Saturday, May 28, 2016

Part #5 - The Proposed Fixes


To improve the poor milling quality 3 areas need addressing, table flex, table ringing, and column flex. I looked at ways of improving these areas while keeping a functioning drill press.

The Table Brace
I need to reduce the flexing of the cast iron table with minimum permanent changes. Here is what I have come up with.


The rods are 3/4" solid thread-all. The rest is machined aluminum. The end of the rods attached to the bottom of the table via some blocks epoxied to the bottom of the table. That is the only permanent change with this mod (really semi-permanent since I'm sure a hammer would knock them off). The the rods terminate into the vertical brace which attaches to the pivot joint of the table which on mine is not adjustable but does provides a suitable mounting point. The lower end of the brace is fixed to the column via a collar which is a snug but sliding fit allowing the table to be raised and lowered. The rods are adjustable length and will be used to tram the table surface. The force on the rods is directed to the center axis of the column so there won't be any torque on the brace.

This setup should substantially reduce the flexing and ringing and provide a stable table surface. Also will allow the table to safely hold the weight of the XY table + rotary table + work piece + what ever else I want to pile on there.

Column Stiffening
After many hours of looking around I have determined my best bet is to fill the column with polymer concrete, also known as epoxy granite. This stuff is highly damped, adds mass and should add some stiffness. I have read conflicting theories on the stiffness improvement and it was I believe all conjecture, so since I took some stiffness measurements I'll be able to report in improvement quantitatively.
I'll be ordering a 5 gallon bucket Fillit (tm) from Castinite (www.castinite.com/precision.html). It has to be mixed, it's about 10% epoxy and 90% quartz aggregate from 1/2" and down. Then will be "poured" down the tube and then tamped to remove air pockets. My plan is to seal one end by welding or screwing a cover on the end and scoop in a small amounts, tamping and repeat until full. This should be fun. Details to follow.

And if I have some left over I might add a layer to the bottom of the table, not sure about that yet.

First Steps
The only tricky part is these parts will have to be machined on this drill press. To reduce the table flexing and vibration I will rig up some adjustable length poles under each of the 2 outer corners of the table to the base. This will reduce the table flexing and I'm hoping allow for reasonable some machining to be carried out.

But to do this machining I'm going to need an accurate quill stop and depth indication. So my first milling project is adding a quill stop/lock and a DRO for the depth.


This will be a permanent modification since the drill press really needs this. The stop rod is a piece of 1/2"-20 SS threaded rod and I have a couple of threaded stop nuts from a Bridgeport setup. The DRO is an e-bay special for about $20. This additional will help me control the depth of my cuts/holes.

3D CAD Drawings
By the way the 3D drawings were made in Fusion 360. I have almost no experience with real 3D CAD programs (Solidworks, SolidEdge, etc) and it took about 40 hours to go from zero to a parametric driven model of the drill press and with all my additions. The best part is Fusion 360 is free for enthusiast use (Fusion-360). Just in case you were wondering.
 

Friday, May 20, 2016

Part #4 - The First Milling Test



I figured it would be safe now to try some milling. The posts I read claimed there is too much flex in a drill press to mill well. After tying it the finish is not great and could definitely use some improvement. The head needs to be trammed and the lack of a real spindle lock also caused some uneven surface milling.

Rough Surface Finish.
There was some chatter causing some scalloping of the finish. Not horrible for a first test but room for improvement. I learned the hard way how much to tighten the collet nut and work clamps. While trying out my brand new rotary table the bit grabbed the work, pulled it into it which caused the bit to drive into the table face. Now I have a reminder to tighten things well.
You can see where the bit grabbed and milled down to the table surface.
  And the reminder.

Flex Testing
So after some goofing around milling this and that I figured I should measure the amount of flex I was getting between the bit and the work piece. Using a dial indicator, a scale, and block and tackle I pulled down on the table and measured from the table surface to the spindle and with 50 lbs I had 8.5 mil of flex. I also noticed that if I rapped on top of one of the corner of the table closest to me it rang like a bell. Those are big problems.
 The Table Flex Test.
I also set the table height with 16" of column between the table and the head and pulled down on the head with 50lbs and saw 7.5 mil of movement. That would be the column flexing.
The Column Flex Test.

At some point I still need to measure the column twist. I need to think about how to best do that so I haven't do it yet but I will.

Now that I know how much flex I have I have to figure out a way to reduce it. 

Part #3 - Drawbar Addition

I have in the past tried some milling on a drill press. Usually by using the side of a drill bit (crude to say the least) and I was never aware of the danger in doing that. Since then I have read many posts that strongly state how dangerous this is. The Morse taper in the spindle and Jacob taper in the chuck will come loose if side pressure is applied. I didn't even know these tapers were present and so I have reached my first hurdle. How do I keep the chuck in the spindle? Like a milling machine does, with a draw bar. That required drilling a hole through the center of the spindle. I had no idea how that is done but after some research the answer was a gun drill. My press has an MT3 taper and mills with that taper use a 3/8"-16 draw bar. I didn't feel that the diameter of my spindle shaft would be happy removing that much material so I went with 1/4". My rough calculation says I would still have over 90% of the shaft strength left which like plenty of margin. After some web research, e-mails and phone calls I found a shop that would drill it for the shop minimum of $75. I considered this the riskiest part of this whole project because if the spindle was damaged the press would become scrap. He said he could keep the hole within a few thousandths of center so I went for it.


The spindle ready to be drilled


Still plenty of shaft left

 
Hole in the top of the belt guard.

Close up showing the drawbar end and 12 belleville washers.

The ER32 collet, drawbar and threaded adapter.
The hole came out within the 0.010" of center. So with the spindle drilled, add a hole drilled in the top of the belt guard, a long piece of 1/4"-20 grade 8 thread all ($5), a cool 1/4" to 3/8" adapter (e-bay $2 for 2) and an ER32 collet holder ($60) and I could now safely apply side force to the spindle without fear of spinning parts flying around the shop.

Thursday, May 19, 2016

Part #2 - The General Rebuild

As I stated this journey started with a quick clean up but ended up a nearly full rebuild.
The rebuild included wire wheel the rust, Rust-Oleum Rusty Metal Primer and some Rust-Oleum Gray Enamel. Also replacing the wiring with some 14ga SO cable and new strain reliefs, a new safety power switch (in place of the dangerous double rocker switches, motor / light, which is which?) and added a LED ring light which I am still trying to work out the mounting. And after the table was painted I discovered that the t-slots had casting issues and would not allow the dogs to move in them smoothly. I had to grind off the snags and I still haven't gotten around to repainting it. That ship may have sailed.

After it was all back together I determined the bearings didn't sound good at the max speed (4200 RPM) so back apart to install some new bearings. But before I did that, I researched comments I had read earlier about how a drill press did not have the right bearings for milling. I looked into installing some angular contact bearing since those seemed to be better for milling applications. But after reading bearing manuals I determined that my bearings are fine. There is an deep groove bearing at the bottom followed by a full thrust bearing and finally another deep groove bearing at the top. People claimed that the deep groove bearings are not good for radial loads which I believe is not correct. They are in fact designed for radial loads (they are used on axles after all) and handle some radial load but since I have a full on thrust bearing that is not an issue. So I'm confident my bearings will not be a problem, I guess we'll see.


 
Post Rebuild


Wednesday, May 18, 2016

Part #1 - Drill Press to Milling Machine, Hopefully

I bought an older (1980s) Taiwanese Drill press off Craigslist. It's a 20" with a 1HP motor and 3/4" chuck with a production table. Since this was a pretty beefy machine I got the idea to see if I could do some milling on it. So I went from "I'll just clean it up a little" to a full blown milling machine conversion project. My motivation is lack of money for a real milling machine, several thousands of dollars, a simple lack of room and of course I like making things. I have spent many hours reading posts about people saying how these kind of conversions are a waste of time but those people usually have a Bridgeport. Hopefully I can make enough improvements to allow decent milling of aluminum and maybe a little soft steel. If I fail at least my path will be well documented for all to see and learn from and I'll have some fun.

I have never used a milling machine before and perhaps my ignorance has embolden me. I'm avoiding any modifications that would render the drill press less useful and therefore really don't have much to lose except time and some money. I paid $350 for the press and I figure will spend about that on these upgrades, excluding tooling.