i try to build a drawer that is inside a box with locking mechanism.. using Shopbot CNC for milling MDF wood.

design parameters:

im using shopbot CNC with the following materials:

• ¹⁄₄” drill bit (~6.4 mm)
• 12mm MDF wood plate

design limitation:

to start with i’ve used the following joints in the design:

dog bone

the dogbone is used in CNC because the drill-bit is round and cant make Inside straight corners.

designing gears

gears are big subject i will briefly talk about the design here you can search about any gears tutorial im using solidworks add-in to place the gear. you can access it through the right panel:

• make an assembly: file-> new -> assembly
• right panel -> tool box -> ANSI Metric -> Power Transmission -> Gears -> Spur Gear
• then i put the parameters for the gear (module and number of teeth)
• save file as STEP
• open it again as part and modify

important parameters for gear:

im not mechanical engineer and i’ve very little knowledge but these are the parameters that i found important

• model (both gears have to be same Model - M)
• pitch circle: (the two circle of two gears must touch to transmit the power)
• number of teeth

for the CNC it has to drill the gears with ¹⁄₄” drill bit, i’ve to set the smallest inner area to be exactly same as the drill-bit.

the gear selected was:

• module: 5.5
• teeth number: 10
• hence the base diameter: M x T = 5.5 * 10 = 55

distance in arc:

now the arc distance is larger than drill-bit so i can make this as hole in the cnc..

gears distance:

the gear distance are larger than base which of course have backlash and inaccuracy this is beacause im using them as bevel gear while they are just spur gears

C = r+R

for me im going to use it as bevel gear although its not.. so i put more distance to avoid problems .. yes it does gives inaccuracy and backlash ..

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Design:

it really takes too much time to document all the design however i found the gear is the most one that is important to document so i did here are some pictures of the design you can download it here Download

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Shopbot:

the shop-bot software accepts DXF files which i exported all the parts in one file i set layers for different depths:

fixing the mdf plate to the workarea..

Zero (homing .. and running) ..

the cut parts .. you can see that one tooth has a problem of the gear due to small piece that pop from its place and the drill-bit damege it ..

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Assembly:

i assambled the box to Test gear motion:

i really didnt take care of shaft holding in the design and i had mistakes:

Design issues:

• fit joints: the joints were to tight that i have to press the parts hard which can damage them .. so most of joints were filed manually filed later
• pin holding: the pin holding is not properly designed so i sticked a 6mm shaft with a glue
• pin alignment: this is really bad design where i should put the lock-gear to be behind the gear holder so the pin is holded from two sides .. to solve this issue a new 3d printed pin holder was designed and mounted near the locking gear.

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Result:

didnt complete it, another shaft i’ve to work on.. but im illustrating the mechanism here:

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References:

cnc joint:

1. press-fit desk
2. creating cnc dog bone
3. cnc cut wood joints

gear:

1. spur gear spacing