Thread: has this happened to you

Originally Posted by hughie

My teknatool screw chuck and the chuck part company the other day much to my surprise as it has never happened before. On closer inspection it looks like it had fractured sometime previously and this was the last straw.

That is why I have maintained that some turners who use screw chucks on large bowl blanks with high speeds in a public demo setting are very dangerous people!

Strength of materials again, may have been a QA issue with a flaw in the material, or manufacturing, just normal wear & tear, or abuse. How often do you use a screw chuck on a blank with an uneven mating surface to the faceplate or chuck when using the screw chuck? Each time you flex the screw out of alignment you are weakening the material & creating micro stresses in the material, it will eventually fail! Don't get me wrong they have their place and are safe to use when used appropriately.

The old excuse - it has never happened yet - is a pretty poor excuse, it does happen & it is predictable & known about, its just that its not publicised, so turners do not hear of it often.

Can't say it has, but I'm not surprised. I guess I'm over cautious, because I drill a slightly undersized hole and then have to really wrench it tight to screw it in. I guess one of these days it will shear right off.

Strength of materials again, may have been a QA issue with a flaw in the material, or manufacturing, just normal wear & tear, or abuse. How often do you use a screw chuck on a blank with an uneven mating surface to the faceplate or chuck when using the screw chuck? Each time you flex the screw out of alignment you are weakening the material & creating micro stresses in the material, it will eventually fail! Don't get me wrong they have their place and are safe to use when used appropriately.

Its done a fair bit of work, so I doubt QA, probably me pushing it . I do have a couple backing plates I use when turning larger diameters to back up the screw chuck.

But looking at it I reckon it could be improved upon design wise also use a different material. Making my own here would mean not being constrained to make a profit. But I am fairly busy at this time of year so it may not get done.

I have seen it on a club lathe

Hi Hughie,

This certainly is a timely reminder to those of us who become complacent from time to time. If this can happen to an experienced turner such as yourself, then what about novices/learners. They may not realise the importance of using the tailstock early on when roughing down, although this could have happened at any time after removing the tailstock.

I won't go into the physics, but even a piece such as this, doing 1000 rpm could have a lineal velocity that would be scary, not to mention the energy tied up in rotational motion. The irony of using 'deadfinish' does not escape me. Having EVS control as on your lathe is also much safer for those who are a bit slack about changing speed manually.

A novice turner at our club had a large unbalanced block on a centre screw/parallel screw etc but turned on the lathe without checking speed or bringing up the tailstock. The lathe, although having EVS, was left on a high speed. The end result: one flying mass that went through a lexen screen, a screw that bent by about 35 degrees and a massive influx of adrenalin from all those close by.

Cheers mate and see you soon,
Paul

Hi Hughie
About ten years ago I had two brake in a week they were from Carba Tec one was brand new the other had been around for a while ,I believe the problem was that the hole wasn't drilled true to the face of the blank and when tightened this caused some stress and eventually snapped.
Cheers Tony

[About ten years ago I had two brake in a week they were from Carba Tec one was brand new the other had been around for a while ,I believe the problem was that the hole wasn't drilled true to the face of the blank and when tightened this caused some stress and eventually snapped.

Thats highly likely and or as it pulls up to the blank its skewed off square. Further to my woes I glued a block left it over night and darn thing come apart as well Comes of being in a hurry and not giving enough attention to the details

Neil, I bent a Woodfast screw and mentioned it at the club, just in time for one of the boys to tell me that a machinist mate of his, was going to make him a 25mm screw chuck and would I want one. I can supply some pics if you want, it's very hard to bend or break the screw.

Neil, I bent a Woodfast screw and mentioned it at the club, just in time for one of the boys to tell me that a machinist mate of his, was going to make him a 25mm screw chuck and would I want one. I can supply some pics if you want, it's very hard to bend or break the screw.

Absolutely Pat I have a design for a slightly bigger one, but 25mm sounds the way to go.

Hughie,

Yes I had it happen to me with a Teknatool screw but I was using it in my Vicmarc chuck when it happened. Broke in the same place and I also had evidence of a stress fracture or previous crack.

I had never abused it and always had been careful to use the tailstock and had never spun really huge chunks as I have the rings for that

I suspected a stress fracture from when the threads had been rolled.

Trevor

I dont use mine a lot and only on small stuff when I do. Guess I,ll be using it less now after seeing that.
Regards
John

Some pics31102012062.jpg31102012058.jpg31102012056.jpg31102012055.jpg

Apologies for pic quality, I am a worse picture taker than turner.
Mine is M30x3.5 bolt welded to 65mm bar that has been turned down leaving a minimum 23mm shank.

Drill a 25x25mm hole with a forstner bit

I dont know if you guys down unda have Oneway stuff but I use the Stronghold chuck(the only one that would fit on the 1 1/2" headshaft )and I use the screw that came with it, the 1 1/2" Woodworm and it works a treat! I just did this platter on it to get the bottom ready for a chucking recess. Its 14" in diameter and I was running it at 1650. Jimmy Clewes taught me to turn fast and it works. I just entered this in the monthly comp on UK workshop. Its done in mahogany and ash.

Originally Posted by Pat

Neil, I bent a Woodfast screw....

I've snapped quite a few of the smaller diameter Woodfast screws, but none of the Vicmarc or Novas, yet.

I agree that drilling off square to the face of the blank is the cause. The flexing of the screw as it is pulled tight against the blank eventually fatigues the metal. It's just quicker to drill the hole freehand than use some sort of jig to keep it square. While in the workshop by myself I can justify doing it that way. In company I would need to use a fresh screw and do it properly!