the best method to get out the Centre bit

[lordedmond]

Ok the problem I have a 5 inch disc 1 inch thick of en1a and I need to get the middle out ( its not critical dimension wise say 3.5 inches )


I have a lathe thats ML super 7  , a SX3 mill with 6 inch rotary table

The bit I need is the bit that is like a polo mint  so I can drill the middle to clamp it down on the rotary table


Before you ask its for a flywheel rim and the tube that I can get hold of is to thin


Stuart

[Jasonb]

Stitch drill all round just inside of the line you want that way you get to use the bit in the middle for something else and should just be able to clamp it to the R/table which saves drilling a cental hole.

Once its drilled out put it in the lathe to finish off

[picclock]

You may have already thought of this. Centre rotary table under head (I use a dti magnetically attached to the spindle). Lock the axis. Loosely clamp outside of disc to rotary table as near as you think is correct. With dti attached to mill table and rotary table on free rotate  by hand to find high spot and tap it with a soft hammer . Repeat until disc is centred.

With practice I've found I can centre 131mm discs (to better than 0.5 thou) in less time than its taken to type this.

Hope this helps  

picclock

[lordedmond]

Stitch drill all round just inside of the line you want that way you get to use the bit in the middle for something else and should just be able to clamp it to the R/table which saves drilling a cental hole.

Once its drilled out put it in the lathe to finish off

Thanks for the input its going to take a lot of drilling , but hey I forgot to mention I have time as well as tools 

[JimM]

Not the most elegant way but could you use a holesaw to remove the centre portion then use the RT and mill for clean up. Only suggest this as I've got a cheapy marketstall  holesaw set that I picked up when installing some downlighters and not yet found a use for the other sizes 

Cheers

Jim

[andyf]

When chain drilling, it might help to spot for each hole, then go round the first time drilling every other hole, then go round again filling in the gaps. Otherwise, the drill can tend to veer off into the hole you have just drilled. Having a hole on each side as you go round the second time evens things up, so the drill can't make up its mind which way to veer off.

Incidentally, there's a nice little graphical calculator to work out the number and location of the holes which can be downloaded from here: http://iwr.ru.ac.za/~iwdf/lathe/pcd.html . It can pay to fiddle around with the number of holes, or reduce the diameter of the "big hole", until it comes up with a drill size which you have available.

Andy

[lordedmond]

Thanks all for the input

Looks like its lots o drilling  


Stuart



andyf  thanks for the link its an unfortunate thing that I do not have a windows pc , I use  a Mac so exe do not work

[mike os]

put it in the lathe & holesaw most out... no need for a pilot hole so you get a useable blank, then bore out to size

[lordedmond]

thanks I will look into the costs of a hole saw


Stuart

[picclock]

Hi Stuart

I thought you just wanted to get it centred  .

FWIW I have cut through 1/2" with 1/8" carbide 3 flute mills - so its possible but takes a while (my machine only goes up to 2000 rpm) and you have to clean out the groove every circle. You would have to turn it over and repeat to get the depth, although with a programmable table it wouldn't be too bad. I've done this with 131mm blanks in 10mm S275 steel OK, and your only doing 3.5"

You may be better off turning it on the lathe. If you turn a groove from the end about 1/2" wide and deepen it with repeated passes allowing it to narrow as you proceed.  

But the fastest way (and the most boring   ) is probably to drill but you have to add the 'tidy up time'.

Best Regards

picclock

 

[lordedmond]

Thanks

[Jasonb]

Or pop down to your local water jet cutters

http://www.sciss.co.uk/epages/sciss3.sf/en_GB/?ObjectPath=/Shops/sciss3/Categories/Gallery

[andyf]

....andyf  thanks for the link its an unfortunate thing that I do not have a windows pc , I use  a Mac so exe do not work

Wish I didn't have to get involved with Windoze!

The PCD program is easier to use in metric. You said more or less 3.5", which is about 90mm. If you do decide on  chain drilling, an 8.5mm drill making 30 holes at 12^o intervals at a PCD of 81.4mm would do it. So would a 6.5mm drill making 40 holes at 9^o intervals at a PCD of 83.45mm. 

And

[lordedmond]

Or pop down to your local water jet cutters

http://www.sciss.co.uk/epages/sciss3.sf/en_GB/?ObjectPath=/Shops/sciss3/Categories/Gallery

I would need a packed lunch to get there

I live in south Derbyshire

Thanks anyway   

Stuart

[Bogstandard]

If you can hold it on your lathe Stuart, the easiest way would be to grind youself up a trepanning tool of just over 1/2" cutting depth. Cut one side, turn around and then cut the other side. Centre falls out.

Or just clamp it central onto your RT, and using something like a 6mm slot drill, machine thru it. Don't forget to put something sacrifical underneath, otherwise you would end up with a new circle in your RT top.

I do have a commercial tipped trepanning tool, but it is a bit far for you to travel just to get it done. But the offer is there.


John

[BillTodd]

Drilling holes will get the job done !

http://www.mycncuk.com/forums/showthread.php/1165-Drill-Mill-Spindle-brake-%28RF30%29?p=9000&viewfull=1#post9000

Bill

P.S. The centre of this block is now my QCTP:
http://www.homemodelenginemachinist.com/index.php?topic=5153.msg102066#msg102066

[lordedmond]

Thanks john for the Offer  and advice
Same for you Bill


I am now spoilt for choice


Stuart

[Jonny]

Used to do this all the time in an ML7 but with a 5"+ chuck.
Will take an eternity just drill hole through with largest available then bore out.

Have done same on rotary table with chuck off the scrapped ML7.

The cheap holesaws from the market i would give 10 secs on that material and dont think the ML7 will power with a 3"+ decent holesaw.

Best way borrow a bigger lathe for 10 mins., welcome to use either of mine.

[Anzaniste]

If you can hold it on your lathe Stuart, the easiest way would be to grind youself up a trepanning tool of just over 1/2" cutting depth. Cut one side, turn around and then cut the other side. Centre falls out.

Or just clamp it central onto your RT, and using something like a 6mm slot drill, machine thru it. Don't forget to put something sacrifical underneath, otherwise you would end up with a new circle in your RT top.

I do have a commercial tipped trepanning tool, but it is a bit far for you to travel just to get it done. But the offer is there.


John

What do they look like and who makes them. I regularly trepan the middle out of sprocket chain-wheels and could be interested in a better tool than mine. I always approach the job with knocking knees!!!

[Bogstandard]

Not cheap by any means, but can be had for reasonable prices on eblag, if you are willing to wait for when they come up.

They are made by Mircona.

You will need to wait until I can get a piccy of mine tommorow, to show what they look like.


Bogs

[Bogstandard]

Here is my new 20x20 one


You can see how they support the tip with a curved member. This is what feeds into the material.




A view from the top. I can use tips from 3mm to about 5mm wide, at a push. This one will penetrate about 14mm, it says on the code spec 12mm plunge depth with a 4mm wide tip, but those figures can be extended a little. They are available in both right angle, like this, or straight on.




The last part of the code, 60 - 75. The trepanning diameter must start within that range, but once to depth, you can go both larger and smaller.



Normally you can pick them up off eblag for around 25 squid, a fraction of the cost if bought thru a retailer or Mircona direct.

I hope that explains it ok.


Bogs

[lordedmond]

Thanks for the info John



Stuart

[Anzaniste]

Thank you Bogs, helpful, clear and concise as ever.  I do enjoy this web site.

[Anzaniste]

I have patiently trawled fleabay for a Nircona trepanning tool but to no avail and then I had a Sprocket to modify like this:-

http://www.flickr.com/photos/74168320@N00/5613441927/sizes/z/in/photostream/

http://farm6.static.flickr.com/5187/5613441927_58db44dd7a_z.jpg

Dash it I seem to be having trouble getting the BBthingey code from Flickr, I'll try again later.


Big hole - Copy by Anzaniste, on Flickr

Good grief that took some doing, why do they keep changing things. Rant over back to the subject. The sprocket is 10" diameter and the hole is 6 1/2" diameter and 7/16" thick. Nervously I dug out my trepanning tool, a war scarred veteran of quite a few sprockets.


IMG_0373 by Anzaniste, on Flickr

This is it, note the curve on one side and the straight side nearest the centre of rotation to give clearance. Some how or other the front clearance had disappeared (!!!!!) in the drawer and luckily I realised what was going on before I broke the tool.

To mount the sprocket I drilled 4 off 8mm holes to coincide with the slots on my face plate and packed it out from the face plate using a couple of repair washers on each bolt and centred it, using a dial gauge as per 4 jaw technique, on the 1 1/2" pilot hole in the centre of the plate. My lathe is a CVA which is good and solid so it does not give trouble if you are careful.




Cutting action by Anzaniste, on Flickr

Here is some cutting action to initially achieve this, a 5.5 mm deep grove. The cutting tool is 4.75 mm wide so I am removing quite a wide piece of swarf.


Swarf by Anzaniste, on Flickr

This is the groove.


5.5mm deep groove - Copy by Anzaniste, on Flickr

To achieve this without breaking the tool requires certain techniques. Firstly the stance, knees slightly bent, buttocks firmly clenched, tong half out and a grim facial expression; this appearance has the effect of keeping anyone in the vicinity well away 'cos they know I'm concentrating and have had the benefit of an attitude assesment when I have been interrupted in the past!!! 
On the more technical side, correct cutting speed, buckets of suds, continual smooth slow in-feed of the tool (a two handed job ensuring continuous rotation of the hand wheel) and listening very carefully to the sound of the cut, you are listening for any change in the tune of the song it is singing. I also take frequent rests as these days I cant keep it up for long. I have never been brave enough to power the tool into the cut though I am sure that would be the best way on a very fine feed.

As part of the operation is thinning the bit of the sprocket that I want to use that is the next operation. This then means that I do not have to plunge a full 11mm with the attendant risks. A second groove is plunged nearer the periphery .


2 grooves - Copy by Anzaniste, on Flickr

And then the waste is turned off and the final plunge is taken leaving the bit in the middle which is where this thread started off.


And here it is showing the packing washers.


Packing washers by Anzaniste, on Flickr

Sorry if this posting is a bit long winded but I have been so impressed with other folks project threads I thought I would have a practice on some thing simple to start with.
Surprisingly it has taken me all of an hour, I could have made a lot of swarf in that time!!
 All the pictures were taken on my Iphone using available light of which there was not much.

[John Hill]

I found it essential to firmly clamp the entire piece to the faceplate as if the outer ring breaks free things can get broken!