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Work Holding For CNC Milling

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Roderick Jenkins14/05/2019 15:46:41
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This is the crankshaft for my Farm Boy engine.

con rod.jpg

The big and little ends still need to be bored out to 9/16" and 5/32" respectively. It's about 5" long and 1" wide, 3/8" thick. Having made this on my Denford/Sherline mill I'm not terribly happy with it, partly due to the software - which I can address but also to my work holding method.

So, how would you go about mounting this on a CNC mill, bearing in mind that it needs to be flipped over since both faces are the same?

Cheers,

Rod

JasonB14/05/2019 16:14:38
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I think I would hold it much as I would with the manual mill.

First machine the two parts manually to overall size, drill and tap then bolt together.

First thing on the CNC holding it in the vice would be spot, drill and ream the holes, couple of different drill sizes for the big end or it could be milled out then reamed. This coulld be done on teh manual mill if you wanted to use a boring head for the big end.

Next put a piece of flat ali bar in the vice and use the CNC to skim it and drill tapping holes. If you can tap with the CNC then do that, if not position the spindle over the holes by jogging and use it to guide the tap for hand tapping.

Without removing that tooling plate screw the rod to it using a previously turned bush for the big end.

Adaptive clearing to remove most of the waste either side & around small end and reduce thickness of rod.

Finish profile with same cutter

Change to radius corner cutter and finish the thickness which will leave a fillet in the internal corners

Change to ball endmill to shape the web with 3D path.

Flip it over to thickness rod and do the web

J

PS I may be inclined to add a bit more shape to the big end so it looks more liked something found on the farm rather than something from inside a barstock IC engine.

Edited By JasonB on 14/05/2019 16:15:22

Edited By JasonB on 14/05/2019 16:18:11

Andrew Johnston14/05/2019 16:23:40
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What's the problem? According to the experts all you need to do is press the button marked 'GO'! smile

How would I do it, assuming rotational symmetry? For a start I wouldn't split the big end at this stage, and I'd leave the material over length. I'd drill and/or ream the little end. I'd also drill a hole in the big end, but of a size and in a position that will disappear when the big end is opened out. I'd set the work co-ordinates to zero in X & Y on the little end bush. Then mount the work with bolts and washers in the little and big ends. If one is particular about accuracy you could bush the little end or provide a tapped spigot. Machine the profile (except the outer end of the big end, so to speak) and then relieve and flute one side. Turn over and relieve and flute t'other side. I'd clean up the outer edge of the big end, and drill and start the tap, for the big end bolts while the stock is still rectangular; makes life easier.

Once all that's done I'd split the big end, clean up as required and bore the big end hole.

Andrew

Martin Connelly14/05/2019 18:33:52
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Connecting rod?

Martin C

John Pace14/05/2019 20:12:07
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149 photos

I use these simple fixtures for these sort of jobs seen in an article
in Mew January 2015 " Cnc fixings" .The pillar attached to the
slotted part is sacrificial can be made to any size and has a
hollow dowel and typically 4mm thread for fixing ,placing these
on the machine table can be part of the cnc cutting file ,i hold these
in a chuck and let the machine locate the position.The slotted part
must obviously be large enough to pick up the nearest tee slot.
The first photo shows four of these fixtures the second shows some
brass connecting links for a hot air engine being cut.

John.cnc fixings 1.jpgcnc fixing 2.jpg

Roderick Jenkins15/05/2019 11:53:32
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Posted by Martin Connelly on 14/05/2019 18:33:52:

Connecting rod?

Martin C

Brain fart - sorry

Thanks, guys, for the comments. I drilled the 2 holes using the DRO on the manual mill and then transferred to tee bolts on the CNC mill table - not accurate enough needs reamed holes and accurate pins but I basically used Jason and Andrews recipe. I like John's fixtures, I'll look out the MEW back copy.

The 2 1/2 D CAM from Denford is pretty basic and I had to fiddle around with variations of the DXF file to get the CAM to understand where I wanted to cut. Fusion 360 has a Denford post processor so I need to watch some more of Lars Christensen's videos and dive in.

I'm wondering whether the Blue Tape and Superglue method (video here ) might work with a blank which could be drilled (with the end mill) to reaming size, profiled, recessed, removed for reaming and then flipped over onto John's pins (which would need to be set up exactly on the X axis). I might try that out on something cheaper than my precious H15.

Cheers,

Rod

Confession : something went wrong, probably with my depth setting, on the final cut and the end mill drilled a hole in the middle of the web before I could stop it. I photoshopped it out to avoid complicating the initial question. blush

Hopper15/05/2019 12:13:45
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Posted by Roderick Jenkins on 15/05/2019 11:53:32:

Confession : something went wrong, probably with my depth setting, on the final cut and the end mill drilled a hole in the middle of the web before I could stop it. I photoshopped it out to avoid complicating the initial question. blush

Once you work out how to combine Photoshop with a 3D printer you'll have those kinds of problems licked. cheeky

Roderick Jenkins15/05/2019 14:27:49
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Posted by John Pace on 14/05/2019 20:12:07:

I use these simple fixtures for these sort of jobs seen in an article
in Mew January 2015 " Cnc fixings" .

Yikes! - that's the one with me on the front cover blush

JasonB15/05/2019 14:33:54
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Posted by Roderick Jenkins on 15/05/2019 14:27:49:
Posted by John Pace on 14/05/2019 20:12:07:

I use these simple fixtures for these sort of jobs seen in an article
in Mew January 2015 " Cnc fixings" .

Yikes! - that's the one with me on the front cover blush

You will have to take it out of the frame now so you can read the other articlesmile p

John Pace15/05/2019 20:39:26
151 forum posts
149 photos


An additional photo shows the set up for placing the fixtures down to
the mill table ,the dowel is held in the drill chuck and providing that the chuck
is not rotated during this procedure the fixings will be in the correct place
relative to each other.In Rod's case of the connecting rod it would obviously
be wise to check the alignment of the position of each fixture .

A little off topic with this part but for those of you who may have read the original article an opportunity to see the end result.

In the original article the subject matter was the production of a pair of
shears to make some dummy cover tapes for a model aircraft.
I am pleased to show here the model that was featured in the article
now completed with the owner David Knott in the centre of the
photo.He is the current British national scale model champion with
this model at last years national championship.

Johncnc fixing 3.jpghurricane.jpg

Andrew Johnston15/05/2019 20:56:44
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Posted by John Pace on 15/05/2019 20:39:26:

In the original article the subject matter was the production of a pair of
shears to make some dummy cover tapes for a model aircraft.

Miniature pinking shears?

Andrew

John Pace16/05/2019 18:20:23
151 forum posts
149 photos

Hi Andrew

Should have said pinking shears ,here they are ,one of the first
cnc type jobs that was done back in 2006 .The blades are
gauge plate hardened and only sharpened on the face ,there is
no clearance behind the cutting edge but they worked well enough
to cut the required tapes from paper.
The model Hurricane i think was started somewhere around 2004,
when i wrote the article in 2009 all of the main structure was completed.
The model was only finished last year such is the level of detail
of models of this type ,it has full cockpit detail and many other
features that would only be seen on the real thing.
David Knott the builder of the model has been British champion
many times and has been in the GB team several times representing this
country, maybe with this model he will win the world championship ,hope so.

Johncnc fixings 4.jpg

Nick Hulme18/05/2019 08:45:22
698 forum posts
37 photos

I usually fixtures in the same CAD/CAM file as the part, just in a different layer, it's easy to create a fixture in the same file as the work piece using the same co-ordinate system and so knowing exactly where everything is.

John Haine18/05/2019 09:17:23
2591 forum posts
133 photos

I have started to use the blue tape/superglue approach at least for brass and ali. It doesn't work as well as you might hope, in particular when profiling the piece you are cutting out can become unglued at inconvenient moments. It works best for things where the work has one or more holes that can be used to screw it down so it doesn't move, with the majority of the blank held down with superglue and tape. It's the tape/substrate joint which goes, getting the superguled tape off the work (or the back of the blank) is a horrible job. I tend to use small bits of "Valchromat" as the substrate, which has a somewhat matt surface, it would probably work better using aluminium.

Andrew Johnston18/05/2019 09:53:16
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4787 forum posts
539 photos
Posted by John Pace on 16/05/2019 18:20:23:

Should have said pinking shears ,here they are ,one of the first
cnc type jobs that was done back in 2006 .The blades are
gauge plate hardened and only sharpened on the face ,there is
no clearance behind the cutting edge but they worked well enough
to cut the required tapes from paper.

Thanks for the elucidation. Very neat looking job. As I recall full size pinking shears didn't have much, or any, clearance. We used them on Irish linen when recovering the Tiger Moth and on madapolam for gliders. For full size aircraft you could buy narrow rolls of cloth with pinked edges, thus saving a lot of hand cutting!

Andrew

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