How to Fix or Hot-Rod Your Jacks

How to Fix or Hot-Rod Your Jacks

I'm a tool geek; I love to try different tools by established makers, have custom tools made to my specs as well as make my own when I can't find something that meets my needs.  

As most toolmakers aren't glassblowers, they aren't using their own tools and need to rely on feedback from their customers.  But since glassblowers have a range of styles and personal preferences for tools, not all tools are ideal for everyone.  I've learned to modify many tools to better suit my needs and jacks seem to be the ones I've most needed to tweak.  

The problems I've run into with jacks are usually the following:

  1. A tendency to corkscrew, making it difficult to create a single, uniform jackline.
  2. Blades that suck too much heat out of the glass, shortening working time.
  3. Too stiff a strap, making it hard to squeeze a neckline and feel the glass as you do so.
  4. Scratchy blades leaving gouges or scratch marks on the glass.

Here's how I've addressed each:

Corkscrewing on a jackline/neckline - Sometimes this is user error--if you tend to dig in and squeeze your jacks hard when first starting a jackline, you'll raise your risk of corkscrewing.  But if you're gentle in first establishing the line and you still get corkscrews, often the blades of the jacks are too sharp.  Sharp-bladed jacks look cool and would seem to be a good thing, but in practice they work poorly.  A sharp blade is able to cut into the glass better than a dull one, which seems like a good idea, but if sharp jacks are even slightly misaligned on a hot neck it's more likely each blade will cut a slightly different line with each rotation rather than slide and establish a uniform groove.  This is how corkscrewing happens.  I've found that dulling the edge of the blade doesn't sacrifice any ability to put in a crisp neckline and really helps if you're having trouble with corkscrewing your jacklines.  Slightly duller edges on your jacks are also less likely to overheat and get scratchy while necking.  A nice blade style is to round out the blade a bit on the bottom 3" of the jack blades but leave the top section of the jacks a bit sharper.  This way when you use the bottom part of the blade to establish your jackline it is less prone to corkscrewing and if you want a sharp line you can still use the thicker and sharper part of the blades.   

Heat Sink Blade - If your jacks have super-beefy blades, you're trading longevity and heft for working time.  It's simple physics that thick-bladed jacks (either too wide, too thick or both) will draw the heat out of the glass more quickly than thin-bladed jacks.  The blades of my favorite large jacks are actually on the thinner side, which makes them look a bit delicate, but they are plenty stiff and strong for necking big work as long as it's hot.  Thinner blades (less mass/heat sink) means that I can get necklines in quickly and move the glass more than if I had heavy-duty blades that sucked out more heat.  I've had good success taking thick-bladed jacks to a large wet belt sander (106 x 4) and thinning and re-profiling the blades.  Obviously you want to take a great deal of care in sanding down your blades, but if you take your time and do it progressively, checking your work as you go, it's possible to remove a good amount of metal from the blade and improve the performance of your jacks.  Take care and have a clear vision for what you're going for, don't do too much at once and keep the blades from overheating while you sand.  I check the blades with digital calipers as I go to ensure they're even.

Stiff Strap - The stiffness of a pair of jacks is a function of the thickness and width of the metal that forms the strap as well as how it has been heat-treated.  The strap is not only the spring for the tool, but it's also a mini-marver and provides alignment to keep the blades in the same plane.  To adjust the stiffness of the spring, there are three obvious approaches--remove metal from the strap by reducing its width, remove metal from the strap by reducing its thickness or soften the spring through heat treating.  Since I feel the stability/alignment and width of the strap are important, I haven't attempted to remove metal to adjust stiffness, but I know some makers do this in the final tuning of a set of jacks.  I have had success by re-heat treating the strap in order to relax the springiness in the metal.  This needs to be done very gently and evenly to prevent warping the strap and throwing your jacks out of alignment.  I've used MAP gas on the very center of jack straps until I see a very dark cherry glow.  Then just letting the jacks air cool has given me great results.  I like hitting only the middle of the strap, leaving a stronger spring in either edge of the strap which I think helps maintain alignment.  This heat treating will tend to blacken the strap of your jacks so you have to be ok with that or plan to sand off the surface discoloration.  I've only done this with large jacks that were too stiff--I would be extra careful with a more delicate tool like goblet or cup jacks.  

Scratchy Blades - This is most often the result of dirty wax on the blades or the surface of the blades not being smooth.  If the blades have gotten rusty or been dinged by rough handling and impacts from other tools, you're likely to have scratchy jacks.  To diagnose what's up, first use heat to liquify all the wax on the blades and wipe them off with a paper towel.  After they're cooled (don't quench them!) feel the metal carefully to see if there are obvious burrs or dings.  If there isn't, re-wax with some clean wax and try using them again.  If there are scratches or dings in your blades, you can get these out with #220 or a new #400 sandpaper--I use a 106 x 4" belt sander to do this.  Once you've smoothed the dings out of the blades, re-burn wax on them and give them a try.  This is usually the solution.  If this doesn't work, either you're not conditioning them (burning wax on them) properly, there's still a flaw in the metal, or the blade steel just isn't great for working glass.  Some types of steel slide more smoothly than others' on hot glass.  

Be careful in modifying any glass tool and remember that you can't put the metal back once you've taken it away, so check your work as you go.  I've improved many jacks but I've also seen people screw up a perfectly nice pair of jacks by not being careful enough.