Work Holding

Work Holding

Introduction
It is often the case that the most difficult stage in manufacturing a product is working out how to hold a billet of material while it is machined. The tool must not foul the working holding system. The work holding system must not get in the way of the machining operations. And how do you hold a part so that you can machine right round its profile?

Options
The F1/WoodWorker mill is supplied with a pair of clamping bars and a machine vice which can hold billets up to 60mm wide. These alone may be a bit limiting. For an overview of other options have a look at the working holding equipment shown in the catalogue of a company like J&L Industrial Supply, 7 Pacific Avenue, Wednesbury, West Midlands WS10 7WP, freephone 0800 663355

The USA company, Mitee Bite, have a range of ingenious work holding solutions: see www.miteebite.com Their products are available in the UK from various sources including J&L Industrial Supply: see also www.hillcliff-tools.com

Glue
Mitee Bite have an adhesive product, available in several forms, called Mitee-Grip. It enables a billet to be glued to a flat surface which is parallel to the X-Y plane of the mill. It is then possible to machine the top surface of the billet and all the way round its profile. The billet and flat surface have to be heated to between 85 and 90 degrees Celsius for the adhesive to hold the two together. When they are cool, machining takes place. When machining is complete the part and flat surface are separated by reheating.

See the Mitee Bite web site, above, for further information.

Sacrificial Bed
This system has proved effective for holding wood, man-made boards, such as MDF, and plastic sheet.

Cut a slab of 18mm MDF the same length as the mill table and 2 to 3mm narrower. The slab will be a sacrificial bed.

Near each of the four corners of the slab drill a clearance hole to suit the Tee bolts used to secure the machine vice or the clamping bars. Position the four holes so that they line up with the Tee slots in the mill table. The holes may need to be counter-bored so that nuts and washers can be put on the Tee bolts.

Use the Tee bolts from the machine vice or clamping bars to secure the slab to the mill table. Ensure that the slab does not overhang the back of the table.

Put the 40mm diameter shell cutter in the milling head and, under manual control, lightly skim the slab so that it is flat and parallel to the mill table. Move the cutter to the full extent of its X - Y travel. (Strictly speaking, the shell cutter is not meant to plunge into material but it will cope with going 0.5mm straight into MDF.)

Put a 3 or 4mm diameter slot drill in the milling head. Manually move the cutter to each of its four extreme X - Y positions and drill a short distance into the slab.

With a ruler and fine marker pen, draw a rectangle whose corners are at the centre of each of the four drilled holes. The rectangle is the working envelope boundary.

Attach a Billet to the Sacrificial Bed
Cover the whole of the flat underside of a billet of wood, MDF or plastic with office double sided adhesive tape. The underside of the billet must be flat. Leave narrow gaps between strips of tape. Do not let strips of tape overlap to form a ridge.

Ensure that the surface of the MDF sacrificial bed is flat, clean and free of dust and the residues of previous use.

Place the billet on the sacrificial bed symmetrically within the marked working envelope. Give it a thump with a hide-faced mallet to make the double-sided tape stick. Take care not to crack brittle plastics.

In the middle of each edge of the billet put a short (20 - 30mm) fillet of glue-gun glue to further secure the billet to the sacrificial bed.

The billet is ready to be machined.

Machining Round a Part
If it is necessary to cut right round the outer profile of a part, make this the last machining operation. If inner machining is done after the profile has been cut there is a real risk of the part breaking free of the sacrificial bed and being spoiled.

Even when machining round the profile of a part there is a risk of the part breaking free at the last moment. To avoid this, do not cut to the full depth of the part. Cut 0.1mm short of full depth. For example, if the full depth is 12mm, cut to a depth of 11.9mm.

To remove the part from the sacrificial bed, use a 25mm chisel. Put the chisel flat on the sacrificial bed and ease the part up in several places until it is free. Do not dig the chisel into the sacrificial bed or the part.

Pop the part out of the surrounding billet like popping a pill out of its packaging. Pop it in a direction which will not tear the part's under-surface, ie push it out through the back of the billet.

Very little work is needed to remove any rough edge at the bottom of the part: a quick touch on a belt sander will clean up wood or MDF items.

The double-sided tape can be difficult to remove, especially from plastic. Place the plastic in a bath of white spirit for a few minutes: the tape will float off. Clean off any residues with a cloth moistened with white spirit.

Make sure that the surface of the sacrificial bed is clean and free of dust and other residues before re-use. When the surface becomes chewed up, skim it flat and clean with the 40mm shell cutter.

Alternative Double Sided Tapes
Ordinary office double sided tape can be difficult to remove. TechSoft UK Ltd stock a better one as do Trend Ltd: see www.techsoftuk.co.uk
and www.trendmachinery.co.uk and www.techsupplies.co.uk

DIY stores stock wide double sided tape intended for securing carpets to floors. There are various grades of adhesive ranging from fairly easy to remove, through fairly difficult, to almost impossible to remove. Make your choice with care!

Toggle Clamps
Trend, and no doubt other suppliers, have a range of toggle clamps which may be useful for holding work, www.trendmachinery.co.uk

Vacuum Clamp
If anyone knows of a supplier who has a vacuum clamp which fits the mill table, please let WELsoft know so that we can let others know. Similarly, if you have a plans for a DIY vacuum clamp, please share them with us.

Billet Size and Working Envelope
Safety
Setting the Origin for Machining