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Dunc
01-21-2009, 01:12 PM
First, I have to say a huge thank you to all board members for their patience and coaching of a newbie. Working my way up gradually from getting the feel of... to actually doing something productive. At the same time, I know that there is a lot of knowledge that comes from experience and practice and is rarely found in the books.

I have a mini-lathe & mini-mill. Lathe headstock & mill spindle use MT3 tapers. When using tooling in them, how tight does the drawbar need to be? I am asking, I know, for the definition of an unknown - ie., how to put into words "as tight as it needs to be." Does the taper itself provide the "hold" and the bolt merely prevents the pieces separating due to cutting forces or vibration? If yes, then the bolt would only need to be "snug-tight." Otoh, if the bolt is intended to hold the tool against rotation within the taper then I will have to exert some amount of force. What is realistic: pull it tight with a combination wrench (drawbar is 3/8-16) or do I need more - like a long-handled socket wrench? Is there such a thing as "standard machinist practice?"

I know (well, maybe I think I know) that I should wipe the mating surfaces to remove dirt before assembly. That done, do I then apply an oil film... Cutting fluid... WD-40 or similar? Should the surfaces be dry (but leave the residual oil film after wiping) or super-dry - ie, wipe down with an oil removing solvent to allow literally bare metal to bare metal?

If I need to use the tooling over a few days, should I slack the drawbar and loosen the tool in the MT? Saw a post about a stuck lathe chuck ... granted, different tooling and mounting but if it becomes really jammed up then it will take a lot of ? that can't help bearing accuracy and long life.

Again, thanks for your patience and sharing of knowledge.

Fasttrack
01-21-2009, 01:27 PM
Hello Dunc

Good questions! Morse Taper is a self-holding taper. For practical purposes, this means two things. First, the taper is driving the cutting tool, not the draw bar. Second, it means you will have to use a hammer on the draw bar or a morse taper drift to remove the taper from the socket. The purpose of virtually every taper I can think of on a machine tool spindle is to drive the cutting tool. They may not all be self-holding, (R8 or 5C or etc) but the taper is still what does the driving, the drawbar is there just to keep the collet (or drill chuck or what-have-you) from pulling out.

The draw bar only needs to be snug, not super tight. Like you guessed, its purpose is to prevent the taper from coming unseated due to cutting vibration. Once the taper is set, you could even remove the draw bar and it is unlikely that the tool would come out of the spindle on a mini-machine since the side loads are so minimal. DO NOT, however, try this on a mill spindle! You don't want something sliding out at 1000 rpm or even coming loose and scoring up your spindle. As an example, consider the morse taper in your tailstock. To seat the morse taper, all it takes is a quick, firm "push", almost a "slam" to seat the center/drill bit/drill chuck in the socket. Once it is seated, it holds itself in place and won't spin. Most mini-machines don't have drive screws to hold a tang, and we don't want to start that discussion here anyway! ;)

Finally, do not oil the tapers. You can wipe them down with oil if they are not going to be used for awhile to protect them from rust. The tapers will not seat properly if they are oily, though. Generally, a good wipe down with a clean towel is sufficient. I've never had to clean a taper with solvent.

edit: Don't worry about the morse taper getting stuck. No matter what, it will take a "bop" from a hammer or similiar to unseat the taper. You can leave them in the tailstock or spindle for as long as you want, so long as they don't rust in place, and still remove them. Usually tapers get stuck due to extreme conditions - either extremely heavy thrust loads, seating the taper when there is a big temperature differential, etc. The tapers that usually get stuck in the spindle are Brown and Sharpe. Those you would want to remove when you are done with them. Lathe chucks that get stuck are usually the threaded type, meaning they thread onto a register on the spindle. A heavy load will tighten the chuck on there. Also leaving it on there for a period of time and allowing rust to form will make removal that much more difficult. I suspect your machine has a backplate and three bolts holding your lathe chuck on, so no problems there.

Teenage_Machinist
01-21-2009, 09:19 PM
If you have a really stuck taper, try putting penetrating oil down the spindle, and running it fast to warm and expand it.

wierdscience
01-21-2009, 09:54 PM
Tigthening the drawbar puts the spindle in compression and the drawbar in tension,at that point the drawbar is a spring.If you snug the drawbar up by hand and then torque it to 25ft lbs it will apply approximately 4100 lbs of tension to the system.So even a small amount of torque is capable of significant holding power.

That said usually 1/8 of a turn (approx 15 ftlb)is most times enough to hold an endmill in a MT3 collet.Definately wipe both the tool and the socket clean before use.

I would avoid running tools without a drawbar thread in the mill.Any slip and you can trash the runout of the mill spindle by galling.

tattoomike68
01-21-2009, 10:19 PM
My trick with my morse taper mill is tightien the draw with a wrench with one finger tip , snug but not crazy.

On an R8 taper I use 2 finger tips on the wrench, if I can strip a draw bar with 2 fingers then it was junk anyway.

So thats my trick, use one or two finger on the wrench so you cant tork the piss out it.

Over the years my friends have told me "damn you machinist tighten the piss out of everything" we try not to do that but we machinist know what a tight screw is.

lwalker
01-21-2009, 10:35 PM
My mini mill has the R8 taper, so I don't know how much better/worse it holds than MT. I never use the spindle lock pin: I grab the spindle by hand and a quick twist of the wrench tightens or loosens the drawbar and a cutter has never slid down or came loose since I've been doing this.

I used to use the lock pin until I realized the drawbar didn't need to be that tight, and using the pin was slowing me down. Hand tight is plenty good enough for anything I've done.

aboard_epsilon
01-21-2009, 10:43 PM
when you start to feel the spanner or wrench whatever it is ...spring back when you tension and let go ......that's it you're too tight.

get a feel for this .......

this will only work on small drawbars under 1/2 inch thick.

all the best,,,,,,,markj

aboard_epsilon
01-21-2009, 10:47 PM
My mini mill has the R8 taper, so I don't know how much better/worse it holds than MT. I never use the spindle lock pin: I grab the spindle by hand and a quick twist of the wrench tightens or loosens the drawbar and a cutter has never slid down or came loose since I've been doing this.

I used to use the lock pin until I realized the drawbar didn't need to be that tight, and using the pin was slowing me down. Hand tight is plenty good enough for anything I've done.

R8 lock pin is there only to help you locate the collet........and assist you tightening the draw bar .........it should be never be used to hold a collet against the turning forces of cutting.........youre going to come unstuck with this method .the pin will break off one day ..tear into the taper and jam your collet.

all the best.markj

darryl
01-22-2009, 04:18 AM
Well now this seems like a good HSM project- a torque wrench made for tightening drawbars. What WS suggests, 15 ft/lbs or so seems about right to me. You don't need a long handled wrench to achieve this, so the tool could be made compact and would take the guesswork out of it everytime it's used. It could be made adjustable from say 5 to 20 ft/lbs, and might be made to slip at the desired torque level in the tightening direction, but not slip at all when loosening the drawbar.

Fasttrack
01-22-2009, 12:55 PM
Well now this seems like a good HSM project- a torque wrench made for tightening drawbars. What WS suggests, 15 ft/lbs or so seems about right to me. You don't need a long handled wrench to achieve this, so the tool could be made compact and would take the guesswork out of it everytime it's used. It could be made adjustable from say 5 to 20 ft/lbs, and might be made to slip at the desired torque level in the tightening direction, but not slip at all when loosening the drawbar.


Personally, I don't think a torque wrench is really neccessary. You get a feel for it pretty fast. What I have seen and think is pretty nifty is a wrench/hammer for the draw bar. It is fairly compact and has a small brass hammer and steel wrench combo. I think someone here posted a pic of it at one point.

lwalker
01-22-2009, 04:30 PM
I don't mean the little pin on the inside of the spindle, I'm talking about the spindle lock pin (http://www.littlemachineshop.com/products/product_view.php?ProductID=1392&category=6) that fits into a slot on the outside of the spindle so you can lock it in place to turn the drawbar.

That thing is pretty useless to me.


R8 lock pin is there only to help you locate the collet........and assist you tightening the draw bar .........it should be never be used to hold a collet against the turning forces of cutting.........youre going to come unstuck with this method .the pin will break off one day ..tear into the taper and jam your collet.

all the best.markj