Here's another nice mess I've gotten myself into...

I'm pretty bad at planning machining sequences - I have a lamentable tendency to just get stuck in and think 'I'll cross that bridge when I come to it'. It rarely works out well. Latest is this:

(no idea why pic is rotated 90 degrees?). I cut the pocket (2.5mm deep) at the crotch of the fork with a 12mm endmill (biggest I've got) whilst the work was still on the mill, but I need to enlarge it to 20mm -ish. I've tried mounting the part in the 4-jaw on the lathe to bore, but my attempts to get it square and centered have been defeated by entropy. Any thoughts on how to proceed - or how I could have avoided getting myself into this nice mess by more forethought?

Rob.

Edited By Neil Wyatt on 17/02/2018 11:28:28

My $0.02 worth - bear in mind I have limited experience in these matters

Would it make sense to start with the 20mm hole and then finish the rest of the piece using the hole circumference for reference?

I'll get my coat...

Back in the mill, centre on the hole and if you have then a boring head or knock up a fly cutter and bore with that..

I would make a stepped mandrel to fit the the hole and the recess. Clamp the mandrel in the tailstock chuck and use this to hold your inplace whilst you set the four jaws to the job. Hope this makes sense.

If you have a small vice that could be mounted on a faceplate it may be easier than setting up in a 4 jaw. A vee block on a faceplate may be another possibility.

Mike

Either clamp it to the lathe faceplate and bore it, or make a "sub-faceplate" out of a piece of flat bar or plate. I use 10mm thick aluminium alloy flat bar about 50mm wide and 150mm long for this. Then drill and tap a couple of pairs of 6mm holes in the plate, strategically located so that a pair of clamping bars, -- cut from 20mm x 5mm or so steel flat bar with two holes drilled to match the tapped holes in the base plate -- can be held held down with 6mm screws.

It looks like you will need to put a bit of packing under your job so it sits on the square face. I use makeshift "parallels" cut from a length of 6mm square keysteel or bar for this.

Attach the job to the plate, then bolt or clamp the plate to the faceplate. I have a hole drilled each end of the plate so a bolt goes straight through and thtrough the faceplate. Then I use a clamp on the other end to allow movement to adjust it to get the job centred.

A bit of mucking about, but I use that sub-faceplate over and over again for drilling and boring small parts in teh lathe where I want more accuracy than my cheap Chinese drill press will give me (I found its table is half a millimetre out of square with the spindle!) so its well worth the effort to make up. You could use a piece of steel flat bar if no ally in the scrap box.

A pitcher's worth a thousand words. Here's what I was rabitting on about:

Counter-boring a recess in the middle of the brass sector arm for the Fabricated Versatile Dividing Head.

In you case, you would not use the bolt through the middle but a second clamping strap and bolts over the two unused screw holes to the left of centre. Again, its worth making this simple device. It is just the handiest gizmo and gets used a lot. My son and I recently used it to drill and ream the holes in the connecting rod small end and big end bearings for a Potty Horizontal Mill  engine. Much tidier than using the drill press.

Edited By Hopper on 17/02/2018 03:58:26

ER collet block to hold by the round spigot and then fix that to a face plate and bore out to 20mm if you only have the tooling to cut on the lathe. No ER block then just bore a hole in a bit of square stock and drill & tap a cross hole for a grub screw to lock the part in position.

If on the mill hold by spigot and use a boring head or home made cutter.

Edited By JasonB on 17/02/2018 07:41:57

Jason you read my thoughts re square block. I often make up little jig blocks to hold awkward shapes.

if you want it absolutely central in the lathe, why not take a piece of bar, mount it in the 3 jaw, and bore it out to be a really tight fit on the diameter of the stem. Mark the position of No 1 jaw, remove and slit along the axis, with a hacksaw.

Deburr, replace in the 3 jaw with the mark aligned on No 1 jaw.

Fit stem into this newly made collet, clamp up hard.

Bore out to 20mm.

HTH

Howard

Thanks to all who have replied - and to Neil for silently reorientating my photo!

It looks like either a faceplate job or boring on the mill. I have a faceplate which came with the lathe, but have never actually had need to use it. LIkewise I I have a boring head - bought to turn long tapers on a lathe with a fixed tailstock - but have never used it on the mill. So either way there's a learning curve...

I had thought about boring on the mill, which would be easier from a workholding/centering POV as it's all set up, but I can't see how I would get a decent finish on the bottom of the 'pocket'. When i do this sort of thing on the lathe I cut axially to near diameter and a tad under depth then take a radial swipe across the bottom and up the side to finish. I can't see how I can do anything similar with a boring head in the mill .Is there a technique for this?

Thanks again to all for suggestions - much appreciated.

Rob.

As you say its would be difficult to get a flat bottom using a boring head in the mill, unless it was the boring and facing type, and it would be an OTT method. For speediness and expediency my money would be on using a four jaw in the lathe.

Once you can hold it (4 jaw or faceplate) then holding on a rotary table in the mill is another possibility. By using an end mill less than half the recess diameter, getting a flat bottom is easy. I have a simple manually rotated table that I use frequently to create large counterbores. Sitting an already drilled hole on a spigot easily ensures the counterbore is concentric. The same technique works without a rotary table, just by hand rotating the part as long as its size and proportions make it amenable to hand rotation.

Ian P

OK, looks like a faceplate job. So I'm going to have to get to grips the thing - a PITA or a learning opportunity! I think the latter - it's potentially a very versatile thing. I'm a slow learner.

I'm going for Hopper's gizmo and Jason''s suggestion of a bored block mounted on that - I have only an ER25 collet block (the spigot is 19.050(+/-0.002)mm diameter ).

My faceplate (250mm) isn't too friendly because the slots are short and close to the periphery - so some sort of adaptor has anyway to be made to work on smaller work close to the axis.

Rob.

Edited By Robin Graham on 20/02/2018 00:25:59

Hi Robin, you may find it useful to drill and tap some holes in the faceplate to enable parts to be fixed for turning, especially useful for holding smaller parts for turning.

Emgee

Take up Baking ? Or gardening, , perchance?

It's a classic, another "useful" and "informative" post from one of our leading armchair users.

Andrew

Hi Robin, your faceplate will work fine if you use Hopper's gizmo and follow his instruction words reprinted below:

"Attach the job to the plate, then bolt or clamp the plate to the faceplate. I have a hole drilled each end of the plate so a bolt goes straight through and thtrough the faceplate. Then I use a clamp on the other end to allow movement to adjust it to get the job centred."

Drill a couple of holes in outer ends of your plate to suit your faceplate's outboard slots, use some through-bolts and rear side clamps (just some steel bar with a plain or tapped hole in centre) behind your plate, and you are in business. JD

Not quite sure what I did to deserve that!

Anyhow, in an interval between my baking activities (making a properly formed 70% hydration baguette is at least as challenging as turning metal to size actually - bakery is an art not to be sneezed at!) I made the Hopper gizmo and a block to hold the work as per Jason's suggestion.

Andy Carruthers - I think you are right in saying that I'd have been better off starting with the 20mm pocket and referencing everything off that. It's sort of counter-intuitive, for me at least - I see the the raw bar of metal, think I'll do the 'major features', in this case milling the fork, then attend to details... I need to take a different view.

Thanks again for helpful responses.

Robin.

.

Nothing, Robin

MichaelG.

Thanks Michael.