Silhouette Curio-sity

Here we have our general purpose die cutter, stencil, etching, marking machine.

Silhouette Curio. Die cutter and etching machine.

By all marks a reliable, accurate, and best of all inexpensive piece of equipment.  Pair it up with their Silhouette Studio Designer Edition, and you have a platform to go from CAD to anything that requires surface markup.  We are constantly pushing the boundaries of what this machine can do.

Now, the machine only has one tiny flaw and it is strictly associated with how we are using it.  No one (at least in their right mind) would be trying to create cut elements down to .001″ accuracy from a unit available from a craft store, but we are.  To achieve this,the only part we had to modify was the deck hold downs.

Silhouette Curio original build deck with quick release snaps.

The original design is beautiful and functional with a distinct usage pattern in mind.  It was setup for the user to be able to quick change parts from the deck, simply undo the snaps and replace.  Great concept, to be able to setup or work material while the unit is busy and then quick swap.

Where the quick swap gets us into trouble is the amount of play they require.  Now while cutting a stencil of your name you won’t notice .050″ shift in the pattern, but an optical encoder will.

So this mod focuses replacing the snaps with hold down bolts.  While we may lose some efficiency in the quick swap, we get a massive boost in accuracy when cutting small dimension structures.

First up we need to measure the build deck pad snap holes, and select an appropriate size of through bolt.

Silhouette Curio build deck pad hold down hole. Measured to 0.195″.

The build deck pad hole(s) measured 0.195″, so a #10 hex bolt at 0.180″ average shaft diameter gives us 0.015″ of play for mounting.

#10 x 5/8th’s hex head bolt for build deck hold down.

A 5/8th’s length bolt will give plenty of stock to work with for any size trimming required without excessive waste.

Next task is to drill the through holes into the deck that line up with the quick snap hold down posts.

Close up of hold down snap and left alignment post.

Fortunately the hold down snaps are designed to be replaced, so we can make this mod and revert to the original configuration if desired.

Sharpie strike marks aligned with hold down snap post.

With the snap hold down in place, strike alignment marks freehand with a sharpie or pencil.  Given the free floating nature of the build boards, perfect hole alignment isn’t required…. but getting close helps.

Center mark for through bolt hole.

Remove the hold down snap.  Then use a straight edge to form a cross center mark based on your outside markings.  This will be the drill point for the through bolts. Repeat this for all four hold down points.

Layout of drill bits and through bolts. 3/32nd’s (0.0937″) pilot hole bit. 3/16th’s (0.1875″) primary hole bit. #10 x 5/8th’s hex head through bolts and nuts.

Since plastic surfaces are slippery in general it is advisable to select a small drill bit to create a pilot hole.  This will minimize “walking” when drilling the primary hole.  Went one step further by using a spring loaded punch to dimple the center mark for the pilot hole.

3/16th’s through hole, and #10 x 5/8th’s hold down bolt.

Drill holes on all four marks and you’re ready to trim the bolts.

Insert only the upper left bolt, along with one build pad spacer.  Lay a flat edge across the alignment post and against the side of the through bolt.  Use a sharpie to mark where the flat meets the bolt, this will be trim height.  Must be done to insure that all bolts clear the inside of the machine.

Marking through bolt for height trimming.
Measure of strike marks results in bolt shaft length 0.435″.

Measure and strike all four bolts for cutting to height.  If you don’t want to trim things you can probably get away with 1/2″ length bolts to start, but has not been tested.

Using a machinist vice and hack saw, trim all four bolts to the strike marks.  It is handy to put one nut on before cutting, so if threads get damaged you can back the nut off to straighten.

Trimmed bolt with nut. Inserted and tightened to build pad.

Last item is the head height of the bolts.

The two hold downs along the left side of the board are situated above a honeycomb structure.  This provides excellent clearance for the stock bolt heads.

Board underside honeycomb structure and bolt head clearance.

Unfortunately, beneath the right hand side of the board hold downs is where several motion components travel.

Motion component travel lanes, showing new bolt head obstruction.

To fix this we need to shave the bolt heads to equal or less than the well depth.  This conveniently is also the same thickness as the solid part of the quick snap hold down, so measuring is easy.  The resulting bolt head should be 0.045″ +/- 0.005″.

Trimmed #10 bolt with nuts for mounting in 3 jaw lathe. Note keep space between nuts so they don’t jam during clamping and cutting operations.
Facing cuts on #10 bolt head, trimming to 0.045″ height.
Trimmed right hand bolt head, with normal and quick snap for comparison.
Trimmed bolt heads showing clear way for movement components.

If you don’t have access to a lathe of grinder, you can probably establish sufficient precision with just the two left hold down bolts and right quick snaps.

Mod is complete, tested for freedom of movement and ready for use.

Silhouette Curio build deck with finished bolt down modifications.

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Lathe Trick Out – Gear Box Cover Release

One of the two most ignored and discarded safety features of the Central Machine 33684 lathe, is the gear box cover.  The other is the clear plastic spring loaded spindle chuck safety guard, but will save that discussion for a later time.

Now this machine comes in many variants under different names and color schemes, but they all came off the same design.  Anyone who operates these machines will tell you the same, changing gears is time consuming in no small part because of the cover box.

Central Machine 33684 lathe gear box uncovered.
Central Machine 33684 lathe gear box cover.

Now it may sound like laziness, because it is.  To get into the cover you have to do the following :

  1. Find your Allen wrenches.
  2. Unscrew the cover and place the bolts aside (and not lose them).
  3. Remove the cover.
  4. Remove gears.
  5. Insert desired gears.
  6. Replace the cover.
  7. Find the cover bolts.
  8. The bolts started by fishing around for the holes and turn a couple times.
  9. Tighten bolts the rest of the way with the Allen wrench.
  10. Put your Allen wrenches were you can find them again.
  11. Resume operations.

If you have already selected and laid out your gear set before hand, this whole process takes about 15 minutes. Without the cover it knocks it back to about 10, so most people just leave the cover off.

Why you may ask, would an operator omit a piece of safety equipment which protects fingers from getting sucked into gears?

It’s about time, and getting back to work.  Lets take a look at the gear changes for something simple like a round bar.

  1. Change out current set for roughing cut set.
  2. Change out roughing cut set for finishing cut set.
  3. Repeat for next.

In here is 10 minutes wasted (and hassle) with just replacing the gear box cover.  Multiply that by how ever many changes you have to do, and it’s easy to see why the cover is left off.

So how are we going to fix this?  Well besides replacing the entire back end with a transmission, we will just make the cover easier to remove.  Replacing the Allen screws with studs and simple knobs, will greatly simplify cover handling.

First task is to model the original components so we can work out proper dimensions for the new.

Central Machine 33684 lathe back cover model.
Central Machine 33684 lathe spindle head left, with 5mm-0.8 Allen screws installed to normal insertion depth.

Note : To figure out the normal installation depth, you replace the cover and tighten screws till secure while keeping track of the number of turns.  Remove cover then install screws to same depth and measure distance to surface.

Next up is modelling the replacement studs and jam nuts.

New 5mm-0.8 studs and jam nuts.

The studs work out to 2.25″ inches long, with 0.65″ inch threading from each end.

Now you may notice that the jam nuts are not flush with the mating surface.  This is to model a mounting bracket which goes between the nuts and back plane for another project.

Also, the nuts at the tip of the studs are for integration inside the operator knobs.

So rather than trek to the fastener supplier or wait for delivery, we used some 3/16th” (0.1875) round stock to make our own.  Since 5mm equals 0.19685″ and the stock had an average size of 0.1900″, this would work out just fine.  Also helped that the lathe was between projects, so we could jump on for some quick cuts.

Layout and strike 2.250″ line on 3/16th” bar stock with a caliper and sharpie.
Parting tool and stock. Remember, the smaller the stock the closer to the lathe chuck you have to get to prevent flexing.
Once your 2.25″ blanks are parted, measure for excess material to be removed. Then perform a facing cut to remove the parting nub, and excess.
Blank tip after facing and sizing.

Now that we have our stud blanks, time to add the threads.  Decided to use a die rather than the lathe for these simple cuts.  While it would have been a good demo of small diameter threading, will save that for later.

Using a machinist vise, 5mm-0.8 hand die, and light oil to manually cut threads. Be careful and patient not to cross thread or bend the shaft.
Completed threaded studs.
Once complete. test fit into lathe using same number of turns as the original screws. Note the black sharpie stripe to keep track of turns.

Next up is the knobs.  A simple form built to mate with the bolt wells in the cover and contain a 5mm-0.8 nut.

.STL ready for the printer.
Printed product with integrated 5mm-0.8 nut.

It took a couple iterations to tweak the sizing for a user pleasing stand off from the back cover.

Completed studs, jam nuts, and knobs for installation. Note original Allen cap screws for comparison.

Next step is to modify the inside of the gear box cover.  The two stand offs are designed to mate up against the back plane of the spindle head.  Since we will have jam nuts and a future mounting bracket, they need to be shaved down by 0.325″.

Central Machine 33684 lathe gear box cover interior.
Using a stand off and bolt through the end stock opening, firmly secured to the milling table.
Using a medium end mill, shaved 0.325″ from cover stand offs.

When done, shake out shaving into a trash can then blow remaining out with compressed air.  Clean as well as possible so you don’t get muck in your gear box.

Finally assembly of our new quick access studs and knobs.

Remove the studs and spin on the jam nuts.  Apply some grease to the threads, this prevents them from becoming permanent via rusting in place.  Tighten the jam nuts against the back plane, doesn’t require much torque.

New studs and jam nuts installed.
Cover installed with knobs for easy access.

So with the job complete we have enhanced the access and safety on our Central Machine 33684 lathe. Reducing the access time to a few seconds will inspire use during run time and increase productivity.

Side note : This addition is part of an overall modification project, which will reduce or eliminate the need to access the gear box.  Until then, this is a great addition.

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Lathe Trick Out (overview)


So here is our workhorse bench top lathe.

Central Machine Tools (Harbor Freight) 33684 7″ x 10″ lathe.

By every stretch a capable manual project machine, durable and in the right hands very accurate.

So why would we want to mess around with a good thing?

Beyond an incessant need to tinker with things, we want to increase it’s utilization.

The only way to achieve this is with automation controls, or CNC as it’s known.

This gets us the following;

  • Flexibility of cut types and movement.
  • Increased precision and repetition.
  • Lower machinist tasking and interaction.

The machine operator or machinist is the great brain of any manual tool.  As diligent and precise as they can be, they are still human.  Two parts made by the same person on the same machine can end up having subtle variations.  CNC greatly reduces these variations, but still requires the accurate monitoring of the machinist.  So instead of replacing the brain, we want to give it time to do other things like monitor the mill at the same time.

Now we have a great tool but when the machinist doesn’t have hands on it, it’s a paper weight.  Hovering over a spinning piece of metal, hands at the ready to push buttons and twist knobs.  Insanely time consuming, and yet necessary to achieve a good finished product.  This works great for a one off article, but will drive the machinist batty making ten more just like it. Still we want to retain the basic abilities of our great tool.

That brings us to the core requirements of this project;

  1. Retain the basic manual abilities of the lathe.
  2. Enable CNC level control of all functions.
  3. No downtime during conversion.

So what does all this mean?  Where there’s a dial and switch, there will still be a dial and switch.  The computer’s control will have to overlay on the human input devices.  We can’t take the unit out of service at all during the conversion.

Whew!  That sounds like a lot of work to achieve something you could buy off the shelf?

Fair point.. but learning by doing is way more fun.

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