Making a precision pivot point thoughts?

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dml66
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Making a precision pivot point thoughts?

Post by dml66 »

The easiest way to explain what I want to do is to imagine a pair of scissors. I'll have two pieces of rectangular steel about a 1" wide, 1/8" thick, 14" long that must pivot at one end, like a pair of scissors without the handles.

I'd like to make a precision round hole through both pieces at one end, then turn a pivot stud to connect the two. The absolute size of the hole is not important as I'll turn the stud for a tight fit. What is important is the roundness and perpendicularity of the holes.

Ideally, I'd like to use a boring bar to make the holes but there's no practical way to spin a 14" long workpiece with a 7" swing lathe, at least not that I know of. So, I got to thinking why not chuck the boring bar and hold the workpieces in the tool post? I have a milling attachment with a vise that rides on the cross-slide, the compound slide would control the cut depth. The milling attachment would provide vertical positioning, the cross-slide would provide horizontal positioning.

I guess the $6M question is, even if this scheme worked, how much more round and perpendicular of a hole might I get versus just drilling the holes with a properly trammed drill press and a stubby twist bit?

Thank you.
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Bill Shields
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Re: Making a precision pivot point thoughts?

Post by Bill Shields »

Drilling does not always make a round hole.

What you are proposing has been done for decades using a milling attachment on a lathe carriage...pretty standard fare with a 9":south bend or Myford lathe.

Getting the exact size of the hole can be tedious...but since you will be making the pin to fit...it matters not.

Have you considered drilling then reaming?
Too many things going on to bother listing them.
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Harold_V
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Re: Making a precision pivot point thoughts?

Post by Harold_V »

It is quite common for holes drilled with a twist drill in thin material to come out three sided. Even reaming won't guarantee a round hole, although it will generally be of much nicer quality than just drilling. Taper can be a problem with reamers, as can bell-mouthing and multiple sided holes---generally one more side than the reamer has flutes.

It is hard to beat the quality of a bored hole.

H
Wise people talk because they have something to say. Fools talk because they have to say something.
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tornitore45
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Re: Making a precision pivot point thoughts?

Post by tornitore45 »

How important is perpendicular on a 1/8" thickness? I am too lazy to do the math now but can't be much.
I would drill progressively to avoid the dreaded 3 sided hole. I have seen 5 sided holes on larger bits.
Mauro Gaetano
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dml66
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Re: Making a precision pivot point thoughts?

Post by dml66 »

How important is perpendicular on a 1/8" thickness?
I haven't done the math either but, errors at one end of 14" are likely to be magnified at the other end of the piece. Since the lathe has an ER32 spindle, I should be able to "tram" the workpiece against the spindle face (with the collet nut removed) which, while not a perfect reference, should be at least as good, if not better than tramming a drill press vise.

If memory serves, I did measure spindle face flatness with a DTI, it was unremarkable considering the platform, and then some.
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liveaboard
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Re: Making a precision pivot point thoughts?

Post by liveaboard »

I made many holes like that, I drilled 21.75mm and then used an adjustable reamer to get the fit I wanted with a 22mm pin.
I was using 60x20mm bar, thick enough that the pin has a lot of surface area for grease. load is about 1000 kg.
Although the holes were definitely not up to Harold's standards, machines I made are still working well 20 years later.
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tornitore45
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Re: Making a precision pivot point thoughts?

Post by tornitore45 »

How important is perpendicular on a 1/8" thickness?
I missed the leverage effect at the end of 14", was thinking error across the 1/8" axis
On the other hand, if the "scissor blades" so to speak are 1/8" thick is hard to expect to be flat across the 14"

Actual scissor work on a different principle. The blade are flexed into contact at the share point.
Big heavy shares count on rigid flat blades. All depends on the final application.
Mauro Gaetano
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dml66
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Re: Making a precision pivot point thoughts?

Post by dml66 »

You are right toritore45, it all depends on the final application.

I was indoctrinated by a teacher in my woodworking past; he insisted every cut be measured in 64ths for accuracy. When I protested because I was making something for my dog, and the dog didn't care if the finished piece was perfect or not, his standard retort was "find another teacher", lol. Good man he was, every piece he made was exquisite.
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