Introduction: Dremel Tool Post Grinder
While working on a custom lionel rivet punch for my dad I ran into the need for a tool post grinder. Since this is one off project I did not want to purchase a new one. So decided to see what I could build out of my scrap bin. I happen to have a old Dremel and some aluminum scrap so decided to go with that and got started.
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Attached is a sketchup of my design. Make sure to modify to to fit your machine and available scrap. Make sure your support block is thin enough to fit your tool holder while as thick as possible to minimize flexing. The only key dimensions is the distance from the center of the Dremel hole to the surface of the mounting block (Mine is 0.925"). If that is off either the Dremel will not fit at all or will need a shim between it and the mounting block.
Step 1: Materials and Tools
For this build you will need:
- A Dremel
- Some aluminum scrap (I started with 2 scrap blocks 2"x3"x0.5", and 1.4"x6.3"x0.5")
- 2 1/4"-20 x1.5" bolts
- 1 1/4"-20 x2 bolt
- 2 1/4" lock washers
- 2 1/4" plain washers
- 1 1/4"-20 nyloc nut
- Large metal washers (3/4" or smaller fender washers with a outside diameter greater than 1")
- 3/4" wide metal pipe strap (the kind with 1/4" bolt holes every inch or so)
- cutting fluid
- Basic hand tools
- Mill vice
- machinist square
- Large end mill for facing the stock (exact size unimportant best if wider than the thickness of the stock)
- Dial indicator and magnetic stand
- Table bolts / clamping kit
- mill collets
- deburring tool
- edge finder
- #3 center drill
- 1/8" drill
- #7 drill
- #F drill
- 1/4-20 tap and tap handle
- Tap guide
- countersink bit
- 3/4"-12 / 19mmx2mm Tap and 17mm drill OR 4 jaw chuck, boring bar + cutter, internal threading bar + cutter, 3/8", 1/2", 5/8" drill bits, and thread gauge
- tail stock center (live or dead)
- test indicator and magnetic stand
- 3/8" boring bar + inserts
- 1-2-3 block
- center punch
Step 2: Machining the Mounting Block
Start out by tramming your mill. This greatly varies by mill design so will not go over it here. Next mount your vice to the mill table and true it up on the mill using a dial indicator. To do this set up the dial indicator perpendicular to the fixed jaw of the vice. Move the vice into the indicator for half the dial indicators travel. Next move the vice from one edge to the other in the direction perpendicular to the dial indicator. When the vice is true the indicator will not move (exact measurement is unimportant). Easiest way to adjust the vice is to loosen the mounting bolts (leave them finger tight) and tap on the vice with a mallet as you move the axis. If the speed of change of the dial increases tap the opposite side of the vice. As you get close to true the speed of the dial indicator movement will decrease and slow to a stop even though the axis is moving at the same speed. Once that is done tighten the vice mounting bolts and run the full distance of the axis again to verify the vice did not move during the bolt tightening.
Load a end mill larger than the thickness of the block in the mill using the appropriate collet. Next mount the block in the vice with the best rough edge down supported on a parallel. Take a few light passes unlit there is a uniform surface over the entire top edge of the block. Deburr all edges then rotate the block 90 degrees and using a machinist square set the side just cut perpendicular to the vice. Take light passes until the end of the block has a uniform surface. Deburr then verify the 2 cut sides are square to each other using the machinist square and mark these side for future reference. The condition of the remaining 2 edges is unimportant to this build. But for visual appearance it is recommended to take a skim cut on both to clean them up. For these cuts just set the milled edge of the block on a parallel in the vice then take light passes on the rough side to clean it up. After cutting deburr all edges.
Set up the block with a machinist square so the short edge marked above is up and the long edge marked above is perpendicular to the vice jaws. Using a edge finder find the center of the block in the narrow direction and lock the axis down. (In my setup this is the X axis.) Then use the edge finder and zero out the remaining axis on one of the edges. (In my setup this is the Y axis.)
Load a #3 center drill in the mill using a collet. Move the Y axis over to the location of the first hole and lock it down. Using plenty of cutting fluid pilot drill the first hole. Only drill down till the counter sink portion of the drill just touches the hole and leaves a slight bevel around it. Unlock the axis and move to the second location and repeat this operation.
Without unlocking either axis remove the center drill and replace it with a #7 drill. Peck drill this location to a depth of ~1.5" using plenty of cutting fluid. Those unfamiliar with peck drilling the process is quite simple. First start the mill / drill and slowly lower the bit until it comes in contact with the bottom of the hole. Proceed to drill down normally for 1/4" to 1/2". (This distance is dependent on the drill size. I normally use 2 to 3 time the diameter of the drill.). Next raise the drill and allow all the chips to escape the flutes. Then repeat the process until you hit the desired hole depth. The main reason for using peck drilling is to prevent the chips from jamming in the flutes of the drill. Once that hole is drilled unlock the Y and move back to the first hole and repeat this process.
The next step is to tap the holes. This can be done in the mill vice but I do mine on a bench vice since there is more clearance. First load a 1/4"-20 tap in a tap handle. Next apply a generous amount of cutting fluid to the tap and the hole. The key step is starting the tap is to make sure the tap is parallel to the barrel of the hole. I typically use a tap guide (shown in the photos) to start the tap. To use it place the correct sized hole in the guide over the hole to be tapped. Make sure the face of the guide is flat on the surface of the block. Insert the tap through the guide and rotate while applying light pressure. Once the tap is started and has gone ~6 rotations the tap guide can be removed. When tapping best practice is for every half rotation forward reverse the tap one quarter rotation. This action breaks off any chips forming on the cutting edge and prevents them from jamming the tap in the hole. As your tapping pay attention to the force it takes to turn the tap. As the flutes fill up with chips the force will increase. Make sure to fully remove the tap, clean the flutes, and reapply cutting fluid when the force increases. Also make sure to stop tapping before the tip of the tap reaches the bottom of a blind (open on one end only) hole. Easy way to know when your close is to put a piece of tape on the tap 0.1" short of the depth of the hole. If you fail to clear the chips as needed or run the tap all the way to the bottom of the hole tap breakage is VERY likely. Getting a broken tap out of a hole can be very difficult and or ruin a otherwise good part. Once both holes are tapped all that is left is to remove the bur on the hole. I use a standard countersink bit by hand to remove the raised edge and add a slight bevel to the hole.
Step 3: Machining the Face Block
First square up the edges of the block. This is done the same way as the previous step.
Mount the block face up on a pair of parallels. Using a edge finder find the center of the block in the narrow dimension and lock down the axis (X in my setup). Next find and zero out the Y on one of the edges.
Load up a #3 center drill in a collet and move the Y axis to the location of the first hole and lock it. Using cutting fluid drill the pilot hole until the counter sink on the pilot drill just bevels the parts. Repeat this for the next 2 holes.
Leave the Y locked in the final position and load up a 1/8" drill in a collet. Peck drill (see previous step for instructions) all the way though the part using cutting fluid. Repeat this for the other 2 holes. Next load up a #F drill (1/4"-20 close fit clearance drill) and peck drill all 3 holes using the same procedure as the 1/8" drill.
Depending on the tools you have available there are 2 ways to cut the threaded hold for the Dremel tool. The first and more difficult choice is to single point the threads. The second choice is to use either a 3/4"-12 or 19mmx2mm tap. Both of these are odd sizes and can be difficult to locate.
If you do not have a tap and are going to single point the threads skip to the "Deburr and fit" section below. If you have the tap continue below.
Move the Y axis to the location of the Dremel mounting hole and lock it down. Progressively drill out the hole with the following sizes 3/8", 1/2", then finally 17mm. Once the hole is drilled out using the method outlined in the previous step (guide, fluid, 1/2 turn forward then 1/4 turn back) tap the hole out using either 3/4"-12 or 19mmx2mm. Once this is done check the fit with the Dremel nose and verify the threads are tight but do not bind when the Dremel is fully threaded in.
Deburr and fit:
Once all the holes are drilled (and possibly tapped depending on method used) lightly deburr the holes using a countersink bit by hand. Next assemble the face and mounting blocks together with the marked faces of the mounting block facing the face block and the Dremel hole. Next hold the Dremel in position so the center of the collet lines up with the center of the mounting hole and the side opposite of the switch is facing the mounting block. Verify the gap between the Dremel and the mounting block is very small. If it is too large make and attach shims to the face of the mounting block. If there is no gap and the Dremel is unable to line up with the mounting hole machine the mounting block down.
If you used a tap to cut the threaded hole for the Dremel nose skip ahead to the "Spacer construction" step. If your going to single point thread continue to the next step.
Step 4: Single Point Threading
First step is to set up the lathe with a independent 4 jaw chuck. This varies greatly from machine to machine so consult your owners manual.
Once the chuck is mounted open the jaws wide enough to give clearance on all 4 sides of the face block. Using a center in the tail stock pin the block to the chuck as shown using the pilot hold for the Dremel tool. Gently snug up the 4 jaws so they hold the block in place. Remove the center and slide the tail stock out of the way. Set up a test indicator in a magnetic stand so the probe of the indicator rides on the inside of the pilot hole for the Dremel. Rotate the 4 jaw chuck so a pair of jaws are in line with the point of the test indicator. Zero out the dial of the test indicator. Rotate the 4 jaw chuck 180 degrees and note the movement of the dial. Next adjust the 2 jaws to move the part half the distance indicated. The easiest way to do this is to use 2 chuck keys at once. As you tighten one side with one key you can simultaneously loosen the other side. Note only a small amount of change (usually less than a full turn) of the chuck keys is normally needed also do not fully tighten the jaws down at this time. Once the adjustment is made zero the test indicator and rotate the chuck 180 degrees again. If there is little ( I allowed up to 0.002" for mine) or no movement of the indicator rotate the chuck 90 degrees and repeat the entire process. Once both dimensions are zeroed out rotate the part 360 deg and verify the test indicator does not move. Now fully tighten opposing jaws at the same time using 2 keys. The goal is to tighten down the part with out shifting it. Once all the jaws are tight re verify the hole is on center by rotating the part 360 deg while watching the needle of the test indicator. If the needle does not move (or moves very little) proceed on. If it is out of center (needle moves excessively) repeat the steps above.
First double verify the block is tight in the 4 jaw chuck. If it is not properly tightened down it can be thrown from the chuck while machining. Set up a drill chuck in the tail stock. Drill out the Dremel hole using the following sizes 3/8", 1/2", 5/8" using the tail stock and plenty of cutting fluid. Once done remove the drill and drill chuck and slide the tail stock out of the way.
Square up the tool post to the face of the chuck. To do this loosen the tool post hold down nut. Then take a 1-2-3 block and place it between the flat/square surface on the tool post and the face of the lathe chuck. Verify no dirt or chips are trapped between the 1-2-3 block and the face of the chuck or the tool post. Apply light pressure on the 1-2-3 block using the apron slide so the block in pinched in place. Tighten the tool post nut and remove the 1-2-3 block.
At this point verify the following dimensions of the threaded nose on the Dremel tool:
1.) Thread minor diameter, This is the smallest diameter at the valley of the thread (mine was ~0.65")
2.) Thread major diameter, This is the largest diameter at the peak of the thread (mine was ~0.745")
3.) Outside diameter of the "Shoulder" between the end of the threads and the start of the main body (mine way ~0.745")
4.) Width of the "Shoulder" from the end of the end of the thread to the start of the body (mine was ~0.1")
5.) Thread pitch using a thread gauge (mine was 12 TPI)
6.) *Optional* location of the start of the thread next to the shoulder. Depending on the machine and operator skill this can be very hard to do correctly. But if the threads are started in the wrong location it can be corrected with use of the spacer in the next step.
Load a 3/8" boring bar and cutter in the tool post. Set the stick out so it will extend 1/4" past the back side of the face block. With the machine off bring the boring bar in and though the face block. Rotate the chuck by hand and verify none of the jaws or other items collide with the tool post or cross slide. Back the boring bar out and verify the tip of the cutter is on the horizontal center line of the part. Adjust the cross slide for a light cut and lock it down. Start the lathe and take a light cut over the entire length of the hole. Stop the lathe and measure the ID set the dial/DRO to this dimension (make sure NOT to unlock the cross slide until this is done). Proceed to take medium cuts the full length of the hole until the ID of the hole is equal to the minor diameter measured above. Next cut the pocket for the Dremel's unthreaded shoulder. Take medium passes for the depth of the shoulder's width until the ID of this pocket matches the shoulder diameter measured above.
Next set up the change gears / levers for the thread pitch measured above. This varies greatly from machine to machine so consult your owner's manual. Also set the lathe speed to the slowest option available. Set the compound slide angle to 59 degrees with respect to the face of the chuck and retract it. Square the tool post to the face of the chuck using the 1-2-3 block method above. Load the inside threading boring bar and bit in the tool post. Verify the tip of the cutter is on the horizontal center line of the hole. Move the cross slide so the tip of the cutter is just shy of cutting the side of the hole. Lock the Y axis down. Engage the power feed / half nut on the apron. NOTE once you start cutting the threads do NOT disengage the power feed / half nut until your finished. Instead use the lathe power to start and stop the advance of the cutter.
Coat the inside of the hole and the shoulder in marking fluid. Apply cutting fluid then advance the compound slide so the cutter will take a light cut on the inside of the hole and lock it down. Turn on the lathe and allow the power feed to draw the cutter through the part. Once the cutter exits the other side of the hole turn off the lathe to stop the cutter advance. Unlock and retract the compound slide so the cutter will not contact the walls of the hole. Failure to retract the cutter will ruin the threads due to the backlash in the lead screw / half nut. Reverse the lathe and allow the power feed to extract the boring bar from the hole. Using a thread gauge double check the pitch of the groves cut by the first pass. If they do not match the thread pitch measured above check your change gears / levers settings. If correct repeat the following operations until the thread depth matches the minor diameter measured above.
1.) Apply cutting fluid to the hole and advance the compound slide to take a medium cut and lock it down.
2.) Turn on the lathe and allow the power feed to draw the cutter through the part.
3.) Turn off the lathe to stop the advance of the cutter.
4.) Unlock the compound slide and retract it so the cutter will not contact the part.
5.) Reverse the lathe and allow the power feed to back the boring bar out of the hole.
Remember at no time should you disengage the power feed / half nut. Continue to leave the power feed / half nut engaged and back the boring bar out of the way. Test fit the Dremel tool in the threaded hole. If the threads are too tight / bind take another pass though the steps above. If the treads are snug but thread in/out smoothly remove the face plate from the lathe and deburr the Dremel mounting hole.
Step 5: Spacer Construction
For this instructalble I am using the following terms for the sides of the Dremel:
- Front = long side with the power switch and spindle lock button
- Back = long side opposite of the power switch and spindle lock button
- Nose = end with the collet
- Tail = power cord end
- Left = long side with a brush holder on the left when tool viewed from the front and nose pointing up
- Right = long side with a brush holder on the right when tool viewed from the front and nose pointing up
If single point threading was used try test fitting the Dremel with no spacers. Screw in and hand tighten the Dremel into the face block. Do not over tighten as it can strip the plastic threads. If the flat back surface is facing the mounting block skip on to then next step as no spacer is needed. If a spacer is needed or no recess for the Dremel shoulder was made continue on.
For spacer material I used normal flat washers. 3/4" washers can be used as can any fender washer with a outside diameter equal to or greater than 1" and a inside diameter of 3/4" or smaller. If appropriate washers cannot be found shims can be made out of sheet metal. Note check the various grades of 3/4" washers as the thickness normally varies between them. If needed stack enough washers to cover up the unthreaded shoulder of Dremel. Next thread in the Dremel to the face block hand tight. Unscrew the Dremel just enough so the back of the Dremel faces the mounting block. Do not unscrew more than 1 full turn. Once lined up test fit washers under the edge of the Dremel lip and the mounting block. Find a washer or combination of washers that are a tight fit between the Dremel lip and the face block.
If the ID of the stack of washers is 3/4" use them as is. If the ID is less than 3/4" they will need to be enlarged. Clamp the washer in a vice and use either a step bit (shown) or a grinding drum in a Dremel to enlarge the hole to 3/4". Once all the washers are enlarged deburr the hole, stack them on the Dremel, and test fit it again. If when hand tight the back of the Dremel is facing the mounting block continue on. If not adjust the washer stack as needed.
Next try assembling the mounting block to the face block. If any of the washers are in the way dismantle and cut the washer in a D shape as shown. Clamp the washer in a vice and use a fiberglass reinforced cutoff wheel to slice off the side of the washers. Once done deburr the cut edges and reassemble everything with the flat side of the washers facing the mounting block. Hand tighten the Dremel down and install the mounting block.
Step 6: Body Strap Construction
First mount the tool post grinder in the lathe tool post or quick change bit holder. You want to mount the grinder with the holder as close to the face block as possible.
Next take some 3/4" wide metal pipe strap and bend it into a U shape centered on one of the 1/4" holes around the Dremel body. Position the strap as far away from the face block as possible but still remain on the round portion of the Dremel body. Verify there is a 1/4" hole in the pipe strap on both sides of the mounting block in a straight line though the mounting block perpendicular to it's surface. Cut the strap off just past the 1/4" hole used for mounting to the body block. Make sure the strap and mounting bolt hardware do not interfere with the tool post holder or other part of the lathe. Mark the location of the 1/4" hole in the pipe strap for mounting on the mounting block. Dismantle the mounting block from the rest of the tool post grinder.
Since I was just free handing the location of the hole I decided to just use my drill press to drill the hole. A mill could be used to drill the hole but was overkill precision wise and I did not want to set it back up again. First use a center punch and dimple the center of the hole to be drilled. Load up a #3 center drill in the drill chuck using cutting fluid drill the pilot hole until the counter sink on the pilot drill just bevels the part. Load up a 1/8" drill in the chuck and peck drill (see previous steps for instructions) all the way though the part using cutting fluid. Next load up a #F drill (1/4"-20 close fit clearance drill) and drill though the part. Use a countersink bit by hand remove any burs on either side of the hole.
Reassemble the mounting block to the face block. Assemble the body strap in this order: flat washer, one side of the pipe strap, mounting block, other side of the pipe strap, flat washer, nyloc nut. Tighten down the nut and bolt so it pulls the ends of the pipe strap towards the mounting block. Continue to tighten until the body of the Dremel is pulled tight against the mounting block. Do not over tighten as it could damage the Dremel. Manually spin the shaft of the Dremel if there is roughness or binding that is a sign that the body strap is over tightened or the gap between the dremel and the mounting block was too large before the strap was added. If all feels good try plugging in the Dremel and running it. If all sounds normal the tool post grinder is ready for use. If there is any grinding or squealing noise (that was not there before) immediately turn off the Dremel and check the body clamp / gap.
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