J-Head Mk V-BV hot-ends shipping Monday!

On Monday, December 3rd, the first J-Head Mk V-BV hot-ends start shipping.  Above, on the left, is the J-Head Mk V-BV and, on the right, is the J-Head Mk V-B.  The only differences are all thermistor-related and they are as follows:  The thermistor itself, the mounting location, and the size of the thermistor hole.  Otherwise, both hot-ends are identical.

I decided to add the trailing "V" to the J-Head model number solely in an attempt to avoid confusion and bring attention to the fact that the thermistor changed.  Historically, the J-Head hot-end has always used the type 1 (EPCOS) thermistor and this is the first time the thermistor has had to change.  The "V" has been added to denote that thermistor 5 will have to be selected in most firmware configurations.

In addition, the thermistors are shown separately, above, in the center.  The new thermistor is on the left and the original thermistor is on the right.

The J-Head Mk V-BV was created due to the fact that EPCOS is no longer manufacturing the original type 1 thermistor.  While a substitute appears to be available, the lead time is excessive and I have noticed that it is only available in large quantity orders.  After some testing, I selected the type 5 thermistor as it is already used in other hot-ends and has been proven to be able to perform the job at hand.

The information, on the new thermistor, is as follows:


Part Number: 104GT-2
Manufacturer: Semitec
Description: NTC 100k ohm 3%
Tolerance: 3%
Data Sheet: High Heat-Resistance and High Sensitive Thermistor
Diameter of Glass Bead: 1.35mm +-0.15mm
Length of Glass Bead: 3mm +-1mm
Lead Length: 45mm +-1mm
Lead Diameter: 0.15mm to 0.18mm
If using Sprinter firmware, select temperature table #5.

Please note that I do have a limited supply  of brass nozzles that are already drilled out for the original thermistor.  During the transition, to the new thermistor, these nozzles may have the new thermistor hole added and, as a result, will be able to handle either thermistor.  If the thermistor fits the small hole, it will be the new thermistor.

J-Head Clones!

It's been quite a while since I updated the blog, so I figured I'd post the results of my examination of two J-Head hot-ends that are for sale online.  So, within the last 2 weeks, I purchased a Mk IV clone off of e-bay and Mk IV-B from MakerFarm.  I'll talk about the e-bay clone, first.

The E-Bay J-Head Mk IV Clone

The e-bay clone arrived, straight from China, and came fully assembled with a heater cartridge.  (The heater cartridge is held in by a set-screw.)  The most obvious difference is that this clone has 4 cooling vents instead of the specified 5.  Looking at the cooling vents it was apparent that the machining is a little rough. 

On my way to my machine shop, I dropped the hot-end and the PTFE liner fell out.  This should not happen as it is supposed to be secured with a hollow set-screw.  No hollow set-screw was installed and there are not any threads in which to install one.  So, the PTFE liner will only be held in place by the extruder body.  This was tried, on the J-Head Mk I, and it would result in the failure of the hot-end if the liner moved.  As the PTFE liner is not projecting out of the hot-end, as per the J-Head Mk I specification, it is apparent that the liner will not stay in it's proper place.

Looking more closely, at the PTFE liner, it has a smaller OD, than the liner that is specified.  Also, the tapered end is roughly finished and the cold end, of the liner, looks like it was just cut off with a razor knife.  However, the cold end does have an internal taper.  I don't understand why this end wasn't cleaned up properly when the taper was cut.

The PEEK nozzle holder has 4 cooling vents instead of the usual 5.  They are bigger and deeper than the specified cooling vents.  I would say that using a larger end-mill, to create these vents, is possible due to the PTFE liner having a smaller OD.  (Using a larger end-mill also should reduce the machining time and cost of making this part.)  The mounting groove fits nicely into an aluminum mounting plate.  On the hot end, of the PEEK, there is a shoulder for the optional mounting washer.  The original specification, of the root of the shoulder, is 13mm and this is 12mm.  In addition, the flats to tighten the nozzle holder with a wrench are completely missing.  Since I was able to loosen the PEEK nozzle holder, by hand, it wasn't very tight.

The aluminum/PEEK junction is an improvement as there is no longer a taper between the two.  In addition, some sort of thread sealant was applied to the aluminum threads.

The aluminum nozzle/heater is made to different specifications and really has too many minor differences to mention.  There are 3 major problems with it, however.  The first is that the interior is based upon the original design and does not have a melt chamber section.  So, the liner goes all the way to the tip.  Another issue is that the nozzle orifice is supposed to be 0.50mm; however, it is closer to 0.53mm in diameter.  The final issue is that the nozzle orifice length is a whopping 3mm long.  I would bet that the extrusion pressure is pretty high.

Do to all of the noted differences, this hot-end is more of a hybrid between a J-Head Mk I and an early J-Head Mk IV.  In addition, some effort was taken to cheapen the design.

The MakerFarm J-Head Mk IV-B

The first thing I noticed about the J-Head Mk IV-B, from MakerFarm, is that the machining is very nice.  After taking some measurements, it is obvious that care was taken to ensure that it is made to the J-Head specification.

I only noticed two very slight differences.  The first is that there is a dimple in the brass next to the thermistor.  It could be for locating, work-holding, or something else.  I don't know why it is there; but, it will not affect the functioning of the hot-end.  The other, minor difference, is that the taper is missing on the ID on the cold end of the PTFE liner.  This may cause the filament to slightly catch when it is initially threaded and will not make any difference to the normal functioning of the hot-end afterwards.

The orifice measured 0.50mm and was 1.9mm long.  These measurements were expected and have been proven to work fine.

In summary, MakerFarm sells a very impressive J-Head Mk IV-B.

Initial testing of a thermally fused concept nozzle.

I finished machining a concept nozzle for the J-Head Mk VI-B this evening.  Since some people have had the PEEK nozzle holder melt after the hot-end exceeded 248 degrees Celsius, for one reason or another, I decided to try a hot-end with a thermal fuse wired in series with the heater resistor.

For this purpose, thermal fuses were purchased with a "Rated Functioning Temperature" of 240 degrees.  Unfortunately, I didn't realize that 240 degrees was the upper temperature limit and that the "Holding Temperature" was really 200 degrees and that is the temperature, above which, the circuit is broken.

In order to test the thermal fuse concept, the hot-end was suspended, from the ceiling, and hooked up to a Sanguinololu board.  At 185 degrees, the fuse holds fine.  When the temperature was set to 230 degrees, however, the hot-end didn't quite reach 210 degrees before it started to cool.  The thermal fuse opened up and shut the heater resistor down as it was supposed to do.

I am planning on mounting it in a printer and testing it with PLA in the near future.  However, until a higher temperature thermal fuse is available, it clearly would not be able to print ABS unless the heater resistor is wired without the thermal fuse.

As it appears that the thermal fuse manufacturer is testing thermal fuses, with "Holding Temperatures" up to 230 degrees, maybe this concept will be more useful at some point in the future.

Drill Bit Sizes Used For J-Head Nozzles

At one time, about 2 years ago, I used metric sized drill bits to drill the nozzle orifice.  It was discovered that these drill bits were drilling holes larger than the rated size.  For example, a 0.50mm drill bit would drill out a hole about 0.55mm in size.  I then made a decision to use slightly smaller drill bits in order to ensure that the orifice would be as close to the rated size as possible.  The drill bits used are as follows:

Orifice Size		SAE Drill Bit		Actual Metric Size
0.35mm		0.0135		0.343mm
0.40mm		0.0157		0.399mm
0.50mm		0.0189		0.480mm

Aluminum J-Head Mk V, 1.75mm conversion, and Mini J-Head Updates

The 13 gram aluminum J-Head Mk V has been tested and proven to work fine.  While the only advantage is it's light weight, I am planning on making it available in the near future.

Many people have asked me if and/or when I will have 1.75mm hot-ends available again.  While I was planning on producing the mini J-Heads, that was when the 3mm to 1.75mm conversions were a lot more involved than they are now.  With the Mk V design, a conversion is a lot simpler than with the Mk IV design.  As a result, I am planning on making Mk V-B 1.75mm conversions available in the very near future.

Pondering the Nozzle Orifice Drill Bits

From time to time I see people mention that the J-Head nozzle extrudes slightly under what is expected.  This may be because I have been drilling out the nozzle orifice with a similar concept to what is done with machine tools.  A quality lathe will be built with the tailstock that is about 0.001" high.  This way, as it wears, it will eventually line up perfectly before wearing to a point under the center line.  Taking this concept to the hot-end, the J-Head nozzle orifice is drilled out slightly smaller than the specification in order to provide room for wear. 

A 0.50mm nozzle is therefore drilled out with a 0.4572mm drill bit.

This concept has been creating issues.  For example, in Slic3r, the nozzle diameter needs to be set to a lower size in order to generate the proper gcode.

At this point, I am considering abandoning strategy of providing room for wear, and using exact size drill bits.  While there will be less room for wear, this should result in a hot-end that is easier to use.

Comments?

Machining Mk V-B brass nozzles.

Today, I made a video of the first turning operation performed on the J-Head Mk V-B brass nozzle blanks.  This is the 4th step undertaken when machining these nozzles and is performed on a Hardinge DSM-59 turret lathe.