mandrel-tangs pull out :-(

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mandrel-tangs pull out :-(

Postby gavin » Sat Aug 22, 2009 3:49 pm

I glued with 2-part epoxy glue 4.5 mm diameter nails into 4.5 mm holes, and they were 25 mm proud of the mandrel-end.

Sadly, when I whacked the core off the mandrel, the tangs came with them. :oops:
Image

Any suggestions to better fix tang to mandrel?

Robin Fawcett aka Treewright uses nails for his mandrel tangs
Image

Perhaps I could try shorter tangs - you'll see Robin F's are 5 or 10 mm proud of his mandrel and approx 6 mm diameter. I could also try using green wood for the mandrel which will then shrink onto the tangs as it dries, thus gripping the tang more tightly. Or drill 5.5. mm holes to allow more glue to bind to the nails...

I usually use 40 mm long M8 engineers bolts and beat the head into a screwdriver or tang shape, and these do stay secure, esp when glued. Does anyone know how Geo Lailey fixed his tangs into the mandrel? He did not have access to 2-part epoxy glue! I think the Lailey tangs were rectangular in section.
Here are his mandrels:
Image
Image
Image
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Re: mandrel-tangs pull out :-(

Postby Bob_Fleet » Sat Aug 22, 2009 4:06 pm

It seems a bit obvious to ask why do you make the holes 4.5mm for a 4.5mm nail.
That means there is no grip and they are slack so the only thing holding them in is the glue.
I'd be tempted to make guide holes about 2.5 mm and then the wood will grip them. Making it bigger as you suggest is the wrong way to go.
You've got a metal collar on your mandrel so it shouldn't split.

You could also try annular nails which also seem to hold better or you could use a triangular file to put some tangs on them.

Alternatively try soaking it once they're in to swell the wood and grip them.

Epoxy sets solid with no flexibility so any pressure will make mini cracks in it then it releases the tangs and defeeats the object.
Could you try something "softer" like rubber cement?
Whole new world of adhesives to explore here - another excuse to buy "Rubber Monthly".

I usually use 40 mm long M8 engineers bolts and beat the head into a screwdriver or tang shape, and these do stay secure

Why change if it works?
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Re: mandrel-tangs pull out :-(

Postby Ian S » Sat Aug 22, 2009 4:10 pm

Hi Gavin

Mandrell-tangs pull out - ooh, sounds painful!

Some points which spring to mind-

1) The epoxy hasn't held the face of the dark wood at all. Would you be better to make sure the steel collar is lower so you get the two pieces of wood to adhere over the whole surface?

2)
gavin wrote:4.5 mm diameter nails into 4.5 mm holes


Actually two points here - 4.5mm dimensions - no room for the epoxy and no to minimal friction for the nails to bite. I'd be tempted to either try either bigger holes and a metal filler epoxy so that the epoxy is holding or much smaller pilot holes so that the nails are holding.

Caveat - I've never made a mandrel....

More experienced minds will no doubt add their wisdom.

Cheers

Edit - Bob got there before me!!
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Re: mandrel-tangs pull out :-(

Postby gavin » Sat Aug 22, 2009 4:29 pm

Ian S wrote:Hi Gavin


1) The epoxy hasn't held the face of the dark wood at all. Would you be better to make sure the steel collar is lower so you get the two pieces of wood to adhere over the whole surface?


Sorry - you may have misunderstood my method.
- I just drive the tangs into the dark wood for a friction fit. No glue is intended to unite the blank and mandrel.
- I have the collar proud of the mandrel so I know the mandrel will sit dead square on the blank - it saves guess work.
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Re: mandrel-tangs pull out :-(

Postby Ian S » Sat Aug 22, 2009 4:33 pm

Hi Gavin

Ah, is the dark wood the piece being turned then?

I think that what has happened is that the nails have held the turning piece with sufficient friction to pull the nails out of the mandrel, then. I think that my points about the holes drilled in the pale wood (the mandrel body?) would still hold true.

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Re: mandrel-tangs pull out :-(

Postby paul atkin » Sat Aug 22, 2009 4:35 pm

why dont you just stick to using threaded bar its a lot easier???
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Re: mandrel-tangs pull out :-(

Postby gavin » Sat Aug 22, 2009 6:04 pm

paul atkin wrote:why dont you just stick to using threaded bar its a lot easier???

Because nails are more common than threaded bar or M8 bolts. I want to make bowl turning more accessible to more people.
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Re: mandrel-tangs pull out :-(

Postby paul atkin » Sat Aug 22, 2009 6:53 pm

gavin wrote:
paul atkin wrote:why dont you just stick to using threaded bar its a lot easier???

Because nails are more common than threaded bar or M8 bolts. I want to make bowl turning more accessible to more people.

i use m10 threaded bar, available for about £1.50 a metre available from ironmongers, electrical wholesalers and most diy shops. cut it into short lenghts grind on some flats and screw it into a tight hole i sometimes dab in a bit of glue, they wont pull out and the thread spreads the load, so not as prone to splitting the mandrel, just make sure you drill in about 2in, if you want to use nails use annulars and make sure you grind off the ribs that go into the blank
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Re: mandrel-tangs pull out :-(

Postby Bob_Fleet » Sat Aug 22, 2009 6:58 pm

Here's a link to a company that sell things like this;
Image
You make a little hole then put them in with some kind of device.
Then you file the bit sticking out to a point.
You can even get them in different sizes for different jobs.
If you want them to go further into the wood you can get longer ones

Anything to be helpful
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Re: mandrel-tangs pull out :-(

Postby gavin » Sat Aug 22, 2009 7:19 pm

paul atkin wrote:i use m10 threaded bar, available for about £1.50 a metre available from ironmongers, electrical wholesalers and most diy shops. cut it into short lenghts grind on some flats and screw it into a tight hole i sometimes dab in a bit of glue, they wont pull out and the thread spreads the load, so not as prone to splitting the mandrel, just make sure you drill in about 2in, if you want to use nails use annulars and make sure you grind off the ribs that go into the blank

What size hole do you use for your M10 bar to screw into?
What glue do you use?
How many tangs do you use?
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Re: mandrel-tangs pull out :-(

Postby paul atkin » Sun Aug 23, 2009 11:32 am

gavin wrote:
paul atkin wrote:i use m10 threaded bar, available for about £1.50 a metre available from ironmongers, electrical wholesalers and most diy shops. cut it into short lenghts grind on some flats and screw it into a tight hole i sometimes dab in a bit of glue, they wont pull out and the thread spreads the load, so not as prone to splitting the mandrel, just make sure you drill in about 2in, if you want to use nails use annulars and make sure you grind off the ribs that go into the blank

What size hole do you use for your M10 bar to screw into?
What glue do you use?
How many tangs do you use?

drill an 8mm hole, a dab of pva swells the wood and holds them tight, i use two tangs, and i dont bother with ferrules, my lathe gets used by lots of other folks and the first thing they do when starting to undercut the centre is to take a lump off the mandrel, my mandrels all show the scars but tis better than taking lumps out of good tools, In ten years i have worn a few mandrels away but only broken about 3, and at about ten mins to make a new one i dont see the point.
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Re: mandrel-tangs pull out :-(

Postby robin wood » Sun Aug 23, 2009 1:35 pm

This seems to be an attempt to reinvent the wheel and I don't see the benefit.

Beginners can spindle turn a mandrel in seconds and push fit it into their bowl blank. See this thread viewtopic.php?f=14&t=679&p=4711&hilit=+mandrel#p4711 This was commonly used throughout history by serious production turners though I prefer now to use spiked mandrels.

I see two potential problems with Gavin's nails, one is the pyramid points which in my experience work loose, all old mandrels have parallel sided spikes. The other reason to use threaded bar, coach screws, spring steel etc rather than nails is that they have a bit of carbon in and are much tougher nails are so soft and bendy I don't think they would last any longer than a simple turned push fit mandrel.
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Re: mandrel-tangs pull out :-(

Postby Donald Todd » Wed Sep 02, 2009 9:42 pm

What Lailey may have done is heat the tangs before driving them into the holes, just as fitting a chisel handle. The secret to this is, to heat only sufficiently to melt the "Gum" in the wood, not to char it; this forms a natural glue. He used Sycamore, I believe. Gean is much more gummy; it tends to clog up bandsaws!

Heating the tang will also make the wood more malleable.
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Re: mandrel-tangs pull out :-(

Postby Donald Todd » Thu Nov 05, 2009 10:18 pm

I have just been making a mandrel with some guidance from Paul Atkin. It has occurred to me that Lailey et al. may have put the tangs into a green mandrel. They could still true up the mandrel if the turning centres were left. Do Lailey's mandrels still have them?
You could try filing one or more flats onto threaded rod before screwing into a green mandrel. The flats will make the thread cut like a tap. Align the flats to the shrinkage and it should grip it without glue being needed. True up the mandrel once dry and put a ferrule on if you want.
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