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knurling knowledge kneeded

Posted by rocket_scientist 
knurling knowledge kneeded
September 29, 2010 12:39PM
I traded Wade some experimental glass nozzles for some wade geared extruder parts, and am now trying to build a complete extruder to help testing the various glass nozzles I make. The bearing went in fine, a second bolt cut off in the no thread portion nicely holds the idler bearing on its separate block, the 4mm bolts are long enough to go through the base, gap, and idler block, the springs are to big but can be cut down, the 3mm bolts nicely fit the stepper motor mount. But the bolt that goes through the main gear and drives the filament I am having trouble with. It does not yet fit in the hexagon recess, but that should be fixed when I heat it up and melt it into its final position. I made lots of measurements and chucked it into a drill press and made the 3mm wide grove at about the spot. But when it comes to adding the knurling, I keep wearing out tools and getting very little edging on the bolt.

First off, I must confess that so far I have used Harbor Freight taps to try making the ridges parallel to the length of the bolt in the notch that I have cut. But the bolt is an ordinary grad 5, not a super strong grade eight. I initially thought I would have to make a jig for holding the side of the tap hard enough against the bolt to cut the ridges. Then I realized that the extruder base itself has bearing in the right spot to hold the bolt in line but allow it to rotate, and that another bearing presses against the other side of the tap to press it hard enough to cut ridges. But when I tried that, it tried to climb up out of the slot, and rounded off all the cutting edges of the tap. The tap also went so deep that it was mostly passed the cutting teeth. So added spacer washers on the head side to make up for the missing plastic gear, and washers and a locking nut on the other side to hold the bolt at exactly the right depth. Then I used a slightly larger, stronger tap, and added a spacer on it so that it would stay in the mid region where the flutes are deep and the cutting edges large. That got a little more roughness on the bolt, but not even a full circle of the bolt before the tap was completely worn down where it touched the bolt.

Later, thinking about the movement of the parts, I realized that the third bearing is in the proper orientation for feeding filament, but 90 degrees off from the best position for allowing the tap to rotate freely. Now I am back to thinking about drilling and notching a block of aluminum to make a jig for putting the ridges on the notched section of the bolt. I can also see now why it is a good thing that reifsnyderb is talking about make whole geared extruder assemblies. If he has a good jig for knurling the bolts, and high quality taps that can knurl many bolts before needing to be replaced, it can save everyone else the bother and expense of doing that one small job.

But that still leads me back to my problem. How do I complete the knurling on my bolt (actually, I plan to start a new bolt, this one is getting to be a bit messed up)? I have ordered TiN coated tapes from Mcmaster-Carr, which should help, but is there an easier way than making a full extruder like jib to hold the bolt and tap together properly? What have other people done when they built theirs? I can not even see how a lathe or end milling machine could help with cutting the steel this way.

Re: knurling knowledge kneeded
September 29, 2010 01:33PM
I used 8mm brass rod so hobbing with a tap was easy. I use a jig in my lathe but I have seen people use a printed jig on Thingiverse. I put a thread on one end and screwed a nut on and secured it by drilling a small hole through it and putting a steel pin through. The pin was actually part of a broken drill. The plastic gear stops it from coming out.

I have used brass for my pinch wheels and not seen any sign of wear so I think steel is overkill.

Re: knurling knowledge kneeded
September 29, 2010 01:42PM
I did not realize that brass was strong enough. I will pick up some brass bolts and try again. When the TiN coated taps come in, that should shift the advantage in my favor. Thanks for the reply.

Re: knurling knowledge kneeded
September 29, 2010 02:18PM
Oh boy where to start...disclaimer I guess, I am a machinist by day so forgive some of the overly #[email protected] retentive tendencies in this.
Number one you aren't knurling, as knurling is a different process. Similar result but totally different. Most common knurls use pressure to form metal, cut knurls are big $$$. You wouldn't want a knurling tool for this job regardless of type.
Now on to the taps. The taps you get at the local big box store are junk, the taps you get at harbor freight are junk. Don't bother with them, they will just break off in your workpiece, cost you time and aggravation and not work as well as the standard HSS uncoated taps you can get from MSC, McMaster, or any one of a number of places that carry good quality tools. I prefer US, Japan or Euro tools personally.
Tool coatings are one of the most misunderstood things when it comes to the casual user. Straight up, if you are using these things at home they are a waste of money. You aren't cutting at the speeds and feeds needed to take advantage of the coatings, and they won't make the surface hard enough to make a difference for what you are doing. They look pretty, and people market them to you, but they are not worth it. I cut metal every day, from 5052 aluminum sheet (soft and gummy) to hardened die steel. 99% of what I do an uncoated endmill works just fine. TiN coated taps are not going to make a difference that you will notice, but good quality taps absolutely will.
You have absolutely no need for a grade 8 bolt, nor for that matter a grade 5. Grade 2 is going to be stronger than you need. Grade 8 is harder than you want to cut, and will only make things more difficult. Fasteners are another greatly misunderstood thing. Carol Shelby wrote an excellent book detailing how and why to use what grade fastener, a great read if you like building things. As others have said brass is fine. I've seen some 303 stainless ones, and they work well, and 303 cuts very nice with HSS tooling, but you aren't going to find a 303 bolt that I know of.
I actually cut my roller this morning on the lathe. I used a brass roller, and an uncoated 2 flute HSS tap. Fed in ~.002" per turn of the roller. No tap breakage and no flex, and this is a tap that has a reduced shank after the flutes. You cannot cut this kind of groove in one pass no matter what material or tap you use, you have to ease into it.
Re: knurling knowledge kneeded
September 29, 2010 04:23PM
I understand where your coming from koko. I used to sell camping equipment at a retail store, and people would always waste money on cheap crap instead of making a good investment in something much much better.

On the other hand, I have no need for taps and dies (especially metric (stupid SAE)) other than for reprap. I'm probably only use it on plastic or soft metals, with the exception that I too have to make a wade's. So i got a cheap 40 piece set from sears for $20. I'm sure they are crap, but they will work fine until i push them too far, at which point i'll invest in something better, piece by piece.

I appreciate the info because you taught me something, but Mendel is expensive enough and i don't want any potential reprapers reading this thread to think that they need an $80-100+ tap and die set. (of course if my crappy set breaks in abs i'll be back here to eat crow :-) )
Re: knurling knowledge kneeded
September 29, 2010 05:01PM
How about a $6-8 single tap that's exactly what you need? I don't advocate buying any kind of set, just the ones that you need at the time. Then you will always have the good tool, and if you do find a project you need it for the good one is waiting for you.
The average cordless drill sold at Home Depot or Lowes has an engineered lifetime of 12-13 hours of drilling time. People buy disposable tools there, and wonder why things turn out poorly.
Everyone has to judge their own threshold for a tool. I've tried it both ways, and this way I have a lot of quality tools I know are ready for the job, and I don't get frustrated when the cheap bit I bought broke in the middle of a part at 11PM Saturday night winking smiley YMMV of course, but this works for me.
Re: knurling knowledge kneeded
September 29, 2010 05:35PM
While I have little experience as a machinist, I have a strong background in physics and engineering. I understand that coating only affects the sharpness and how long it remains sharp, it is the quality of the metal underneath that keeps it from breaking in the whole, or failing underneath the coating. However, I did think that the coated would have an easier time of getting into the steel because it is further up the hardness scale. I did not realize that the coating rarely matters if you are not going at production cutting feeds/speeds. And sorry for the knurling pun. I know that knurling is pressing a pattern into a surface to cause the rest of the surface to bulge out slightly, like knurling valve stems so that they fit tighter int he guides. And the grade 5 bolt is simply that standard bolt in that size/thread sold by mcmasters. I was joking about grade 8, I realize it is way beyond what is needed in this application

I am still trying to visualize your machine setup for making your own roller. You used a brass roller (disk, not threaded rod?) instead of steel, so that cut should be much easier. Was the tap chucked into the lathe and the brass roller mounted on a bearing on the tool stock, and you advanced the tool stock 0.002" each time the thread on the tap caused the roller to rotate once around? Or was the brass roller mounted on the lathe and the tap brought in slowly?

Re: knurling knowledge kneeded
September 29, 2010 06:09PM
... most coatings reduce the sharpnes of cutting edges, so coated tools apply more pressure while cutting and have some other issues too ...

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Re: knurling knowledge kneeded
September 29, 2010 08:35PM
I bought cheap M8 shoulder bolts, fairly soft, and used equally cheap taps on it. I didn't even use a real jig; just two skate bearings held firmly (but not firmly enough to lock the bearings), and spun the tap in a hand drill. Details are on the wiki page; I've made several now without too much trouble. The one thing I would be careful about is hobbing the threaded part of the bolt or rod - on my attempts, the threading caused the resulting slot to be slightly off center. Proper tools will probably help that.

I really look forward to the day when I have space for some proper tools - drill press, lathe, etc - can't wait!

Re: knurling knowledge kneeded
September 29, 2010 09:29PM
I'll put up a pic in a bit, you can see my work so far at teamnar.blogspot.com . I'm doing a different setup than most, timing pulley reduction with the large pulley bored out to take a 5/8 diameter brass rod. I bored the center of that to take a couple needle roller bearings. Whole thing spins on a shoulder bolt across the frame of the extruder. I dislike live shafts except where absolutely needed, and I figured I could reduce an overhung load on the pulley too.
Setup to cut it was to put the tap in a collet in the spindle. The bored out brass roller and it's bearings turn free on a shaft that I mounted to the tool post. If you had to rig things with a drill press you might get away with using the screw and moveable vise jaw to advance the work. At least you could then advance things in a little more controlled fashion. I put the lathe in low gear, and brought in the part slowly until the tap started to catch. Then I made a sharpie mark on the roller, and advance the carriage .002" towards the center with each turn. For pics of a similar setup search for diy worm gear. They usually don't use a tap, they make a hob from threaded stock, but the tap cuts better.
On the coatings, the thing is that you can lay down a film cheaply which looks like it's right but has no benefit. Then you pair it with a super cheap substrate and don't bother to heat treat it right. Looks just like the good coated ones but is sold at your local store. Coated tools have their place but you need a machine that can take advantage of them. For the record I've never had a bad tool I bought at mcmaster. They are almost never the cheapest, but I never worry about crap. Harbor freight on the other hand....
Anyway I realize not everyone has the tools available that I do, I just hope I can give people ideas to use with the tools they have to make things work better. I started with a drill press and a hacksaw too smiling smiley
Re: knurling knowledge kneeded
September 30, 2010 11:57AM
Well, I have two taps on their way from mcmasters, so that should solve part of the problem. I think I understand your worm gear cutting setup, so I will try to duplicated it here. I have a cheap lathe/mill/drill combination that is current buried behind some storage shelves and partially un-wired. I was trying to convert it to 220, but decided the included wiring diagram was not detailed enough to ensure getting it right. I may still need to get a collet for the tap and build a holder to mount the bolt and bearings on the tool post. Thanks again for the advice and details.

Re: knurling knowledge kneeded
September 30, 2010 04:42PM
I've had the same problem rocket_scientist, and have made a couple of bolts this week.

With good help from #reprap this is how I do it:

  • First I mount the bolt in the extruder and mark where to the cut. (I like to use the nuts on the gear side for easier disassembly).
  • Then I secure the bolt in a power drill, with the head sticking out.
  • Without the drill spinning, I file a V-shaped groove in the bolt with a square (or triangular) file.
  • When I have made a nice groove for the file to follow, I mount it in the extruder to confirm it's in the right spot.
  • If it's okay, I turn on the drill and file away, sometimes stopping and filing a bit by hand.
  • I often measure the depth with a piece of filament to get to the right diameter. You don't want the groove too deep, as the filament won't grip (been there).
  • When I'm almost at the correct depth, I use a round file to get a nice smooth groove for the tap.
  • Then I mount the bolt between two 608 bearings and insert the tap (so far I prefer m4) in the drill.
  • Go slow at first with medium pressure until the bolt starts spinning, then just let it spin for a while until the teeth looks and feels nice.
  • Test the grip of the teeth by dragging a piece of filament along it.

Beside being easier to "machine", I also got a lot better grip and smoother movement by cutting the groove in a bolt which is not threaded all the way. The threads reduce the diameter of the bolt a bit, so I wasn't able to cut teeth with good grip in it.

The tap I used was from a cheap set I have, which works fine on the bolts I've made.
Re: knurling knowledge kneeded
October 01, 2010 10:17AM
This thread has given me some ideas. McMaster has 5/16 brass rod (6 feet) for $15 (8953K47)

A length of this could be cut off, hobbed in the center and threaded at both ends. Since it's brass, you could use even the cheapest taps to hob the center. You would also need a M8-1.25 die to thread both ends.

The exact location of the center hob would not be critical, since you could adjust the nuts on the end to shift it's relative location. Toss in a few washers, a four nuts and two lock washers.

Yield should be 25 parts per 6' rod. The only drawback I see is that it's quite a bit of labor to do the whole rod. But I've seen one hobbed bolt go for $10 + shipping on ebay.


The advantage is that the cost of setting up shop to make these and sell them on ebay is considerably lower. No lathe needed, yet you can reasonable guarantee that it will fit. And your cheap-o taps and die will last longer.

Edited 1 time(s). Last edit at 10/01/2010 10:19AM by pica.

My blog's Reprap feed: [blog.markbova.com]
I'm currently working on a stock Mendel build with a Seeeduino Mega and four Pololu A4983 stepper controllers.
Re: knurling knowledge kneeded
October 01, 2010 11:24PM
i took some photos recently of my process which is pretty much the same as wades instructions.

in a drill press, small triangular file first

then small round file

i use 2 extruder bodies, one that i've ruined by drilling a recess for the bolt head might be hard to see but the space for the bearing has been drilled deeper (i already ruined it in the casting proccess).

the body with the recess gets tied down with two screws, the other body just 1 screw to allow it to swing out

cheap m3 tap in hand drill

the bolt wants to move as you do this, but here it just butts up against the extruder body tied down properly. got to have the drill direction going the right way, anti clockwise here i think.

Edited 1 time(s). Last edit at 10/01/2010 11:25PM by mlagana.
Re: knurling knowledge kneeded
October 01, 2010 11:59PM
malgana: Thanks for the photo. I wondered what this discussion was about.

Just a thought - could a fine toothed gear be used which fits inside a plastic part? or has this been tried? Maybe it is essential to have the knurling rounded (hollow)?
Re: knurling knowledge kneeded
October 02, 2010 01:13AM
by the way i'm using 5/16 bolts because the only metric ones i could find are coated in something. These are mild steel and have worked so far just doing calibrating stuff not printing yet.
Re: knurling knowledge kneeded
October 07, 2010 02:05PM
I had so many problems with soft ABS slipping, shredding and clogging those little teeth. I got frustrated and started experimenting with larger taps. I ended up using a 5/16-18 three flute. I haven't had a problem with slipping since. I also reversed the bolt in the housing to center the track better on the shank. I press a nut into the gear and lock it into the threads with jamb nuts on both sides.

This is the setup that I used.

Edited 1 time(s). Last edit at 12/19/2011 09:31PM by Bruce.
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