Two quotes come back for the same acrylic panel. One is $11 a part, the other is $19. Nothing on the drawing changed between them, so the natural assumption is that the cheap shop is just more efficient. Then the first batch arrives, half the parts have hairline cracks around the mounting holes, and a call to the shop reveals they ran the job in extruded acrylic because “it looks the same” and nobody specified otherwise.
Acrylic hides its problems better than metal does until the part is in your hands. It’s optically clear, so people assume machining it is straightforward. It isn’t. The material stresses easily, cracks around drilled or tapped features if a shop doesn’t compensate for it, and comes in two base forms that machine and price very differently. Most of the quote variance and the field failures trace back to a handful of decisions a buyer never asked about.
This is what to check before you hand your drawing to a new acrylic machining shop, and where a quote is quietly cutting a corner you’ll notice later.
Why Two Acrylic Quotes Can Differ by 40% on the Same Drawing
The biggest swing usually comes from which base material got quoted. Cast acrylic is poured and cured slowly, which gives it more uniform internal structure and better resistance to stress cracking when you drill, tap, or machine tight features into it. Extruded acrylic is pushed through rollers at speed, runs 20 to 30% cheaper per sheet, and machines fine for simple flat cuts, but it’s more prone to crazing and cracking the moment you introduce a hole, a sharp corner, or a threaded feature.
A shop quoting the cheaper number without asking about your application may simply be pricing extruded stock. That’s a reasonable choice for a flat display panel with no fasteners. It’s a bad one for a housing with mounting holes that will see repeated assembly and disassembly.
Cast or Extruded: Decide Before You Ask for a Price
| Property | Cast Acrylic | Extruded Acrylic |
| Relative Cost | 20–30% higher | Baseline |
| Optical Clarity | Higher, more consistent | Slightly lower, batch variation |
| Crack Resistance (holes/threads) | Better | Weaker, prone to crazing |
| Machining Precision | Holds tighter tolerance | More prone to drift/warping |
| Best For | Housings with holes, threaded features, optical parts | Flat panels, signage, low-stress covers |
If your part has any drilled, tapped, or press-fit feature, specify cast acrylic on the drawing rather than leaving it to the shop’s default stock. That single line saves the argument about who’s responsible when a part cracks in the field.
What Optical Clarity Actually Costs
As-machined acrylic typically comes off the tool at Ra 1.6 to 3.2 µm, which is fine for housings and structural parts but leaves visible tool marks on any surface people will look through. Getting genuine optical clarity means a polishing pass afterward, and that’s a separate line item, not something that happens automatically because the material is clear.
If the part is a light-diffusing panel, a lens, or a display cover where surface haze matters, say so explicitly and ask whether polishing is quoted separately or bundled. A quote with no mention of surface finish, on a part where clarity clearly matters, usually means the shop is planning to ship it as-machined and let you find out.
Does the Tolerance You Specified Match What the Part Needs?
Standard tolerance on acrylic runs about ±0.05mm to ±0.10mm depending on part geometry and size, with hole diameters typically held to around ±0.05mm. Shops with the right fixturing and finishing passes can push toward ±0.005mm to ±0.1mm on critical features, but tighter tolerance on acrylic costs more than the equivalent tightening on metal, because the material moves more during cutting and needs slower finishing passes and more frequent inspection to hold the line.
Before tightening a callout below the standard band, check whether the mating part actually requires it. A cosmetic panel edge rarely does. A snap-fit or press-fit feature usually does.
Seven Things to Confirm Before You Send the Drawing
- Cast or extruded acrylic, named explicitly, not left as “acrylic” on the quote.
- Minimum wall thickness the shop will commit to. Most competent shops hold at least 1.0mm without a design review flag.
- Whether threaded holes are cut directly into the acrylic or use heat-set inserts (direct threading is a common source of stripped or cracked holes).
- As-machined or polished finish, and which one the quoted price assumes.
- Standard tolerance band, and the cost difference for tightening it.
- Whether the shop runs dedicated fixturing for acrylic or reuses generic metal setups.
- Lead time at your actual quantity, not a generic range.
Where Acrylic Parts Actually Fail
Cracking rarely happens where people expect. It shows up around drilled holes, sharp internal corners, and threaded features, because those are the spots where stress concentrates in a material that doesn’t flex much before it fails. A shop with real experience machining cnc machined acrylic parts will already have low-vibration fixturing, sharp single-flute tools, and generous corner radii worked into their process instead of running acrylic through a setup built for aluminum and hoping it holds.
Threads are the other common failure point. Cutting threads directly into acrylic works for a handful of light-duty assemble/disassemble cycles, but it strips or cracks faster than a heat-set brass insert would. If your part gets assembled more than once or twice, ask for inserts instead of direct threading.
Lead Times, Volume, and Rush Charges
Prototype quantities on acrylic typically ship faster than production runs because there’s less finishing and inspection involved, but rush charges on acrylic tend to run higher than on metal, since the finishing steps (polishing, annealing, stress relief) don’t compress well under time pressure. A quick-turn request might add 15 to 25% to the price; a true rush can add considerably more. If a shop quotes rush pricing without mentioning finish or annealing time, ask what got skipped to hit that date.
If your product also needs metal brackets or an injection-molded housing alongside the acrylic panel, sourcing everything from one custom CNC machining supplier instead of juggling separate vendors for each material removes a coordination step you don’t need, especially when the parts have to fit together on assembly day.
Signs a Quote Is Cutting a Corner
A quote for “acrylic” with no material grade named is the first sign. It’s either an oversight or room to substitute extruded stock without telling you.
A quote with no finish specified, on a part where appearance or clarity obviously matters, is the second. Clear plastic doesn’t polish itself.
A quote that treats a threaded, drilled, or snap-fit feature exactly like a flat panel, with no comment on stress relief or insert options, is the third. That’s usually the shop that hasn’t machined much acrylic beyond simple sheet cutting.
What to Ask Before You Commit
You don’t need to know acrylic’s material science to buy it well. You need four answers: which base material, what finish, what tolerance, and how threaded or drilled features get handled. A shop that answers all four without prompting has run enough acrylic jobs to know where they fail. One that just quotes a price per square inch hasn’t, and you’ll find out which one you picked when the parts show up.
FAQs
Is cast or extruded acrylic better for CNC machining?
Cast acrylic holds tolerance better and resists cracking around drilled or threaded features, making it the better choice for anything beyond a simple flat panel. Extruded acrylic costs 20 to 30% less and works fine for signage or covers with no holes or fasteners.
What tolerance can you expect on CNC machined acrylic parts?
Standard tolerance runs about ±0.05mm to ±0.10mm depending on part size and geometry, with hole diameters typically around ±0.05mm. Tighter tolerances down to roughly ±0.005mm on critical features are achievable with the right fixturing, at a cost premium over the standard band.
Can you tap threads directly into acrylic?
You can, but it holds up poorly beyond a few assembly cycles because acrylic doesn’t flex before it stresses and strips. For anything that gets assembled and disassembled more than once or twice, a heat-set metal insert is the more reliable option.
How do you get optical clarity on a machined acrylic part?
As-machined acrylic comes off the tool at roughly Ra 1.6 to 3.2 µm, which shows visible tool marks. Genuine optical clarity requires a separate polishing pass, so specify it explicitly rather than assuming it’s included because the material itself is transparent.
Why does acrylic crack during or after machining?
Cracking concentrates around drilled holes, sharp internal corners, and threaded features because stress builds up faster than the material can absorb it. Using cast rather than extruded stock, generous corner radii, and the right fixturing reduces the risk significantly.
What’s a normal lead time for custom acrylic parts?
Prototype quantities often ship faster than production runs since less finishing is involved, but rush charges on acrylic tend to run higher than on metal parts because steps like polishing and stress relief don’t compress well under time pressure.
Does part thickness affect what tolerance a shop can hold?
Yes. Thinner sections are more prone to vibration and deflection during cutting, while very thick sections risk heat-related stress if machined too aggressively. A shop should adjust cutting parameters and expected tolerance based on your actual wall thickness, not quote one number for every job.
