[gtiworks]

There are several web sites showing the reinforcement of mill bases and towers with polymer concrete. What is not generally shared is that aggregate based concretes are at their best when used in compression not in tension. To get the most from your polymer concrete, you need to put the mill base under the tower in compression. Fortunately, this is relatively easy by using a steel compression plate and tower hold down studs. The pictures below show the ¼ inch thick steel plate I had cut by eMachine Shop and that I embedded in the base polymer concrete. The tower hold down studs were cut from high-strength steel threaded rod. The large nut and washer in the center of the photo is for the additional 1 inch diameter tower hold down bolt. For inside the tower, I cut a ¼ inch thick aluminum plate and embedded it in the tower’s polymer concrete to distribute the compressive forces created by the central 1 inch bolt. Since the area inside the tower is small, I chose the aluminum plate since I could mill it easily on my Taig mill. Using compression plates and tower hold down studs should maximize the potential stiffening of your mill base and tower combination.

[johnohara]

Nice job gtiworks. Do you have more pics?

~john

[jetflatline]

Do you have some before cuts you can compare to after cuts? I didn't do this fill while I had my mill apart for CNC installation, but would like to see some "why it's worth it tests" to see if I should take it apart (again).

[gtiworks]

Below are some more photos of the base and my polymer concrete fill. I was primarily interested in increasing stiffness with vibration dampening as a secondary benefit. The tubes for the studs and feet are simple electrical conduit and the tube for the central tower bolt is a pipe union. These were bought at a local home improvement outlet.

[gtiworks]

There are several polymer concrete formulas available on the Internet for use in filling mill bases and towers. I substituted crushed granite for about half of the gravel aggregate. I bought aquarium crushed granite as it comes very clean and thoroughly washed. The same goes for my "large" gravel which was again washed aquarium small round river rocks of about 1/2 inch in size. I bough "crushed" gravel at my local home improvement outlet, washed it thoroughly, and screened it through a 1/4 inch hardware cloth. This gave me a good distribution of aggregate size. I chose not to use the "washed" sand I bought at the local home improvement outlet as it was still "dirty." I chose to use clean silica blast media. Lastly, I added glass microspheres to fill in any voids left in the aggregate mix. Most other formulas do not use microspheres, but rely on the variation in sand particle sizes. The white surface on my polymer concrete is some of the microspheres that floated to the surface during the curing process. Be sure to use a good marine grade. low viscosity, epoxy resin to get good wetting of the aggregate mix. I used 16% by weight of epoxy in my polymer concrete mix. I cast a piece of polymer concrete about 1 inch thick in a plastic bucket and after curing broke the piece in half with a sledge hammer. The polymer concrete broke cleanly through the aggregate with no epoxy bonding failure.

Aggregate formula by weight:

Glass Microspheres 2.94% Silica Blast Media 13.6%
Crushed Granite 31.36% Screened Crushed Gravel 36.45%
Small Round River Rocks 15.65%

Polymer Concrete Mix:

Aggregate 84%
Low Viscosity Epoxy Resin 16%

[gtiworks]

As an engineer by training, I chose to follow the tradition of filling machinery bases with polymer concrete as it makes good sense to me from a strength and vibration dampening point of view. I do not have test results as my mill was never used in its original "as delivered" state. I immediately dissembled the whole mill for inspection, cnc conversion, and polymer concrete treatment. The best discussion on polymer concrete for mill bases and towers is given in the CNC Cookbook:

CNCCookbook: Epoxy Granite Fill

[johnohara]

Thank you for the pictures. Very nice work.

~john

[FannBlade]

subbed for updates

[philbur]

Unfortunately the central compression bolt and the high tensile hold down studs will add nothing to the rigidity of the column. They will increase the strength, but then ultimate strength is not really an issue.

Phil

[gtiworks]

Because of the paucity of bolts clamping the tower to the base, the first issue for me is the stiffness of the joint between the tower and base. The central bolt will increase joint stiffness for the assembled mill. Additionally, filling base and the lower portion of the tower with polymer concrete will increase the rigidity of the base and the lower portion of the tower.

This makes the best of the as-manufactured square column mill capabilities.

One will never be able to increase the rigidity and stiffness of these mills to that of a typical Bridgeport type mill which weight 4 to 6 times as much.

[philbur]

As an engineer have you calculated the % improvement in overall rigidity of each of the modifications. Whether it "makes the best of it" is dependent on the degree of effort and cost in comparison with the degree of improvement.

Phil

Because of the paucity of bolts clamping the tower to the base, the first issue for me is the stiffness of the joint between the tower and base. The central bolt will increase joint stiffness for the assembled mill. Additionally, filling base and the lower portion of the tower with polymer concrete will increase the rigidity of the base and the lower portion of the tower.

This makes the best of the as-manufactured square column mill capabilities.

One will never be able to increase the rigidity and stiffness of these mills to that of a typical Bridgeport type mill which weight 4 to 6 times as much.

[gtiworks]

The tower/base joint is 185mm long and the bolts are 16mm in diameter and from bolted joint theory, the optimum number of bolts for this joint is 4 bolts on each side of the tower to achieve maximum practical tower/base clamping stiffness. Since these mills come with only 2 bolts on each side, they are far from their maximum practical joint stiffness and adding the large central tower bolt and replacing the cap screws with through bolted studs is an upgrade to overall joint stiffness. The total cost of this up grade is about 2% of total mill cost. You do the math and decide if the polymer concrete and through bolted studs are worth it for you.