Power drawbar

[John McNamaraParticipant @johnmcnamara74883]

Hi All

 

I noticed a question re power drawbars in the “How to remove a Morse taper” thread.

I had seen these videos a while back so thought it would be good to share the link here.

 

A lot of the “Add On” units use an impact wrench as the tightening method. It simply screws the old draw bar in and out. An air cylinder is used to engage the unit as needed or hold it clear of the nut

 

Commercial units tend to use spring loaded claws that pulll the special stud screwed to each tool via the threaded clamp hole up, (Disk springs..A.K.A. Bellville washers) are used to apply a static (and importantly constant force, so the tool length does not change) to the pull stud up. an air cylinder positioned above the spindle but not touching the the stud in the the clamped position pushes the stud down releasing the claw(s) when needed.

 

There are lots of different pull stud types.

 

The catch is there may not be room for the commercial and I believe far better method using an existing spindle. Unless very small profile clamping claws are designed. to fit in the existing space. The disk springs are not a problem they are at the top of the drawbar. in open space, And a small air cyclinder is no problem at all

 

I would be very pleased to hear if such a design has been made to work.

 

There are a few videos here……

 

Re CNC Pull studs….

 

I do like this link.. an example of a commercial type DIY made. (down the page a bit) 

Looks like he is from my side of the pond too….

 

Cheers

John

Edited By John McNamara on 19/12/2011 05:04:02

Edited By John McNamara on 19/12/2011 05:09:50

[John McNamaraParticipant @johnmcnamara74883]

Methods of making a power drawbar.

[Ex contributorParticipant @mgnbuk]

I have not seen a power drawbar used on a morse taper. I supsect that the force required to break such taper would be large – even the normal steep taper tooling can stick if the tool gets hot in use – some drawbars are designed to contact the head of the pullstud in the last millimeter or so of travel to break the taper free, particularly on toolchanger machines where a stuck tool can damage the mechanism.

 

I have designed replacement spindles incorporating screw type & collet type drawbars. The collet clamping element & cylinder were bought in from a German company – Berg Spanntechnik. The datasheet for the clamping element is here :

 

http://www.berg-spanntechnik.de/wEnglish/produkte/werkzeug/pdf/ssk.pdf

 

This gives details of the spindle internal features required to make it work. We used them with hydraulic clamping cylinders, not Belleville spring packets. More reliable clamping & almost impossible to pull a tool out, as the cylinder features non-return valves that generate a hydraulic lock when clamped. Not cheap. though – IIRC a 50 taper cylinder (3.5T pull) was around £3500 10 years ago. The collet was around £200

 

For the screw type units we used Align Air-power units (same company that makes the add-on power feeds), usually bought from XYZ Machine Tools. Drawback to these is the need for a spindle brake, as the spindle spins when the tool touches the taper otherwise. The tool also has to be guided into the spindle as the drawbar rotates, there is a risk of cross threading & the threads must be kept free from debris & well lubricated (molybdenum grease) or they can seize.

 

During rebuilds of CNC machines we frequently found broken Belleville springs on machines equipped with them.. To get the required pull strength and travel can make for quite lengthy stacks (longer spindles). The stack needs supporting to keep the spindle balanced. Many release cylinders put the release pressure through the spindle bearings (not good), though it is not difficult to design an arrangement that doesn’t.

If using air to release the tool, many machines require tandem cylinders to get sufficient thrust. Assembling the things can be a bit fraught, as the stack has to be pressurised in order to screw the clamping element into the release shaft.

 

Another variation uses ball bearings to clamp the pullstud instead of a collet – a sleeve with 4 holes drilled “not-quite-through” holds the balls captive in the spindle. An groove in the spindle allows the balls to move away from the pullstud when the sleeve is moved to release the tool & the pullstud is released. When the sleeve moves to clamp the tool, the balls move out when they contact the end of the groove & catch under the pullstud head. A Wadkin vertical machining centre where I did my apprenticeship had this arrangement – one operation on stainless ball valve bodies was a back counterbore & a tough casting (or operator on bonus with the feed override on 150% !) could pull the tool out of the spindle. And the ball bearings with it. . Apprentice maintence person (me) would have to lie under the spindle wearing goggles & attempt to replace the balls in the sleeve with a long screwdriver & a dollop of grease. I don’t know if the sleeve was drilled through (which allowed the balls to escape) of it pulling the tool out had opened the holes out. A hydraulic clamp cylinder would not have allowed it to pull out in the first place !

 

If I were designing a compact system “from scratch” I would investigate internal clamping – as used on modern high speed machines (HSK, Sandvik Coromant Capto or Kennamental KM) like this :

 

http://www.berg-spanntechnik.de/wEnglish/produkte/werkzeug/hsk_auswahl.shtml

 

A much stiffer system, as the flange on the front of the tool is pulled in to contact with the spindle face as well as the taper . The industrial products detailed are tri-lobed designs (The Sandvik system requires a licence to incorporate it, not sure about KM, but I think HSK is “open source”). A low power system for home use & manufacture would probably work fine with a straight cone.

 

HTH

 

Nigel B.

[John McNamaraParticipant @johnmcnamara74883]

Thank you Nigel for the feedback

 

You are right that if I was designing a machine from scratch and it was for industry HSK tooling may be the way to go. For high KW (Often 20 or more on the spindle) and high speeds (20k+rpm) it has many advantages, and by using the face of the spindle and the taper to locate the tool the accuracy is better in the Z axis and of equal radial accuracy to steep taper tooling like NT 30 40 50 etc.

 

The proprietary Sandvik or any other “system” that try’s to limit you to a particular supplier is not for me. Open standards are the way forward.

 

For the DIY constructor HSK is not necessarily the way to go. The extreme accuracy required to harden then grind to spindle nose to a perfect fit in two planes is no mean feat. and once done there is the cost of new HSK tool holders, not widely available second hand.

 

The older NT series steep taper tooling is available second hand at reasonable prices as are many existing milling and other machines that use it. A big advantage.

 

The berg-spanntechnik PDF link you posted is very useful the drawings are excellent and will assist. I am going to study it in detail. A really compact clamp would make it easy to retrofit existing machines. If the clamp can be made to fit in the existing hole in the spindle.

 

I have not written off Disk springs; and they are not all of the same quality, the link below is a useful reference. For a workshop producing small batches the machines we use are not likely to do the millions of cycles that production machines are subjected to; they should last a lifetime. Hydraulic clamping would be nice but the cylinder you mentioned…Ouch. Air will have to do and nearly all workshops have air.

Shnorr handbook….

 

I agree Morse taper spindles pose a different set of problems; I wonder if others have solved the quick change problem for them.

 

Cheers

John

 

 

 

 

 

[Billy MillsParticipant @billymills]

John, there is a simple design for a clamping spindle at:Buildyouridea.com from 2004. The site is worth looking at, I like his home cast tripple ballrace assembly and very simple spindles.

 

regards,

Billy.

[KWILParticipant @kwil]

I have made and am using a “impact wrench” type PD on my Bridgeport, which has a R8. I do not think it would release a MT tool however. Not had any trouble picking up or releasing the thread on the actual draw bar to tool interface. The INT 30 on a smaller mill can be a ***** sometimes.

[Ex contributorParticipant @mgnbuk]

John,

 

I was not particularly advocating using HSK tooling – rather that a home-made solution along the lines of a simplified commercial design could be feasable. For home use, hardening may not be required for a reasonable life & a gauge or two should allow repetative production of blanks within useable limits without too much hassle. I must admit to thinking from my own machinery perspective here – my Super 7 came fitted with the taper tuning attachment.

 

I take your point on ISO 40 tooling being relatively cheap & available – my prefered tooling supplier at work has a standing promotion for 5 off ISO40 ER collet chucks or endmill holders for £99. We frequently buy from auctions & see used toolholders go for more than that !

 

There is a cheap way of getting the benefits of hydraulic operation – air-over-oil intensifiers. Only suitable for “closed” systems with no leakage (so not suitable for the Berg cylinder, which lubricates the rotary union from leakage back to tank), you can get high pressures from shop air. The American milling machine manufacturer Monarch used these extensively on their vertical machining centres for tool release & axis clamping functions. Another (more expensive) version is the Spencer Franklin intensifier, which uses a large diameter air cylinder to push a small diameter oil piston – the “intensification” being the ratio of surface areas of the pistons, with 20:1 being a popular size (2000 psi hydraulic from 100 psi air). Jones & Shipman used these for clamping CNC rotary tables.

 

Disc springs fail regardless of manufacturer, in my experience. We used to de-grease then, spray with a dry molybdenum anti-friction coating (Rocol, I think – around £25 for an aerosol tin) & assemble with molybdenum grease. Some still broke eventually.

 

Regards,

 

Nigel B.

[John McNamaraParticipant @johnmcnamara74883]

Hi Nigel

 

That’s a pretty good deal on ER holders…. Down under in OZ where I live Quite a few small shop owners use the net and buy from a company called “CTC tools” in HK Worth a browse if you use tooling. (I have no connection with them).

 

You do not mention the type of holder BT ISO, The old NT, or CAT etc. They are all slightly different. Some have a shank some do not. and the grip area if there is one, differs.

 

A very useful link re machine tapers….

 

My objective here is to design a quick change system the will work at least with BT and NT tooling. Because one has a shank and one does not, the pull stud on a BT holder will need to be longer than the stud on an NT holder. To enable the pull claw to engage at the same point.

 

In OZ these are the two types I see most often second hand.

 

The berg-spanntechnik link has proved that it is feasible to make a gripper that works within a 12mm hole (last page) that is very encouraging. I plan to use 4 pieces of 1/4 inch square gauge plate, maybe soft soldered together then turned inside and out as a whole to make the claws. They can be unsoldered and hardened later. That will be a real test of skill.

 

For the pull studs maybe a standard bolt turned as the base part that screws into the tool. then as the gripped part a high tensile cap screw; screwed and lock nutted to the standard bolt used as the base. That will give some length adjustment.

 

For those that are wondering this work is part of a small CNC mill design I am working on.(Made from Epoxy aggregate, Mineral Casting) I will post it here as it progresses.

 

My existing manual turret mill a Shizuoka VHRG and 30 years young, will be a good test bed for the power drawbar. It uses NT tooling.

 

There will be plenty to do over the holidays…….

 

Cheers

John

 

[JonParticipant @jon]

I think consideration should be given when using MT tooling, the drawbar actually needs to be removed when using drills.

 

 

[Ex contributorParticipant @mgnbuk]

You do not mention the type of holder BT ISO, The old NT, or CAT etc.

 

The basic taper is the same on all of them. At work the Bostomatic is DIN 69871, while the Kira & 2 Cincinnati Sabres (a 750 & a 2000) are BT40. The Correa is a 50 taper to DIN69871

 

Only the DIN2080 form has the parallel stump behind the taper – a Marwin Max-E-Mill we scrapped earlier this year used that (50 taper). That machine came with some DIN69871 tooling that could be used with “special” pullstuds that had the parallel stump included. I have a couple of specialist DIN2080 holders away at a spark eroders having the stumps cut off to allow the tooling to be used on the Correa (which replaced the Marwin).

 

Shizuoka were well built machines – you see them here (UK) badged as Matchmakers, frequently fitted with Posidata stepper motor CNC controls and Summit toolchangers. These used an electric impact wrench on a screw drawbar. Unusually they functioned with DIN2080 tooling, the tool exchange gripper fingers having an internal groove that clamped over the flange, rather than fitting into the vee-grooves on DIN69871 / BT holders. The air-operated change arm “followed” the tool in and out of the spindle as the drawbar was screwed in or out. No drive dogs were fitted, as the spindle could not be orientated.

 

Standard design pullstuds are very cheap here – the tooling supplier I mentioned supplies new from around £4 each.

 

The Berg collets are a one-piece turned part that is subsequently cut in to 4 pieces – with a slitting saw, I presume, from the marks on the edges.

 

My previous employer dabbled (unsuccessfully) with an epoxy aggregate construction vertical borer. Horrible stuff – the Technical Director was no where near careful enough working with the epoxy resin & became sensitised to it – he had to wear light cotton gloves impreganated with special cream for weeks to clear up the skin problems & subsequently only had to get a whiff of epoxy fumes to break out in running sores. Be careful !

 

Regards,

 

Nigel B.

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