Bent Spindle

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sambo
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Bent Spindle

Post by sambo »

Boo Hoo,

i've bent my spindle - AGAIN ! on my 3/4 inch BP core burner set.

it's always the very tip that seems to hook over. As I a) can't fit a long enough length of stock on my lathe and b) can't turn a taper it's back to the maker for a replacement. That put a stop to my November the 5th Rocket plans. Balls !
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Lloyd
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Re: Bent Spindle

Post by Lloyd »

I'm confused as to why you can't turn taper on your lathe. I've owned a number of lathes during my span here on the planet, and have never found a single one that couldn't turn a taper!

What about your machine precludes that?

(Show me a picture of the lathe. I can't believe it's incapable of turning-taper... seriously!)

Lloyd
"Pyro for Fun and Profit for More Than Sixty Years"
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Boophoenix
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Re: Bent Spindle

Post by Boophoenix »

Mr Forster I believe it was came up with an idea to help with bending spindles. By inserting a small tube into the rammer that would keep the spindle aligned during pressing if that happens to be some of you issue.
sambo
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Re: Bent Spindle

Post by sambo »

ah ok ? I have no lathe training of any kind and all I have learnt is from online video or websites. I bought a model engineering lathe book but didn't find it of any use. Perhaps I need to rephrase that then - " I don't know how to turn a taper, assuming it can be done on my lathe" : here is the lathe in question, and I find it is too small to do anything really useful :

http://www.axminster.co.uk/axminster-mo ... the-505101

I'd be very pleased to hear if it can be done and keen to give it a go.

Boo, I'm not sure exactly what's going on, which is even more frustrating than the issue itself as I have done the exact same thing twice now. I use a firesmith collet tube support so the whole thing is really held in tight and true. My ram moves however as it engages, it's not welded to the frame and I think this might cause a small amount of shearing force if it doesn't align itself perfectly.
Locky Smith
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Re: Bent Spindle

Post by Locky Smith »

Hi Sam, I notice one of the reviews on your lathe stating that the reviewer needed to purchase a 'compound top slide' to taper turn.
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Lloyd
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Re: Bent Spindle

Post by Lloyd »

NO! The 'classic' way to turn tapers is by offsetting the tailstock, and turning 'between centers' with a lathe dog driving the stock.

A couple of years back, I spoke with a machinist (who's a professional machinist... I only do it because it's the only way to do my chosen job! <grin>). He only would turn tapers via the compound. I recommended using the 'old way' by offsetting the tailstock, and he vehemently refused because "it was impossible to get the tail-stock properly re-centered, once you'd moved it."

I showed him a FAST, EASY way to reset the tailstock to 'perfect center' once it had been offset for turning tapers. It takes about a minute, period, and it's 'dead-on', every time!

He was both amazed at how simple it was, and regretful that he'd never been shown that method in his entire career.

Lloyd
"Pyro for Fun and Profit for More Than Sixty Years"
sambo
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Re: Bent Spindle

Post by sambo »

Lloyd, I don't know whether to love you or hate you ( <grin> as you would say ) - As Ben said, I always thought I needed a compound slide and Axminster no longer stock them, I found the original Chinese importers but was put off by yet another huge lathe cost for the slide, which along with all my other hobbies was just getting ridiculous...

so, now you tell me there is a neat trick. I'll go and do some reading as you have me intrigued - are you able to elaborate or point me in the direction I should head ?

no, joking aside, you're brilliant, thanks.

Sam.
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Lloyd
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Re: Bent Spindle

Post by Lloyd »

Sam,
There's no "neat trick" to turning tapers. For as long as lathes have been in use (until CNC), the only accurate way to turn tapers has been to offset the tailstock by a measured amount, and turn the taper 'between centers'. There are some fairly minor 'tricks' to making sure the offset is just exactly right, but they are well-documented in almost any lathe manual. I have a .pdf of the Atlas 6" and 12" lathe manual which discusses it, if you're interested. Surprisingly, they do not expose the way to re-align the tailstock to center, again!

The only 'trick' I meant is in how to quickly and easily re-align the tailstock to 'center', once it's been offset. Some otherwise VERY experienced machine folks are so afraid of tailstock misalignment, that they're unwilling to use the best way to turn tapers, for fear they won't be able to get the tailstock back to center. In fact, it's so easy it's almost painful to tell folks who don't believe it. And most won't believe it until they've actually tried it.

Boo saw that conversation I had with a professional machinist who's been doing it for 45 years, and who was similarly deterred from tailstock offset methods because of the 'impossibility' of getting it back to center. My only 'upfront' question to such folks is, "How do you know it's accurately-aligned NOW?" They seldom have any answer except "it seems to be turning cylinders accurately". Yeah... well... there is a quick way to make sure it will.

In the meanwhile, I suggest you read-up on how to turn tapers between centers, attending to how to accurately set-over the tailstock. That, too, is easy. Atlas lathe .pdf is available at your request.

When you get to the point of being willing to attempt it, I'll dig up the short treatise I posted that allows you to re-center it accurately once you're done.

Lloyd
"Pyro for Fun and Profit for More Than Sixty Years"
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Boophoenix
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Re: Bent Spindle

Post by Boophoenix »

Here's a simple little vid that shows the taper process Lloyd is talking about

https://youtu.be/z3iYhKFPHKc


Here is an alignment method I believe is pretty close to what Lloyd is talking about

https://youtu.be/PKvlWZ3rScU
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Lloyd
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Re: Bent Spindle

Post by Lloyd »

Boo,
The first vid accurately shows how tapers are turned between centers, although it doesn't go into some of the finer details of 'refining' the taper to an exact value, should you wish to do things like make more tailstock/headstock tapers or tapered tool holders. But the representation they give is fine.

The alignment method you showed is 1) The final solution, when you need alignment within a couple of tenths, 2) easy to make yourself, and 3) not as quick an easy as the method I showed 'that guy'. The quicky-method involves pinning a piece of rigid flat stock between the headstock and tailstock tapers, and simply adjusting the tailstock until the piece of material is dead-square to the bed. It takes mere seconds, and will get the 'gross alignment' within a half-thou in one try. Then, if you're doing hyper-critical work, you can use an alignment bar to refine it.

Lloyd
"Pyro for Fun and Profit for More Than Sixty Years"
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Boophoenix
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Re: Bent Spindle

Post by Boophoenix »

I recall the conversation when you described the process, but haven't quite wrapped my head around yet. Maybe on a visit we could do a short video of the process. Hopefully this coming year will allow for more visiting than this year has.
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Pyro-Gear
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Re: Bent Spindle

Post by Pyro-Gear »

Using the offset tailstock method as suggested by Lloyd I have been turning out some nice spindles, this is a ¾ set I knocked up for fun!
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Boophoenix
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Re: Bent Spindle

Post by Boophoenix »

Looking pretty good. Hoping things work out to where I can get back on my machines a little pretty soon too.
Dave F
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Re: Bent Spindle

Post by Dave F »

I have had some issues with mis-aligned black powder spindles in the past, even with a collet-type tube support. One problem can arise when the tube slides down in the tube support while pressing the first increment. If the tube is at all loose or the inside of the support is slick, the tube compresses and then re-expands when the pressing force is released. This makes the motor able to 'rock' on the spindle base. It would be great if the tube support could lock to the spindle base and maintain square. One thing I found helpful is to put a chipboard disc or two up against the bottom of the tube when loading it, and then snugging up the support. When closed snugly on the tube, the bottom of the tube is now 2-3mm from the bottom. Part of the energy of the first pressing of nozzle or propellant increment is used to pull the tube down to the bottom, but it usually doesn't end up sticking out. So the support stays square.

The insert Bradley mentioned works very well. It is sized to fit fairly snugly inside the first drift. It could be a flexible plastic, silicone, or even rolled up gummed tape. The inner hole in the insert is sized to accept the spindle tip. When the first increment is pressed, the spindle tip is being cradled, which keeps it centered from the beginning. I got the idea from the rubber boot inside a spark plug socket.
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Lloyd
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Re: Bent Spindle

Post by Lloyd »

"It would be great if the tube support could lock to the spindle base and maintain square."
----------------
Dave, I'm going to have to plead ignorance on this one. I've never used or made a gerb mold or rocket mold that DIDN'T 'lock' the spindle to the base in some fashion (and didn't also firmly compress the tube vertically in the mold!).

I guess I don't immediately see how a mold that didn't hold a tube rigidly would work -- more-especially with snugly-fitting press rods and/or sticky comps.

Lloyd
"Pyro for Fun and Profit for More Than Sixty Years"
Dave F
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Re: Bent Spindle

Post by Dave F »

Hmm, I guess we have some crossed wires. And maybe you have only had the luxury of using tooling designed the best way ;)

With black powder rocket tooling I have, the spindle locks into the base, yes. Tube supports/moulds are sold separately to rocket makers. No support (including Ben Smith's collette style support) that I have seen locks to the spindle/base combination. So, when the ram comes down, there is nothing holding the whole setup square, which perfectly centers the spindle in the tube.

A couple of bolts (or some other arrangement) coming up through the spindle base into the bottom of the support would take a little longer, but would eliminate some of the problems that occur when such a 'loosey goosey' setup is used to make a lot of rockets. It is true that hundreds of years ago they knew to do this. I have an old picture of such a setup. I think it's in Mr. Lancaster's book. I got the eyeroll when I mentioned it to a couple of toolmakers, and I dropped it.
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Boophoenix
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Re: Bent Spindle

Post by Boophoenix »

Interesting idea Dave. A bolt might not be necessary guide pins should suffice I’d think. Unless the suport is riding up the tube? Which is why you mentioned the disks? Not having dealt with many rockets yet and only using a suport on one so far and if I recall correctly it wanted to walk up a little on me. I don’t recall if I rammed it or pressed it though. I do know that one would have been spiral tubes though.

I wanna try one day to make my own support. I can’t recall if the concept I have in mind came from you or where, but I’ve thought about in similar sized rockets an Al outer shell with a Delrin insert. Inserts could be designed to fit two or three different tube sizes into the support housing and is much easier material to machine. Since Delrin will give just a smidge they could be built to a little tighter tolerance possibly helping aid in some of the tube variables we experiance from batch to batch and from humidity differences ( if thet matters ).

Oh, another idea just hit! How about a stop at the end of the support on the inside? The tube never contacts the base at this point just the support. That might not prevent any walk up, but a couple of guid pins to keep everyth8ng centered and maybe a lock pin in the vertical path could prevent walking. Now hand ramming isn’t an issue with a support at all either. Hmm
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Lloyd
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Re: Bent Spindle

Post by Lloyd »

Dave,
You said it correctly. I was the one who said 'spindle to base', when I meant 'tube-support to base'.

ALL my gerb and rocket molds over the decades have had a 'twist-lock' design, which not only 'locked' the tube support to the spindle/base assembly, but which also put positive compression (down-force) on the tube to keep it square over the spindle, and to prevent 'pullup' when pulling out a press rod. My various spolette molds were all similarly-equipped.

I wouldn't even consider purchasing one that didn't have that feature.

I'll plead ignorance when I say I didn't even know anybody built them without a positive lock on the tube support.

Lloyd
"Pyro for Fun and Profit for More Than Sixty Years"
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Boophoenix
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Re: Bent Spindle

Post by Boophoenix »

I like the twist lock that’s simple and solves all issues. That’s why you get the big bucks, lol
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Lloyd
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Re: Bent Spindle

Post by Lloyd »

Bradley,
With two twist-lock pins on the base, and two slots in the tube support, there's not a LOT of room for 'fooling around'. However, I've seen one twist-lock idea I liked a lot.

Instead of plain 'spiral' grooves catching the pins, it had a 'stepped' spiral, so there were several locking-points as the support tightened down onto the tube. That would (I think) permit the use of a single support for tubes that varied slightly in length, while still locking positively at each length increment.

LLoyd
"Pyro for Fun and Profit for More Than Sixty Years"
Dave F
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Re: Bent Spindle

Post by Dave F »

Bradley yes, I was talking about the support 'walking up', and causing the whole setup to be able to wobble. I forgot to mention that the discs were just used to set the distance from the bottom of the tube to the bottom of the support, and are removed before pressing. The ones I used fit loosely, so they just fall out. I like the small shoulder in the tube support idea. Yes, it was me that suggested the inserts in the tube supports. This was when NEPT tubes were regularly not fitting supports, and it was left to the customer to solve the problem.

Lloyd, I'm glad you agree. Maybe a toolmaker would try it if YOU suggested it ;)

Sambo, when I had my collette-type support, I found it annoying to handle the collettes. I sold it to a different toolmaker, with the suggestion that he mill grooves around the outside of the assembled support. The grooves were to accept O-rings, which kept everything together as a unit, rather than having to align a handful of metal parts every time. As far as I know he did it and it worked well.
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