Posts Tagged ‘machining’

A lot of what we offer here on the “DIY” blog is material information; however, as the hobbyist, you actually have to cut, drill, and machine the plastic yourself. It’s beneficial to educate yourself on practical fabrication matters. Most hobbyists grow up working with metal and wood; however, industrial plastic is only usually encountered when one knows to look for them – usually when one is a teenage “DIYer”.

The prevalence of CNC machines in recent years makes machining knowledge even more critical. In the video linked below, Instagrammer Tom Zelickman educates on issues he’s had with machining plastics and how to overcome various challenges including his stressing about one “aggressively” machining plastics and cleaning them.

You can view the whole video on YouTube here:

For those not very familiar with plastics, it is sometimes difficult to tell the plastics apart. A plastic rod feels like, well, just plastic and we don’t consider sometimes the nuances of each material. One major consideration for those of us in the DIY community is how a material is fabricated or machined. Acetal is a very hard plastic and machines very well and can be held to tight tolerances (+/- 0.005″) whereas realistically (+/- 0.05″) is the best you can get out of UHMW in a DIY setting. UHMW is much softer and less dimensionally stable; however, it is slicker and more economical.

A short video on YouTube we found shows both plastics being machined and offers a comparison, take a look below:

Often members of the DIY community are unsure how to machine engineering plastics. It’s one of the main questions that comes up. Redwood Plastics and Rubber has a handy sheet that gives both tooling and run speeds for various common industrial plastics. You can see those below:

 

 

 

Quadrant Engineering Plastics is a major manufacturer of industrial plastics. While they do not sell directly to end-users, and instead sell through distribution, they still invest in many resources for end-users to help them reach their goal with their applications. One of the best is the “Machinists Toolkit” which is available by clicking here. This gives a variety of tips from what coolant to use, to tool tips and even troubleshooting specific issues that come up. It also gives feed rates and end milling/slotting guides for various plastics.

One handy section the toolkit has is a rating of the machinability of various plastics on a rating system. Acetal is usually the best where tight, critical tolerances are involved. But like with all plastics, there are situations where acetal is not ideal. In those cases when reviewing the various options you need to know what the “next best option” is. More useful still, is on the left side of the toolkit page is a link to the chemical resistance chart. This will allow you to look up alphabetically various plastics and their resistance to various chemicals. Please do not think that just because plastics in general are resistant to many chemicals, that a given plastic will be resistant to chemical exposure in your application! And do not assume that just because a chemical is “household” that it will not attack your plastic – it might, so do not assume!

Finally, Quadrant has also released a short video giving some machining tips. While the video says it is geared for a few high performance materials, much of the advice given is relevant to any home machinist working with plastics. That video can be seen here:

 

UHMW polyethylene is probably the most popular engineering plastic for use in DIY projects. But how do you work with it? People often get overly concerned with fabrication, especially in regards to the “tolerances” needed for most home projects, but regardless here are some tips to get the most out of your UHMW application.

Sawing:

UHMW can be cut with either a circular or band saw. A band saw is best as it will vent heat away from the plastic and allow for faster cutting speeds. The band saw blades may be 10-30mm wide and about 1-2mm thick with the circular pitch between 3 and 10mm. To prevent the saw blades from becoming jammed, the teeth must be set at approximately 0.5mm. When using circular saws, saw blades with a minimum setting 0.5mm are also preferred. Normal cutting speed for band saws is 1,000-2,000m/min and for circular saws, 3,000-4,000 m/min.

Drilling:

Lower RPM drilling is recommended unless compressed air, water, or cooling oils are used – UHMW melts easily. Twist drills are most commonly used but pointed drills and circular cutters can be used for higher diameter holes.

Welding

Because of its high melt viscosity, friction and butt welding are the only practical methods for joining Redco UHMW by welding.

Machining

Machining is the principal method used to fabricate finished parts from UHMW. UHMW can be sawed, turned, planed, milled drilled, stamped and welded easily on woodworking or metalworking machines. The following general directions should be observed in these operations: To obtain surfaces of high quality, tools should always be sharp. For the most purposes, normal tool steel is satisfactory through many fabricators use special steels.

The optimum cutting speed is between 250 and 1,000 m/min. At lower cutting speeds cooling is not required, but at higher cutting speed range, water cooling or the use of soluble cutting oil is essential. In all cases, care must be taken to avoid heat build-up in the machining operation, so that the work piece does not smear the cutting edges. In milling and turning, the feed should not be too fast and the depth of the cut should be greater than 0.3mm.

Bonding

Questions about solvent bonding are common but unfortunately this is not recommended with UHMW. It resists most solvents and, at best, will create a below-average to poor bond. Mechanical fixation is recommended wherever possible.

drill_close_up

 

There are a lot of flashy “do it yourself” projects that hobbyists get in to with plastic but one application that keeps coming up that you might not think of is the simple gear. The general reasoning to use a plastic gear as a replacement for a metal gear make intuitive sense: the plastic gear will often be mated to a metal component and the fact that you have a plastic on metal connection now should greatly increase the life of the mating components. The problem is hobbyists do not really know what to use: we’ve heard of HDPE, UHMW, nylon and even polyurethane be requested by the DIY community for a home made gear application. But how suitable are these plastics for the application?

Actually – not very. Most plastics cannot be machined to tolerances as tight as metal be but many plastics, especially the polyolefins such as HDPE and UHMW, are very soft and could have the gear teeth quickly lose their shape once that gear starts working. Nylon is better, but it cannot take much impact at all – especially in the cold – meaning it is considered to be fragile. The best material for the application is actually acetal. Acetal is a very hard plastic that machines very well and holds excellent tolerances. It is widely available in rod stock from plastics distributors across North America. The gear in the picture below is made of white, homopolymer acetal. Black copolymer acetal is also widely available and would work well too – its properties are slightly worse than homopolymer in some respects but it is also less expensive.

Acetal

 

 

Home machinists are often bewildered by the wide variety of plastics available. Each plastic has its own advantages and disadvantages. Two that seem similar to the uninformed are acetal and UHMW. However, as the YouTube video we found shows – machining the two materials is very different. Acetal is extremely hard and machines to close tolerances without “gumming” up lathes. UHMW on the other is soft and is difficult to machine and suffers from thermal expansion. Both materials are impervious to moisture and come in FDA compliant forms. Price is a major factor between the two as UHMW is much more economical and this means for projects that do not require maximum precision, UHMW will usually perform very well. An example “diy” UHMW application would be sled tracks, popular in Arctic locations.

Acetal, as you might expect, performs better in precise – typically small – parts. Acetal is also extremely hard and can replace metals (and is often marketed for that reason), such as in the case of the paintball marker bolts highlighted in one of our recent blog postings. For more information check out these acetal and UHMW website sub-pages and watch this video that shows some of the machining differences: