I would saw off the 4 corners

Hold in 4 jaw and turn to 76mm dia for half the 12mm thickness.

Change to 3-jaw and hold by the 76mm dia to taken the remaining 6mm of material to the same 76mm dia over the full 12mm thickness

Drill and bore the hole

Hold by hole to take the last 1mm off diameter to your final 75mm

Drill and tap for grub screw

Remount in chuck or on a bit of 25mm bar so your fingers are away from the jaws and finish with scotchbrite or Emery

Edited By JasonB on 10/09/2022 20:52:04

I would:

• cutoff (3) corners so that the part fits in a 3 jaw chuck. Draw a circle and use that as a guide
• drill a 24 or 24.8 mm hole. Be gentle, the work holding isn't that good.
• turn or ream the hole to 25 mm
• face both sides gently.
• Make an 25 mm arbor to hold the part
• Turn the outer diameter
• Drill and tap the hole for the grub screw.
• Check the length of your tap before you start. 25 mm is on the limit. If you bore the hole thicker over a length of 10 mm, a standard 5mm tap will do

If you have a mill, you could also make the 25 mm hole on the mill.

If you face your 12 mm part, it won't be 12 mm thick after facing.

If this is your first exercise in stainless, take some practise cuts at this diameter on another piece of the same type.

If you use the jaws to hold the 25 mm hole, check if the hole fits the jaw. My 80 mm chuck can hold 23 mm on the outside of the jaws.

if you hold the part in the jaws, check if the jaws don't hit the lathe when turning.

Edited By Huub on 10/09/2022 23:39:19

.

Very ‘forward thinking’ for someone with your alias

MichaelG.​​

The first paragraph states " I have a piece of 304 75mm x 12mm x 80mm stainless bar to make this from". Why buy a second and waste the first piece ?

Simply because 304 is an unpleasant materiel to machine.

Maybe because Old School has been around the course before ?

Rowan, as Chris Evans said your choice of material is not great. 304 stainless is an awful choice for a novice to machine.

Does the part particularaly need to be stainless or is it just that you happened to have some so decided to use it?

By no means an expert, but what about using a rotary table on a mill, or CNC to do the job? I normally make things out of acrylic and other plastics (mainly because I can get offcuts FOC from my ex place of work!) My Siieg SX2.7 makes short work of maching 12mm aluminium bar, and my little 3018 CNC router cuts plastics up to 19mm easily, just takes a while.

Although 304 machines reasonably well when cutting, it work-hardens in a blink if the tool fails to cut for any reason; blunt tool, rubbing, depth-of-cut, feed-rate or rpm too slow etc. Trepanning is high-risk for all of these, and 12mm being a deep cut will multiply the problem.

A fair amount of skill is needed to cut 304 and similar stainless steels without work hardening them. My first attempt to drill stainless was a disaster because I started with a blunt old twist-drill in a hand-powered drill. It work-hardened amazingly, and the hand-drill wouldn't touch it. Then it instantly blunted a new TiN coated brand-name drill driven by a power drill. Another new drill moved slightly to fresh metal, went through easily. No problem if 304 cuts first time, nightmare if it work-hardens. The difficult part is keeping the cut going.

Dave

O.K. Jason … I probably got it all wrong

I will keep out of it for now

MichaelG.

Similar to Peter Cook,s suggestion.

1. make an arbor with a 25mm stub to mount the job. There should be a shoulder on the arbor which will keep the workpiece running true so there is no side to side wobble. The shoulder should be 10mm or so i.e. the o.d. around 45mm. The arbor should be turned in one set up so the shoulder and 25mm stub are true make the stub 0.5 mm shorter than the workpiece say 11.5mm, I would under cut the shoulder so only the outter 5mm supports the workpiece. Tap the the end of the stub 8mm or similar. Make a second ‘washer same dia as the shoulder with the centre hole clearance on your chosen clamping thread. Ideally undercut one side like the shoulder. Set this aside.

2. Find the centre of the the square/ rectangle and centre pop or spot drill it this will help in setting up. Remove any burrs from the edge to aid seating and chuck he square workpiece in the 4 jaw. Use the centre mark to centre the job. Bore the 25mm hole; centre drill then use the largest drill your lathe can safely handle, there is no point in doing excessive step drilling when you will be boring the hole to dia. as this will correct any error from the last drilling op.

3. knock off the corners by whatever method suits but don’t get it hot as the job might distort.

4. Re Chuck the arbor and get it running true on the stub dia. and shoulder face. Mount the job and clamp it up, get it running as true as you can i.e. minimise the side to side wobble.

5. Remove the corners, take it slow and watch out as the corners are near invisible when spinning. I would use an HSS tool as carbide won’t like the interrupted cuts. You can swap to carbide once it’s round if needed.

as for the grub screw it sounds like you are drilling radially, that’s going to be tricky to tap and I am not convinced that is a great method given the ratio of the disc dia. to its thickness.

Hope this helps.

Tony