Metal Shaping Using A Sand Bag

Bossing Mallet and Sand Bag

I started metal shaping using a sand bag. Another first for me. It goes hand it hand very well with the English wheel and can save quite a bit of time.

I was fairly sure what to do with the bag, but a little Youtube guidance never hurts.

I used an ancient maple bossing mallet and a 18″ round sand bag obtained from an auto jumble. The nose cone was originally one piece but that plan was ditched quickly. Just a little tension in each side, was causing massive amounts of twisting and distortion. By simply cutting the nose into 2 halves, the tension was instantly released and each half became much more manageable. I guess 180º of 3D curves was too much for a sand bag newbie.

The wife and I spent an hour filling the sand bag with ‘play sand’. Being October, the sand was damp so we had to dry it on the log burner first before it would pass through the funnel. Drying it, also showed us that the sand wasn’t pure and it fact had grit in it. This was taken out as we filled the bag.

Having a warm, dry bag, with very fine sand maybe helped get quite a good finish. I could have planished it really smooth but a quick pass through the English Wheel got it super smooth in seconds. 

Smoothing the Sand Bag with the English Wheel

Sand Bag and English Wheel

The edges of the piece were shrunk with my deep throat shrinker. By shrinking the bottom edge, it helps curl the sides under. 

The nose had to be tall enough to just clear the MG TF bonnet latch mechanism, but still allow a gradual sweep from the bonnet top into the grille.

Bespoke Bonnet

Once I was happy with the shape of one side, I marked out points every 1″ along the outer circumference. From these, I drew radial lines, so I could use a profile gauge. 

Metal Shaping using a Profile Gauge

I also made sure the length of each line was also identical on both sides.

Getting Both Sides To Match

Metal Shaping using a Sand Bag

The first side took around an hour to make. Getting the left side to match the right is taking considerably longer.

Metal Shaping with a Sand BagI’ve cut the left side oversized, to allow for shrinkage, which so far hasn’t been a problem; in fact I have stretched the steel by about 3mm.

You can just about see in the above photo, the teeth marks left by the shrinker. The right side had very little shrinker work, however the left side, maybe because I cut it over 1cm larger than it’s twin has needed more. As the ‘Paul’s Garage‘ video says:
‘Anywhere that needs to come up, needs to be stretched.
Anywhere that needs to go down, needs to be shrunk’.

The two halves are reasonably close now, but the left still needs work. To tweak the shape to match the profile gauge, I’m now using a combination of the mostly the shrinker, hammer and dolly (anvil) and occasionally the English Wheel to lightly planish the results. 

I’ve numbered the radial lines from 1 to 18. Whilst straightening, you can get the first 10 lines perfectly matched. You then move on to the 11th which is already quite close and apply a small ‘tweak’, which distorts the whole panel drastically. That tweak maybe wrong or maybe it’s the 12th line needing tweaking as well. Every line has to be carefully analysed.  

Once the two side match 100%, I’ll run over them with the DA sander, then I have the central ridge section to fabricate.