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Printrbot Talk Forum • View topic - Machining Flats on Motor shafts (How to)

Machining Flats on Motor shafts (How to)

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Machining Flats on Motor shafts (How to)

Postby CL1 » 2012-May-Fri-04-May

Flat surfaces machined into the shaft of stepper motors are used for several reasons.

1) They increase the contact area between the setscrew and the motor shaft, which helps to keep the setscrew tight, and allows the setscrew to do a better job of resisting the rotational forces. Setscrews on un-flatted shafts will need to re-tightened REGULARLY!

2) The flat lies beneath the periphery of the shaft surface. A hardened setscrew will properly make an indent and ridge in the shaft. With a flat, you can still remove the pulley (PULY) since the raised area is not at the shaft surface.

3) When the flat is less than the full length of the shaft, it can provide locating/alignment help in the axial direction (along the length of the shaft).

For these reasons, It is a very good idea to use flatted motor shafts. But it is also very easy to cause motor problems or even failure by improperly flatting the shafts! To achieve the results listed in number 1 above, the flats also need to be FLAT.

The single greatest danger in filing the stepper motor shafts of your PrintrBot is the possible introduction of metal filings into the motor bearing or interior. The easiest way to address this problem is to use a tightly fitting *shield* over the shaft. A ziploc sandwich bag is ideal for this purpose, backed up with a sheet of printer paper.

Put your motor inside the plastic bag and push the shaft through the plastic wall of the bag so it makes its own hole and protrudes. Then close the bag seal with the motor inside and the shaft sticking out.
Now take a piece of paper and press it over the shaft in the same manner, so that it also makes a hole and lets the shaft poke through. We have accomplished step one of flatting the shaft, as the motor is now protected from the unwanted entry of anything foreign, provided we are careful not to tear the paper and plastic, and provided we do not allow any filings to enter the motor as we remove it from the coverings!

The actual flat can be created using a wide variety of tools and methods. Many of these will require specialised or expensive tools. Others will demand a relatively high skill factor. Among these are using a lathe or milling machine to create the flats. Sanding them on a belt sander or grinder, either stationary or portable. Using a Dremel-style rotary grinder. Hand filing. It is the last two of these which will receive our attention.

But first, we need to consider how to prevent the shaft from turning while we create the flat. And we need to consider where and how much of the shaft we will need to make flat. Most flats are approximately 1/4 of the shaft diameter. The PrintrBot pulleys are used with the setscrew close to the motor body and also towards the shaft end. We can either make a full length flat on all motors, or only at the top or bottom of the shaft where the actual setscrew will contact each particular motor. If you are hand-filing, it will probably be easier to file the whole length of the shaft. Using a greater surface area of the file will help to make the area you file truly flat. If you use a powered grinding tool, it might be easier to confine your work to the actual area needed for the setscrew.

The simplest way to hold the shaft for filing or grinding is to use a table-mounted vise. At PBHQ, that is how the BotFarm motors were flatted, using hand-filing with an inexpensive bastard file from HarborFreight. If you have such a vise available, it makes sense to use it! Be careful not to scar the shaft by over-tightening. And be doubly careful not to bend the shaft!
A second choice is vise-grip pliers. These can be clamped around the shaft and when used on a table top, will keep the shaft from rotating while grinding or filing the flat.
A 3rd choice is to use a YBAR end, screw and nut; with a helper to hold the motor and YBAR as you file or grind. The YBAR lets you control the rotation, and see the current angle by its *flag* nature. Simply place on the shaft at top or bottom as needed and tighten the nut against the YBAR end until it is holding the shaft securely, If you don't have a helper, consider how the flag part could be made to come against a stack of PBot pieces to keep it from rotating. As you hand file, the force will serve to keep the flag pushed against the stack of parts.

Hand-filing should be in ONE direction only. LIFT the file on the return strokes! Don't use a file that is worn-out or too fine. Also avoid one which is too coarse.
If you use a rotary grinder, take care to make a single flat plane. With either file or grinder, You should see a square or rectangle being formed. Not a trapezoid!

Once you have created the flat, be sure to remove the motor from the bag in such a way that yo do not get the filings you just creating ANYWHERE NEAR IT!The whole process is pretty simple, and it can be quite fulfilling to see the results of careful, accurate handwork done by you! Perhaps more importantly, your PrintrBot will perform better, and work longer between maintennance tasks!

Good Luck! CL1
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Machining Flats on Motor shafts (How to)

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Re: Machining Flats on Motor shafts (How to)

Postby augspark » 2012-Jun-Fri-22-Jun

Thanks for this post! This is something I have been keen to learn more about. I just got my kit and I have been trying to find out more information on this, as it is one of the steps I am least clear on how to do.

Is here any way you could post a picture of what a properly flatted shaft looks like? A picture of how it should look in the vise would be greatly appreciated as well.

Thanks again!

A
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Re: Machining Flats on Motor shafts (How to)

Postby jr57k » 2012-Jun-Fri-22-Jun

Do you recommend flatting ALL motors? Brook only really mentioned the Y axis.
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Re: Machining Flats on Motor shafts (How to)

Postby Marcus » 2012-Jun-Sat-04-Jun

Thank you for writing this!
Why not move it back and forth / with a fine file?
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Re: Machining Flats on Motor shafts (How to)

Postby marmil » 2012-Jun-Sat-11-Jun

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Re: Machining Flats on Motor shafts (How to)

Postby scantrontb » 2012-Jun-Sun-03-Jun

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Re: Machining Flats on Motor shafts (How to)

Postby drawcut » 2012-Jun-Sun-08-Jun

Any motor with a setscrew on the shaft should be flatted. Absolutely, positively. Set screw with no flat is just begging for trouble. The Z motors don't need flatting because of the coupling method (no set screws) - unless you switch to some other coupling that uses a set screw. I don't see any harm in flatting the Z motors - just not really needed.
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Re: Machining Flats on Motor shafts (How to)

Postby CL1 » 2012-Jun-Sun-23-Jun

Do NOT flat the shafts of the Zmotors if you are using the printed couplers!

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Re: Machining Flats on Motor shafts (How to)

Postby scantrontb » 2012-Jun-Mon-01-Jun

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Re: Machining Flats on Motor shafts (How to)

Postby extent » 2012-Jun-Mon-01-Jun

The purpose of cutting a flat on the shaft is to give the setscrew a solid face to land for best contact. Where is the setscrew in the couplers?

Or conversely, what is the gain in cutting away part of the surface of an interference fit?
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Re: Machining Flats on Motor shafts (How to)

Postby scantrontb » 2012-Jun-Mon-01-Jun

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Re: Machining Flats on Motor shafts (How to)

Postby extent » 2012-Jun-Mon-04-Jun

The material of the tubing already fits snugly around the end of the shaft, removing material from the shaft isn't going to make it fit more snugly, just will require more force from the coupler to then push the tubing in the amount that's missing. The difference there is probably only academic. More importantly I suspect, unless you redesign the couplers to mate to a flattened shaft they are going to want to squeeze down and center themselves over the "center" of the remaining bit of the shaft. That could push the rotation center of the coupler possibly 1 or 2mm off from center from the shaft right from the get-go.

Pulleys with setscrews don't have this problem because the bore of the pulley is solid, not flexible, so it maintains a concentric relationship with the shaft on it's own. If you twist the shaft around so that the flattened section is perpendicular to the coupler split so that it maintains it's center then you're only back to having removed surface area from your friction fit for the most part.
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Re: Machining Flats on Motor shafts (How to)

Postby drawcut » 2012-Jun-Mon-17-Jun

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Printrbot original. Major mods: Y axis extended to ~8", Z extended to ~8.5". 5mm SS Z threaded rods w/ flex couplings. E3D hotend. Purchased Acetel gears. Glass bed with Elmers' purple glue stick for most prints. Top of Z axis rods have added cross structure similar to a Prusia i3.
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Re: Machining Flats on Motor shafts (How to)

Postby CL1 » 2012-Jun-Tue-01-Jun

Extent well covered it in the potential for non-concentric offset joining and even mentioned the means to minimize it if you already have flat shafts (flat perpendicular to coupler join plane or a re-designed coupler).

But I'd add that the lack of full motor shaft support for the vinyl tube does matter. It's no longer concentric circles of motor shaft and vinyl tube mating to arcs of printed coupler halves. By flatting the shaft, you have added all kinds of variation on compression and offset and fit and shape to the interface between the three components You do not want ANYthing to be different at different rotations of the Z motors if you are striving for the best quality prints. Except an equal, repeatable and discrete axial linear movement.

FWIW, the highest grade couplers are not setscrew, but clamp types.

Fewer microsteps is worth looking into as part of your Z axis optimization. The stock 1/16 setup is using *roughly* 226 steps for a .1mm layer height! Most of these WILL be empty steps.

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Re: Machining Flats on Motor shafts (How to)

Postby drawcut » 2012-Jun-Tue-17-Jun

Interesting. Hard to reconcile with Brook's comments on the Z axis thd rods (it's OK if they wobble, the Z axis really rides on the smooth rods). I'd believe that it may be a factor when pushing the PB to it's resolution limits, but still, I'd like to see some side by side experience - not just theory.
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Printrbot original. Major mods: Y axis extended to ~8", Z extended to ~8.5". 5mm SS Z threaded rods w/ flex couplings. E3D hotend. Purchased Acetel gears. Glass bed with Elmers' purple glue stick for most prints. Top of Z axis rods have added cross structure similar to a Prusia i3.
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Re: Machining Flats on Motor shafts (How to)

Postby extent » 2012-Jun-Wed-00-Jun

Go ahead and do it, it's not going to break your bot. Even if you don't mind the wobble I just don't see any reason to go out of your way to make the wobble worse, with no other benefits.
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Re: Machining Flats on Motor shafts (How to)

Postby drawcut » 2012-Jun-Wed-17-Jun

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Printrbot original. Major mods: Y axis extended to ~8", Z extended to ~8.5". 5mm SS Z threaded rods w/ flex couplings. E3D hotend. Purchased Acetel gears. Glass bed with Elmers' purple glue stick for most prints. Top of Z axis rods have added cross structure similar to a Prusia i3.
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