Model Engineers’ Workshop – August 2019

70 http://www.model-engineer.co.uk Model Engineers’ Workshop

All the inner rolls were made from
the 65mm bar which was drilled as per
the cheeks, but with 6.1mm clearance
holes. After returning it to the lathe,
the end with the 28mm bore was then
machined to create a shallow 50mm
recess, approximately 0.4mm deep
before parting off a 4.5mm slice. This
was repeated for the matching 4mm roll.
Both slices were then mounted on the
expanding mandrel with the recesses
together, to allow another 50mm shallow
recess to be machined on the parted-off
face. This recess was sufficiently deep
(approx. 1.5mm) to remain a recess after a
later operation reduced the slice thickness

to 4mm. The slices were then reversed to

process the other one. See photo 15.

It was then necessary to machine each

slice to 4mm, but the expanding mandrel

was not suffi ciently accurate for this, so

another mandrel was needed.

This was made from a short length of 2”

bar, faced at both ends with a short 28mm

shoulder at one end. The bar was then

mounted vertically in the mill and three

equally spaced 4.5mm holes were drilled

on a PCD of 39mm. The holes were then

tapped M5, see photo 16.

Each slice in turn was then mounted on

the mandrel and screwed securely, rough

machined side outwards, to the shoulder by

means of three M5 screws – see photo 17.

The purpose of the recesses is twofold.

1. The slice can be bolted in direct contact
   with the turned surface of the shoulder.


2. Because the M5 retaining screws are
   within the recess, the slice can be accurately
   trimmed to precise thickness without
   having to worry about the retaining screws.
   The next job was to machine the semi-
   circular groove on the edge of one of the
   rolls. The prospect of grinding a semi-
   circular parting-off type tool, exactly 4mm
   wide, was not attractive, especially since I
   needed a 3.5mm one as well. So, I decided
   to obtain two HSS drill rods (4 and 3.5mm

diameter) grind one end rather like a

tangential turning tool and make a suitable

holder. It worked really well – see photos

18 & 19.

The fi nal part of making the rolls was

to cut an 8mm keyway. Since neither roll

would experience torque very much and

be turned very slowly, there was no real

requirement for a proper rectangular

keyway, so I decided to cut a small slot with

an 8mm end-mill. To ensure consistency

I made and used the simple jig shown in

photos 20 & 21.

With all the parts made it was time to

test the theory. I assembled both new rolls,

photo 22, rolled a piece of copper to 4mm

square in the V-rolls, and put it through

the new rolls. It seemed to roll nicely, but -

horror of horrors - I couldn’t remove it from

the “die” roll. It was well and truly stuck.

I could see one end of it, which looked

exactly right, but it just wouldn’t budge.

The considerable pressure needed to

mould the copper to the required shape had

jammed it onto the sides of the deep groove.

Serendipity smiled. I just removed the

roll from the mill and undid the screws

holding it together. The copper then came

out easily, and it was just the right shape.

Had I made the rolls by machining them

from one big lump I would have been well

Had I made the rolls by

machining them from

one big lump I would

have been well and

truly stuff ed.

17 18

19 20