters above said already, obviously I was late to the party!**

 

[John Brown]

Whatever the ultimate resolution, I think everybody agrees that tapping the holes in the plate is a bad idea.

 

[Lonsdale73]

Thanks all. Been sidetracked but will look at it again and get back to you.

 

[AES]

Thinking about your job Lonsdale, I suggest that you do exactly as CHJ has drawn but instead of tapping and using a bolt, you rivet it. This assumes that A) the plate doesn't have to rotate on the bar, and B) a permanent fix is OK.

If the plate is already tapped, drill the thread out to clear it, then countersink, but ideally use a 100 degree c/s instead of the usual 90 degree (these are common in the aircraft industry and can be bought from Snap On amongst others. The reason for 100 is because although it's ss, your plate is only 2.5 mm thick and this will make a much better flush head that won't distort the plate or break it in use).

Choose, ideally a steel c/s head rivet, or a hard "aircraft alloy" 7000 series rivet, NOT a "standard" pop rivet, and make sure it's a tight fit in the hole. Cut off the length so that when entered into the hole it's between 2 and 3 times the rivet dia above the combined plate and bar. If you're used to riveting you can form the head with only a ball pein hammer, but best is probably to buy a rivet snap and dolly to at least start & finish the head forming on the other side.

HTH.

But if the pieces are small you can send them both to me and I'll do the whole job for you as I have all the rivets and gear necessary. Your only cost would be 2 ways post, but unfortunately the Swiss PO are rather (to say the least) ridiculously expensive, sorry. The job'll take about 10 mins!

Alternatively, IF I can find it, I can send you a scan of the appropriate pages of my initial apprenticeship training manual - I think I've still got it somewhere - so you can have a go yourself - assuming you haven't already learnt hand riveting. It's quite easy once you get the knack, and really useful.

 

[CHJ]

Thinking about your job Lonsdale, I suggest that you do exactly as CHJ has drawn but instead of tapping and using a bolt, you rivet it. This assumes that A) the plate doesn't have to rotate on the bar, and B) a permanent fix is OK.

If the plate is already tapped, drill the thread out to clear it, then countersink, but ideally use a 100 degree c/s instead of the usual 90 degree (these are common in the aircraft industry and can be bought from Snap On amongst others.....

I have a 2.5mm sheet of stainless steel that I want to attach to an aluminium mitre bar that came pre-drilled with 6mm threaded holes. .

Andy, with stainless sheet that is not going to 'flow?' and a substantial alloy pre-threaded mitre slot compatible bar I don't think he needs to go to the trouble of AC skin flush riveting.

 

[AES]

Maybe not Chas, it depends on the ss, it may/may not "flow", especially if I used monel metal rivets (but I haven't got any)!!!!

I think I agree with everyone else though, a simple sketch showing EXACTLY what the job is would be better For example, one fixing or more, length n thickness of bar, and why a ss plate, etc, etc. Perhaps "just" an Araldite joint - with the addition of a couple of simple locating pins for absolute location would do? Dunno.

 

[Lonsdale73]

Okay, hadn't planned to but decided to tackle it today. I started by using double-sided tape to hold the slider to the plate so I could get the position for the pilot holes which I then drilled out. Perhaps not the best idea as it seems some of filings for into the threads making it a bit tight on the bolt. I then enlarged the hole to 5.5mm, thinking maybe the bolt would cut it's own thread - no chance. So I took it up to 6mm and now the bolts slip through. That ruled out fitting bolt through the bar and into the plate so I went through the plate into the bar. Quite naturally, this left the bolt a little proud of the plate. My countersink bit came from a super cheap set from Wickes, maybe 14 years ago so wasn't sure it would be up to the task. Much to my surprise it worked much better on steel than it ever has on wood! Ideally, I'd like the heads a smidgeon lower but didn't want to push my luck any further . It did mean the nice short bolts I'd bought specifically for the job now protruded below the slider but that was soon remedied with the angle grinder. This is what I got:

After sacrificing a fingertip to the woodworking gods I bought some Magswitch gear, not knowing my new table saw had a top cast not in iron but in aluminium! My thinking was I could circumvent this minor inconvenience by fitting a steel plate to a slider, using the expansion discs on the slider to secure it the mitre slot then attach the Magswitch to the plate thus:

 

[AES]

OK Lonsdale. Does it work? If yes, job done!

 

[Lonsdale73]

Couldn't resist a test cut and yes, it seemed to do what I hoped it would. Think I could do with a wider sheet of metal so I can move the Magswitch further left but guess I'm going to be restricted by the distance from blade to mitre stop as there's now a 2.5mm step.