Hex holes

[Trebor]

Well I only have a myford s7 and I need to machine a hex through hole about 12mm (hex bar) into a piece of brass round bar 20mm long. I'm trying to find out which ways to go about doing this, if it works I could end up doing 100s. As yet I have no rotary table, indexing system, vertical / horizontal slides. I only have a three and four jaw chuck but looking into collets for the future. What's my best route to take ? Help I have a headache 

[DMIOM]

Have a look at this thread which may give you some food for thought.

Dave

[Trebor]

If only I had the tools or the money to buy one, just now I have to be a bit careful just what I buy. I was thinking about drilling 6 holes and machining out the centre to start with. Only it could be very repetitive.

[drmico60]

Hi Trebor,

If you have many of these to make then rotary broaching is probably the easiest way to go. I have developed a very simple rotary broaching tool which is described here:
http://mikesworkshop.weebly.com/rotary-broaching.html
The version I describe would be too small for what you want to do but you could probably scale it up to accommodate a bigger tool.

Mike

[Jasonb]

Or just use a hex broach and press you could cut several hundred in the time it takes to make  a rotary broaching tool

http://www.steelmans.co.uk/?130,12mm-hexagon-broach

They can be homemade from silversteel (drill rod) if you just want a try.

[Trebor]

I'd seen them but £200.00, if it didn't pass the test what would I use it for again and I'm stuck to that size so the hex bar would always have to be spot on. At least if I went and machined the first couple I could still use the tools again. What would be the least tooling I would need just to make a couple.

[Jasonb]

A drill and a file

[Trebor]

 

[Lew_Merrick_PE]

Back in the Dark Ages when I served my apprenticeship, one of the standard tests was to make male and female hex by hand.  I was given a piece of 1 inch HRS round bar X 3 inches long that I was allowed to file into a (male) .750 (+.000/-.002) inch hex.  It had to pass through a (female) hex Herr Meister Muller had (I believe he made it himself) in all 6 orientations without binding.  It took me four tries to meet that standard.

I was then given a piece of 1/2 inch CRS bar through which I had to cut and file a (female) .750 (+.002/-.000) inch hex.  I drilled (ø1/16, as I recall) holes with an "eggbeater" drill just inside the corners of the hex and cut out between them using a fretsaw.  I ground down (3, as I recall) mill files to .375 inch wide which were used to file the first side of the hex square to the face of the bar.  (I spent 4 hours/day for a week getting that face to me Herr Meister Muller's approval.)  I then made (4 or 5 as I recall) a set of files that were .375 wide with a 120° safe edge on one side and a 90° safe edge on the other.  I used these to file out the other 5 sides of the hex.  A master .750 hex gage pin had to pass through my (female) hex in all 6 directions without binding or tipping (.005 inch maximum side motion at .500 inch from the surface of the bar).  I completed that the first time -- though I spent the better part of a month doing so...

There is always that approach...

[Trebor]

I'm not sure what to say to that, well done, sod that, I'm sure I would cock that up although I'm not bad with a file !! I remember casting an alloy block at school to make a rotating cake stand (top marks for that) we used to cut half crown sized disks for the cig machines.

[awemawson]

Blast it out with a die sinker edm machine

[Trebor]

 

Blast it out with a die sinker edm machine

What's one of those ?

[awemawson]

Electro Discharge Machining : EDM for short. A controlled electrical discharge is created between an electrode (that is usually copper or graphite) and the work piece, and the electrode holder is fixed on a servo controlled ram that lowers it into the material being cut. All this happens immersed is a dialectric fluid chosen to insulate so that the discharges happen above a critical voltage. Frequently used for forming the dies for press tooling hence the term 'die sinker edm' Process has the happy feature that you can cut precise shapes in very hard materials using easily shaped soft electrodes.

Mine is a Eurospark H425 P25 like this one, but this one is just one I found on the web for you:

[chipenter]

Drill a 20 mm hole in a piece of hex bar putt the brass bar in the hex with a set screw , set up some stops on the cross slide and drill the corners out , then drill the centre out mount a piece of tool steel in a four jaw chuck lock the head stock and use the saddle as a slotting tool .

[hopefuldave]

if it works I could end up doing 100s

If youll be.doing hundreds, definitely the rotary broach, it'll go like this:
Face end - 5 seconds;
Centre drill - 5 sec;I
Drill to a/f diameter - 5 sec;
Broach hex - 10 seconds

Total 25 seconds cutting..

On a rotary table youll spend at least that long just cranking between positions! Drilling and filing (accurately) will take minutes per item just in go/no-go gauging. Broaching on a press... A press will cost 100 local currency units or more, the broach itself won't be cheap...
The rotary broach holderr will take an evening in the shop to build, and you can grind the cutters any size and shape you like, evermore, so it's a good use of your time

[Trebor]

I wouldn't have to file anything, the six initial holes would act as a vent as long as the flat spots are good, not too worried about the corners.

[Trebor]

Just been looking at this but the dial handle will face the back of my machine with the way I would need to use it.
http://chronos.ltd.uk/acatalog/info_CG34_.html

[Jasonb]

If you are thinking that it has to stand vertical and you tilt the mill head whild drilling your six holes then thats not how to do it they also mount horizontally so teh handle can face you and the mill head does not need to be touched.

[Trebor]

I am only using it on a lathe if it fits, I don't have anything else yet. Not sure how it would attach as I've never seen one. I need to mount a chuck facing my lathe chuck with it on cross slide, I don't have a vertical slide.

[hopefuldave]

I think.Bob said he only.has a (Myford) lathe and a couple of chucks etc? If so, the rotary table would need to stand vertically on the cross-slide as.he suggests...

It could br done, but I can't see an alternative to milling the flats (other than filing) with a small, delicate cutter in a less than rigid setup - recipe for broken cutters and damaged workpieces!

A local engineering works went from CNC machining hex sockets in parts to rotary broaching, saved upwards of.a.minute per part, lot less breakages of small carbide endmills,  they reckon it saved £1 or more per part, £200 a day up in profit...
A rotary broach would.be worth the.time in construction, I think.

[hopefuldave]

I am only using it on a lathe if it fits, I don't have anything else yet. Not sure how it would attach as I've never seen one. I need to mount a chuck facing my lathe chuck with it on cross slide, I don't have a vertical slide.

They have a.couple of slots for bolting into the cross-slide T-slots, youll want to put a.DTI on the.chuck/collet and sweep.the table surface to check for squareness etc, .allot 10 mins or.more to get everything lined up right! My rotary broach holder's built into a boring bar holder for my QCTP, takes about 30 seconds to fit.and line up, let's.me use power feed and stops for depth... ;)