Author Topic: How do I....bore out a hollow section leaving the centre intact?  (Read 8097 times)

Offline AdeV

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How do I....bore out a hollow section leaving the centre intact?
« on: December 22, 2009, 09:00:04 AM »
Here's a picture Darren posted recently of his slitting saw arbor:



Liking the idea very much, and having recently acquired a slitting saw with no arbor, I decided to more or less copy his design. But.... I don't know how to bore out the central section of the bottom of the arbor, whilst leaving the threads intact... I did try using a cut-off tool, but it just dug in & threw the work out of the chuck (which destroyed any concentricity I was hoping to maintain - lining stuff up in my 3-jaw chuck is next to impossible, and I've not yet got around to making the chuck key to remove it from the 4-jaw - and anyway, the 4 jaw has outside jaws not inside, so I'm not sure how useful that'd be with 2" bar stock).

So, I used the simple route which was to make the 1" stub short enough that a flat-section base would sit on it. All very well, but because I'd had to re-chuck, it ended up not quite flat, so I've got a few thou run out. No prob - I've got lots of spare metal, and plenty of time over Christmas, so I'll have another go: But it would be nice to cut a hollow, like Darren's above.

BTW, I do have a couple of boring bars, but I'm not sure they'll cut on the front face (in fact, one of them definitely won't)
Cheers!
Ade.
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Offline andyf

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Re: How do I....bore out a hollow section leaving the centre intact?
« Reply #1 on: December 22, 2009, 09:52:11 AM »
It would be a horrible job, going in from the front with a trepanning/face parting tool, like a normal parting tool but with relief on its LH side (looking from the tailstock) so it didn't rub. The overhang would be pretty big, too. Being lazy, I would just drill and tap a hole for a bit of threaded rod to be Loctited in.

BTW, on 4-jaw independent chucks, you usually only get one set of jaws, to be reversed when you want to reverse the grip. There is no scroll inside such a chuck, so the teeth on the jaws aren't staggered from jaw to jaw as they must be with a scroll to get each jaw equidistant from the centre.

Andy
Sale, Cheshire
I've cut the end off it twice, but it's still too short

Offline Bernd

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Re: How do I....bore out a hollow section leaving the centre intact?
« Reply #2 on: December 22, 2009, 10:10:39 AM »
If your talking about the threads in the bottom part of the holder, that is usally a bolt. The bottom part has a hole in it to pass the bolt through and into the upper part of the arbor.

I believe our thinking that the threaded part is all part of one piece of the bottom?

Bernd
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Offline Darren

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Re: How do I....bore out a hollow section leaving the centre intact?
« Reply #3 on: December 22, 2009, 10:21:20 AM »
Bernd is correct, the bottom part is in fact two parts one of which is a normal Allen headed bolt.  :thumbup:
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Offline andyf

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Re: How do I....bore out a hollow section leaving the centre intact?
« Reply #4 on: December 22, 2009, 10:33:05 AM »
Oops, silly me - didn't look at the picture properly :hammer: 
Andy
Sale, Cheshire
I've cut the end off it twice, but it's still too short

Offline AdeV

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Re: How do I....bore out a hollow section leaving the centre intact?
« Reply #5 on: December 22, 2009, 11:36:18 AM »
Bernd is correct, the bottom part is in fact two parts one of which is a normal Allen headed bolt.  :thumbup:

Aaaahhhh, sneaky!   :bow:

However, I do have a problem with getting both ends exactly flat & at 90 degrees with respect to the sides; I can do one side easily enough, but if I have to unchuck the work, I'm buggered, because I'll never be able to guarantee getting it exactly flat & concentric. OK, I can use a parallel in the chuck to get close on the flatness... would that be enough? I've found with my 1st arbor attempt that the bottom piece doesn't run true when the saw is in place - but it DOES run true when the saw is left off... implying it's one (or both) of the holding faces that's out of true.


Andy - I'll check that out on my 4-jaw. It'd be great if that's the case :) I just have to make the chuck key first....
Cheers!
Ade.
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Offline Darren

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Re: How do I....bore out a hollow section leaving the centre intact?
« Reply #6 on: December 22, 2009, 11:50:33 AM »
1, Turn down the shank that fits into your arbour on the mill. (mine was 20mm)

2, turn the part around and machine all the steps and thread the end,

3, finally face the end of the first part and de-bur.

4, face another piece if the stock, drill centre through to match bolt size for first part.

5, bore centre out to match the boss on the arbor.

6, remove second part from chuck and fit it to the first part and re-chuck the first part.

7, Now face off the second side of second part.


I suppose you could make the second part first so the first part never needs to be removed from the chuck until the whole job is finished for even greater accuracy.


Like the cups trick you need to keep in mind which is the first and second part all the way through not forgetting if you swap the order they are made the first part becomes the second part and the second part becomes the first part respectively.

Just read it again ... slowly  :lol: :lol: :lol:
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Offline AdeV

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Re: How do I....bore out a hollow section leaving the centre intact?
« Reply #7 on: December 22, 2009, 03:05:36 PM »
Your steps 2 & 6 cause problems for me... as soon as I remove the part from the chuck, it's a dead cert it won't go back in without runout and/or wobble. If I had a collet chuck, then I'd have no bother following your instructions, 'cos the arbor could be chucked in that once the shank was cut.

The way I made the part (or, would have done, had the bore cutting worked) was as follows:

1) Face & turn stock so it's running true & faced flat. Turn down to main arbor size (1.5", or maybe 1.25" for Mk2).

2) Centre drill, drill, ream a 3/4" deep hole to accept a suitable thread (I chose 1/2" BSW, as I have both taps & dies for that). Cut about 1/4" to clearance size, then tap the remaining material in the hole.

3) Turn a small portion of the end of the stock down to 1" to hold the saw. Once close, take fine cuts & test the fit often. Face the outer section to ensure it's flat.

4) Use the cut off-tool to make a couple of relief areas at top & bottom of shank. Turn shank down to size, then part off.

5) Turn down about 1/2" to the appropriate size for your thread. Cut threads as close to the shoulder as possible (I cheated: I cut them to 0.040" depth, then finished off with a die - this left quite a loose fit thread which I wasn't that happy with). Bore out some stock around the thread, to an outer (inner?) diameter of 1.003" or thereabouts. It should fit neatly over the saw, without too much slop. This is the bit that failed for me...

6) Once you're happy with the fit & finish, screw the two parts together & turn the outer diameter to whatever you're happy with (did that bit in Step 1!). Remove the first part, then part off the 2nd part from what's left of the stock. Job done!

If I can make Step 5 work, it never needs to come out of the chuck, which should guarantee next to no runout (less than 0.001"). I do need a left & a right handed turning tool (only got RH tools in the toolpost just now)

« Last Edit: December 22, 2009, 03:09:19 PM by AdeV »
Cheers!
Ade.
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Offline Darren

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Re: How do I....bore out a hollow section leaving the centre intact?
« Reply #8 on: December 22, 2009, 03:36:29 PM »
I suppose I'm fortunate in having an accurate 5C collet nose in the spindle.

Another option would be to use the four jaw and dial it up true.

You will find it a distinct help… if you know and look as if you know what you are doing. (IRS training manual)

Offline John Rudd

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Re: How do I....bore out a hollow section leaving the centre intact?
« Reply #9 on: December 22, 2009, 04:23:16 PM »
Unless I'm mistaken....

The important part is the shoulders for the saw blades being concentric to the diameter of the main body....The retaining part doesnt matter...its only for aesthetics..

Having made a saw arbor( spelling?) I noticed that saw blades dont run true any way......
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Offline kvom

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Re: How do I....bore out a hollow section leaving the centre intact?
« Reply #10 on: January 05, 2010, 11:24:25 AM »
I see three other options for getting a very accurate result wrt concentricity.

The easiest is to turn the parts between centers, so that you can receverse the parts without losing registration.  You just need to have appropriate size lathe dogs and turn a dead center in the chuck before starting (and don't remove this center until finished).

Another option is to use soft jaws with an appropriate size pocket milled into the jaws.

Third, you could cut both sides of the part without unchucking.  You'd need a LH turning tool, and might waste more material than with the other methods.

I plan to make a slitting arbor "one of these days" and will most likely use the soft jaw method.


Offline Alan Haisley

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Re: How do I....bore out a hollow section leaving the centre intact?
« Reply #11 on: January 30, 2010, 11:28:20 AM »
Darren,

Given:

2, turn the part around and machine all the steps and thread the end,

What is the best way to undercut the facing corner where the blade hub hole will sit? Certainly if that is not done, the blade will never seat square and true to the spindle.

Alan

Offline Darren

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Re: How do I....bore out a hollow section leaving the centre intact?
« Reply #12 on: January 31, 2010, 07:29:28 AM »
Hi Alan,

I used a very sharp small nose radius index tip and undercut the face slightly. Not the diameter  :thumbup:
You will find it a distinct help… if you know and look as if you know what you are doing. (IRS training manual)