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Self extracting drawbar for mill

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old mart20/10/2019 21:46:16
3772 forum posts
233 photos

_igp2476.jpgI happened to notice messages and when I looked at them, one was asking how I went about making a captive self extracting drawbar for a mill. Now I have read it, the message has disappeared and the name of the sender has vanished. So I thought I had better explain how I went about this modification.

Not all vertical mills will be suitable to fit a captive drawbar to. I was lucky because there was space inside the spindle to fit a nut close to the lower end of the drawbar and also not foul the tooling.Some measuring will be necessary before any work is carried out. I cut the thread on the 7/16" en19T drawbar blank on the lathe, and put about 2 1/2" on either end. I made the bronze nut (in black) long enough to Loctite securely with a flange thick enough to not be in any danger of breaking off. At the top end, I used two half nuts for locking and a piece of threaded hexagon bar large enough to ensure that the upper locknut didn't foul the ring spanner that tightened and released the tooling. I was lucky to find a stub ring spanner with a handle of just 6" long, which is plenty. Due to the design of the mill, I had to put spanner flats on the bottom of the spindle, as there was no lock,or any way to hold it. With a minimum endfloat on the drawbar, it breaks the taper in 1/4 turn._igp2436.jpg

Chris Mate12/05/2022 08:26:22
146 forum posts
32 photos

Hi, thanks for showing this.

I spend some time trying to do this on my mill, however the mill I got do have a space, but the space is not squared, it goes V shape towards its top, so I machined a nut to fit there, screw it and lock it to the new drawbar, however if I try to loosen the MT4, the nut jams in the V somewhat, which throws off my torque feeling for loosing the MT4......It does work overcoming this, however it does not feel right. I can sort of seeing whats going on there through the 2x Wedge holes in the spindle.

My selution would be to machine the spindle square in that area if I can....I will think about this.

KWIL12/05/2022 09:40:58
3554 forum posts
70 photos

I made an air driven drawbar, spindle does not need to be locked as "inertia" holds it enough for the drabar to tighten or release. 'Twas important to fit this as reaching the top with a spanner is stretching it a bit!Air Power Drawbar

noel shelley12/05/2022 10:16:35
1344 forum posts
21 photos

Hey KWIL, thats neat ! Yes on a bridgeport it's a bit of a climb up to the top ! Noel

old mart12/05/2022 18:05:07
3772 forum posts
233 photos

I did this with the R8 conversion of the Tom Senior light vertical. R8 tapers were designed for mills with drawbars and it is easy to break the taper. The MT3 taper requires more force and I wonder if a bronze taper breaking nut would be the answer, assuming yours is steel. The one I made (shown in black in the drawing) is bronze. Also I used some moly grease when fitting the spindle.

I am thinking of doing away with the R8 location pin in the drill mill, and making that a captive drawbar also, an excuse to fit some SKF taper roller bearings and mill some spanner flats near the bottom end of the spindle.

bernard towers12/05/2022 21:18:04
617 forum posts
109 photos

Chris mate, is it possible to have a vet shaped washer above the nut?

Chris Mate12/05/2022 21:37:25
146 forum posts
32 photos

Bernard, very little space actually.

Thanks for suggestion, I will look into re-turning such a shaped nut a bit thicker then, not sure how rough the top it needs to rest and bearing against actually is.......I wonder if a trustbearing could work with enough space and machined such ballsl/needles cant drop inwards, that would eliminate any drag as it presses the MT4 out.

old mart13/05/2022 15:44:36
3772 forum posts
233 photos

If there is a shortage of space for an extraction nut, it might be possible to shorten the ends or the MT3 tooling, provided there is at least 2 diameters of internal thread left afterwards. This could be done by hand on a bench grinder. Depending on how hard the spindle is, a little bit of boring could well also work. If the pressure face is rough, a cone shaped thick washer to give a flat thrust face might work.

KWIL13/05/2022 16:23:49
3554 forum posts
70 photos
Posted by old mart on 12/05/2022 18:05:07:

I am thinking of doing away with the R8 location pin in the drill mill, and making that a captive drawbar also, an excuse to fit some SKF taper roller bearings and mill some spanner flats near the bottom end of the spindle.

Location pin is not that important, I have run my R8 without one quite satisfactorily.

old mart13/05/2022 17:10:12
3772 forum posts
233 photos

Having the Tom Senior R8 without the pins has had certain benifits, one is that you can clean the internal tapered portion with one of the BT30 taper cleaning wipers. The only disadvantage is if using an R8 collet, it can sometimes turn and there is nothing to hold onto. I make sure if using one, that the 7/16" thread is clean and lubricated with moly grease. The head end of the collets could be drilled with a small solid carbide drill axially in 3 places to use a ring tool with pins to stop the collet from turning while being tightened or loosened.

Chris Mate13/05/2022 21:18:31
146 forum posts
32 photos

Hi, Old Mart.....I do not fully understand how you did exactly the following...

I bought a steel threaded bar from farm workshop 16mm same thread as original toolbar(I determined the olenght to accommodate all, hacksaw it off). At the top I added a long nut and lock it with a short nut just like you, in my case those go down in the spindle hole no problem. However I can only insert the bar from the top, its too long from the bottom, so if I want to copy what you did, I must hacksaw a short piece of drawbar, screw Bronze Nut on it, then position it with loctite into the spindle, then unscrew that, and screw the drawbar from the top.

1-Where exactly you cut the threads blank...I am not sure how you exactly went about this...
----In my case I think I dont need to cut threads blank.......? Its 16mm right through.

2-I assume your drawbar screw from the top through the Bronze Nut with a space as indicated into the in my case MT4 tool and you can tighten lock it as usual. If you unscrew the drawbar the pressure is in the treads of the Bronze nut forcing out the MT4 tool in my case....?



Some measuring will be necessary before any work is carried out.

I cut the thread on the 7/16" -(11.11mm)-en 19T drawbar blank on the lathe, and put about 2 1/2" -(50.8mm)-on either end.

I made the bronze nut (in black) long enough to Loctite securely with a flange thick enough to not be in any danger of breaking off.

old mart13/05/2022 22:32:27
3772 forum posts
233 photos

With the Tom Senior light vertical it is just possible to lower the knee far enough to fit the drawbar from the bottom without swinging the head. It would be a problem if the drawbar couldn't be fitted that end of the spindle as that nut inside has to be loctited so it cannot slip when it is used for breaking the taper. Removing the bed might give enough clearance, or even the Y axis part. Otherwise the head would have to come off. It can be done if you are determined enough and sure that the mill can be reassembled ok. Another way would be to remove the quill if there is one.

Chris Mate14/05/2022 06:28:15
146 forum posts
32 photos

Hi OLd Mart, it seems the part I dont understand correctly is how you assemble your new Drawbar with the Brass Nut, and if the threads in Brass Nut is different than the rest of the drawbar not like in my case the threads in the MT4 section you draw into the MT4 of the spindle.

Your Drawbar....
Threads for taper tool......small space....threads for Brass Nut(Now glued in spindle pocket) of drawbar through spindle......Top of Drawbar ......Nuts plus Long Nut.

If its the same, I may make the Brass Like part and use a short temporary bar screw it on, and insert/glue it to spindle small pocket............

If I can understand how exactly your drawbar components/threads arranged on it, I can figure out if it will work in my situation......There is no way I can insert it from the bottom at the required lenght without removing parts of the mill, however if I can use a temp short bar with screwed on Brass Nut, I can insert and glue it that way-??


Edited By Chris Mate on 14/05/2022 06:33:25

old mart14/05/2022 18:20:45
3772 forum posts
233 photos

The nut is made from bronze, and the lower end of the drawbar has whatever threads are appropriate for the tooling. These threads extend as far as the nut needs to be screwed onto the drawbar. After degreasing the threads on both parts, I used Loctite 620 as a thread locker, even though it is intended as a sleeve and bearing fit. This is because it is very slow curing and the length of the nut can cause slight friction which makes most other grades such as 270 threadlocker harden before the nut is in its final position.

If there was enough size of cavity to fit a long nut with sufficient wall thickness, then top and bottom parts of the drawbar could be loctited into the nut. You mentioned boring the spindle, but that would involve much more dismantling than just making room for inserting the drawbar from the bottom.

The nut has to rotate with the drawbar, it is not glued into the spindle.

Chris Mate15/05/2022 14:25:10
146 forum posts
32 photos

Hi Old Mart, it seems for me seeing I have to insert it from the bottom with your successfull option, which is problematic for me. I did make an option that worked, but I had to tighten it with 4x 4mm grubscrews after inserting the drawbar from the top town, but it does not have the easy torgue yours would have, and seeing the drawbar is in centre of taper its difficult to clean the the taper.

I did setup a kennametal boring bar vertically mounted in a square block(I made one for each boring bar I got for toolpost), I can see I can bore this carefully, it might take very long because of stickout and somewhat dangerous operation. I abondioned this, because I dont know if the swarf could end up in the spindle bearing, seeing the manual says oiling/greasing it from the hole in the spindle for drawbar.

I am back at the One - Wack method, and to make this more satifying, I used an smaller handwheel from old tablesaw, machine it to take socket for original drawbar, it reminds me of the wheel on an old Singer knitting machine. So I tighten the drawbar whith handwheel take it off and use mill. Then mount it, loosen drawbar(no spanners), take it off, One WAck and MT4 is out. I think I have to stick to this then. I really liked your method, but not prepared to take mill apart yet.

old mart15/05/2022 14:49:26
3772 forum posts
233 photos

It would be helpful if you could tell us whick mill you have.

Another option which would require very careful measuring would be to put the drawbar in position and then screw the nut on and loctite it into position. You would have to clean the bore of the spindle first, degrease the drawbar and nut when ready to screw them together and put loctite on the male threads when the nut has about 25mm left to screw on. Then leave the drawbar resting on the bed for 24 hours before throughly cleaning the surplace loctite off. Then put moly grease on the top part of the nut and lower drawbar threads, push it up and install the upper nuts. If this did not work, a hacksaw would be needed and you would have to resort to using the original drawbar.

Many mill owners have modified the top end of the spindle to provide captive drawbars. If you started a new thread entitled " types of captive mill drawbars" there might be some very good suggestions.

Chris Mate16/05/2022 14:01:17
146 forum posts
32 photos

I have ZX-45(ZX-7045) Drill/Mill a drill modified to be a mill, its better than trying to mill with a drill press, also manual=ZAY7045. I have a lot to buy before I can do all sorts of milling, waiting for a Walter Shoulder mill 40mm, it seems everything is out of stock now, was promised 7 days then after I paid its a month.

I will see if I can come back to this much later on if I need to. Thanks for all the info.

Edited By Chris Mate on 16/05/2022 14:02:09

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