Stock Height Adjuster for Benchrest

[rsterne]

I recently saw something similar, and redesigned it for home 3D printing, and improved it.... On the original you had to hold the base from turning when adjusting the height, on mine I used a keyway to allow the stock cradle to adjust for height but prevent the base from turning with the stock in the cradle.... I want to be able to primarily use it for my new Dual Tank Benchrest rifle, where the buttstock will be a straight piece of 2x4, with the bottom parallel with the bench, so the cradle is 40mm wide and flat in the bottom.... However, I also have V-block inserts to fit stocks with a rounded toe, and different heights of inserts to choose from as the vertical adjustment is only 10-15 mm (there are 20mm of threads).... Here is what it looks like....



Here are the main parts disassembled.... The thread is 75mm x 2mm pitch.... You can see the keyed cradle support, and the keyway in the post in the middle of the base.... The fits are good, and basically there is no significant side play in the assembly.... There is a small hole for a round bubble level (not yet arrived), and three adjusting screws to level it before use.... The threaded holes for the 1/4"-20 NC 1" long bolts need chasing with a tap.... If you tap from the top, and don't quite go through, the bolts will be tight enough to not self-adjust....



Here is the tallest insert in place, and the adjuster cranked up to maximum height.... At it's lowest, the bottom of the stock is 2" above the bench, with over 1/2" adjustment up from there.... The inserts each add 10mm, and you can make them any height and shape you want.... they just drop into the cradle....



I am attaching the .stl files below.... NOTE, you have to scale the base and screw adjuster UP by 50%.... The largest I could draw the threads in TinkerCad was 50mm, so those two parts MUST be scaled up to 150% (in all 3 directions) for the cradle and inserts to fit.... The cradle and inserts are to be printed at 100%, no scaling required.... I used PETG, nozzle at 250*C and the PEI build plate was at 80*C.... I printed on an Ender 5 - S1 with a 0.4mm nozzle, using the Cura slicer, at "100 mm/sec", but set the infill speed to 80, and the walls were done at 50.... I used "Adaptive Layers" to make the threads from more layers (using 0.2mm layer height, with 0.08mm variance, so the layers for the threads were printed at 1.2mm thick.... I used a 40% Cubic Infill, 4 walls and 8 top and bottom layers, and it took 15 hrs. to print the base, about 8 hrs. for the screw adjuster, 4 hrs. for the cradle, and a couple hrs. each for the inserts.... Supports are needed on the Cradle, but not anything else (the screw adjuster is printed upside down).... You can reduce the number of walls, top and bottom layers, and infill density on the inserts, they don't need as much strength....

I undersized the diameter of the male threads by 0.5mm (0.75mm after scaling up), and the height of the male thread profiles was scaled down by 5%, both of these to allow clearance.... After careful deburring and cleaning of the male and female threads, and a slight sanding of the OD of the male threads to get rid of the pointed crests, the thread are a great fit.... I used SuperLube Silicone/PTFE grease on the threads and sliding post with key, and the OD of the cradle where it slides through the adjusting wheel.... You have to remove any ridges formed by the "Z height" layer change, of course, I used a medium diamond file, followed by 220 grit sandpaper for all post-processing.... Make SURE none of the parts are too tight a fit, or they could stick together, ruining both parts (yep, ruined the first base and cradle because of ridges on the key and keyway).... 

If you have any questions, please ask.... Files below.... BTW, "UP" is the direction of impact on target (like a scope elevation dial), and is opposite to the direction the cradle moves....

Bob

[subscriber]

Those are clean looking parts, Bob.    Well designed and well printed.

[rsterne]

Thanks.... A small learning curve on the PETG, need 250*C and 80*C on the bed, and slower on the Infill to avoid getting voids....

Bob

[daveb59]

Really nice job

[TorqueMaster]

I am attaching the .stl files below.... NOTE, you have to scale the base and screw adjuster UP by 50%.... The largest I could draw the threads in TinkerCad was 10 turns of 50mm, so those two parts MUST be scaled up to 150% (in all 3 directions) for the cradle and inserts to fit....

Very slick!  I was tempted by a similar Caldwell front rest Midway had half-ish price, but still seemed excessive for my needs.  This should work great, thanks for posting it.

I'm curious why you couldn't/didn't scale the parts in tinkeracad?  When a ruler is laid down, you can see the xyz dimensions of them, couldn't you for example change 2 x 4 x 6 to 3 x 6 x 9?  I don't think any precision is saved by letting the slicer do the scaling. 

[subscriber]

The other thing I notice is how efficient Bob's attached print files are.  Some people go overboard with print file resolution, when a much smaller file will do just fine.  I am reminded of a gent on this forum, rendering a simple part at extremely fine resolution.  He thought he would end up with a gas tight part, without needing post machining the important surfaces.  Meanwhile FDM printing is the limiting factor for a finish that an O-ring could turn gas tight.

This is a bit like crime shows on TV where they take a low-res image and keep zooming in, to reveal detail that could not possibly be there.

When I generate STLs, I leave the resolution to whatever Solidworks has as a default.  I once tried to generate print files based on the idea that more resolution must be better.  They were just much larger, and made upstream processing and file handling choke; with no upside.

[rsterne]

Bob, the scaling in Cura was necessary because the thread generator in TinkerCad has a maximum diameter of 50mm.... I wanted a 70-80mm diameter for the jack threads, so the easiest way was just to design at the largest size for TinkerCad and then scale up by 50% to get 75mm threads.... I had tried designing the threads in FreeCAD, and also had problems with the thread count on large diameters, it just crashed every time I tried.... plus TinkerCad was so much easier to use.... For a front rest, you could easily change the cradle dimensions and/or shape to fit your forestock, and even allow for padding, such as a piece of leather belt in the cradle.... I might make another one for the front of my Dual Tank Benchrest, good idea!....

Sub, the one resolution related thing I was not happy with was the number of segments in the threads, which is a maximum of 32 (ie 11.25 degrees each).... This causes fairly large "flats" in the threads in such a large part.... It is too bad the guy who made the thread "widget" didn't allow 64 segments like almost all the other circular shapes in TinkerCad.... I looked at a very enlarged preview of the thread profile in Cura, and at 0.2mm layer height, it was pretty coarse.... I could have used 0.12mm for the entire part, but the printing time would have been even longer.... Using the "Adaptive" layer height feature, where Cura decides on a layer height based on the slope and complexity of the profile for that area of the print, was the best solution.... The biggest "surface finish problem" in FDM printing, IMO, is the bump in the outer wall caused by the "Z" layer change.... If you line them all up, you get a seam, which is ugly but easy to file off.... If you select "random", then you get small bumps everywhere, which you may not have to clean up, but if you do, they are hard to find because they can be anywhere on the part.... FDM is great, but it certainly has its limitations in terms of surface (im)perfection(s)....

Added thought.... Maybe I should make either the male thread with 31 segments (a prime number) so that it wouldn't tend to "index" a flat at a time when rotated in the female thread?....

Bob

[subscriber]

Bob Sterne,

If you have the ability to merge parts then you might be able to smooth out your threads:

Take the STL you have now into Cura.  Bring in another part that is just the threaded section.  Rotate the threaded section by half a "flat"; and translate it vertically to mate the phase of the thread in the full part.  Tell Cura to merge the parts (assuming there is such a function)  Else, tell Cura to print the two parts and ignore the fact that they are superimposed.  After all there is such a thing as printing nested parts that have a clearance between them. In this case, the clearance is negative.

We can't let the machines win.  We must never surrender to software.   

[subscriber]

Making the male threads with an odd number of facets, and the female thread even, will probably make rotating the threads smoother.  On the other hand, and even number of facets on both might make the system able to lightly detent into discreet positions, if that were useful.

[rsterne]

Indexing is not desirable, as the vertical height change for 1/32 turn is 0.0025", which for 2 feet between front and rear rests would be 0.37" at 100 yards (a bit too coarse I think).... I have designed an alternate "Adjuster" (the wheel with the male threads, with 31 segments and have reduced the OD by 0.1mm (which once scaled up will make the part 0.9mm smaller than the female thread, instead of 0.75mm).... I think this will improve the part, and if I make another one, I will try it, but I don't see the necessity, as the (post processed) one I have works great.... However, here is the new .stl file in case anyone wants to try it instead of the original.... I am pretty sure it will be an improvement....

Bob

[TorqueMaster]

Bob,

I get that the thread generator is the limiting factor -- it cannot make the larger size you want, so you chose smaller (66%) with the intent of scaling up.  Those threads would be combined (grouped) into the final part, which is also at 66%.  Then the whole part, threads and all, should be scalable up to 100% in Tinkercad. 

I also have used the trick subscriber mentioned -- to get smoother circular objects -- duplicate the part, rotate it 1/2 of a flat, (and raising Z if  needed) and merging the two into one part.  The main issue is that threads are very complicated objects to begin with, and doubling them up adds to Tinkercad's workload.  Sometimes it gets to be too much.  "Save Shape" should remove the part's construction history, making it easier on Tinkercad.

The Z seam is a fact of life.  I've spent many hours tweaking retraction distance, retraction speed, prime speed, and trying wipe and coasting, too, to get the seam as reduced as possible, but never completely gone.  I've gotten it down to a faint distortion, which doesn't need filing off.  Tell Cura to put the seam on corners works... if there are corners.  And you can direct it to a certain side of the part if that helps.  I've read that klipper, or other FDM firmware that supports Pressure Advance, is the best way to go to, rather than fiddling with Cura's settings, some of which sometimes seem to do nothing.

[rsterne]

I did not know how to scale things up or down in TinkerCad.... All you have to do is hold the shift key and pull a corner of the box and all dimensions scale equally!.... Here are the two new Base and Adjuster files, which are already scaled up by 150% and do not need scaling in the slicer....

Having the Z Seam in the corner on the keyed part was a problem.... I have never managed to shrink the seams to the point where they don't need post-processing.... Perhaps sharing your method in a thread here?.... Please?....

Bob

[WhatUPSbox?]

Thanks Bob, great design. I don't need a bench rest but was curious about a fine adjustment platform. I wanted a little more resolution so I added an extra threaded interface (I reused the thread on the adjuster) and nudged the size. As printed, I get 0.27 mm (.011") per full turn of the adjuster. The threads in the base didn't clean up for me as well as I hoped so I may reprint it with a coarser thread.
Thanks again for posting this.

[TorqueMaster]

I did not know how to scale things up or down in TinkerCad.... All you have to do is hold the shift key and pull a corner of the box and all dimensions scale equally!.... Here are the two new Base and Adjuster files, which are already scaled up by 150% and do not need scaling in the slicer....

Having the Z Seam in the corner on the keyed part was a problem.... I have never managed to shrink the seams to the point where they don't need post-processing.... Perhaps sharing your method in a thread here?.... Please?....

Bob

Oh, ok wow I can't imagine using tinkercad without re-dimensioning things.  Have you been dropping a ruler on the workplane?  The L-shaped item from the upper right menu area.  With that in place, you can click on any dimension box or distance box, and type in a new value.  The shift drag works, but doesn't always let you stop exactly at the value you want. 

Getting rid of seams was interactive for me -- watching the outer wall of a layer begin and end, to see where/when extra material is being deposited.  I was printing an LDC, a cylindrical object -- any thin cylinder would be ideal, with cura set to put the seam on the front, front right, or front left, where it's easier it see.

Assuming the extruder is calibrated so 100mm command pulls 100mm filament into the extruder (or out the disconnected output.)  Assuming flow percent is calibrated within a percent or so  -- for me it's a 2-line thick wall printing at the correct thickness (measured on the top few layers only, to avoid most mechanical "slop" error,) and/or printing 1cc cubes until the volume mulitplies out closely, and there's not obviously extra height or extra material getting pushed around on the top of the cube.

From that state of tune, I saw *my* printer squirting out extra filament at the beginning of every line, as if the extruder was faster to start extruding than the XY was to start moving the head.  I print a little hot, so it is a bit more drippy.  Ultimately I use/used a bit of "outer wall wipe distance" to clear some drool out of the nozzle at the end of a line before starting a new one, and I increased the travel speed and acceleration in Cura.  I remember going into the "control" menu on my ender 3 to increase the hardware XY max speed and acceleration, but I don't remember if I needed to actually change those settings, or if they were already high enough.  "Coasting" is another cura tool to try, but it did not seem to have any effect on my situation.

And certainly before most of those adjustments, I worked with the retraction speed, retraction distance, and prime speed to try to reduce that pre-blob I was getting.  I did a lot of testing on retraction, and there is a hot drippy temperature point where it becomes ineffective -- requiring way more distance than it should, and too much retraction distance can cause clogging... my current setting isn't perfect, but it's safe.  You may simply need to retract a bit more, or a bit faster -- it's tedious doing lots of test prints, changing one parameter at a time, but ultimately worth the time invested.

Sorry I can't be more specific on how to get rid of your seams, but hopefully the above list of applicable settings will give you some starting points.

[rsterne]

Stan, how did you reduce the resolution on the adjuster, by using different pitches on the two threads?.... Doesn't that reduce the total travel available?.... I looked at using a left hand thread in the inner one to double the travel, but that would lose the keying function, and require you to hold the base with one hand while turning the adjusting wheel.... As designed, the stock in the cradle prevents it, and the base, from turning....

Bob, I knew how to stretch and shrink boxes in one direction at a time, but not how to do all three simultaneously.... I haven't used the ruler, does it do only one at a time, or can you do all three at once?.... Thanks for the tips on things to try for the Z seam, much appreciated!....

Bob

[WhatUPSbox?]

Stan, how did you reduce the resolution on the adjuster, by using different pitches on the two threads?.... Doesn't that reduce the total travel available?.... I looked at using a left hand thread in the inner one to double the travel, but that would lose the keying function, and require you to hold the base with one hand while turning the adjusting wheel.... As designed, the stock in the cradle prevents it, and the base, from turning....

Bob

Yes, I use two different pitches (differential screw configuration). The nice thing about 3D printing is that you are not limited  to existing thread pitches. I just took the thread you made for the adjuster, shrank it in X-Y so it fit the interior of the adjuster, and arbitrarily (as an experiment) nudged it 20% in Z and that got me the .27mm/turn difference in the two threads.
Yes, as designed you lose total travel (finite number of turns on the adjuster). However, for my use as a platform, for version 2.0, I would separate the male thread from the saddle (platform part) and castle or key the top edge. The saddle/platform part would then drop in and engage the post in the base (as well as the top of the male threads) once the coarse height was set. From then on it would have fine adjustment.

I think for version 2.0 I would look for coarser easier to print threads since the pitch difference can be set in CAD. Maybe match the adjuster lobes to .001" height (came close in the first version) or print a scale on the adjuster.
 

[TorqueMaster]

...
Bob, I knew how to stretch and shrink boxes in one direction at a time, but not how to do all three simultaneously.... I haven't used the ruler, does it do only one at a time, or can you do all three at once?.... Thanks for the tips on things to try for the Z seam, much appreciated!....

Bob

You're welcome, hope you find the magic settings for your seam situation.

Ok...definitely use the ruler.  It lets you type in the exact dimension you want for the X,Y,and Z dimensions, one at a time, but the end result is the same if you change all three.  You may need to use a calculator to upscale them all to 150%, so in that situation, the grab and drag is easier *if* it has the precision you need.  I almost never drag to do dimension changes.

The ruler also gives you XYZ coordinates of object edges, great for getting things aligned perfectly.
The ruler has a "Use midpoint" mode so you can easily center cylinders in cylinders, etc., etc.  If you are in midpoint mode, and you change the dimension of a cylindrical hole, it happens around the cylinder midpoint, rather than expanding in one axis, and then having to shift it back to where you wanted the hole centered.

Tinkercad isn't super intuitive, but once I got a grasp of its toolset, I feel it can do almost anything I need. 
There's also the "duplicate" tool which can not only duplicate an existing object, but also remembers how the duplicate was changed/moved, so when you hit "duplicate" again, it creates another one, with the exact same moves and changes.  Add a box, duplicate it, move the box up and to the right, duplicate again several times...boom, "staircase."  Works with rotations too, so you could make circles or spirals out of any shape. 

[rsterne]

So much to learn with any CAD program, but TinkerCAD is sure a lot easier than FreeCAD....

Bob